Shane M
oran
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TURUN YLIOPISTON JULKAISUJA – ANNALES UNIVERSITATIS TURKUENSIS
SARJA – SER. AI OSA – TOM. 731 | ASTRONOMICA – CHEMICA – PHYSICA – MATHEMATICA | TURKU 2025
WHERE ARE THE STARS OF 
YESTERYEAR?
A Reflection on Astrophysical Transience
Shane Moran
WHERE ARE THE STARS OF
YESTERYEAR?
A Reflection on Astrophysical Transience
Shane Moran
 
 
 
 
 
TURUN YLIOPISTON JULKAISUJA – ANNALES UNIVERSITATIS TURKUENSIS 
SARJA – SER. AI OSA – TOM. 731 | ASTRONOMICA – CHEMICA – PHYSICA – MATHEMATICA | TURKU 2025 
University of Turku
Faculty of Science
Department of Physics and Astronomy
Astronomy
Doctoral Programme in Exact Sciences (EXACTUS)
Supervised by
Doc. Rubina Kotak
Department of Physics and Astronomy
University of Turku
Turku, Finland
Prof. Seppo Mattila
Department of Physics and Astronomy
University of Turku
Turku, Finland
Assist. Prof. Morgan Fraser
School of Physics
University College Dublin
Dublin, Ireland
Reviewed by
Dr. Stacey Habergham-Mawson
Astrophysics Research Institute
Liverpool John Moores University
Liverpool, United Kingdom
Dr. Heidi Korhonen
Technical Departments
Max Planck Institute for Astronomy
Heidelberg, Germany
Opponent
Dr. Jonathan Mackey
Dunsink Observatory
Dublin Institute for Advanced Studies
Dublin, Ireland
The originality of this publication has been checked in accordance with the University
of Turku quality assurance system using the Turnitin OriginalityCheck service.
ISBN 978-952-02-0052-7 (PRINT)
ISBN 978-952-02-0053-4 (PDF)
ISSN 0082-7002 (PRINT)
ISSN 2343-3175 (ONLINE)
Painosalama, Turku, Finland, 2025
In memory of David Tims
UNIVERSITY OF TURKU
Faculty of Science
Department of Physics and Astronomy
Astronomy
MORAN, SHANE: Where Are the Stars of Yesteryear?
Doctoral dissertation, 104 pp.
Doctoral Programme in Exact Sciences (EXACTUS)
January 2025
ABSTRACT
This thesis concerns a number of kinds of astrophysical transients, including inter-
acting supernovae (SNe), gap transients and tidal-disruption events. There are three
papers included and they address differing phenomena, each of which has implica-
tions for stellar evolution more broadly.
The first paper is a detailed study of the long-lived slowly evolving type IIn
SN, SN 2017hcc. The paper covers a five-year follow-up campaign, with extensive
optical and near-infrared imaging and spectroscopy. The object was very bright,
peaking at−20.78± 0.01 mag in ATLAS 𝑜 band after an eight-week rise, suggesting
significant interaction between the ejecta and circumstellar material. The paper also
covers evidence for pre-existing dust as well as new dust formation. The second
paper is a study of the intermediate-luminosity red transient (ILRT), AT 2022fnm.
This ILRT displayed certain features consistent with luminous red novae (LRNe) and
the overlapping observational features of ILRTs and LRNe are discussed. The third
paper concerns the fast-evolving transient SN 2017fwm/Gaia17byh. This object had
an extremely fast rise time (≲5 days) as well as a small ejecta mass (0.01M⊙). One
particularly interesting feature of this object is that it displayed a peculiar velocity
of approximately −5000 km s−1. A number of origin scenarios for SN 2017fwm are
explored in the paper, including its being an ultrastripped SN and its resulting from
the tidal disruption of a white dwarf.
Additionally, upcoming astronomical surveys are discussed as well as the future
of astronomical modelling and multi-messenger astronomy.
v
TURUN YLIOPISTO
Matemaattis-luonnontieteellinen tiedekunta
Fysiikan ja tähtitieteen laitos
Tähtitiede
MORAN, SHANE: Where Are the Stars of Yesteryear?
Väitöskirja, 104 s.
Eksaktien tieteiden tohtoriohjelma (EXACTUS)
Tammikuu 2025
TIIVISTELMÄ
Tämä väitöskirja käsittelee useita eri tyyppisiä astrofysikaalisia transientteja, mukaan
lukien vuorovaikuttavat supernovat (SN), gap-transientit ja tähtien hajoamiset mus-
tan aukon vuorovesivoimien vaikutuksesta (TDE, tidal-disruption event). Väitöskir-
jaan sisältyy kolme artikkelia, jotka käsittelevät erilaisia ilmiöitä, joilla on vaikutuk-
sia tähtien kehitykseen laajemmin.
Ensimmäinen artikkeli on yksityiskohtainen tutkimus pitkäikäisestä, hitaasti
kehittyvästä tyypin IIn SN:sta, SN 2017hcc. Artikkeli kattaa viiden vuoden seu-
rantakampanjan, johon sisältyy laajaa optisen ja lähi-infrapuna-alueen kuvan-
tamista ja spektroskopiaa. Kohde oli erittäin kirkas, saavuttaen huippu kirkkauden
−20.78± 0.01 mag ATLASin 𝑜-kaistalla kahdeksan viikon nousun jälkeen, mikä
viittaa merkittävään vuorovaikutukseen ulosvirtaavan ja ympäröivän aineen välillä.
Artikkeli käsittelee myös todisteita jo olemassa olleesta pölystä sekä uuden pölyn
muodostumisesta. Toinen artikkeli on tutkimus keskikirkkaasta punaisesta tran-
sientista (ILRT, intermediate-luminosity red transient), AT 2022fnm. Tämä ILRT
osoitti tiettyjä ominaisuuksia, jotka olivat yhteensopivia kirkkaiden punaisten novien
(LRN, luminous red nova) kanssa, ja artikkelissa käsitellään ILRT ja LRNe havain-
tojen yhteneväisyyksiä. Kolmas artikkeli käsittelee nopeasti kehittyvää transienttia
SN 2017fwm/Gaia17byh. Tällä kohteella oli poikkeuksellisen nopea nousuaika (≲5
päivää) sekä pieni ulosvirtausmassa (0.01M⊙). Yksi erityisen mielenkiintoinen
piirre tässä kohteessa oli sen poikkeuksellinen nopeus ∼ −5000 km s−1. Artikke-
lissa tutkitaan useita SN 2017fwm:n syntyskenaarioita, kuten ultrastripped SN, tai
valkoisen kääpiön aiheuttama TDE.
Lisäksi työssä pohditaan tulevia tähtitieteellisiä havainto-ohjelmia sekä tähtiti-
eteellisen mallinnuksen ja moni-ilmaisinhavaintojen tulevaisuutta.
vi
Acknowledgments
My sincere thanks to my supervisors, Rubina Kotak, Morgan Fraser and Seppo Mat-
tila, for their invaluable guidance on research and academic matters throughout the
PhD process. I would also like to thank the staff and students at the Nordic Optical
Telescope in La Palma for an unforgettable time.
I wish to thank my pre-examiners, Stacey Habergham-Mawson and Heidi Ko-
rhonen, for their careful reading of the manuscript, as well my opponent, Jonathan
Mackey, for his willingness to take part in my defence. The supernova group in
the School of Physics at University College Dublin and the supernova group at the
University of Turku also have my gratitude for very productive discussions and as-
sistance. I also thank Christian Vassallo for translating the abstract of this thesis.
I acknowledge the Magnus Ehrnrooth Foundation and the Vilho, Yrjö and Kalle
Väisälä Foundation for providing financial support for the research contained in this
thesis.
Finally, I must thank my friends and family, in particular, my mother, Teresa,
whose unwavering encouragement I have cherished.
17th December 2024
Shane Moran
vii
Table of Contents
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii
Table of Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii
Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . x
List of Original Publications . . . . . . . . . . . . . . . . . . . . . . xi
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Stellar Evolution . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Mass Loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3 Core-Collapse Supernovae . . . . . . . . . . . . . . . . . . . 7
1.3.1 Power sources . . . . . . . . . . . . . . . . . . . . . . 9
1.3.2 Nucleosynthesis . . . . . . . . . . . . . . . . . . . . . 9
1.3.3 The P-Cygni profile . . . . . . . . . . . . . . . . . . . 11
1.3.4 Dust . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
1.4 Layout of the Rest of the Thesis . . . . . . . . . . . . . . . . 13
2 Classical Type IIn Supernovae: Iron Core-Collapse Super-
novae within Hydrogen-Rich Circumstellar Material . . . . 14
2.1 Astrophysical Interpretation . . . . . . . . . . . . . . . . . . . 17
2.1.1 Environments . . . . . . . . . . . . . . . . . . . . . . 18
2.2 Type IIn-P Supernovae . . . . . . . . . . . . . . . . . . . . . 19
2.2.1 SN 2020nub . . . . . . . . . . . . . . . . . . . . . . . 20
3 Weak Explosions within Dense Circumstellar Material . . . 23
3.1 Luminous Red Novae . . . . . . . . . . . . . . . . . . . . . . 23
3.2 Intermediate-Luminosity Red Transients . . . . . . . . . . . . 26
4 Fast-Evolving Transients . . . . . . . . . . . . . . . . . . . . . . 30
4.1 Peculiar Stripped-Envelope Supernovae . . . . . . . . . . . 30
4.1.1 Calcium-dominated supernovae . . . . . . . . . . . . 30
4.1.2 Ultrastripped supernovae . . . . . . . . . . . . . . . 31
4.2 Tidal-Disruption Events . . . . . . . . . . . . . . . . . . . . . 33
viii
5 Summary of the Papers . . . . . . . . . . . . . . . . . . . . . . . 37
5.1 Paper I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
5.2 Paper II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
5.3 Paper III . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
6 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
6.1 Surveys and Facilities . . . . . . . . . . . . . . . . . . . . . . 40
6.2 Modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
6.3 Multi-Messenger Astronomy . . . . . . . . . . . . . . . . . . 41
List of References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
Original Publications . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
ix
Abbreviations
4MOST 4-metre Multi-Object Spectrograph Telescope
AGB Asymptotic giant branch
BH Black hole
CCSN Core-collapse SN
CDS Cool/cold dense shell
CSM Circumstellar material
ECSN Electron-capture supernova
ILRT Intermediate-luminosity red transient
IMBH Intermediate-mass black hole
LRN Luminous red nova
LSST Legacy Survey of Space and Time
NOT Nordic Optical Telescope
NS Neutron star
sAGB Super-asymptotic giant branch star
SMBH Supermassive black hole
SN Supernova
SoXS Son of X-Shooter
TDE Tidal-disruption event
TiDES Time-Domain Extragalactic Survey
USSN Ultrastripped supernova
WD White dwarf
ZAMS Zero-age main sequence
x
List of Original Publications
This dissertation is based on the following original publications, which are referred
to in the text by their Roman numerals:
I Moran, S., Fraser, M., Kotak, R., Pastorello, A., Benetti, S., Brennan, S. J.,
Gutiérrez, C. P., Kankare, E., Kuncarayakti, H., Mattila, S., Reynolds, T.
M., Anderson, J. P., Brown, P. J., Campana, S., Chambers, K. C., Chen, T.-
W., Della Valle, M., Dennefeld, M., Elias-Rosa, N., Galbany, L., Galindo-
Guil, F. J., Gromadzki, M., Hiramatsu, D., Inserra, C., Leloudas, G., Müller-
Bravo, T. E., Nicholl, M., Reguitti, A., Shahbandeh, M., Smartt, S. J.,
Tartaglia, L., Young, D. R. A long life of excess: The interacting transient
SN 2017hcc. Astronomy & Astrophysics, 2023; volume 669, A51.
II Moran, S., Kotak, R., Fraser, M., Pastorello, A., Cai, Y.-Z., Valerin, G.,
Mattila, S., Cappellaro, E., Kravtsov, T., Gutiérrez, C. P., Elias-Rosa, N.,
Reguitti, A., Lundqvist, P., Filippenko, A. V., Brink, T. G. and Wang, X.-
F. Red eminence: the intermediate-luminosity red transient AT 2022fnm.
Astronomy & Astrophysics, 2024; volume 688, A161.
III Fraser, M., Moran, S., Kotak, R., Benetti, S., Jonker, P., Levan, A., Kun-
carayakti, H., Lyman, J., Moriya, T., Pastorello, A., Smartt, S., Tucker, B.,
Wyrzykowski, L. Going nowhere fast: an ultra-stripped supernova with a
large velocity offset from its host. To be submitted to Astronomy & Astro-
physics, 2025.
The original publications have been reproduced with the permission of the copyright
holders.
xi
1 Introduction
Transient astronomical phenomena (transients) are objects that appear in the sky and
then disappear or fade away, lasting between milliseconds and years. They include
asteroids and comets, as well as high-energy events such as supernovae (SNe) and
tidal-disruption events (TDEs).
In the current era transients are typically detected by wide-field surveys such
as the Asteroid Terrestrial-impact Last Alert System (ATLAS, Tonry et al. 2018;
Smith et al. 2020), the Zwicky Transient Facility (ZTF, Bellm et al. 2019), the
Gravitational-Wave Optical Transient Observer (GOTO, Gompertz et al. 2020; Steeghs
et al. 2022) and the All-Sky Automated Survey for Supernovae (ASAS-SN, Shappee
et al. 2014; Kochanek et al. 2017).
An overview of the parameter space of transient phenomena is offered in Fig.
1, with a plot of timescale versus peak optical luminosity, displaying their varia-
tion. In this thesis I will only focus on a small subset of the objects in the diagram,
namely interacting core-collapse SNe (CCSNe), Ca-dominated SNe1, ultrastripped
SNe, TDEs, intermediate-luminosity red transients (ILRTs) and luminous red novae
(LRNe). SNe are typically divided into two primary groupings: type I (hydrogen-
poor SNe) and type II (hydrogen-rich SNe) (Minkowski, 1941). Further division
takes place in accordance with additional spectroscopic and photometric observa-
tional features. The so-called "gap transients" occupy the luminosity range between
core-collapse SNe, discussed in Section 1.3, and classical novae. The gap transients
are a diverse group, with various physical origins, but they display some overlap
in terms of duration and luminosity. They include the aforementioned ILRTs and
LRNe, weak explosions within dense CSM, both of which will be further discussed
in Chapter 3, as well as the Ca-dominated SNe, to be discussed in Chapter 4.
Transients are not studied merely for their own sake: many are connected to
the formation of compact objects and are a key source of nucleosynthesis in the
universe, chemically enriching the interstellar medium (Thielemann et al., 2011).
Additionally, some can be used as cosmological distance indicators (Kirshner and
Kwan, 1974). Transients can also be used to provide indirect evidence about the
nature of high-redshift stars (Tanvir et al., 2009).
1commonly referred to as "calcium-rich SNe"
1
Shane Moran
Figure 1. Diagram showing the parameter space of transient astrophysical phenomena. The time
for the luminosity to decrease by 0.1 dex is shown on the x axis and the peak optical luminosity is
shown on the y axis. Only some of the categories shown will be addressed in this thesis. Source:
Cai et al. (2022a).
1.1 Stellar Evolution
In Fig. 2, I present the Hertzsprung-Russell diagram, which shows the relation be-
tween the luminosity of stars and their temperatures, where the luminosity can be
expressed as,
𝐿 = 4𝜋𝑅2𝜎𝑇 4 (1)
where 𝜎 is the Stefan-Boltzmann constant, 𝑅 is the stellar radius, and 𝑇 is the ef-
fective temperature. The Hertzsprung-Russell diagram groups stars into different
spectral classes depending on their temperatures. It is clear that in plotting the stars
we observe on this diagram, a structure appears. The structure is indicative of the
various phases of stellar evolution. In particular, we can see a roughly diagonal
structure extending from near the upper left (hot and luminous) to the bottom right
(cool and faint). This is the main sequence, the largest population of stars; it is the
area on the HR diagram occupied by stars through most of their lives, extending from
the point at which they initially begin nuclear fusion until approximately 10% of the
original hydrogen content of the star has been converted to helium (Hansen et al.,
2004; Carroll and Ostlie, 2017). Stars are often referred to in terms of the zero-age
2
Introduction
Figure 2. The Hertzsprung-Russell diagram from Gaia DR2, a plot of stars by their luminosity or
absolute magnitude against their colour or effective surface temperature. Adapted from
ESA/Gaia/DPAC original (CC BY-SA 3.0 IGO).
3
Shane Moran
main-sequence (ZAMS) mass, which is their mass when they first enter the main se-
quence, that is, when the fusion of hydrogen in the stellar core becomes the primary
source of energy (Kippenhahn et al., 2013).
The more massive the star, the less time it spends on the main sequence, and a
star’s position on the main sequence is governed by its mass, 𝑀 , which is coupled to
its luminosity, 𝐿, by the mass-luminosity relation,
𝐿 = 𝑀𝑎 (2)
where the exponent 𝑎 varies depending on the stellar mass (Kuiper, 1938). Taking the
mass-luminosity relation and comparing the energy released from the nuclear fusion
of 10% of a star’s hydrogen content with the main-sequence luminosity allows us to
estimate the time a star will spend on the main sequence (Hansen et al., 2004),
𝑡𝑀𝑆 ≈ 1010(𝑀⊙
𝑀
)𝑎−1 years (3)
As such, the Sun itself is expected to spend approximately 1010 years on the main
sequence. Once a star ceases to fuse hydrogen in its core, it begins to evolve off the
main sequence, and its mass will determine whether it becomes a supergiant, a red
giant or a white dwarf (WD) (Karakas, 2017). With the cessation of the fusion of
hydrogen in the core, the star contracts, heating the core and leading to sufficiently
high temperatures to initiate the fusion of helium (Karakas, 2017). In smaller stars we
instead get contraction until electron-degeneracy pressure re-establishes hydrostatic
equilibrium, where the outward force due to the radiation pressure is balanced with
the inward gravitational force,
𝑑𝑃
𝑑𝑟
= −𝐺𝑀(𝑟)𝜌
𝑟2
(4)
where 𝑟 is the radius, 𝐺 is the gravitational constant, 𝑀(𝑟) is the mass inside the
radius 𝑟, 𝜌(𝑟) is the density and 𝑃 (𝑟) is the pressure (Chandrasekhar, 1935; Karakas,
2017). Very low mass stars (≲0.23 M⊙, assuming solar metallicity) are fully con-
vective and evolve into helium (He) WDs after they finish burning hydrogen (Adams
and Laughlin, 1997; Kippenhahn et al., 2013). Low-mass stars (≲8 M⊙) ultimately
become carbon-oxygen WDs, intermediate-mass stars (∼ 8−10M⊙) eventually gen-
erate oxygen-neon WD cores before undergoing collapse via electron capture (to be
discussed further in Chapter 3), whilst massive stars (≳10 M⊙) instead end their lives
as CCSNe (which will be discussed in more detail in Section 1.3), leaving behind a
neutron star (NS) or black hole (BH) (Nomoto, 1984; Wheeler et al., 1998; Adams
and Laughlin, 1997; Karakas, 2017). Note that these terminal scenarios concern
single stars, whilst binarity can affect the endpoints of stellar evolution in different
manners (Nomoto, 1984). Though supergiants are a key phase in the evolution of
4
Introduction
massive stars before core-collapse, they are exceedingly rare on a population level,
as can be seen in Fig. 2.
As a massive star evolves and progressively burns heavier elements, it builds up
a number of onion-like layers, with lighter elements on the outside (Hansen et al.,
2004). This is because the core is denser and hotter, and so can fuse heavier elements
than the outer layers (Hansen et al., 2004). The composition of a massive star imme-
diately before core collapse can be seen in Fig. 3, but note that this omits the stellar
wind extending beyond the surface of the star and any circumstellar material (CSM)
lost earlier in the star’s evolution (Sieverding et al., 2021). More massive stars have
denser, hotter cores and so nuclear fusion takes place more rapidly, exhausting their
fuel at a higher rate and, as such, they move off the main sequence more quickly
(Adams and Laughlin, 1997).
1.2 Mass Loss
The effects of mass loss on stellar evolution can be profound, so it is important to
address the topic. In fact, massive stars often experience enhanced mass loss in the
months to centuries prior to a SN explosion, which can have important effects on
the photometric and spectroscopic evolution of the SNe (Fraser, 2020). Mass loss
can be wind-driven, but it can also result from binary interaction (Smith, 2014).
Wind-driven mass loss can itself be line-driven or continuum-driven (Owocki and
van Marle, 2008). The mass-loss rate, ?˙? , in the case of wind-driven mass loss with
a steady wind can be described by the following equations,
?˙? ∝ 𝐿𝑅
𝐺𝑀
(5)
?˙? = 4𝜋𝑅2𝜌𝑤𝜈𝑤 (6)
where 𝐿 is the luminosity, 𝑅 is the stellar radius, 𝐺 is the gravitational constant, 𝑀 is
the stellar mass, 𝜌𝑤 is the wind density and 𝜈𝑤 is the wind velocity (Reimers, 1975;
Blinnikov, 2017).
Metallicity has a profound effect on mass loss (Maeder and Meynet, 2000; Heger
et al., 2003). So-called line-driven mass loss is metallicity dependent: in this kind
of mass loss, radiation is absorbed and scattered by metals in the stellar atmosphere,
and the resultant momentum transfer leads to gas being unbound from the star and
lost (Lucy and Solomon, 1970). A somewhat simplified view is that line-driven mass
loss 𝑀𝑙𝑑 scales as follows,
𝑀𝑙𝑑 ∝ 𝑍0.5 (7)
where 𝑍 is the metallicity (Nugis and Lamers, 2000). In reality, stellar winds are not
homogeneous, the picture being complicated by the "clumpiness" of the wind; how-
ever, models that take such inhomogeneities into account produce results comparable
to the simpler models (Smith, 2014).
5
Shane Moran
Figure 3. Stellar profile of a 15 M⊙ star at core collapse. The upper panel shows the temperature
and density profile of the star, whilst the lower panel shows its mass fraction for a number of
important isotopes. Source: Sieverding et al. (2021).
Another form of mass loss is continuum-driven mass loss. In this case the winds
are near the Eddington limit or even super-Eddington, meaning that the star is not in
a state of hydrostatic equilibrium, the outward radiation pressure instead exceeding
the inward gravitational pressure (Humphreys and Davidson, 1984; Owocki et al.,
6
Introduction
2004). The Eddington limit is as follows,
𝐿𝐸𝑑𝑑 =
4𝜋𝑐𝐺𝑀𝑚𝑝
𝜎𝑇
(8)
where 𝑐 is the speed of light in a vacuum, 𝐺 is the gravitational constant, 𝑀 is the
stellar mass, 𝑚𝑝 is the mass of the proton and 𝜎𝑇 is the cross section for Thomson
scattering (Eddington, 1926; Branch and Wheeler, 2017). In such cases momentum
is transferred from the radiation to the gas via electron scattering (Humphreys and
Davidson, 1984; Owocki et al., 2004). Continuum-driven mass loss results in far
greater mass loss than in line-driven cases (Owocki and van Marle, 2008). This kind
of mass loss is believed to be the primary source of the giant eruptions in luminous
blue variables (LBVs; massive unstable stars that undergo violent mass loss), though
non-terminal hydrodynamic explosions may also account for some of these eruptions
(Humphreys and Davidson, 1994; Owocki and van Marle, 2008; Smith and Owocki,
2006).
Binary interaction has large effects on stellar systems. Stellar rotation rates can
be significantly altered by angular-momentum transfer: the star gaining mass will in-
crease in angular momentum, the accretion leading to it being "spun up" by the other
star, with the star losing mass correspondingly losing angular momentum (Langer
et al., 2003; Langer, 2012). Extreme mass loss can result through Roche-lobe over-
flow, which will be discussed further in Chapter 3 (Ivanova et al., 2013). Further-
more, even the nuclear burning in a star can be impacted by binary interaction (Pod-
siadlowski et al., 2004).
1.3 Core-Collapse Supernovae
Sufficiently massive stars (≳10 M⊙) end their lives as core-collapse SNe (see Fig. 4)
once the mass of iron-group elements occupying the core exceeds the Chandrasekhar
limit (approximately 1.44 M⊙, Heger et al. 2003). This is because the fusion of
elements heavier than those in the iron group is not energetically favourable (Janka
et al., 2007). The iron core is formed, but energy is still lost in the form of neutrinos
and through electron capture, the high densities leading to the capture of electrons
by nuclei, reducing the electron degeneracy pressure, which had been the primary
counterbalance to gravitational pressure (Janka et al., 2007). Additionally, the high
temperatures also lead to photodisintegration, with gamma rays leading to the decay
of the iron atoms through the emission of alpha particles and neutrons (Janka et al.,
2007).
As a result of these processes, the core no longer produces sufficient radiation
pressure to balance against gravitational pressure, and hydrostatic equilibrium is lost.
The core contracts, decoupling from the other layers of the star and falling until it
forms a proto-NS with a radius of approximately 30 km (Woosley and Janka, 2005).
7
Shane Moran
Figure 4. Representation of the steps from core collapse to SN explosion and nucleosynthesis.
First the core develops a gravitational instability (top left), then we have the core bounce leading to
the formation of the shock wave (top right). This is followed by the stalling of the shock wave
(middle left), only for the shock wave to be reinvigorated (middle right). The explosion takes place
(bottom left), with continuing formation of radioactive nickel as well as other kinds of
nucleosynthesis in the neutrino-driven wind (bottom right). Source: Janka et al. (2012).
The outer core falls towards the proto-NS that the inner core has become, collides
with it and is thrown outwards. The rebounding material collides with other material
that is still falling inwards, and a shock wave is generated. However, the outwards
8
Introduction
travelling shock wave is stalled before this material can be thrown off and if this stall
is not counteracted, the proto-NS collapses to become a BH (Myra and Burrows,
1990).
Alternatively, it is thought that the shock wave may be re-energised by neutrino-
driven convection with the proto-NS contracting to a radius of∼10 km and becoming
a NS whilst the explosion and concurrent nucleosynthesis (to be discussed below)
proceed (Woosley and Janka, 2005; Janka et al., 2012). Though the precise details of
the re-energising mechanism are not well understood, type II CCSNe release around
∼ 3×1053 erg of gravitational binding energy in the form of neutrinos over the course
of a few seconds during NS formation, which is two orders of magnitude higher than
the kinetic energy of the explosion (Bionta et al., 1987; Hirata et al., 1987; Burrows,
2000).
1.3.1 Power sources
SNe can be powered in a number of ways, including the decay of radioactive ele-
ments, interaction with CSM, energy release from a magnetar (a NS powered by an
extremely strong magnetic field) and fallback accretion onto a compact object re-
sulting from the explosion (Mereghetti et al., 2015; Kaspi and Beloborodov, 2017;
Kasen, 2017).
The initial electromagnetic emission from a SN results from shock breakout
(Waxman and Katz, 2017). This occurs when the shock-wave optical depth falls
to a point where photons can escape (Waxman and Katz, 2017). In the absence of
CSM the process lasts less than an hour (and perhaps just seconds), but the presence
of CSM can extend the duration to days (Waxman and Katz, 2017). The connection
between shock-CSM interaction and luminosity will be discussed further in Chapter
2.
As seen in Fig. 4, 56Ni is synthesised during the explosion process (Colgate and
McKee, 1969; Arnett, 1982; Janka et al., 2012). This has a half-life of six days and
decays to 56Co, which has a half-life of 77 days, itself decaying to 56Fe (Colgate and
McKee, 1969). High-energy photons are generated during the decay processes and
these photons deposit energy in the envelope (Colgate and McKee, 1969; Podsiad-
lowski, 2017).
1.3.2 Nucleosynthesis
The importance of CCSNe to nucleosynthesis was first brought to light by Burbidge
et al. (1957) in the so-called "B2FH paper" (following the initials of the authors).
The nuclear burning process in the progenitor star generates progressively heavier
elements. The SN explosion itself can generate yet heavier elements by means of the
rapid neutron-capture process (r-process). The r-process is the primary process for
9
Shane Moran
the generation of nuclei heavier than iron and is solely responsible for the produc-
tion of elements heavier than lead and bismuth (Thielemann et al., 2011, 2017). The
r-process involves the capture of neutrons by nuclei with sufficient rapidity that con-
secutive neutrons are captured before radioactive decay can take place (Thielemann
et al., 2011). In order for the process to occur, there must be far more neutrons than
seed nuclei (Thielemann et al., 2011).
The precise location in the SN where r-process nucleosynthesis actually occurs
remains unclear, though suggestions include a high-entropy neutrino-rich wind and
jets (Wheeler et al., 1998; Thielemann et al., 2011). Solar abundances of r-process
elements can be well reproduced in simulations of nucleosynthesis in high-entropy
neutrino winds (see Fig. 5; Kratz et al. 2014). Nevertheless, it is unclear whether
such entropies can be achieved in actual core-collapse scenarios, though electron-
capture SNe, to be discussed in Chapter 3, may be promising for the nucleosynthesis
of elements up to europium (Thielemann et al., 2017).
Figure 5. Plot of actual solar abundances of r-process elements compared with abundances
generated in simulations of nucleosynthesis in high-entropy winds. FRDM(1992) and FRDM(2012)
represent calculations based on two different nuclear-structure databases. 𝑌𝑒 is the electron
fraction of the wind, 𝑣𝑒𝑥𝑝 is its expansion velocity and 𝑆 is the radiation entropy in units of
𝑘𝐵/𝑏𝑎𝑟𝑦𝑜𝑛. Source: Kratz et al. (2014).
10
Introduction
1.3.3 The P-Cygni profile
Figure 6. Left: P-Cygni profile showing the blueshifted absorption from the portion of the shell
approaching the observer and the emission from the other portions. Right: a physical
representation of the different regions of emission and absorption in the shell, with labels
corresponding to the left part of the figure. Source: Carroll and Ostlie (2017).
A key observational feature observed in certain CCSNe is the P-Cygni profile
(see Fig. 6), a composite spectral feature where both emission and blueshifted ab-
sorption are seen (Kirshner et al., 1973). The feature is named after the LBV P Cygni,
which displays the prototypical example of the feature, the profile being visible in
almost all the spectral lines of the star (Israelian and de Groot, 1999; Humphreys and
Davidson, 1994; Kotak and Vink, 2006).
The absorption arises from the continuum passing through the cooler low-density
gaseous envelope moving towards the observer, whilst the emission instead origi-
nates from hot diffuse gas with a component moving perpendicular to the line of sight
(Lamers and Cassinelli, 1999; Carroll and Ostlie, 2017). SNe IIn, to be discussed in
Chapter 2), are a subgroup of type II (i.e. hydrogen-rich) SNe, and display narrow
(< 1,000 km s−1) hydrogen emission lines in their spectra owing to CSM interaction
(Schlegel, 1990). P-Cygni features in H𝛼, common in type II SNe, are sometimes
observed in SNe IIn, though the greater and more enduring the CSM interaction,
the less likely that such features will be seen as the photosphere will be concealed
(Chugai et al., 2004; Smith, 2017). The superluminous (peak absolute bolometric
magnitude <−21) type IIn SN 2006gy, for example, which peaked at least −21.8
in 𝑅 band, showed a P-Cygni profile with outflow velocities of −130 km s−1 in the
trough and −260 km s−1 at the blue edge, likely associated with dense unshocked
11
Shane Moran
CSM (Chomiuk et al., 2011; Smith, 2017).
1.3.4 Dust
Figure 7. Condensation temperatures for graphite, silicon carbide (SiC) and titanium carbide (TiC)
as a function of pressure. The effect of altering the C/O ratio is displayed in the case of graphite.
Solar ratios for Ti/O and Si/O are assumed with a C/O ratio >1.05. Source: Tielens et al. (2005).
Another indicator in observations of SNe that is important to consider is evidence
of the presence of dust as this can give us insight into the environment of the SN and
the evolutionary history of the progenitor star. Depending on the conditions of the
explosion, SNe dust can be pre-existing or newly formed. There can be pre-existing
dust from the interstellar medium or the progenitor star as well as dust condensation
in SN ejecta (Hoyle and Wickramasinghe, 1962; Lucy et al., 1989; Kotak et al.,
2009).
When the temperature of an element in the ejecta falls below its sublimation
temperature, then dust condensation can take place (Tielens et al., 2005). The precise
sublimation temperature depends on the kind of dust, the gas pressure and the C/O
ratio, as is illustrated in Fig. 7 (Lodders and Fegley, 1995; Tielens et al., 2005). Dust
can also form in the cool or cold dense shell (CDS, to be discussed in Chapter 2) that
forms at the interface between the ejecta and CSM in interacting SNe (Matsuura,
2017). Type IIn SNe, to be discussed further in Chapter 2 are significantly over-
represented (between around five and 20 times) when it comes to SNe presenting
12
Introduction
late-time near-infrared excesses, indicative of the presence of warm dust (Fox et al.,
2011; Eldridge et al., 2013; Li and Morozova, 2022).
Another factor to consider is pre-existing dust from the host galaxy itself, which
can obscure SNe, preventing them from being detected in the first place (Mattila
et al., 2012). Approximately 19% of SNe in the local universe are undetected at
optical wavelengths owing to high levels of extinction from dust, and this rises to
38% at a redshift of ∼1.2 (Mattila et al., 2012).
Photometry allows us to determine the amount of energy radiated by an event
as well as the colour and photospheric temperature and radius evolution (Dessart
et al., 2022). Light curves also reflect radioactive-nickel yields and ejecta properties
(Dessart et al., 2022). Spectroscopy gives insight into the chemical composition of
transient events whilst also allowing us to constrain their geometry both by means of
the velocities determined from emission and absorption lines, as well as by probing
asymmetries along the line of sight (Dopita and Tuohy, 1984; Leonard et al., 2000).
Considering these data together, we can start to build a clearer picture of the
formation of NSs and BHs and gain insight into the aforementioned nucleosynthesis
and dust production. For example, photometrically, dust formation will lead to a
near-infrared excess (i.e. more emission at near-infrared wavelengths than would be
expected, given the blackbody temperature) and a change in the decline rate of the
light curve (Lucy et al., 1989; Fassia et al., 2000). Spectroscopically, dust formation
can reveal itself by the apparent blueshifting of spectral emission lines, the redder
parts being obscured by the dust (Lucy et al., 1989; Gehrz and Ney, 1990; Wooden
et al., 1993). We can also develop a better understanding of Galactic structure and,
ultimately, cosmology.
1.4 Layout of the Rest of the Thesis
In Chapter 2 I discuss classical SNe IIn (iron core-collapse SNe within hydrogen-
rich CSM) and in Chapter 3 I describe weak explosions within dense circumstellar
material, in particular LRNe and ILRTs. In Chapter 4 I give an overview of peculiar
stripped-envelope SNe. In Chapter 5 I summarise the included papers and in Chapter
6 I address the future of the field and its challenges.
13
2 Classical Type IIn Supernovae: Iron
Core-Collapse Supernovae within
Hydrogen-Rich Circumstellar Material
As briefly addressed in Chapter 1, SNe IIn are a subgroup of type II (i.e. hydrogen-
rich) SNe which display narrow (< 1,000 km s−1) hydrogen emission lines in their
spectra as a result of CSM interaction (Schlegel, 1990). There is a great deal of
observational diversity in SNe IIn: they show a wide range of peak luminosities, and
their lifetimes vary significantly (Smith, 2017). The details of this heterogeneity will
be examined in the following subsections.
It should be noted that the precise delineation of SNe IIn can be somewhat un-
clear; there can be difficulty, for example, in disentangling SNe IIn and SNe Ia-
CSM (Smith, 2017), as they appear spectroscopically similar. Many SNe Ia-CSM
are thought to be thermonuclear SNe exploding into pre-existing CSM, hence their
designation, but this is not clear in all cases (Smith, 2017). Additionally, from a
certain perspective, SNe Ia-CSM could be considered to be a kind of SN IIn, as the
IIn designation is purely observational and SNe Ia-CSM display the required narrow
lines. Furthermore, any kind of SNe or even any kind of non-SN explosive outflow
could, in theory, appear as a IIn if the environmental features were appropriate for
the relevant narrow-line formation (Kotak et al., 2004; Smith, 2017). In this chapter I
will focus primarily on iron core-collapse SNe within hydrogen-rich CSM, and I will
address other phenomena that technically satisfy the classification criteria for SNe IIn
(namely, narrow hydrogen lines) in Chapter 3, as they are fundamentally different.
However, SNe IIn-P, which are another kind of event showing features of interaction,
are briefly addressed at the end of this chapter, along with the presentation of some
related preliminary data.
Estimates vary in the literature as to the precise rates of SNe IIn as a fraction of
CCSNe found in the local universe, with values ranging from 2.4%± 1.4% (Eldridge
et al. 2013) to 6.5% ± 2.5% (Li et al. 2011), but these values are highly sensitive to
the sample chosen, particularly given the overall rarity of SNe IIn (Cappellaro et al.,
2015).
Many SNe IIn display near-infrared excesses owing to the presence of dust (Ger-
ardy et al., 2002; Fox et al., 2011). Such excesses can be produced in multiple ways:
for example, pre-existing dust grains can scatter light emitted by the SN, reprocessing
14
Classical Type IIn Supernovae: Iron Core-Collapse Supernovae within Hydrogen-Rich
Circumstellar Material
Figure 8. Three SN IIn spectra at pre-peak, post peak, and late times, showing the typical
appearance of such objects at different points in their evolution. Note the presence of Balmer
emission lines in each case. The pre-peak spectrum shows a blue continuum, the post-peak
spectrum’s Balmer and helium emission lines display extended wings, whilst a pseudo-continuum
is present at late times. Source: Gal-Yam (2017).
it into redder wavelengths (Bode and Evans, 1979; Graham et al., 1983). It should be
noted that the formation of dust can also result in an increasing near-infrared excess.
Other signatures of dust formation are blueshifted line profiles and more rapid fading
of the optical continuum (Lucy et al., 1989; Gehrz and Ney, 1990). There is a great
deal of variation seen in the light curves of SNe IIn. Many display long-lasting and
slowly evolving light curves, as in the case of SN 2010jl, which remained visible for
years; however, others evolve far more quickly and can fade within months in the
optical, as in the case of SN 1998S (Fassia et al., 2000; Ofek et al., 2019).
The peak magnitude of IIn light curves, as well as their overall shape, can give
insight into the nature of the CSM in the environment of the SN. Indeed, many SNe
IIn are very luminous owing to the CSM interaction that powers their light curves
(Arcavi, 2017). SNe IIn peak luminosities cover a large range, spanning approxi-
mately six magnitudes (Arcavi, 2017). Additionally, pre-explosion outbursts, known
as SN impostors, are seen in many SNe IIn, arising from non-terminal mass ejections
(Reguitti et al., 2024).
We shall now address the nature of the spectra found in SNe IIn. As can be
15
Shane Moran
Figure 9. Three Gaussians fitted to the H𝛼 profile. Showing the characteristic narrow line, but also
a broad component arising from the fast-moving ejecta and an intermediate component with
multiple possible origins including emission from the CDS or photons from interaction being
scattered by thermal electrons, leading to broadening. Source: Ransome et al. (2021).
seen in Fig. 8, the spectra of SNe IIn show the presence of narrow (< 1,000 km s−1)
hydrogen lines by definition (Schlegel, 1990). These result from interaction, the
ejecta impacting slow-moving massive and dense CSM and the ionisation of the
CSM by the shock leading to the narrow emission (Chugai, 1997). Intermediate
emission lines (∼ 1,000 to 2,000 km s−1) can arise from electron scattering or from
emission from the CDS, whose formation is discussed below, whilst broad emission
(> 5,000 km s−1), when seen, instead arises from the fast-moving ejecta (see Fig. 9;
Filippenko 1997; Ransome et al. 2021).
The nebular phase occurs when the photosphere recedes in mass space suffi-
ciently to reveal the inner ejecta (Jerkstrand, 2017). In physical terms, this means
that the density is low enough for the formation of forbidden lines (Jerkstrand, 2017;
Inserra, 2019). Some SNe IIn, however, maintain CSM interaction in the long term,
never reaching the nebular phase (Smith, 2017).
16
Classical Type IIn Supernovae: Iron Core-Collapse Supernovae within Hydrogen-Rich
Circumstellar Material
Figure 10. A simplified diagram of the basic structure of interacting SNe (i.e. SNe IIn and Ibn,
where SNe Ibn, lacking hydrogen, instead show narrow helium lines). Zone 1 corresponds to the
pre-shock CSM, zone 2 to the shocked CSM, zone 3 to the shocked ejecta and zone 4 to the freely
expanding ejecta. FS - forward shock, RS - reverse shock. Source: Smith (2017).
2.1 Astrophysical Interpretation
SNe IIn constitute an observational class and a diverse one at that, but we can never-
theless attempt to provide a physical interpretation of these objects grounded in the
observational properties we have already addressed.
Given the presence of narrow lines in the spectra of SNe IIn, we can infer the
presence of an optically thin CSM that has been photoionised (Chugai, 1997). A
simplified view of the structure of an interacting SN is offered in Fig. 10. In par-
ticular, we can see that a CDS forms between the forward and reverse shock of the
SN. The CDS results from the cooling of the shocked CSM and the shocked ejecta
that have been mixed owing to Rayleigh-Taylor instabilities, and its mass, greatly
affected by the radius of the CSM, can be estimated as follows,
𝑀𝑠 ≈ 2× 10−4 ( 𝑅500𝑀⊙ )
2( 𝜈𝑠𝑏104 𝑘𝑚 𝑠−1 )
−1( 𝜅0.4 𝑐𝑚2𝑔−1 )
−1𝑀⊙ (9)
where 𝑅 is the radius of the CSM, 𝜈𝑠𝑏 is the velocity at shock breakout, and 𝜅 is the
opacity of the material where the shell is formed (Chevalier, 1981; Blinnikov, 2017;
Smith, 2017). The fast ejecta is travelling at approximately 10,000 km s−1, whilst
17
Shane Moran
the CSM is moving far more slowly, at roughly 10− 500 km s−1(Smith, 2017). The
cooler conditions of the CDS can allow for the formation of dust (Mattila et al.,
2008).
The forward shock (see Fig. 10) will sweep mass up at the following rate,
?˙?𝑓𝑠 = 4𝜋𝑟2𝜌𝑤𝜈𝑓𝑠 (10)
where 𝜌𝑤 is the density of the wind and 𝜈𝑓𝑠 is the velocity of the shock (Branch and
Wheeler, 2017). Plugging that into Equation 1, we get the luminosity radiated away
by the forward shock,
𝐿𝑓𝑠 =
?˙?𝑓𝑠𝜈
2
𝑓𝑠
2 = 2𝜋𝑟
2𝜌𝑤𝜈
3
𝑓𝑠 =
?˙?𝑤𝜈
3
𝑓𝑠
2𝜈𝑤
(11)
where ?˙?𝑤 is the wind mass-loss rate and 𝜈𝑤 is the velocity of the wind (Chevalier
and Fransson, 2017; Branch and Wheeler, 2017). The luminosity radiated away by
the reverse shock can be described similarly,
𝐿𝑟𝑠 =
?˙?𝑟𝑠𝜈
2
𝑟𝑠
2 = 2𝜋𝑟
2𝜌𝑤𝜈
3
𝑟𝑠 =
?˙?𝑤𝜈
3
𝑟𝑠
2𝜈𝑤
(12)
where ?˙?𝑟𝑠 is the rate at which mass is swept up by the reverse shock and 𝜈𝑟𝑠 is the
velocity of the reverse shock (Chevalier and Fransson, 2017). This radiation can take
a number of forms, including inverse Compton scattering in the shocked CSM, free-
free emission in the forward and reverse-shocked regions and x-ray line emission
from metals in the reverse-shocked ejecta (Branch and Wheeler, 2017). If the CSM
is optically thick, then ionisation of the unshocked CSM by radiation from shock
breakout or from x-rays from the region where interaction is taking place can lead to
further emission (Branch and Wheeler, 2017).
2.1.1 Environments
Whilst most SNe II are believed to arise from red-supergiant progenitors, given the
presence of massive CSM, SNe IIn must arise from stars with high mass-loss rates
(Chevalier and Fransson, 1994). The mass-loss rates of the progenitor stars can be
described as follows,
?˙? = 2𝐿𝜈𝐶𝑆𝑀
𝜈3𝐶𝐷𝑆
(13)
where 𝐿 is the luminosity of the SN, 𝜈𝐶𝑆𝑀 is the velocity of the CSM, and 𝜈𝐶𝐷𝑆 is
the velocity of the CDS (Smith, 2017). The velocity of the CDS can be determined
by measuring the intermediate lines in the spectra at late times and the velocity of
the CSM can be determined by measuring the narrow lines (Smith, 2017).
18
Classical Type IIn Supernovae: Iron Core-Collapse Supernovae within Hydrogen-Rich
Circumstellar Material
Steady mass loss from massive stars is insufficient to provide CSM of the densi-
ties necessary to produce typical SNe IIn; such events instead require eruptive mass
loss or episodic mass loss with only the former being sufficient in the case of super-
luminous SNe IIn (Smith, 2014). In particular, most kinds of SNe IIn are thought to
originate from massive LBV-like stars, as the eruptive mass-loss of such stars can ex-
plain the high-density CSM apparent in such SNe, whilst steady stellar winds cannot
(Kiewe et al., 2012; Smith, 2017). Furthermore, the mass loss must take place suffi-
ciently close in time to the SN itself for the CSM to remain nearby or else interaction
will not take place (Smith, 2014).
If there is dust present in the environment of the SN, then it can absorb incident
light and re-emit it at longer wavelengths, which can result in the presence of a near-
infrared excess in the light curve of the SN (Crotts, 1988). Depending on the location
and geometry of the dust, light reflected from it may take a noticeably longer time
to reach the Earth than light that has arrived directly, hence the term "light echo"
(Couderc, 1939; Crotts, 1988).
Asymmetry of spectral lines can suggest the presence of dust, though such asym-
metry can also result from asymmetric ejecta (Lucy et al., 1989; Taubenberger et al.,
2009). Polarimetry can offer insight into asymmetric distributions of CSM as it traces
asymmetry in the plane of the sky. This can be combined with spectroscopy, which
traces asymmetry along the line of sight in order to build up a more complete picture
of the layout of the CSM (Leonard et al., 2000).
2.2 Type IIn-P Supernovae
Another area for exploration is that of the so-called type IIn-P SNe, which through-
out their evolution display the spectroscopic features of the shock interaction with
dense CSM associated with IIn SNe, but also exhibit plateaus in their light curves,
something characteristic of SNe IIP (Fraser, 2020; Mauerhan et al., 2013; Smith,
2013). They decline much more rapidly than would be expected from the decay of
radioactive 56Co, indicating low yields (Mauerhan et al., 2013). Members of this
class include SN 1994W, SN 2005cl, SN 2009kn, SN 2011ht and SN 2020pvb, all of
which featured outbursts before explosion, the material lost during which undoubt-
edly playing a role in the later interaction (Chugai et al., 2004; Kankare et al., 2012;
Mauerhan et al., 2013; Li and Morozova, 2022; Elias-Rosa et al., 2024). It has also
been proposed that SN 1054, the SN responsible for the formation of the Crab neb-
ula, was a type IIn-P SN as it shows, for example, the telltale signs of having been
a low-energy explosion with a low 56Ni yield and also has the expected chemical
abundances for such an event (Smith, 2013).
These events appear to be the result of relatively low-energy explosions (∼ 1049−
1050 erg radiated electromagnetically), perhaps resulting from electron-capture SNe,
which will be discussed in Chapter 3, or the core-collapse of massive stars with
19
Shane Moran
significant fallback of metal-rich ejecta (Mauerhan et al., 2013; Smith, 2013). The
continued disagreement as to the exact natures of the progenitors stresses the need
for further observations and analysis of the precursor outbursts, which could shed
more light on IIn-P origin scenarios (Elias-Rosa et al., 2024). Here I will present
some preliminary unpublished data concerning one such object, SN 2020nub, for
illustrative purposes.
2.2.1 SN 2020nub
SN 2020nub was first discovered by the ASAS-SN team at 02:38 on the 30th of
June 2020 (MJD = 59030.11; (Stanek and Kochanek, 2020)). It was classified as a
type IIn SN at redshift z = 0.017 by Dimitriadis et al. (2020) from the University of
California, Santa Cruz on the 24th of July 2020 on the basis of observations taken
with the LRIS instrument on the Keck I telescope. Here I show the light curve and
spectral sequence of SN 2020nub, which demonstrate the features characteristic of
type IIn-P SNe.
I obtained optical imaging from the ALFOSC instrument on the Nordic Optical
Telescope (NOT) as part of the NUTS2 programme1. In addition, I also make use
of observations taken as part of the Asteroid Terrestrial-impact Last Alert System
(ATLAS) survey (Tonry et al., 2018; Smith et al., 2020). The first spectrum is the
aforementioned publically available Keck+LRIS spectrum, which was obtained from
the Transient Name Server2. I obtained the other spectra from NOT+ALFOSC as
part of the NUTS2 programme.
The light curve, presented in Fig. 11, resembles that of a type IIP SN, displaying
a plateau for approximately one month after peak before dropping sharply. At this
point the SN went behind the Sun, becoming unobservable. By the time the SN
returned, the rate of decline had significantly decreased, from approximately 0.3 mag
per day in the 𝑜 band at the end of season one to approximately 0.02 mag per day in
the 𝑟 band in season two. Both of these rates exceed the approximately 0.01 mag per
day decline rate associated with the decay of radioactive 56Co (Miller et al., 2010).
The spectra, presented in Fig. 12, span 69 days, from +28 d to +97 d. The +28 d
spectrum shows a blue continuum, though the continuum gradually becomes redder
later. The Balmer lines are evident throughout the evolution of the SN. A large
change in the Balmer decrement (i.e. the ratio of the flux density of 𝐻𝛼 to the flux
density of 𝐻𝛽) is apparent over time. The decrement expected from recombination
is 2.85 (Osterbrock, 1989). Higher values can result from the collisional excitation
of 𝐻𝛽 or from reddening due to dust as 𝐻𝛽 is more attenuated by the presence of
dust than is 𝐻𝛼, whilst lower values can result from the 𝐻𝛼-emitting region having a
1https://nuts.sn.ie/
2https://www.wis-tns.org/
20
Classical Type IIn Supernovae: Iron Core-Collapse Supernovae within Hydrogen-Rich
Circumstellar Material
0 20 40 60 80 100 120
Days post explosion (rest frame)
16
17
18
19
20
21
Ap
pa
re
nt
 m
ag
ni
tu
de
59027 59047 59067 59088 59108 59128 59149
ATLAS ALFOSC
18
17
16
15
14
13
Ab
so
lu
te
 m
ag
ni
tu
de
z
i
o
r
c
g
Figure 11. Multiband light curve of the type IIn-P SN 2020nub. The IIP-like plateau, characteristic
of such events, is evident for about a month after peak before a steep drop in apparent magnitude
occurs.
higher optical depth than the 𝐻𝛽-emitting region (Fraser et al., 2013; Levesque et al.,
2014).
He I is visible in emission in the +28 d spectrum, but it is no longer obvious
after this point until +51 d, following which it grows in prominence. The Ca II
𝜆𝜆𝜆8498,8542,8662 triplet is evident from in the +28 d spectrum, but has greatly
increased in prominence by the +66 d spectrum. O I is visible to the left of the near-
infrared calcium triplet at all times, though it decreases in relative prominence as the
SN evolves. The Fe II lines are clearly visible from the beginning up to the +86 d
spectrum (inclusive), after which they are not readily visible above the noise.
The preliminary results shown in this elucidatory example highlight the charac-
teristic plateau seen in such SNe, along with the narrow hydrogen lines also required
for the IIn-P classification. Furthermore, the decline, in excess of that expected from
radioactive 56Co, is also typical of the class. Other objects displaying narrow Balmer
lines, indicative of interaction, will be addressed in the next chapter.
21
Shane Moran
4000 5000 6000 7000 8000 9000 10000
Rest frame wavelength (Å)
No
rm
al
ise
d 
F
 +
 c
on
st
an
t +28d
+35d
+41d
+51d
+66d
+86d
+97d
4000 5000 6000 7000 8000 9000 10000
Rest frame wavelength (Å)
Balmer Series
Fe II
Ca II
[Ca II]
O I
[O I]
He I
Telluric
Na I D
Figure 12. Spectral sequence of SN 2020nub. The flux densities of each spectrum have been
normalised against the peak of H𝛼. A very large increase in the Balmer decrement (i.e. the ratio of
the flux density of 𝐻𝛼 to the flux density of 𝐻𝛽) is evident between the +28 d and +35 d spectra.
The initial spectrum also shows a blue continuum not evident at later times.
22
3 Weak Explosions within Dense
Circumstellar Material
In this chapter I will address particular kinds of weak explosions (i.e. events reaching
a peak absolute magnitude of no more than around −15 mag) taking place within
circumstellar material. This can encompass a broad range of event types, but only
the kinds most relevant to my research will be treated here. The emphasis on the fact
that these are weak explosions serves to differentiate these objects from the SNe IIn
covered in Chapter 2.
3.1 Luminous Red Novae
LRNe display double-peaked light curves (see Fig. 13) with peak magnitudes cover-
ing a broad range of approximately−4 to−15 (Blagorodnova et al., 2017; Pastorello
and Fraser, 2019; Pastorello et al., 2019). LRNe show a slow rise over months to
years before a rapid increase in luminosity just before the first peak (Pastorello et al.,
2023).
Certain binary stars spend a short period orbiting one another inside a shared
envelope (Ivanova et al., 2013). The Roche lobe of a star in a binary system is the re-
gion around the star in which matter is gravitationally bound to it (Paczyn´ski, 1971;
Ivanova et al., 2013). The expansion of one of the stars or the decrease in orbital
separation between the stars leads to Roche-lobe overflow, with mass transfer tak-
ing place from one of the stars, the donor, to the other star, the accretor (Paczyn´ski,
1971; Ivanova et al., 2013). Should the accretor be unable to accrete all of the over-
flowing material from the donor star, then a common envelope will be formed and
the drag generated as the stars move through this envelope will slow them, reduc-
ing their orbital separation even further over time (Paczynski, 1976; Ivanova et al.,
2013). Eventually, the stars will either merge directly or eject the common envelope
(Paczynski, 1976; Ivanova et al., 2013).
LRNe are generally believed to arise from stellar mergers and, indeed, display
signatures of common-envelope ejection (Pastorello et al., 2019). The initial blue
peak is often suggested to result from gas outflows or the cooling emission of the
common envelope post ejection, whereas the later red peak results from hydrogen re-
combination, as in SNe IIP, or from radial heating from shock interaction (MacLeod
et al., 2017; Metzger and Pejcha, 2017; Blagorodnova et al., 2021; Matsumoto and
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Shane Moran
Figure 13. Pseudobolometric light curves of seven LRNe. All but SN 1997bs and
UGC12307-20123OT1 were sufficiently well observed for the characteristic double peak to be
seen. Source: Pastorello et al. (2019).
24
Weak Explosions within Dense Circumstellar Material
Figure 14. A series of LRNe spectra at early, intermediate and late times. The early spectra
display the typical Balmer and Fe II emission lines. The forests of metal lines appear in the
intermediate spectra whilst the late spectra show the characteristic molecular absorption bands.
Adapted from Cai et al. (2022b).
25
Shane Moran
Metzger, 2022).
The first, or "blue", peak in the light curve shows Balmer and Fe II lines in emis-
sion (see Fig. 14), reminiscent of SNe IIn (Cai et al., 2019, 2022b). Around the time
of the second, or "red", peak in the light curve, the Balmer lines weaken, and a forest
of metal lines in absorption appears (Pastorello et al., 2019). At late times LRNe
have very red spectra that resemble those of M-type stars, with titanium monoxide
and vanadium monoxide molecular absorption bands (Pastorello and Fraser, 2019).
The key to the classification of these events as stellar mergers came with obser-
vations of V1309 Scorpii. This event showed clear evidence of periodicity, with the
luminosity evolving in a way consistent with a decreasing orbital distance resulting
from binary inspiralling, with the period decreasing exponentially from an initially
observed 1.4-day period (Tylenda et al., 2011). Additionally, the evolution of the
light curve showed the two peaks characteristic of LRNe and the spectral properties
typical of LRNe, such as a forest of metal absorption lines, were also present (Mason
et al., 2010).
Mergers have a much higher rate of occurrence than SNe, with galactic stellar
mergers of absolute magnitude 𝑀𝑉 = −3 or greater occurring at a rate of approxi-
mately one every two to three years (Kochanek et al., 2014). Even events as lumi-
nous as V1309 Scorpii (𝑀𝑉 ≈ −7 mag), can be expected about once every ten years
(Kochanek et al., 2014). Mergers can be expected to greatly outnumber CCSNe: CC-
SNe require massive stars, but most massive stars are found in binaries and low-mass
stars, which, owing to the initial mass function, greatly outnumber massive stars, can
also be found in binaries, though the binary fraction decreases with decreasing mass
(Salpeter, 1955; Chabrier, 2003; Stanway et al., 2020).
3.2 Intermediate-Luminosity Red Transients
ILRTs are another kind of gap transient, and they show a number of observational
similarities with LRNe. Their peak absolute magnitudes are typically between−11.5
and −14.5, and their rises typically last approximately two weeks; however, the
sample size of objects with a constrained rise time remains small, and variation is
present (Cai et al., 2018, 2021; Stritzinger et al., 2020). After peak they tend to
exhibit a plateau, resembling SNe IIP (Cai et al., 2021).
A key feature seen in ILRT spectra throughout their evolution is the [Ca II]
𝜆𝜆7291,7323 doublet (see Fig. 15; Cai et al. 2021). The presence of the Ca II H&K
lines and the Ca II 𝜆8498,8542,8662 near-infrared triplet is also typical (Cai et al.,
2021). The vast majority of stars with a ZAMS mass of 0.9 − 8M⊙ pass through
an asymptotic giant branch (AGB) stage (an evolutionary stage reached after the
end of helium burning in the core) and ultimately end their lives as WDs (Karakas,
2017). 7.5 − 9.25M⊙ is the mass range for super-AGB (sAGB) stars (Poelarends
et al., 2008). Larger stars, up to about 40 M⊙, typically become red supergiants
26
Weak Explosions within Dense Circumstellar Material
Figure 15. Sequence of ILRT light curves (upper panel) and spectra (lower panels). The typical
light-curve rise times of approximately two weeks can be seen in the upper panel along with, in the
case of most of the objects, the characteristic IIP-like plateau. The lower right panel shows a
blown-up view of the characteristic [Ca II] 𝜆𝜆7291,7323 doublet that is present throughout the
evolution of ILRTs. Source: Cai et al. (2022a).
27
Shane Moran
Figure 16. Spitzer Space Telescope images of the site of NGC 300 2008OT-1. The top row is
in the 3.6-micron band and the bottom row is in the 4.5-micron band. The left column shows the
site before explosion and the centre column after explosion, whilst the final column shows the
difference between the two. The difference images illustrate that there is a significant decrease in
brightness between the pre-explosion and post-explosion imagery, suggesting that this was a
terminal event. Source: Adams et al. (2016).
(RSGs) before exploding as CCSNe (Groenewegen and Sloan, 2018; Humphreys
et al., 2020).
Various scenarios have been suggested for the production of ILRTs, but the origin
channel currently favoured is that of electron-capture SNe (ECSNe), terminal events
arising from sAGB stars (Pumo et al., 2009; Thompson et al., 2009), and so I focus
on that here. In particular, ECSNe are far more likely to form in binary systems
than from single stars (Podsiadlowski et al., 2004). At the ends of the lives of stars
with masses in the transitional zone between those of intermediate-mass stars and
massive stars, a SN can take place, but through a different mechanism than core
collapse (Nomoto, 1984; Wheeler et al., 1998). This transitional zone occupies a
mass range between approximately 8−10M⊙; however, the precise range is sensitive
to metallicity (Langer, 2012). Such stars form a degenerate oxygen-neon-magnesium
core rather than an iron core (Wheeler et al., 1998). This core falls in a mass range
of approximately 1.1−1.37M⊙. The lower limit is the point above which C burning
can take place to lead to the formation of the oxygen-neon-magnesium core, and the
upper limit is the point at which ignition of Ne would take place (Nomoto and Leung,
2017). The fusion of H and He in the shells leads to an increase in the mass of the
core, which causes it to contract, ultimately leading to electron capture which leads
to further contraction and, eventually, deflagration (Nomoto and Leung, 2017).
28
Weak Explosions within Dense Circumstellar Material
There is support for the notion that ILRTs may in fact be terminal: progenitors
have been detected in the case of two ILRTs, SN 2008S and NGC 300 2008OT-1,
with post-explosion imaging (see Fig. 16) having shown luminosities lower than
those visible before the explosions (Adams et al., 2016). It should be noted that
other potential origin pathways from ILRTs have been suggested in the literature.
For example, it has been suggested that ILRTs are outbursts of stars of around 20 M⊙
in dusty cocoons powered by super-Eddington winds (Smith et al., 2009). Another
possibility is that ILRTs are instead powered by mass transfer from an AGB star to a
main-sequence star (Kashi et al., 2010).
29
4 Fast-Evolving Transients
There are various kinds of fast-evolving transients (e.g. optical gamma-ray burst
afterglows and fast blue optical transients; Fox et al. 2003; Fang et al. 2019), but
here I am focusing on peculiar stripped-envelope SNe and tidal-disruption events
(TDEs) as they have the greatest relevance to my own research.
4.1 Peculiar Stripped-Envelope Supernovae
Standard stripped-envelope SNe arise from stars that have lost their outer envelopes
of hydrogen or of both hydrogen and helium through binary interaction or through
the activity of intense stellar winds and, as such, they display no hydrogen in their
spectra (Smith, 2014). Their light curves are traditionally taken to be powered by
radioactive decay, though recently, a central-engine power source has been proposed
(Rodriguez et al., 2024). In particular, I will focus on calcium-dominated SNe and
ultrastripped SNe.
4.1.1 Calcium-dominated supernovae
Calcium-dominated SNe1 are a form of gap transient, with peak absolute magni-
tudes falling between −15.5 to −16.5 in the 𝑅 band (Kasliwal et al., 2012). They
have been estimated to occur at a rate approximately 7%± 5% of that of type Ia SNe
(Perets et al., 2010). They show rapid photometric evolution, with rise times of ap-
proximately two weeks, and large photospheric velocities (∼ 6,000 to 10,000 km s−1)
(Kasliwal et al., 2012). They also swiftly move to the nebular phase, from around
one to three months after explosion, and display intense calcium lines in their spectra
(see Fig. 17), something central to their classification (Perets et al., 2010; Kasliwal
et al., 2012).
Additionally, calcium-dominated SNe are notable for their tendency to have pro-
nounced offsets from their host galaxies (Kasliwal et al., 2012; Lyman et al., 2016;
Lunnan et al., 2017). Both thermonuclear SNe and CCSNe have been suggested
as potential origin pathways for calcium-dominated SNe, but in either case non-
1These are commonly referred to as "calcium-rich SNe", but whilst their spectra are
dominated by calcium lines that does not necessarily imply that there is a great deal of
calcium present.
30
Fast-Evolving Transients
Figure 17. Nebular spectra of a selection of calcium-dominated SNe. The characteristic [Ca II]
𝜆𝜆7291,7323 doublet is prominent as is the near-infrared Ca II 𝜆8498,8542,8662 triplet. The
highlighted [O I] emission is far weaker by comparison. Source: Lunnan et al. (2017).
standard channels would be required: for example, the explosion of a white-dwarf
core, potentially with interaction between the shock and previously ejected material,
or, in the case of a massive star, perhaps the fallback of ejecta onto the core resulting
in the formation of a BH (Kasliwal et al., 2012).
4.1.2 Ultrastripped supernovae
Ultrastripped SNe (USSNe) are faint, with peak luminosities < 1042 erg s−1, and
they fade quickly (Tauris et al., 2013, 2015). They display evidence of very low
ejecta masses (< 0.1 M⊙) as a result of extensive stripping, and they produce cor-
31
Shane Moran
Figure 18. Comparison of a selection of USSNe at similar phases. The upper panel shows early
phases, whilst those in the lower panel are later. The later spectra agree particularly well, being
especially similar at their redder ends, though the near-infrared Ca II 𝜆8498,8542,8662 triplet is
weaker in the case of SN 2019wxt. Spectra with low S/N have been smoothed. Note that the
identification of SN 2019ehk as an USSN is contested owing to the potential presence of hydrogen
(Yao et al., 2020; Jacobson-Galán et al., 2020; De et al., 2021). Source: Agudo et al. (2023).
32
Fast-Evolving Transients
respondingly small masses of 56Ni (∼ 0.01 M⊙; Tauris et al. 2013, 2015). Such
intensive stripping is possible as their progenitors have compact companions whose
small size permits the formation of tight binaries where stripping is enhanced (Tauris
et al., 2013, 2015). Given their small ejecta masses, they evolve rapidly, typically
rising to peak over the course of five to ten days (Tauris et al., 2013).
Close binary interaction can result in both iron CCSNe and ECSNe, with models
suggesting that USSNe can arise from either (Tauris et al., 2015). In the case of close
binaries resulting in iron CCSNe, large asymmetric kicks (> 1,000 km s−1) can be
expected (Scheck et al., 2006). The large kicks from the explosion would lead the
stars to become runaway stars thrown away with large velocities (Tauris et al., 2015).
On the other hand, ECSNe are expected to result in practically symmetric explosions
and, hence, should produce minimal kicks (Podsiadlowski et al., 2004).
USSNe are typically thought to be CCSNe resulting from the explosion of bare
1.5 M⊙ metal cores, which is at the lower end of what is possible in terms of mass
(Tauris et al., 2015). The near-infrared Ca II 𝜆8498,8542,8662 triplet is typically
present in the spectra of USSNe at late times (see Fig. 18), and indeed comparisons
between the observed spectra of calcium-dominated SNe and simulated spectra of
USSNe suggest that these classes of events may be connected (Moriya et al., 2017;
Nakaoka et al., 2021). USSNe offer one pathway for the formation of double-NS
binaries, which can merge to produce kilonovae (the electromagnetic transients re-
sulting from the merger of two NS or a NS and a BH; Tauris et al. 2015, 2017). Such
mergers are detectable in the form of gravitational waves (distortions of spacetime
produced by concentrations of mass or energy) by instruments such as the interfer-
ometers LIGO and Virgo and also optically, as in the case of the event AT 2017gfo
(Barish, 1999; Metzger et al., 2010; Tauris et al., 2015; Abbott et al., 2017b).
4.2 Tidal-Disruption Events
TDEs are another kind of phenomenon that can produce fast-evolving optical tran-
sients. They are expected to occur at a rate of approximately 10−5 to 10−6 per galaxy
per year, with the higher end of the range corresponding to active galaxies, though
obscuration by dust can result in non-detection, especially at higher redshifts (Donley
et al., 2002; Mattila et al., 2018).
A classical TDE occurs when a star ventures too close to a supermassive BH
(SMBH) and becomes disrupted by the consequent tidal forces once they exceed the
star’s self-gravity (Hills, 1975). Almost half of the disrupted material will then es-
cape on hyperbolic orbits, but the rest of the disrupted material will be circularised,
forming an accretion disc (Rees, 1988; Gezari, 2021). This material will then be
accreted onto the SMBH resulting in the production of a luminous flare of electro-
magnetic radiation, with the nature of the flare reflecting the structure of the ejected
material and of the accretion disc formed (Rees, 1988; Arcavi et al., 2014; Komossa,
33
Shane Moran
2015; Gezari, 2021). The physical details of the accretion process remain unclear
(Gezari, 2021). It is also possible that the circularisation of the accretion disc pro-
ceeds slowly, in which case the luminosity must originate from another physical pro-
cess, such as emission resulting from interaction from the collision of debris streams
as they fall onto the black hole (Gezari, 2021).
Figure 19. The tidal disruption of a solar-type star by a massive BH. Almost half of the debris
created is not bound and instead leaves on a hyperbolic orbit, whilst the rest is circularised and
forms an accretion disc (Rees, 1988; Gezari, 2021). Source: Rees (1988).
Though the classical picture involves a SMBH (> 106 M⊙), TDEs could also
34
Fast-Evolving Transients
occur under other circumstances, involving intermediate-mass BHs (IMBHs; 103 −
106 M⊙) or even stellar-mass BHs (< 103 M⊙; MacLeod et al. 2016; Mezcua 2017;
Kremer et al. 2021; Gutiérrez et al. 2024). The tidal radius, that is, the radius at
which a star will become disrupted by tidal forces, is as follows,
𝑅𝑡 ≈ 𝑅*(𝑀ℎ
𝑀*
) 13 (14)
where 𝑀ℎ is the mass of the BH, 𝑀* is the mass of the star and 𝑅* is the radius of
the star (Phinney, 1989; Kobayashi et al., 2004). The so-called "penetration factor"
is given as follows,
𝛽 = 𝑅𝑡
𝑅𝑝
(15)
where 𝑅𝑝 is the pericentre distance (Luminet and Pichon, 1989; Rosswog et al.,
2009). Tidal disruption will occur for 𝛽 ≳ 1 (Luminet and Pichon, 1989). As
such, given the compactness of WDs, they cannot be disrupted by SMBHs, which is
illustrated in Fig. 20 (Luminet and Pichon, 1989; Rosswog et al., 2009).
Figure 20. Plot of the penetration factor, 𝛽, against BH mass, 𝐵ℎ, in solar masses. The range of
values of 𝛽 and BH masses for which disruption can take place are shown in the case of both a
1.4 M⊙ WD and a 0.2 M⊙ WD. It can be seen that SMBHs cannot disrupt WDs. Source: Rosswog
et al. (2009).
35
Shane Moran
TDEs are bright in the ultraviolet range, owing to the thermal emission from ac-
cretion, and their spectra display blue continua (Arcavi et al., 2014). However, TDEs
occupy a continuum of spectral types, ranging from hydrogen-dominated to helium-
dominated, with intermediate types showing evidence of both features (Arcavi et al.,
2014).
Given that the light curves of TDEs are expected to be accretion powered, we
would expect a decline rate of t−5/3 (Phinney, 1989). However, it should be noted
that this would specifically be expected for the bolometric light curve, and naturally,
individual bands may display differing rates (Strubbe and Quataert, 2009). Further-
more, a number of other considerations can impact the decline rate, such as the nature
of the density profile (Lodato et al., 2009). Additionally, analytical modelling indi-
cates that the t−5/3 decline rate should only hold at late times after the luminosity of
the TDE has dropped by two to three magnitudes (Lodato et al., 2009).
TDEs are conceivably a way of finding IMBHs, which are of much interest, given
their proposed role in seeding the SMBHs seen at high redshift (Lin et al., 2018; Di
Matteo et al., 2023; Gutiérrez et al., 2024). Moreover, TDEs are observationally
important as they can serve as a probe of otherwise non-accreting BHs and, indeed,
may be the only potential probes of accretion in the case of IMBHs (Gezari, 2021;
Angus et al., 2022). Additionally, they provide great insight into accretion physics,
given the interaction of the debris streams and the circularisation of the material into
an accretion disc along with jet formation (Shen and Matzner, 2014; Gezari, 2021).
36
5 Summary of the Papers
In this chapter I will provide an overview of the contents of the research papers
contained in this thesis. The papers present novel data sets for unique astronomical
objects, helping to shed light on various areas of the parameter space of transients in
terms of duration, luminosity and physical origin. Interacting SNe, USSNe, TDEs
and gap transients such as ILRTs and LRNe are addressed, all having broader impli-
cations for stellar evolution.
5.1 Paper I
The first paper concerns the long-lived type IIn SN 2017hcc and provides the re-
sults of a five-year follow-up campaign of the object with optical and near-infrared
imaging and spectroscopy. SN 2017hcc was a slowly evolving object, both photo-
metrically and spectroscopically. It displayed a long rise time of around eight weeks
and reached peak absolute magnitude of−20.78± 0.01 in ATLAS 𝑜 band, indicative
of significant interaction between massive ejecta and massive and dense CSM. Not
long after maximum light, the object began to display an infrared excess. The object
exhibited blueshifted Balmer lines from a few hundred days until around +1200 days
post explosion. We plotted the evolution of the centroid of H𝛼 over time in order to
demonstrate the evolution in its position.
I was the first author of this paper and wrote the vast majority of the text, reduced
most of the data and was principally responsible for the analysis and overall material
in the paper. I also responded to the referee.
The extensive data set in this paper allows for a thorough examination of a high-
energy explosion in massive and dense CSM. Observations of ejecta-CSM interac-
tion in such events allow for the probing of the production mechanisms and history
of the CSM; such observations offer insight into the evolution of massive stars in
general, as does distinguishing between the presence of pre-existing dust, indicative
of mass loss, and the formation of new dust after the explosion.
5.2 Paper II
The second paper concerns the intermediate-luminosity red transient AT 2022fnm.
The observational overlap between LRNe and ILRTs is addressed. In particular, the
37
Shane Moran
evolution of LRN and ILRT light curves and spectra is compared along with rise
times and decay rates.
The object did not display the double-peaked feature typical of LRNe, nor did
the spectra show the forest of metal lines in absorption or the late-time molecular
bands associated with LRNe. The spectra featured the [Ca II] 𝜆𝜆7291,7323 doublet,
which is characteristic of ILRTs, though it was weak. The Ca II 𝜆8498,8542,8662
NIR triplet found in both ILRTs and LRNe was also present. The presence of a near-
infrared excess is evident from around +100 d, and we suggest that this results from
a dust echo. The spectra also show evidence of ejecta-CSM interaction at late times.
We employed a simple analytical diffusion model to reproduce the pseudo-bolometric
light curve of the event and found it to be compatible with ECSNe, though the param-
eters were degenerate and compatible with a wide variety of scenarios. We concluded
that AT 2022fnm is likely the result of a weak stellar explosion or eruption.
I was also the first author of this paper and wrote almost the entirety of the text.
Additionally, I am responsible for the majority of the analysis and overall material in
the paper and responded to the referee.
This paper serves to comb the parameter space of LRNe and ILRTs, highlighting
the similarities between such gap transients, whilst also clarifying the differences in
their underlying explosions mechanisms. Improved understanding of objects such as
AT 2022fnm helps provide a clearer picture of observationally "transitional" events,
though the paper also emphasises the need for higher cadence optical and near-
infrared photometry and spectroscopy in order to more clearly discriminate between
potential origin pathways.
5.3 Paper III
The third paper concerns the fast-evolving transient SN 2017fwm/Gaia17byh. The
object displayed an extremely fast rise time (≲ 5 days) and a small ejecta mass
(0.01M⊙).
The +17 d spectrum of the object was compared against the +23 d spectrum ul-
trastripped SN 2010X, and a peculiar velocity of −4,980 km s−1 was found after
correction for galactic redshift. Various scenarios were investigated in an attempt to
explain this object’s origins, such as the explosion of a star dynamically ejected by
a SMBH via the Hills mechanism (Hills, 1988), shock-cooling emission from the
explosion of a low-mass helium star and the merger of a WD and a BH or NS. Ulti-
mately, the strongest suggestions are that this object was an ultrastripped SN or that
it was a TDE with the accretion of the material of a tidally disrupted WD leading to
the emission. However, neither of these explanations is entirely satisfying.
I was the second author of this paper. I was responsible for the majority of
the analysis of the photometric and spectroscopic evolution and made significant
contributions to the text and astrophysical analysis.
38
Summary of the Papers
This paper provides an overview of a number of different categories of events
that can produce low ejecta masses and calcium lines, weighing them up against
one another by comparing the physical processes leading to their origins as well
as their observational characteristics. Furthermore, the puzzling high-velocity dis-
cussed presents challenges to our current understanding of the origins of high-energy
transients, spotlighting the need for further research in this area.
39
6 Future Work
6.1 Surveys and Facilities
We are at the dawn of a new age for large sky surveys. In particular, we can ex-
pect great observational progress as a whole new generation of instruments and tele-
scopes see first light. Upcoming wide-field surveys, such as the Legacy Survey of
Space and Time (LSST), expected to begin in August 20251 at the 8.4 m Simonyi
Survey Telescope at the Vera C. Rubin Observatory, will find far larger numbers of
objects than ever before, producing ∼10 million alerts per night (LSST Dark Energy
Science Collaboration, 2012; LSST Dark Energy Science Collaboration et al., 2021;
Thomas et al., 2022; Hambleton et al., 2023). The upcoming Time-Domain Extra-
galactic Survey (TiDES) will use the 4-metre Multi-Object Spectrograph Telescope
(4MOST), which is expected to see first light on the VISTA telescope2 in 2025 and
has a 4.2 square degree field of view (de Jong et al., 2012, 2019; Swann et al., 2019;
Mainieri et al., 2024). TiDES will serve to complement all-sky photometric surveys,
including LSST, by providing spectroscopic follow-up of extragalactic optical tran-
sients, including SNe (Swann et al., 2019). Son of X-Shooter (SoXS), which should
see first light in early 20253, is a medium-resolution wide-band spectrograph to be
mounted on the 3.6 m New Technology Telescope at the La Silla Observatory, oper-
ated by the European Southern Observatory in Chile, which will allow for spectral
classification and follow-up of many more transients (Schipani et al., 2016).
I hope to use the advanced capabilities of these upcoming facilities to enhance
our understanding of events such as interacting SNe, ILRTs and TDEs and to find
counterparts to gravitational-wave events, discussed in Section 6.3 ahead. Addition-
ally, I am eager to use these instruments and surveys to uncover entirely new kinds of
transients, building a clearer picture of the diversity of the transient parameter space
with improved detection and prompt (within minutes) follow-up.
1https://www.lsst.org/about/project-status
2https://www.eso.org/public/teles-instr/paranal-observatory/surveytelescopes/vista/
3https://www.eso.org/public/teles-instr/lasilla/ntt/soxs/
40
Future Work
6.2 Modelling
This observational progress is only one part of the story, however. In order to make
theoretical progress, we need observations that provide much deeper physical insight.
Furthermore, we need improved computational models. The accuracy and usefulness
of the simulation of astrophysical systems can be improved by furthering our physical
understanding of phenomena and by increasing the complexity of the simulations
based on already-understood physics. The primary difficulty is the bottleneck posed
by computing constraints. It takes a significant amount of time (tens of millions of
CPU hours) to model asymmetric events using huge grids (Vartanyan et al., 2019).
The use of three-dimensional magnetohydrodynamic codes for simulation is now
widespread, and whilst there are difficulties in handling angular-momentum trans-
port and magnetic-field amplification when modelling stellar evolution, promising
improvements are being made in this area (Müller, 2024). Excellent work is being
done with regard to modelling of gap transients such as ILRTs and LRNe, but this
needs to be extended to models of higher mass. Currently, the modelling is typically
done for low-mass stars, more suitable for describing red novae than LRNe (Shara
et al., 2010). Additionally, there is a very large parameter space to investigate, includ-
ing various mass ratios, orbital separations, eccentricities and evolutionary stages of
progenitors.
It has consistently been the case that the pace at which observational data has
been gathered has far exceeded the corresponding progress of theoretical modelling,
a problem that may only be exacerbated with the coming observational revolution
(Gezari, 2021). Indeed, as described above, future surveys will find far greater num-
bers of transients; however, next-generation instruments will allow for improved
classification with more photometric and spectroscopic resources and will enable
follow-up with higher cadence. As such, our understanding of the stellar evolution-
ary processes leading to these events will be enhanced, which will allow for the
improvement of the physical underpinnings of the models used to simulate them.
6.3 Multi-Messenger Astronomy
Multi-messenger astronomy, the acquisition and analysis of multiple different kinds
of signals originating from the same source, will be of profound importance in the
coming years. Comparisons are made not only between different parts of the elec-
tromagnetic spectrum, from radio waves to gamma rays, but also including neutrinos
(with the IceCube Neutrino Observatory and others; IceCube Collaboration et al.
2006) and gravitational waves (with LIGO-Virgo and KAGRA; Abramovici et al.
1992; Barish 1999; Somiya 2012). As some events are expected to produce all three,
the ability to collate these different data is of great value.
The neutrinos detected from SN 1987A alongside the electromagnetic detec-
41
Shane Moran
tions made this the first multimessenger transient detection, and to date it is the only
transient event for which there has been a confirmed neutrino detection (DarkSide-
20k Collaboration et al., 2021). However, a high-energy neutrino detected in 2019
may have originated from the tidal-disruption event AT 2019dsg (Stein et al., 2021).
Moreover, neutrinos were detected from the blazar TXS 0506+056 in 2017, offering
hope for future successes with improved instrumentation, such as IceCube-Gen2,
which is currently in the technical-design stage and will have greatly improved sen-
sitivity compared with current-generation detectors (IceCube Collaboration et al.,
2018; Clark et al., 2021; Aartsen et al., 2021). Additionally, future dark-matter detec-
tors, such as the liquid-argon detectors DarkSide-20k and Argo, will also be sensitive
to neutrinos, increasing the likelihood of detections and offering enhanced distance
determination (to a precision of ∼ 5%; DarkSide-20k Collaboration et al. 2021).
The gravitational-wave event GW 170817, corresponding to the merger of a NS
binary, was detected on the 17th of August 2017, by the LIGO and Virgo detectors
and the multimessenger data provided unprecedented insight into the process (Ab-
bott et al., 2017b). Unfortunately, there were no corresponding neutrino detections,
though this may have been a viewing-angle effect (Albert et al., 2017). However,
a corresponding short gamma-ray burst, GRB170817A, lasting approximately two
seconds was detected 1.7 seconds after the initial gravitational-wave detection (Ab-
bott et al., 2017a). Additionally, an optical counterpart, designated AT 2017gfo, was
discovered 11 hours after the gravitational-wave detection (Abbott et al., 2017b).
Electromagnetic counterparts of gravitational-wave events are expected to be par-
ticularly faint and quickly evolving, making them especially difficult to find (Agudo
et al., 2023). This serves to emphasise the importance of high-cadence wide-field
follow-up of sufficient depth. Upcoming surveys, instruments and telescopes are
particularly well positioned to tackle these challenges, so the field can be expected
to undergo significant growth and progress in the near future.
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