We present a data-driven method based on long short-term memory (LSTM) neural networks to analyze spectral time series of Type Ia supernovae (SNe Ia). The dataset includes 3091 spectra from 361 individual SNe Ia. The method allows for accurate reconstruction of the spectral sequence of an SN Ia based on a single observed spectrum around maximum light. The precision of the spectral reconstruction increases with more spectral time coverages, but the significant benefit of multiple epoch data at around optical maximum is only evident for observations separated by more than a week. The method shows great power in extracting the spectral information of SNe Ia, and suggests that the most critical information of an SN Ia can be derived from a single spectrum around the optical maximum. The algorithm we have developed is important for the planning of spectroscopic follow-up observations of future SN surveys with the LSST/Rubin and the WFIRST/Roman telescopes.
The spectrum of a supernova is a summation of numerous overlapping atomic line signatures. Consequently, empirical measurements are limited in application when compound features are assumed to be due to one or two spectral lines. Here I address matters of spectroscopic boundaries between normal and peculiar type I supernovae using multi-component empirical metrics. I discuss some obstacles faced when using supernova spectra to pair model with data and I demonstrate how spectrum synthesis can benefit from fairly complete observational coverage in wavelength and time.
A quantitative data-driven comparison among supernovae (SNe) based on their spectral time series combined with multi-band photometry is presented. We use an unsupervised Random Forest algorithm as a metric on a set of 82 well-documented SNe representing all the main spectroscopic types, in order to embed these in an abstract metric space reflecting shared correlations between the objects. We visualize the resulting metric space in 3D, revealing strong agreement with the current spectroscopic classification scheme. The embedding splits Type Ib supernovae into two groups, with one subgroup exhib…
We present Keck spectroscopic observations of three probable high redshift superluminous supernovae (SLSNe) from the Subaru HIgh-Z sUpernova CAmpaign (SHIZUCA), confirming redshifts of 1.851, 1.965 and 2.399. The host galaxies were selected for transient monitoring from multi-band photometric redshifts. The supernovae are detected during their rise, and the classically scheduled spectra are collected near maximum light. The restframe far-ultraviolet (FUV;$\sim$1000A-2500A) spectra include a significant host galaxy flux contribution and we compare our host galaxy subtracted spectra to UV-lumino…
Investigations of some type Iax supernovae have led to the suggestion that their ejecta must be layered to some degree. Such an ejecta structure has been argued as inconsistent with the well-mixed composition predicted by pure deflagrations. Based on explosion models, we create toy models in which the ejecta are artificially stratified and progressively mixed until a uniform composition is obtained. We find that models that are heavily mixed, containing burned and unburned material at all velocities, produce reasonably good agreement with SN 2012Z, for which a layered structure has been sugges…
We have developed a radiative transfer code, CMFGEN, which allows us to model the spectra of massive stars and supernovae. Using CMFGEN we can derive fundamental parameters such as effective temperatures and surface gravities, derive abundances, and place constraints on stellar wind properties. The last of these is important since all massive stars are losing mass via a stellar wind that is driven from the star by radiation pressure, and this mass loss can substantially influence the spectral appearance and evolution of the star. Recently we have extended CMFGEN to allow us to undertake time-d…
We study a sample of 23 Type II Plateau supernovae (SNe II-P), all observed with the same set of instruments. Analysis of their photometric evolution confirms that their typical plateau duration is 100 days with little scatter, showing a tendency to get shorter for more energetic SNe. The rise time from explosion to plateau does not seem to correlate with luminosity. We analyze their spectra, measuring typical ejecta velocities, and confirm that they follow a well behaved power-law decline. We find indications of high-velocity material in the spectra of six of our SNe. We test different dust e…
The division between stripped-envelope supernovae (SE-SNe) and superluminous supernovae (SLSNe) is not well defined in either photometric or spectroscopic space. While a sharp luminosity threshold has been suggested, there remains an increasing number of transitional objects that reach this threshold without the spectroscopic signatures common to SLSNe. In this work we present data and analysis on four SNe transitional between SE-SNe and SLSNe; the He-poor SNe 2019dwa and 2019cri, and the He-rich SNe 2019hge and 2019unb. Each object displays long-lived and variable photometric evolution with l…
We present $81$ near-infrared (NIR) spectra of $30$ Type II supernovae (SNe II) from the Carnegie Supernova Project-II (CSP-II), the largest such dataset published to date. We identify a number of NIR features and characterize their evolution over time. The NIR spectroscopic properties of SNe II fall into two distinct groups. This classification is first based on the strength of the He I $\lambda1.083\,μ$m absorption during the plateau phase; SNe II are either significantly above (spectroscopically strong) or below $50$ angstroms (spectroscopically weak) in pseudo equivalent width. However bet…
We search for the reasons behind the spectroscopic diversity of hydrogen-poor superluminous supernovae (SLSNe-I) in the pre-maximum phase. Our analysis is a continuation of the paper of \citet{ktr21}, who disclosed two new subtypes of SLSNe-I characterized by the presence/absence of a W-shaped absorption feature in their pre-maximum spectra between 4000 and 5000 Å(called Type~W and Type~15bn, respectively). However, the physical cause of this bimodality is still uncertain. Here we present pre-maximum spectral synthesis of 27 SLSNe-I with special attention to the photospheric temperature ($T_{ …
The existence of multiple subclasses of type Ia supernovae (SNeIa) has been the subject of great debate in the last decade. One major challenge inevitably met when trying to infer the existence of one or more subclasses is the time consuming, and subjective, process of subclass definition. In this work, we show how machine learning tools facilitate identification of subtypes of SNeIa through the establishment of a hierarchical group structure in the continuous space of spectral diversity formed by these objects. Using Deep Learning, we were capable of performing such identification in a 4 dime…
Photometric classification of supernovae (SNe) is imperative as recent and upcoming optical time-domain surveys, such as the Large Synoptic Survey Telescope (LSST), overwhelm the available resources for spectrosopic follow-up. Here we develop a range of light curve classification pipelines, trained on 518 spectroscopically-classified SNe from the Pan-STARRS1 Medium-Deep Survey (PS1-MDS): 357 Type Ia, 93 Type II, 25 Type IIn, 21 Type Ibc, and 17 Type I SLSNe. We present a new parametric analytical model that can accommodate a broad range of SN light curve morphologies, including those with a pl…
We investigate strongly gravitationally lensed type II supernovae (LSNe II) for time-delay cosmography incorporating microlensing effects, which expands on previous microlensing studies of type Ia supernovae (SNe Ia). We use the radiative-transfer code ${\rm \small TARDIS}$ to recreate five spectra of the prototypical SN 1999em at different times within the plateau phase of the light curve. The microlensing-induced deformations of the spectra and light curves are calculated by placing the SN into magnification maps generated with the code ${\rm \small GERLUMPH}$. We study the impact of microle…
Using the largest spectroscopic dataset of stripped-envelope core-collapse supernovae (stripped SNe), we present a systematic investigation of spectral properties of Type IIb SNe (SNe IIb), Type Ib SNe (SNe Ib), and Type Ic SNe (SNe Ic). Prior studies have been based on individual objects or small samples. Here, we analyze 227 spectra of 14 SNe IIb, 258 spectra of 21 SNe Ib, and 207 spectra of 17 SNe Ic based on the stripped SN dataset of Modjaz et al. (2014) and other published spectra of individual SNe. Each SN in our sample has a secure spectroscopic ID, a date of $V$-band maximum light, an…
We study the observables of 158 relatively normal Type Ia supernovae (SNe Ia) by dividing them into two groups in terms of the expansion velocity inferred from the absorption minimum of the Si II 6355 line in their spectra near B-band maximum brightness. One group ("Normal") consists of normal SNe Ia populating a narrow strip in the Si II velocity distribution, with an average expansion velocity v=10,600+/-400 km/s near B maximum; the other group ("HV") consists of objects with higher velocities, v > 11,800 km/s. Compared with the Normal group, the HV one shows a narrower distribution in both …
The classification of supernovae (SNe) and its impact on our understanding of the explosion physics and progenitors have traditionally been based on the presence or absence of certain spectral features. However, current and upcoming wide-field time-domain surveys have increased the transient discovery rate far beyond our capacity to obtain even a single spectrum of each new event. We must therefore rely heavily on photometric classification, connecting SN light curves back to their spectroscopically defined classes. Here we present Superphot, an open-source Python implementation of the machine…
Over the past decade wide-field optical time-domain surveys have increased the discovery rate of transients to the point that $\lesssim 10\%$ are being spectroscopically classified. Despite this, these surveys have enabled the discovery of new and rare types of transients, most notably the class of hydrogen-poor superluminous supernovae (SLSN-I), with about 150 events confirmed to date. Here we present a machine-learning classification algorithm targeted at rapid identification of a pure sample of SLSN-I to enable spectroscopic and multi-wavelength follow-up. This algorithm is part of the FLEE…
Interacting supernovae (SNe) IIn and Ibn show narrow emission lines and have always been a mysterious and unsolved genre in SNe physics. We present a comprehensive analysis of the temporal and spectroscopic behaviour of a group of interacting SNe~IIn and Ibn. We choose SNe~2012ab, 2020cui, 2020rc and 2019uo as representative members of these SN sub-types to probe the nature of explosion. Our study reveals that SNe~IIn are heterogeneous, bright depicting multi-staged temporal evolution while SNe~Ibn are moreover homogeneous, comparatively fainter than SNe~IIn and short lived, but limited in sam…
We present a set of new quantitative classification criteria for major subclasses of Type I Supernovae (SNe). We analyze peak spectra of 146 SNe Ia from the Berkeley Supernova Ia Program (BSNIP), 12 SNe Ib, 19 SNe Ic (including 5 SNe Ic-BL) and 4 SNe Ib/c. All the spectra are within 5 days of maximum light. We study their absorption line depths relative to the pseudo-continuum at around observed wavelength 6150Å, attributed to Si II 6355Å for SNe Ia and Ic and to hydrogen for SNe Ib, and O I $λ7774$Å observed around $λ7500$Å. We found that Type I SNe could be quantitatively classified using th…
The design and analysis of time-domain sky surveys requires the ability to simulate accurately realistic populations of core collapse supernova (SN) events. We present a set of spectral time-series templates designed for this purpose, for both hydrogen-rich (type II, IIn, IIb) and stripped envelope (types Ib, Ic, Ic-BL) core collapse supernovae. We use photometric and spectroscopic data for 67 core collapse supernovae from the literature, and for each generate a time-series spectral template. The techniques used to build the templates are fully data-driven with no assumption of any parametric …
There are now hundreds of publicly available supernova spectral time series. Radiative transfer modeling of this data gives insights into the physical properties of these explosions such as the composition, the density structure, or the intrinsic luminosity---this is invaluable for understanding the supernova progenitors, the explosion mechanism, or for constraining the supernova distance. However, a detailed parameter study of the available data has been out of reach due to the high dimensionality of the problem coupled with the still significant computational expense. We tackle this issue th…
We present the analysis of four hours of spectroscopic observations of NGC 6946 with the SITELLE Imaging Fourier Transform Spectrometer on the Canada-France-Hawaii Telescope, acquired to search for supernova light echoes from its ten modern supernovae. We develop a novel spectroscopic search method: identifying negatively sloped continua in the narrow-band SN3 filter as candidate highly-broadened P-Cygni profiles in the H$α$ line, which would be characteristic of the spectra of supernovae ejecta. We test our methodology by looking for light echoes from any of the ten supernovae observed in NGC…
The light from distant supernovae (SNe) can be magnified through gravitational lensing when a foreground galaxy is located along the line of sight. This line-up allows for detailed studies of SNe at high redshift that otherwise would not be possible. Spectroscopic observations of lensed high-redshift Type Ia supernovae (SNe Ia) are of particular interest since they can be used to test for evolution of their intrinsic properties. The use of SNe Ia for probing the cosmic expansion history has proven to be an extremely powerful method for measuring cosmological parameters. However, if systematic …
Hydrogen-rich (type II) supernovae (SNe) exhibit considerable photometric and spectroscopic diversity. Extending previous work that focused exclusively on photometry, we simultaneously model the multi-band light curves and optical spectra of Type II SNe using RSG progenitors that are characterized by their H-rich envelope masses or the mass and extent of an enshrouding cocoon at the star's surface. Reducing the H-rich envelope mass yields faster declining light curves, a shorter duration of the photospheric phase, broader line profiles at early times, but only a modest boost in early-time opti…
SN 2002cx-like Type Ia supernovae (also known as SNe Iax) represent one of the most numerous peculiar SN classes. They differ from normal SNe Ia by having fainter peak magnitudes, faster decline rates and lower photospheric velocities, displaying a wide diversity in these properties. We present both integral-field and long-slit visual-wavelength spectroscopy of the host galaxies and explosion sites of SNe Iax to provide constraints on their progenitor formation scenarios. The SN Iax explosion site metallicity distribution is similar to that of core-collapse (CC) SNe and metal-poor compared to …
Photometrically-Classified Superluminous Supernovae from the Pan-STARRS1 Medium Deep Survey: A Case Study for Science with Machine Learning-Based Classification
SN 2017hcc and SN 2023usc a comparative spectroscopic study of type IIn supernovae
Indian Institute of Astrophysics（印度天体物理研究所）；Instituto de Estudios Astrofísicos, Universidad Diego Portales（迭戈帕拉莱斯大学天体物理研究学院）；Millennium Institute of Astrophysics（天体物理千年研究所）；Department of Physics, Florida State University（佛罗里达州立大学物理系）；Homi Bhabha Center for Science Education Tata Institute of Fundamental Research（塔塔基础科学教育霍米巴巴中心）
Automated classification of supernovae (SNe) based on optical photometric light curve information is essential in the upcoming era of wide-field time domain surveys, such as the Legacy Survey of Space and Time (LSST) conducted by the Rubin Observatory. Photometric classification can enable real-time identification of interesting events for extended multi-wavelength follow-up, as well as archival population studies. Here we present the complete sample of 5,243 "SN-like" light curves (in griz) from the Pan-STARRS1 Medium-Deep Survey (PS1-MDS). The PS1-MDS is similar to the planned LSST Wide-Fast…
Super-luminous supernovae (SLSNe) are tremendously luminous explosions whose power sources and progenitors are highly debated. Broad-lined SNe Ic (SNe Ic-bl) are the only type of SNe that are connected with long-duration gamma ray bursts (GRBs). Studying the spectral similarity and difference between the populations of hydrogen-poor SLSNe (SLSNe Ic) and of hydrogen-poor stripped-envelope core-collapse SNe, in particular SNe Ic and SNe Ic-bl, can provide crucial observations to test predictions of theories based on various power source models and progenitor models. In this paper, we collected a…
We present extensive optical and near-infrared photometric and spectroscopic observations of the stripped-envelope (SE) supernova SN 2010as. Spectroscopic peculiarities, such as initially weak helium features and low expansion velocities with a nearly flat evolution, place this object in the small family of events previously identified as transitional Type Ib/c supernovae (SNe). There is ubiquitous evidence of hydrogen, albeit weak, in this family of SNe, indicating that they are in fact a peculiar kind of Type IIb SNe that we name "flat-velocity Type IIb". The flat velocity evolution---which …
We present survey results obtained from the UBVRI optical photometric follow-up of 19 bright core-collapse SNe during 2002-2012 using 1-m class optical telescopes operated by the Aryabhatta Research Institute of Observational Science (acronym ARIES), Nainital India. This homogeneous set of data have been used to study behavior of optical light/color curve, and to gain insight into object-to-object peculiarity. We derive integrated luminosities for types IIP, Ibc and luminous SNe. Two peculiar type IIP events having photometric properties similar to normal IIP and spectroscopic properties simil…
SN 2011fe was the nearest and best-observed type Ia supernova in a generation, and brought previous incomplete datasets into sharp contrast with the detailed new data. In retrospect, documenting spectroscopic behaviors of type Ia supernovae has been more often limited by sparse and incomplete temporal sampling than by consequences of signal-to-noise ratios, telluric features, or small sample sizes. As a result, type Ia supernovae have been primarily studied insofar as parameters discretized by relative epochs and incomplete temporal snapshots near maximum light. Here we discuss a necessary nex…
NOMAI : A real-time photometric classifier for superluminous supernovae identification. A science module for the Fink broker
Stockholm University（斯德哥尔摩大学）；Université Paris-Saclay（巴黎萨克雷大学）；Weizmann Institute of Science（魏茨曼科学研究所）；National Centre for Nuclear Research（国家核研究中心）；Liverpool John Moores University（利物浦约翰摩尔大学）；National Central University（中央大学）；University of Washington（华盛顿大学）；California Institute of Technology（加州理工学院）
We present a Hubble diagram of type II supernovae using corrected magnitudes derived only from photometry, with no input of spectral information. We use a data set from the Carnegie Supernovae Project I (CSP) for which optical and near-infrared light-curves were obtained. The apparent magnitude is corrected by two observables, one corresponding to the slope of the plateau in the $V$ band and the second a colour term. We obtain a dispersion of 0.44 mag using a combination of the $(V-i)$ colour and the $r$ band and we are able to reduce the dispersion to 0.39 mag using our golden sample. A compa…
Type Iax supernovae (SN Iax), also called SN 2002cx-like supernovae, are the largest class of peculiar white dwarf (thermonuclear) supernovae, with over fifty members known. SN Iax have lower ejecta velocity and lower luminosities, and these parameters span a much wider range, than normal type Ia supernovae (SN Ia). SN Iax are spectroscopically similar to some SN Ia near maximum light, but are unique among all supernovae in their late-time spectra, which never become fully nebular. SN Iax overwhelmingly occur in late-type host galaxies, implying a relatively young population. The SN Iax 2012Z …
Stripped-envelope supernovae (SE-SNe) show a wide variety of photometric and spectroscopic properties. This is due to the different potential formation channels and the stripping mechanism that allows for a large diversity within the progenitors outer envelop compositions. Here, the photometric and spectroscopic observations of SN 2020cpg covering $\sim 130$ days from the explosion date are presented. SN 2020cpg ($z = 0.037$) is a bright SE-SNe with the $B$-band peaking at $M_{B} = -17.75 \pm 0.39$ mag and a maximum pseudo-bolometric luminosity of $L_\mathrm{max} = 6.03 \pm 0.01 \times 10^{42}…
Stripped-envelope core-collapse supernovae can be divided into two broad classes: the common Type Ib/c supernovae (SNe Ib/c), powered by the radioactive decay of $^{56}$Ni, and the rare superluminous supernovae (SLSNe), most likely powered by the spin-down of a magnetar central engine. Up to now, the intermediate regime between these two populations has remained mostly unexplored. Here, we present a comprehensive study of 40 \textit{luminous supernovae} (LSNe), SNe with peak magnitudes of $M_r = -19$ to $-20$ mag, bound by SLSNe on the bright end and by SNe Ib/c on the dim end. Spectroscopical…
Light-Curve and Spectral Properties of Type II Supernovae from the ATLAS survey
Universidad Nacional de La Plata（拉普拉塔国立大学）；Instituto de Astrofísica de La Plata (IALP), CCT-CONICET-UNLP（拉普拉塔天体物理研究所 (IALP)、CCT-CONICET-UNLP）；European Southern Observatory（欧洲南方天文台）；Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo（东京大学基础物理学与数学宇宙 Kavli 研究所 (WPI)）；Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciências, Universidade de Lisboa（里斯本大学科学学院空间天体物理研究所）；Institut d’Estudis Espacials de Catalunya (IEEC)（加泰罗尼亚太空研究研究所 (IEEC)）；Institute of Space Sciences (ICE, CSIC)（空间科学研究所 (ICE, CSIC)）；Stockholm University（斯德哥尔摩大学）
What makes a supernova truly `peculiar?' In this chapter we attempt to address this question by tracing the history of the use of `peculiar' as a descriptor of non-standard supernovae back to the original binary spectroscopic classification of Type I vs. Type II proposed by Minkowski (1941). A handful of noteworthy examples (including SN 2012au, SN 2014C, iPTF14hls, and iPTF15eqv) are highlighted to illustrate a general theme: classes of supernovae that were once thought to be peculiar are later seen as logical branches of standard events. This is not always the case, however, and we discuss A…
We present 75 near-infrared (NIR; 0.8$-$2.5 $μ$m) spectra of 34 stripped-envelope core-collapse supernovae (SESNe) obtained by the Carnegie Supernova Project-II (CSP-II), encompassing optical spectroscopic Types IIb, Ib, Ic, and Ic-BL. The spectra range in phase from pre-maximum to 80 days past maximum. This unique data set constitutes the largest NIR spectroscopic sample of SESNe to date. NIR spectroscopy provides observables with additional information that is not available in the optical. Specifically, the NIR contains the resonance lines of He I and allows a more detailed look at whether T…
We present optical photometric and spectroscopic studies of three supernovae (SNe) SN 2013bz, PSN J0910+5003 and ASASSN-16ex. UV-optical photometric data of ASASSN-16ex obtained with Swift-UVOT are also analyzed. These objects were initially classified as 09dc-like type Ia SNe. The decline rate parameters ($Δm_{15}(B)_{true}$) are derived as 0.92 $\pm$ 0.04 (SN 2013bz), 0.70 $\pm$ 0.05 (PSN J0910+5003) and 0.73 $\pm$ 0.03 (ASASSN-16ex). The estimated $B$ band absolute magnitudes at maximum: $-$19.61 $\pm$ 0.20 mag for SN 2013bz, $-$19.44 $\pm$ 0.20 mag for PSN J0910+5003 and $-$19.78 $\pm$ 0.2…
Photometric and Spectroscopic Studies of Type Ic Supernovae SN 2020akf and SN 2021mxx
Pt. Ravishankar Shukla University（普特·拉维尚卡尔·舒克拉大学）；Sant Guru Ghasidas Govt. Post Graduate College（圣古鲁·加希达斯政府研究生学院）；The ICFAI University, Raipur（赖布尔英法伊大学）；University of Chile（智利大学）；Indian Institute of Astrophysics（印度天体物理研究所）；Tsung-Dao Lee Institute, Shanghai Jiao Tong University（上海交通大学李政道研究所）
How Low Can We Go? Minimum Spectroscopic Requirements For Supernova Subtype Classification
University of Delaware（特拉华大学）；Vera C. Rubin Observatory（维拉·C·鲁宾天文台）；University of Virginia（弗吉尼亚大学）；NSF NOIRLab（美国国家科学基金会NOIRLab）；West Chester University of Pennsylvania（宾夕法尼亚州韦斯特切斯特大学）
Type II supernovae (SNe) interacting with disklike circumstellar matter (CSM) have been suggested as an explanation of some unusual Type II SNe, e.g., the so-called "impossible" SN, iPTF14hls. There are some radiation hydrodynamics simulations for such SNe interacting with a CSM disk. However, such disk interaction models so far have not included the effect of the ionization and recombination processes in the SN ejecta, i.e., the fact that the photosphere of Type IIP SNe between $\sim 10$-$\sim 100$ days is regulated by the hydrogen recombination front. We calculate light curves for Type IIP S…
Normal or transitional? The evolution and properties of two type Ia supernovae in the Virgo cluster
University of Copenhagen（哥本哈根大学）；INAF, Osservatorio Astronomico di Capodimonte（意大利国家天体物理研究所，卡波迪蒙特天文台）；University of Queensland（昆士兰大学）；University of Illinois at Urbana-Champaign（伊利诺伊大学厄巴纳-香槟分校）；Institute for Astronomy, University of Hawaii（夏威夷大学天体物理研究所）；Kavli Institute for Theoretical Physics, University of California, Santa Barbara（加州大学圣塔芭芭拉分校，卡弗里理论物理研究所）；Las Cumbres Observatory（拉斯坎伯斯天文台）；David A. Dunlap Department of Astronomy and Astrophysics, University of Toronto（多伦多大学，大卫A·邓普林天文学与天体物理学系）
We present extensive spectroscopic observations for one of the closest type Ia supernovae (SNe Ia), SN 2014J discovered in M82, ranging from 10.4 days before to 473.2 days after B-band maximum light. The diffuse interstellar band (DIB) features detected in a high-resolution spectrum allow an estimate of line-of-sight extinction as Av=1.9+/-0.6 mag. Spectroscopically, SN 2014J can be put into the high-velocity (HV) subgroup in Wang's classification with a velocity of Si~II 6355 at maximum light as about 12200 km/s, but has a low velocity gradient (LVG, following Benetti's classification) as 41+…
Fast-declining Type Ia supernovae (SN Ia) separate into two categories based on their bolometric and near-infrared (NIR) properties. The peak bolometric luminosity ($\mathrm{L_{max}}$), the phase of the first maximum relative to the optical, the NIR peak luminosity and the occurrence of a second maximum in the NIR distinguish a group of very faint SN Ia. Fast-declining supernovae show a large range of peak bolometric luminosities ($\mathrm{L_{max}}$ differing by up to a factor of $\sim$ 8). All fast-declining SN Ia with $\mathrm{L_{max}} < 0.3 \cdot$ 10$^{43}\mathrm{erg s}^{-1}$ are spectrosco…
The abundance distribution in the ejecta of the peculiar slowly declining Type Ia supernova (SN\,Ia) SN\,1999aa is obtained by modelling a time series of optical spectra. Similar to SN\,1991T, SN\,1999aa was characterised by early-time spectra dominated by \FeIII\ features and a weak \SiII\,6355\,Å line, but it exhibited a high-velocity \CaII\,H\&K line and morphed into a spectroscopically normal SN\,Ia earlier. Three explosion models are investigated, yielding comparable fits. The innermost layers are dominated by $\sim 0.3$\,\Msun\ of neutron-rich stable Fe-group elements, mostly stable iron…
We study the spectroscopic evolution of superluminous supernovae (SLSNe) later than 100 days after maximum light. We present new data for Gaia16apd and SN2017egm, and analyse these with a larger sample comprising 41 spectra of 12 events. The spectra become nebular within 2-4 $e$-folding times after light curve peak, with the rate of spectroscopic evolution correlated to the light curve timescale. Emission lines are identified with well-known transitions of oxygen, calcium, magnesium, sodium and iron. SLSNe are differentiated from other Type Ic SNe by a prominent O I $λ$7774 line and higher-ion…
