Supernova (SN) explosions, through the metals they release, play a pivotal role in the chemical evolution of the Universe and the origin of life. Nebular phase spectroscopy constrains such metal yields, for example through forbidden line emission associated with OI, CaII, FeII, or FeIII. Fluid instabilities during the explosion produce a complex 3D ejecta structure, with considerable macroscopic, but no microscopic, mixing of elements. This structure sets a formidable challenge for detailed nonlocal thermodynamic equilibrium radiative transfer modeling, which is generally limited to 1D in grid-based codes. Here, we present a novel and simple method that allows for macroscopic mixing without any microscopic mixing, thereby capturing the essence of mixing in SN explosions. With this new technique, the macroscopically mixed ejecta is built by shuffling in mass space, or equivalently in velocity space, the shells from the unmixed coasting ejecta. The method requires no change to the radiative transfer, but necessitates high spatial resolution to resolve the rapid variation in composition with depth inherent to this shuffled-shell structure. We show results for a few radiative-transfer simulations for a Type II SN explosion from a 15Msun progenitor star. Our simulations capture the strong variations in temperature or ionization between the various shells that are rich in H, He, O, or Si. Because of nonlocal energy deposition, gamma rays permeate through an extended region of the ejecta, making the details of the shell arrangement unimportant. The greater physical consistency of the method delivers spectral properties at nebular times that are more reliable, in particular in terms of individual emission line strengths, which may serve to constrain the SN yields and, for core collapse SNe, the progenitor mass. The method works for all SN types.
When supernovae enter the nebular phase after a few months, they reveal spectral fingerprints of their deep interiors, glowing by radioactivity produced in the explosion. We are given a unique opportunity to see what an exploded star looks like inside. The line profiles and luminosities encode information about physical conditions, explosive and hydrostatic nucleosynthesis, and ejecta morphology, which link to the progenitor properties and the explosion mechanism. Here, the fundamental properties of spectral formation of supernovae in the nebular phase are reviewed. The formalism between eject…
We propose that nebular H-alpha emission as detected in the Type Ic superluminous supernova iPTF13ehe stems from matter which is stripped from a companion star when the supernova ejecta collide with it. The temporal evolution, the line broadening, and the overall blueshift of the emission are consistent with this interpretation. We scale the nebular H-alpha luminosity predicted for Type Ia supernovae in single-degenerate systems to derive the stripped mass required to explain the H-alpha luminosity of iPTF13ehe. We find a stripped mass of 0.1 - 0.9 solar masses, assuming that the supernova lum…
We use non-local thermal equilibrium radiative transport modeling to examine observational signatures of sub-Chandrasekhar mass double detonation explosions in the nebular phase. Results range from spectra that look like typical and subluminous Type Ia supernovae (SNe) for higher mass progenitors to spectra that look like Ca-rich transients for lower mass progenitors. This ignition mechanism produces an inherent relationship between emission features and the progenitor mass as the ratio of the nebular [Ca II]/[Fe III] emission lines increases with decreasing white dwarf mass. Examining the [Ca…
We extend the range of validity of the ARTIS 3D radiative transfer code up to hundreds of days after explosion, when Type Ia supernovae are in their nebular phase. To achieve this, we add a non-local thermodynamic equilibrium (non-LTE) population and ionisation solver, a new multi-frequency radiation field model, and a new atomic dataset with forbidden transitions. We treat collisions with non-thermal leptons resulting from nuclear decays to account for their contribution to excitation, ionisation, and heating. We validate our method with a variety of tests including comparing our synthetic ne…
We place statistical constraints on Type Ia supernova (SN Ia) progenitors using 227 nebular phase spectra of 111 SNe Ia. We find no evidence of stripped companion emission in any of the nebular phase spectra. Upper limits are placed on the amount of mass that could go undetected in each spectrum using recent hydrodynamic simulations. With these null detections, we place an observational $3σ$ upper limit on the fraction of SNe Ia that are produced through the classical H-rich non-degenerate companion scenario of < 5.5%. Additionally, we set a tentative $3σ$ upper limit on He star progenitor sce…
Nebular phase spectra of core-collapse supernovae (SNe) provide critical and unique information on the progenitor massive star and its explosion. We present a set of 1-D steady-state non-local thermodynamic equilibrium radiative transfer calculations of type II SNe at 300d after explosion. Guided by results for a large set of stellar evolution simulations, we craft ejecta models for type II SNe from the explosion of a 12, 15, 20, and 25Msun star. The ejecta density structure and kinetic energy, the 56Ni mass, and the level of chemical mixing are parametrized. Our model spectra are sensitive to…
Key information about the progenitor system and the explosion mechanism of Type Ia supernovae (SNe~Ia) can be obtained from early observations, within a few days from explosion. iPTF16abc was discovered as a young SN~Ia with excellent early time data. Here, we present photometry and spectroscopy of the SN in the nebular phase. A comparison of the early time data with a sample of SNe~Ia shows distinct features, differing from normal SNe~Ia at early phases but similar to normal SNe~Ia at a few weeks after maximum light (i.e. the transitional phase) and well into the nebular phase. The transparen…
In this study, we present one-dimensional, non-local-thermodynamic-equilibrium, radiative transfer simulations (using CMFGEN) in which we introduce micro-clumping at nebular times into two Type Ia supernova ejecta models. We use one sub-Chandrasekhar (sub-M$_{\rm Ch}$) ejecta with 1.02 M$_\odot$ and one M$_{\rm Ch}$ ejecta model with 1.40 M$_\odot$. We introduce clumping factors $f=$0.33,0.25, and 0.10 which are constant throughout the ejecta and compared to the unclumped $f=1.0$ case. We find that clumping is a natural mechanism to reduce the ionization of the ejecta, reducing emission from […
We compare analytic predictions of supernova light curves with recent high quality data from SN2011fe (Ia), from KSN2011b (Ia), and the Palomar Transient Factory (PTF) and the La Silla-QUEST variability survey (LSQ) (Ia). Because of the steady, fast cadence of observations, KSN2011b provides unique new information on SNe Ia: the smoothness of the light curve, which is consistent with significant large-scale mixing during the explosion, possibly due to 3D effects (e.g., Rayleigh-Taylor instabilities), and provides support for a slowly-varying leakage (mean opacity). For a more complex light cur…
The light curves of Type Ia supernovae (SNe Ia) are powered by the radioactive decay of $^{56}$Ni to $^{56}$Co at early times, and the decay of $^{56}$Co to $^{56}$Fe from ~60 days after explosion. We examine the evolution of the [Co III] 5892 A emission complex during the nebular phase for SNe Ia with multiple nebular spectra and show that the line flux follows the square of the mass of $^{56}$Co as a function of time. This result indicates both efficient local energy deposition from positrons produced in $^{56}$Co decay, and long-term stability of the ionization state of the nebula. We compi…
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$7774 line and higher-ion…
Multidimensional Nebular-Phase Calculations of Dynamically-Driven Double-Degenerate Double-Detonation Models for Type Ia Supernovae
Queen’s University Belfast（贝尔法斯特女王大学）；Cosmic Dawn Center (DAWN)（宇宙黎明中心）；Niels Bohr Institute, University of Copenhagen（哥本哈根大学尼尔斯·玻尔研究所）；GSI Helmholtzzentrum für Schwerionenforschung（重离子研究中心）；Max-Planck-Institut für Astrophysik（马克斯·普朗克天体物理研究所）；School of Physics, Trinity College Dublin, The University of Dublin（都柏林三一学院物理系）；Heidelberger Institut für Theoretische Studien（海德堡理论研究所）
We examine the evolution of late-time, optical nebular features of Type Ia supernovae (SNe Ia) using a sample consisting of 160 spectra of 27 normal SNe Ia taken from the literature as well as unpublished spectra of SN 2008Q and ASASSN-14lp. Particular attention was given to nebular features between 4000-6000 A in terms of temporal changes in width and central wavelength. Analysis of the prominent late-time 4700 A feature shows a progressive central wavelength shift from ~4600 A to longer wavelengths out to at least day +300 for our entire sample. We find no evidence for the feature's red-ward…
We investigate the origin of the H$α$-like structure seen in late-phase nebular spectra of type IIb supernovae (SNe IIb) at $\sim 200$ days after the explosion. We compare the luminosities of emission lines in the nebular spectra with the light curve peak magnitudes to reveal their power sources. In this work, we analyze 7 SNe IIb, as well as 2 SNe Ib (SN 2007Y and iPTF 13bvn) that show the H$α$-like emission in their nebular spectra. The luminosity of the H$α$-like emission shows a tight correlation with the light curve peak magnitude, sharing the same behavior with other nebular lines. This …
Type II supernovae (SNe) often exhibit a linear polarization, arising from free-electron scattering, with complicated optical signatures, both in the continuum and in lines. Focusing on the early nebular phase, at a SN age of 200d, we conduct a systematic study of the polarization signatures associated with a 56Ni `blob' that breaks spherical symmetry. Our ansatz, supported by nonLTE radiative transfer calculations, is that the primary role of such a 56Ni blob is to boost the local density of free electrons, which is otherwise reduced following recombination in SNe II. Using 2D polarized radia…
Low-Luminosity Type IIP Supernovae from the Zwicky Transient Facility Census of the Local Universe. III: Hunting for electron-capture supernovae using nebular spectroscopy
California Institute of Technology（加州理工学院）；Stockholm University（斯德哥尔摩大学）；Weizmann Institute of Science（魏茨曼科学研究所）；NSF NOIRLab（美国国家光学-红外天文研究实验室）
This work examines the relationships between the properties (flux ratios, full width at half-maximum velocities) of the [O I] $λλ$6300, 6364, [Ca II] $λλ$7291, 7323, and the Ca II near-infrared triplet, emission lines of a large sample of core-collapse supernovae (SNe) and Ca-rich transients (509 spectra of 86 transients, of which 10 transients are Ca-rich events). Line-flux ratios as a function of time were investigated with differences identified between the transient classes, in particular the Type II SNe were found to have distinct line-flux ratios compared to stripped-envelope (SE) SNe. N…
Aims. We present and analyse late-time observations of the type-Ib supernova with possible pre-supernova progenitor detection, iPTF13bvn, taken at $\sim$300 days after the explosion, and discuss these in the context of constraints on the supernova's progenitor. Previous studies have proposed two possible natures for the progenitor of the supernova, i.e. a massive Wolf-Rayet star or a lower-mass star in close binary system. Methods. Our observations show that the supernova has entered the nebular phase, with the spectrum dominated by Mg~I]$λλ$4571, [O~I]$λλ$6300, 6364, and [Ca~II]$λλ$7291, 7324…
Numerous superluminous supernovae (SLSNe) of Type Ic have been discovered and monitored in the last decade. The favored mechanism at their origin is a sustained power injection from a magnetar. This study presents non-local thermodynamic equilibrium time-dependent radiative transfer simulations of various single carbon-rich Wolf-Rayet star explosions influenced by magnetars of diverse properties and covering from a few days to one or two years after explosion. Nonthermal processes are treated; the magnetar-power deposition profile is prescribed; dynamical effects are ignored. In this context, …
Massive stars live fast and die young. They shine furiously for a few million years, during which time they synthesize most of the heavy elements in the universe in their cores. They end by blowing themselves up in a powerful explosion known as a supernova. During this process, the core collapses to a neutron star or a black hole, while the outer layers are expelled with velocities of thousands of kilometers per second. The resulting fireworks often outshine the entire host galaxy for many weeks. The explosion energy is eventually radiated away, but powering of the newborn nebula continues by …
We present nebular-phase imaging and spectroscopy for the hydrogen-poor superluminous supernova SN 2015bn, at redshift z=0.1136, spanning +250-400 d after maximum light. The light curve exhibits a steepening in the decline rate from 1.4 mag/(100 d) to 1.7 mag/(100 d), suggestive of a significant decrease in the opacity. This change is accompanied by a transition from a blue continuum superposed with photospheric absorption lines to a nebular spectrum dominated by emission lines of oxygen, calcium and magnesium. There are no obvious signatures of circumstellar interaction or large nickel mass. …
We present late-time spectra of eight Type Ia supernovae (SNe Ia) obtained at $>200$ days after peak brightness using the Gemini South and Keck telescopes. All of the SNe Ia in our sample were nearby, well separated from their host galaxy's light, and have early-time photometry and spectroscopy from the Las Cumbres Observatory (LCO). Parameters are derived from the light curves and spectra such as peak brightness, decline rate, photospheric velocity, and the widths and velocities of the forbidden nebular emission lines. We discuss the physical interpretations of these parameters for the indivi…
We present SN 2020jfo, a Type IIP supernova in the nearby galaxy M61. Optical light curves from the Zwicky Transient Facility, complemented with data from Swift and near-IR photometry are presented. The 350-day duration bolometric light curve exhibits a relatively short (~ 65 days) plateau. This implies a moderate ejecta mass (~ 5 Msun). A series of spectroscopy is presented, including spectropolarimetric observations. The nebular spectra are dominated by Halpha but also reveal emission lines from oxygen and calcium. Comparisons to synthetic nebular spectra indicate an initial progenitor mass …
Late-time spectra of stripped-envelope CC-SNe are dominated by strong [O {\sc i}] $λλ$6300,6363 emission, caused by thermal electron excitation of forbidden [O {\sc i}] transitions. The permitted O {\sc i} 7774 Å line is also often observed. This line cannot result from thermal electron excitation of the oxygen ground state. In this work tests are performed to verify whether the line can be powered by oxygen recombination alone, using the examples of two of the best studied type Ic SNe, 1998bw and 2002ap. Temperature-dependent effective recombination coefficients for neutral oxygen are calcula…
We discover clear doubly-peaked line profiles in 3 out of ~20 type Ia supernovae (SNe Ia) with high-quality nebular-phase spectra. The profiles are consistently present in three well-separated Co/Fe emission features. The two peaks are respectively blue-shifted and red-shifted relative to the host galaxies and are separated by ~5000 km/s. The doubly-peaked profiles directly reflect a bi-modal velocity distribution of the radioactive Ni56 in the ejecta that powers the emission of these SNe. Due to their random orientations, only a fraction of SNe with intrinsically bi-modal velocity distributio…
The observed diversity in Type Ia supernovae (SNe Ia) -- the thermonuclear explosions of carbon-oxygen white dwarf stars used as cosmological standard candles -- is currently met with a variety of explosion models and progenitor scenarios. To help improve our understanding of whether and how often different models contribute to the occurrence of SNe Ia and their assorted properties, we present a comprehensive analysis of seven nearby SNe Ia. We obtained one to two epochs of optical spectra with Gemini Observatory during the nebular phase ($>$200 days past peak) for each of these events, all of…
The nebular spectra of Type Ia supernovae ($\gtrapprox$ 100 days after explosion) consist mainly of emission lines from singly- and doubly-ionised Fe-group nuclei. However, theoretical models for many scenarios predict that non-thermal ionisation leads to multiply-ionised species whose recombination photons ionise and deplete Fe$^{+}$ , resulting in negligible [Fe II] emission. We investigate a method to determine the collisional excitation conditions from [Fe II] line ratios independently from the ionisation state and find that it cannot be applied to highly-ionised models due to the influenc…
We present an analysis of the nebular spectra of 103 stripped envelope (SE) supernovae (SNe) collected from the literature and observed with the Subaru Telescope from 2002 to 2012, focusing on [O I] 6300, 6363. The line profile and width of [O I] are employed to infer the ejecta geometry and the expansion velocity of the inner core. These two measurements are then compared with the SN sub types, and further with the [O I]/[Ca II] ratio, which is used as an indicator of the progenitor CO core mass. Based on the best fit results of the [O I] profile, the objects are classified into different mor…
One clear observational prediction of the single degenerate progenitor scenario as the origin of type Ia supernovae (SNe) is the presence of relatively narrow ($\approx$1000 km s$^{-1}$) H$α$ emission at nebular phases, although this feature is rarely seen. We present a compilation of nebular phase H$α$ limits for SN Ia in the literature and demonstrate that this heterogenous sample has been biased towards SN Ia with relatively high luminosities and slow decline rates, as parameterized by $Δ$m$_{15}(B)$, the difference in $B$-band magnitude between maximum light and fifteen days afterward. Mot…
Peak 'Nebular' Emission and Early Flux Excesses in Ca-strong Supernovae
Trinity College Dublin, the University of Dublin（都柏林三一学院，都柏林大学）；University of Portsmouth（朴茨茅斯大学）；California Institute of Technology（加州理工学院）；University of Warwick（华威大学）；Queen’s University Belfast（贝尔法斯特女王大学）；Stockholm University（斯德哥尔摩大学）；Purdue University（普渡大学）
The nebular-epoch spectrum of the rapidly declining, "transitional" type Ia supernova (SN) 2007on showed double emission peaks, which have been interpreted as indicating that the SN was the result of the direct collision of two white dwarfs. The spectrum can be reproduced using two distinct emission components, one red-shifted and one blue shifted. These components are similar in mass but have slightly different degrees of ionization. They recede from one another at a line-of-sight speed larger than the sum of the combined expansion velocities of their emitting cores, thereby acting as two ind…
We present multi-wavelength time-series spectroscopy of SN 2013aa and SN 2017cbv, two Type Ia supernovae (SNe Ia) on the outskirts of the same host galaxy, NGC 5643. This work utilizes new nebular-phase near-infrared (NIR) spectra obtained by the Carnegie Supernova Project-II, in addition to previously published optical and NIR spectra. By measuring nebular-phase [Fe II] lines in both the optical and NIR, we examine the explosion kinematics and test the efficacy of several emission line fitting techniques commonly used in the literature. The NIR [Fe II] 1.644 $μ$m line provides the most robust…
Nebular Fingerprints of a Violent White Dwarf Merger: 3D NLTE Modelling of Type Ia Supernovae
Oregon State University（俄勒冈州立大学）；Max-Planck-Institut für Astrophysik（马普天体物理研究所）；Queen’s University Belfast（贝尔法斯特女王大学）；Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA)（天体物理学跨学科探索与研究中心）；Trinity College Dublin, The University of Dublin（都柏林三一学院）；Heidelberger Institut für Theoretische Studien（海德堡理论研究所）；Zentrum für Astronomie der Universität Heidelberg（海德堡大学天文中心）
Reconciling the Tension Between Light Curve Modeling of Type II Supernovae and Neutrino-Driven Core-Collapse Supernovae Models with Late-Phase Spectroscopy
In this work, we present photometric and spectroscopic data of the low-luminosity Type IIP supernova (SN) 2018hwm. The object shows a faint ($M_r=-15$ mag) and very long ($\sim$130 days) plateau, followed by a 2.7 mag drop in the $r$-band to the radioactive tail. The first spectrum shows a blue continuum with narrow Balmer lines, while during the plateau the spectra show numerous metal lines, all with strong and narrow P-Cygni profiles. The expansion velocities are low, in the 1000-1400 km s$^{-1}$ range. The nebular spectrum, dominated by H$α$ in emission, reveals weak emission from [O I] and…
Nebular-phase observations and spectral models of Type Ic superluminous supernovae are presented. LSQ14an and SN 2015bn both display late-time spectra similar to galaxy-subtracted spectra of SN 2007bi, and the class shows strong similarity with broad-lined Type Ic SNe such as SN 1998bw. Near-infrared observations of SN 2015bn show a strong Ca II triplet, O I 9263, O I 1.13 um and Mg I 1.50 um, but no distinct He, Si, or S emission. The high Ca II NIR/[Ca II] 7291,7323 ratio of ~2 indicates a high electron density of n_e >~ 10^8 cm^{-3}. Spectral models of oxygen-zone emission are investigated …
We present a new relativistic radiative-transfer code for $γ$-rays of energy less than 5 MeV in supernova (SN) ejecta. This code computes the opacities, the prompt emissivity (i.e. decay), and the scattering emissivity, and solves for the intensity in the co-moving frame. Because of the large expansion velocities of SN ejecta, we ignore redistribution effects associated with thermal motions. The energy deposition is calculated from the energy removed from the radiation field by scattering or photoelectric absorption. This new code yields comparable results to an independent Monte Carlo code. H…
A large fraction of core-collapse supernovae (CCSNe), 30-50%, are expected to originate from the low-mass end of progenitors with $M_{\rm ZAMS}~= 8-12~M_\odot$. However, degeneracy effects make stellar evolution modelling of such stars challenging, and few predictions for their supernova light curves and spectra have been presented. Here we calculate synthetic nebular spectra of a 9 $M_\odot$ Fe CCSN model exploded with the neutrino mechanism. The model predicts emission lines with FWHM$\sim$1000 km/s, including signatures from each deep layer in the metal core. We compare this model to observ…
We present 1-D non-Local-Thermodynamic-Equilibrium time-dependent radiative-transfer simulations for supernovae (SNe) of type IIb, Ib, and Ic that result from the terminal explosion of the mass donor in a close-binary system. Here, we select three ejecta with a total kinetic energy of ~1.2e51erg, but characterised by different ejecta masses (2-5Msun), composition, and chemical mixing. The type IIb/Ib models correspond to the progenitors that have retained their He-rich shell at the time of explosion. The type Ic model arises from a progenitor that has lost its helium shell, but retains 0.32Msu…
The classic single-degenerate model for the progenitors of Type Ia Supernova (SN Ia) predicts that the supernova ejecta should be enriched with solar-like abundance material stripped from the companion star. Spectroscopic observations of normal SNe Ia at late times, however, have not resulted in definite detection of hydrogen. In this Letter, we study line formation in SNe Ia at nebular times using non-LTE spectral modeling. We present, for the first time, multidimensional radiative transfer calculations of SNe Ia with stripped material mixed in the ejecta core, based on hydrodynamical simulat…
SN2023ixf: ultraviolet-to-infrared radiative-transfer modeling of the nebular-phase evolution until 1000 days
Institut d’Astrophysique de Paris, CNRS-Sorbonne Université（巴黎天体物理研究所，法国国家科学研究中心-索邦大学）；French-Chilean Laboratory for Astronomy, IRL 3386, CNRS and Instituto de Astrofísica, Pontificia Universidad Católica de Chile（中法天文实验室，法国国家科学研究中心和智利天主教Pontificia大学天体物理研究所）；Cahill Center for Astrophysics, California Institute of Technology（加州理工学院卡希尔天体物理学中心）；Steward Observatory, University of Arizona（亚利桑那大学斯图尔德天文台）；Department of Astronomy, University of California, Berkeley（加州大学伯克利分校天文学系）；Institute of Space Sciences (ICE, CSIC)（空间科学研究所（CSIC））；Institut d’Estudis Espacials de Catalunya (IEEC)（加泰罗尼亚空间研究研究所）；Department of Physics and Astronomy, University of California, Davis（加州大学戴维斯分校物理与天文学系）
Geometry of circumstellar (CS) medium around supernovae (SNe) provides important diagnostics to understand the nature of their progenitors. In this paper, properties of CS dust around SN 2012dn, a super-Chandrasekhar candidate Type Ia supernova (SC-SN), have been studied through detailed three dimensional radiation transfer simulations. With the detected near-infrared excess from SN 2012dn, we show that it has a disk-like dusty CS environment whose mass is roughly consistent with a branch of an accreting white dwarf system (the single degenerate scenario). We show that a similar system should …
We measure the strength of HeII$λ$4686 nebular emission in passively evolving ("retired") galaxies, aiming to constrain their populations of hot accreting white dwarfs (WDs) in the context of the single degenerate (SD) scenario of Type Ia supernovae (SNe Ia). In the SD scenario, as a WD burns hydrogen-rich material accreted from a companion star, it becomes a powerful source of ionizing UV emission. If significant populations of such sources exist in galaxies, strong emission in the recombination lines of HeII should be expected from the interstellar medium. To explore this conjecture, we sele…
