We present generalized supernova (SN) light curve (LC) models for a variety of power inputs including the previously proposed ideas of radioactive decay of super(56)Ni and super(56)Co and magnetar ...spin-down. We extend those solutions to include finite progenitor radius and stationary photospheres as might be the case for SN that are powered by interaction of the ejecta with circumstellar matter (CSM). We provide an expression for the power input that is produced by self-similar forward and reverse shocks that efficiently convert their kinetic energy into radiation. We find that this ejecta-CSM interaction luminosity that we derive is in agreement with results from multi-dimensional radiation hydrodynamics simulations in the case of an optically thin CSM. We develop a semi-analytical model for the case of an optically thick CSM by invoking an approximation for the effects of radiative diffusion similar to that adopted by Arnett for SN II and compare this model to the results of numerical radiation hydrodynamics models. This model can give complex LCs, but for monotonically declining shock input, the LCs have a smooth rise, peak, and decline. In the context of this model, we provide predictions of the shock breakout of the forward shock from the optically thick part of the CSM envelope. We also introduce a hybrid LC model that incorporates ejecta-CSM interaction plus super(56)Ni and super(56)Co radioactive decay input. We fit this hybrid model to the LC of the super-luminous supernova (SLSN) 2006gy. We find that shock heating produced by ejecta-CSM interaction plus some contribution from radioactive decay provides a better fit to the LC of this event than previously presented models. We also address the relation between SN IIL and SN IIn with ejecta-CSM interaction models. The faster decline of SN IIL can be reproduced by the diffusion of previously deposited shock power if the shock power input to the diffusive component vanishes when the reverse shock sweeps up the whole ejecta and/or the forward shock propagates through the optically thick CSM. A CSM interaction with forward and reverse shock power input can produce the LCs of SN IIn in terms of duration, shape, and decline rate, depending on the properties of the CSM envelope and the progenitor star. This model can also produce LCs that are symmetric in shape around peak luminosity, which is the case for the observed LCs of some recently discovered peculiar transient events. We conclude that the observed LC variety of SN IIn and of some SLSNe is likely to be a byproduct of the large range of conditions relevant to significant ejecta-CSM interaction as a power source.
The issue of which stars may reach the conditions of electron/positron pair-formation instability is of importance to understand the final evolution both of the first stars and of contemporary stars. ...The criterion to enter the pair-instability regime in density and temperature is basically controlled by the mass of the oxygen core. The main-sequence masses that produce a given oxygen core mass are, in turn, dependent on metallicity, mass loss, and convective and rotationally induced mixing. We examine the evolution of massive stars to determine the minimum main-sequence mass that can encounter pair-instability effects, either a pulsational pair-instability supernova (PPISN) or a full-fledged pair-instability supernova (PISN). We concentrate on zero-metallicity stars with no mass-loss subject to the Schwarzschild criterion for convective instability, but also explore solar metallicity and mass loss and the Ledoux criterion. As expected, for sufficiently strong rotationally induced mixing, the minimum main-sequence mass is encountered for conditions that induce effectively homogeneous evolution such that the original mass is converted almost entirely to helium and then to oxygen. For this case, we find that the minimum main-sequence mass is about 40 M sub(middot in circle) to encounter PPISN and about 65 M sub(middot in circle) to encounter a PISN. The implications of these results for the first stars and for contemporary supernovae are discussed.
Superluminous supernova (SLSN) light curves exhibit superior diversity compared to their regular-luminosity counterparts in terms of rise and decline timescales, peak luminosities, and overall ...shapes. It remains unclear whether this striking variety arises due to a dominant power input mechanism involving many underlying parameters or due to contributions by different progenitor channels. In this work, we propose that a systematic quantitative study of SLSN light-curve timescales and shape properties, such as symmetry around peak luminosity, can be used to characterize these enthralling stellar explosions. We find that applying clustering analysis to the properties of model SLSN light curves, powered by either a magnetar spindown or a supernova ejecta-circumstellar matter interaction mechanism, can yield a distinction between the two, especially in terms of light-curve symmetry. We show that most events in the observed SLSN sample with well-constrained light curves and early detections are strongly associated with clusters dominated by circumstellar interaction models. Magnetar spindown models also show association at a lower degree but have difficulty in reproducing fast evolving and fully symmetric light curves. We believe this is due to the truncated nature of the circumstellar interaction shock energy input compared to decreasing but continuous power input sources like magnetar spindown and radioactive 56Ni decay. Our study demonstrates the importance of clustering analysis in characterizing SLSNe based on high-cadence photometric observations that will be made available in the near future by surveys like the Large Synoptic Survey Telescope, Zwicky Transient Facility, and Panoramic Survey Telescope and Rapid Response System.
Is Betelgeuse the Outcome of a Past Merger? Chatzopoulos, E.; Frank, Juhan; Marcello, Dominic C. ...
Astrophysical journal/The Astrophysical journal,
06/2020, Letnik:
896, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We explore the possibility that the star Orionis (Betelgeuse) is the outcome of a merger that occurred in a low-mass-ratio ( 0.07-0.25) binary system some time in the past hundreds of thousands of ...years. To that goal, we present a simple analytical model to approximate the perturbed internal structure of a post-merger object following the coalescence of a secondary in the mass range 1-4 M into the envelope of a 15-17 M primary. We then compute the long-term evolution of post-merger objects for a grid of initial conditions and make predictions about their surface properties for evolutionary stages that are consistent with the observed location of Betelgeuse in the Hertzsprung-Russell diagram. We find that if a merger occurred after the end of the primary's main-sequence phase, while it was expanding toward becoming a red supergiant star and typically with radius ∼200-300 R , then its envelope is spun up to values that remain in a range consistent with Betelgeuse observations for thousands of years of evolution. We argue that the best scenario that can explain both the fast rotation of Betelgeuse and its observed large space velocity is one where a binary was dynamically ejected by its parent cluster a few million years ago and then subsequently merged. An alternative scenario in which the progenitor of Betelgeuse was spun up by accretion in a binary and released by the supernova explosion of the companion requires a finely tuned set of conditions but cannot be ruled out.
We present the newly incorporated gray radiation hydrodynamics capabilities of the FLASH code based on a radiation flux-limiter-aware hydrodynamics numerical implementation designed specifically for ...applications in astrophysical problems. The implemented numerical methods consist of changes in the unsplit hydrodynamics solver and adjustments in the flux-limited radiation diffusion unit. Our approach can handle problems in both the strong and weak radiation-matter coupling limits, as well as transitions between the two regimes. Appropriate extensions in the "Helmholtz" equation of state are implemented to treat two-temperature astrophysical plasmas involving the interaction between radiation and matter and the addition of a new opacity unit based on the OPAL opacity database, commonly used for astrophysical fluids. A set of radiation-hydrodynamics test problems is presented aiming to showcase the new capabilities of FLASH and to provide direct comparison to other similar software instruments available in the literature. To illustrate the capacity of FLASH to simulate phenomena occurring in stellar explosions, such as shock breakout, radiative precursors, and supernova ejecta heating due to the decays of radioactive 56Ni and 56Co, we also present 1D supernova simulations and compare the computed light curves to those of the SNEC code. The latest public release of FLASH with these enhanced capabilities is available for download and use by the broader astrophysics community.
In certain mass ranges, massive stars can undergo a violent pulsation triggered by the electron/positron pair instability that ejects matter, but does not totally disrupt the star. After one or more ...of these pulsations, such stars are expected to undergo core-collapse to trigger a supernova (SN) explosion. The mass range susceptible to this pulsational phenomena may be as low as 50-70 M sub(middot in circle) if the progenitor is of very low metallicity and rotating sufficiently rapidly to undergo nearly homogeneous evolution. The mass, dynamics, and composition of the matter ejected in the pulsation are important aspects for determining the subsequent observational characteristics of the explosion. We examine the dynamics of a sample of stellarmodels and rotation rates and discuss the implications for the first stars, for LBV-like phenomena, and for superluminous SNe. We find that the shells ejected by pulsational pair-instability events with rapidly rotating progenitors (>30% the critical value) are hydrogen-poor and helium- and oxygen-rich.
ABSTRACT We explore the effects of rotation on convective carbon, oxygen, and silicon shell burning during the late stages of evolution in a 20 M star. Using the Modules for Experiments in Stellar ...Astrophysics we construct one-dimensional (1D) stellar models both with no rotation and with an initial rigid rotation of 50% of critical. At different points during the evolution, we map the 1D models into 2D and follow the multidimensional evolution using the FLASH compressible hydrodynamics code for many convective turnover times until a quasi-steady state is reached. We characterize the strength and scale of convective motions via decomposition of the momentum density into vector spherical harmonics. We find that rotation influences the total power in solenoidal modes, with a slightly larger impact for carbon and oxygen shell burning than for silicon shell burning. Including rotation in 1D stellar evolution models alters the structure of the star in a manner that has a significant impact on the character of multidimensional convection. Adding modest amounts of rotation to a stellar model that ignores rotation during the evolutionary stage, however, has little impact on the character of the resulting convection. Since the spatial scale and strength of convection present at the point of core collapse directly influence the supernova mechanism, our results suggest that rotation could play an important role in setting the stage for massive stellar explosions.
ABSTRACT The recent discovery of the unprecedentedly super-luminous transient ASASSN-15lh (or SN 2015L) with its UV-bright secondary peak challenges all the power-input models that have been proposed ...for super-luminous supernovae. Here we examine some of the few viable interpretations of ASASSN-15lh in the context of a stellar explosion, involving combinations of one or more power inputs. We model the light curve of ASASSN-15lh with a hybrid model that includes contributions from magnetar spin-down energy and hydrogen-poor circumstellar interaction. We also investigate models of pure circumstellar interaction with a massive hydrogen-deficient shell and discuss the lack of interaction features in the observed spectra. We find that, as a supernova, ASASSN-15lh can be best modeled by the energetic core-collapse of an ∼40 M star interacting with a hydrogen-poor shell of ∼20 M . The circumstellar shell and progenitor mass are consistent with a rapidly rotating pulsational pair-instability supernova progenitor as required for strong interaction following the final supernova explosion. Additional energy injection by a magnetar with an initial period of 1-2 ms and magnetic field of 0.1-1 × 1014 G may supply the excess luminosity required to overcome the deficit in single-component models, but this requires more fine-tuning and extreme parameters for the magnetar, as well as the assumption of efficient conversion of magnetar energy into radiation. We thus favor a single-input model where the reverse shock formed in a strong SN ejecta-circumstellar matter interaction following a very powerful core-collapse SN explosion can supply the luminosity needed to reproduce the late-time UV-bright plateau.
We study the effects of rotation on the dynamics, energetics, and super(56)Ni production of pair instability supernova (PISN) explosions by performing rotating two-dimensional ("2.5D") hydrodynamics ...simulations. We calculate the evolution of eight low-metallicity (Z = 10 super(-3), 10 super(-4) Z sub(middot in circle)) massive (135-245 M sub(middot in circle)) PISN progenitors with initial surface rotational velocities of 50% of the critical Keplerian value using the stellar evolution code MESA. We allow for both the inclusion and the omission of the effects of magnetic fields in the angular momentum transport and in chemical mixing, resulting in slowly rotating and rapidly rotating final carbon-oxygen cores, respectively. Increased rotation for carbon-oxygen cores of the same mass and chemical stratification leads to less energetic PISN explosions that produce smaller amounts of super(56)Ni due to the effect of the angular momentum barrier that develops and slows the dynamical collapse. We find a non-monotonic dependence of super(56)Ni production on rotational velocity in situations when smoother composition gradients form at the outer edge of the rotating cores. In these cases, the PISN energetics are determined by the competition of two factors: the extent of chemical mixing in the outer layers of the core due to the effects of rotation in the progenitor evolution and the development of angular momentum support against collapse. Our 2.5D PISN simulations with rotation are the first presented in the literature. They reveal hydrodynamic instabilities in several regions of the exploding star and increased explosion asymmetries with higher core rotational velocity.
Context. A new class of super-luminous transients has recently been identified. These objects reach absolute luminosities of Mu < −21, lack hydrogen in their spectra, and are exclusively ...discovered by non-targeted surveys because they are associated with very faint galaxies. Aims. We aim to contribute to a better understanding of these objects by studying SN 2006oz, a newly-recognized member of this class. Methods. We present multi-color light curves of SN 2006oz from the SDSS-II SN Survey that cover its rise time, as well as an optical spectrum that shows that the explosion occurred at z ~ 0.376. We fitted black-body functions to estimate the temperature and radius evolution of the photosphere and used the parametrized code SYNOW to model the spectrum. We constructed a bolometric light curve and compared it with explosion models. In addition, we conducted a deep search for the host galaxy with the 10 m GTC telescope. Results. The very early light curves show a dip in the g- and r-bands and a possible initial cooling phase in the u-band before rising to maximum light. The bolometric light curve shows a precursor plateau with a duration of 6–10 days in the rest-frame. A lower limit of Mu < − 21.5 can be placed on the absolute peak luminosity of the SN, while the rise time is constrained to be at least 29 days. During our observations, the emitting sphere doubled its radius to ~2 × 1015 cm, while the temperature remained hot at ~15 000 K. As for other similar SNe, the spectrum is best modeled with elements including O ii and Mg ii, while we tentatively suggest that Fe iii might be present. The host galaxy is detected in gri with 25.74 ± 0.19, 24.43 ± 0.06, and 24.14 ± 0.12, respectively. It is a faint dwarf galaxy with Mg = −16.9. Conclusions. We suggest that the precursor plateau might be related to a recombination wave in a circumstellar medium (CSM) and discuss whether this is a common property of all similar explosions. The subsequent rise can be equally well described by input from a magnetar or by ejecta-CSM interaction, but the models are not well constrained owing to the lack of post-maximum observations, and CSM interaction has difficulties accounting for the precursor plateau self-consistently. Radioactive decay is less likely to be the mechanism that powers the luminosity. The host is a moderately young and star-forming, but not a starburst, galaxy.
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