Zheng-Wei Liu

Constraints on single-degenerate Chandrasekhar mass progenitors of Type Iax supernovae

SNe Iax are proposed as one new sub-class of SNe Ia. SNe Iax have been estimated to account for ~5-30% of the total SN Ia rate, and most SNe Iax have been discovered in late-type galaxies. In addition, observations constrain the progenitor systems of some SN Iax progenitors have ages of 3Gyr and low SN rates of ~3e-5/yr are found in our RG donor channel, indicating that this channel is unlikely to produce SNe Iax. We predict that the Galactic rate from the MS (He) donor channel is ~1.5e-3/yr (3e-4/yr), which is consistent with the observed Iax rate. The short delay times in the He channel support the young host environments of SNe Iax. However, the relative long delay times in the MS donor channel are less favourable for the observational constraints on the Iax progenitor ages. Finally, we set an upper limit on the pre-SN mass-loss rate at < 10e-4 M_sun/yr (for V_wind=100 km/s). The delay times in the SD Ch-mass model do not account for that most SNe Iax are located in late-type galaxies. However, at least one Iax event (SN 2008ge) is hosted by a S0 galaxy with no signs of star formation. Moreover, current X-ray observations cannot rule out the SD Ch-mass model. Taking all these into account and considering the uncertainty of the observed rate for SNe Iax, we suggest that some SNe Iax may be produced from weak deflagrations of Ch-mass CO WDs in SD progenitors, especially in the He-star channel. This is consistent with recent analysis of HST observations for the SN Iax SN2012Z. However, this SD deflagration model is still unlikely to be the most common progenitor scenario for SNe Iax.

The interaction of core-collapse supernova ejecta with a companion star

Context. The progenitors of many core-collapse supernovae (CCSNe) are expected to be in binary systems. After the SN explosion in a binary, the companion star may suffer from mass stripping and be shock heated as a result of the impact of the SN ejecta. If the binary system is disrupted by the SN explosion, the companion star is ejected as a runaway star, and in some cases as a hypervelocity star. Aims. By performing a series of three-dimensional (3D) hydrodynamical simulations of the collision of SN ejecta with the companion star, we investigate how CCSN explosions affect their binary companion. Methods. We use the BEC stellar evolution code to construct the detailed companion structure at the moment of SN explosion. The impact of the SN blast wave on the companion star is followed by means of 3D smoothed particle hydrodynamics (SPH) simulations using the STELLAR GADGET code. Results. For main-sequence (MS) companion stars, we find that the amount of removed stellar mass, the resulting impact velocity, and the chemical contamination of the companion that results from the impact of the SN ejecta strongly increases with decreasing binary separation and increasing explosion energy. Their relationship can be approximately fitted by power laws, which is consistent with the results obtained from impact simulations of Type Ia SNe. However, we find that the impact velocity is sensitive to the momentum profile of the outer SN ejecta and, in fact, may decrease with increasing ejecta mass, depending on the modeling of the ejecta. Because most companion stars to Type Ib/c CCSNe are in their MS phase at the moment of the explosion, combined with the strongly decaying impact effects with increasing binary separation, we argue that the majority of these SNe lead to inefficient mass stripping and shock heating of the companion star following the impact of the ejecta. Conclusions. Our simulations show that the impact effects of Type Ib/c SN ejecta on the structure of MS companion stars, and thus their long-term post-explosion evolution, is in general not dramatic. We find that at most 10% of their mass is lost and their resulting impact velocities are less than 100 km s-1.

Revealing the binary origin of Type Ic superluminous supernovae through nebular hydrogen emission

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 luminosity is powered by radioactivity or magnetar spin down. Because a central heating source is required to excite the H-alpha emission, an interaction-powered model is not favored for iPTF13ehe if the H-alpha emission is from stripped matter. We derive a companion mass of more than 20 solar masses and a binary separation of less than about 20 companion radii based on the stripping efficiency during the collision, indicating that the supernova progenitor and the companion formed a massive close binary system. If Type Ic superluminous supernovae generally occur in massive close binary systems, the early brightening observed previously in several Type Ic superluminous supernovae may also be due to the collision with a close companion. Observations of nebular hydrogen emission in future Type Ic superluminous supernovae will enable us to test this interpretation.

PRE-EXPLOSION COMPANION STARS IN TYPE Iax SUPERNOVAE

Type Iax supernovae (SNe Iax) are proposed as one new sub-class of SNe Ia since they present observational properties that are sufficiently distinct from the bulk of SNe Ia. SNe Iax are the most common of all types of peculiar SNe by both number and rate, with an estimated rate of occurrence of about 5-30% of the total SN Ia rate. However, the progenitor systems of SNe Iax are still uncertain. Analyzing pre-explosion images at SN Iax positions provides a direct way to place strong constraints on the nature of progenitor systems of SNe Iax. In this work, we predict pre-explosion properties of binary companion stars in a variety of potential progenitor systems by performing detailed binary evolution calculations with the one-dimensional stellar evolution code STARS. This will be helpful for constraining progenitor systems of SNe Iax from their pre-explosion observations. With our binary evolution calculations, it is found that the non-degenerate helium (He) companion star to both a massive C/O WD (> 1.1 solar mass) and a hybrid C/O/Ne WD can provide an explanation for the observations of SN~2012Z-S1, but the hybrid WD+He star scenario is more favorable.

Early ultraviolet signatures from the interaction of Type Ia supernova ejecta with a stellar companion

The progenitors of SNe Ia are not yet fully understood. The two leading progenitor scenarios are the single-degenerate (SD) scenario and the double-degenerate scenario. In the SD scenario, the collision of the SN Ia ejecta with its companion star is expected to produce detectable ultraviolet (UV) emission in the first few days after the SN explosion within certain viewing angles. A strong UV flash has recently been detected in an SN 2002es-like peculiar SN Ia iPTF14atg by Cao et al., which is interpreted as evidence of an early-time UV signature due to SN ejecta interacting with its companion star, supporting the SD scenario. In this paper, we present the expected luminosity distributions of early-time UV emission arising from SN Ia ejecta-companion interaction by performing binary population synthesis calculations for different progenitor systems in the SD scenario. Our theoretical predictions will be helpful for future early-time observations of SNe Ia to constrain their possible progenitors. Assuming the observed strong UV pulse of iPTF14atg was indeed produced by the SN ejecta-companion interaction, our population synthesis model suggests that the progenitor system of iPTF14atg is most likely a red-giant donor binary system, and it is unlikely to have been a main-sequence or helium-star donor system.

Observable fractions of core-collapse supernova light curves brightened by binary companions

Many core-collapse supernova progenitors are presumed to be in binary systems. If a star explodes in a binary system, the early supernova light curve can be brightened by the collision of the supernova ejecta with the companion star. The early brightening can be observed when the observer is in the direction of the hole created by the collision. Based on a population synthesis model, we estimate the fractions of core-collapse supernovae in which the light-curve brightening by the collision can be observed. We find that 0.19 per cent of core-collapse supernova light curves can be observed with the collisional brightening. Type Ibc supernova light curves are more likely to be brightened by the collision (0.53 per cent) because of the high fraction of the progenitors being in binary systems and their proximity to the companion stars. Type II and IIb supernova light curves are less affected (∼10−3 and ∼10−2 per cent, respectively). Although the early, slow light-curve declines of some Type IIb and Ibc supernovae are argued to be caused by the collision with the companion star (e.g. SN 2008D), the small expected fraction, as well as the unrealistically small separation required, disfavour the argument. The future transient survey by the Large Synoptic Survey Telescope is expected to detect ∼10 Type Ibc supernovae with the early collisional brightening per year, and they will be able to provide information on supernova progenitors in binary systems.

How plausible are the proposed formation scenarios of CEMP-r/s stars?

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Constraining the progenitor of the Type Ia Supernova SN 2012cg

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Three-dimensional Hydrodynamical Simulations of Mass Transfer in Binary Systems by a Free Wind

stars are metal-poor stars with enhanced abundances of carbon and heavy elements associated with the slow (

Can the helium-enriched main-sequence donor scenario hide enough hydrogen to explain Type Ia supernovae?

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