Search

The identity of Type Ia supernova progenitors remains a mystery, with various lines of evidence pointing toward either accretion from a nondegenerate companion or the rapid merger of two degenerate stars leading to the thermonuclear destruction of a white dwarf. In this paper, we spectroscopically scrutinize 24 of the brightest stars residing in the central 38″ × 38″ of the SN 1604 (Kepler) supernova remnant to search for a possible surviving companion star. We can rule out, with high certainty, a red giant companion star—a progenitor indicated by some models of the supernova remnant. Furthermore, we find no star that exhibits properties uniquely consistent with those expected of a donor star down to L > 10 L☼. While the distribution of star properties toward the remnant are consistent with unrelated stars, we identify the most promising candidates for further astrometric and spectroscopic follow up. Such a program would either discover the donor star or place strong limits on progenitor systems to luminosities with L ≪ L☼. [ABSTRACT FROM AUTHOR]

In this paper, we report on our analysis using Hubble Space Telescope astrometry and Keck-I HIRES spectroscopy of the central six stars of Tycho's supernova remnant (SN 1572). With these data, we measured the proper motions, radial velocities, rotational velocities, and chemical abundances of these objects. Regarding the chemical abundances, we do not confirm the unusually high [Ni/Fe] ratio previously reported for Tycho-G. Rather, we find that for all metrics in all stars, none exhibit the characteristics expected from traditional Type Ia supernova single-degenerate-scenario calculations. The only possible exception is Tycho-B, a rare, metal-poor A-type star; however, we are unable to find a suitable scenario for it. Thus, we suggest that SN 1572 cannot be explained by the standard single-degenerate model. [ABSTRACT FROM AUTHOR]

The number of strong (equivalent width >1Å) Mg ii absorbers observed toward gamma-ray bursts (GRBs) has been found to be statistically larger than the number of strong absorbers toward quasi-stellar objects (QSOs). We formalize this "Mg II problem" and present a detailed explanation of the statistical tools required to assess the significance of the discrepancy. We find that the problem exists at the 4σ level for GRBs with high-resolution spectra. It has been suggested that the discrepancy can be resolved by the combination of a dust obscuration bias toward QSOs, and a strong gravitational lensing bias toward GRBs. We investigate one of the two most probable lensed GRBs that we presented in our previous work (GRB020405) and find that it is not strongly gravitationally lensed, constraining the percentage of lensed GRBs to be<35% (2σ). Dust obscuration of QSOs has been estimated to be a significant effect with dusty Mgii systems removing ∼20% of absorbed objects from flux-limited QSO samples.We find that if ∼30% of the strong MgII systems towardQSOs are missing from the observed samples, then GRBs and QSOs would have comparable numbers of absorbers per unit redshift. Thus, strong gravitational lensing bias is likely to make only a modest contribution to solving the Mg II problem. However, if the dust obscuration bias has been slightly underestimated, the Mg II problem would no longer persist. [ABSTRACT FROM AUTHOR]

Type Ia supernovae play a significant role in the evolution of the universe and have a wide range of applications. It is widely believed that these events are the thermonuclear explosions of carbon-oxygen white dwarfs close to the Chandrasekhar mass (1.38 M☼). However, CO white dwarfs are born with masses much below the Chandrasekhar limit and thus require mass accretion to become Type Ia supernovae. There are two main scenarios for accretion: first, the merger of two white dwarfs and, second, a stable mass accretion from a companion star. According to predictions, this companion star (also referred to as donor star) survives the explosion and thus should be visible in the center of Type Ia remnants. In this paper, we scrutinize the central stars (79 in total) of the SN 1006 remnant to search for the surviving donor star as predicted by this scenario. We find no star consistent with the traditional accretion scenario in SN 1006. [ABSTRACT FROM AUTHOR]

We carry out the first multi-dimensional radiative transfer calculations to simultaneously compute synthetic spectra and light curves for models of supernovae driven by fast bipolar outflows. These allow us to make self-consistent predictions for the orientation dependence of both color evolution and spectral features. We compare models with different degrees of asphericity and metallicity and find significant observable consequences of both. In aspherical models, we find spectral and light curve features that vary systematically with observer orientation. In particular, we find that the early-phase light curves are brighter and bluer when viewed close to the polar axis but that the peak flux is highest for equatorial (off-axis) inclinations. Spectral line features also depend systematically on observer orientation, including the velocity of the Si II 6355 Å line. Consequently, our models predict a correlation between line velocity and color that could assist the identification of supernovae associated with off-axis jet-driven explosions. The amplitude and range of this correlation depends on the degree of asphericity, the metallicity, and the epoch of observation but we find that it is always present and acts in the same direction. [ABSTRACT FROM AUTHOR]

We investigate the steepening of the power-law slope beyond the Galactocentric radius of R ∼ 45 kpc in the RR Lyrae (RRL) space density distribution found by Keller et al. They identified 2016 RRL candidates derived from the analysis of archival observations of the Southern Edgeworth-Kuiper Belt Object (SEKBO) survey. Our aim is to verify the completeness of RRLs and ultimately the space density distribution function given by Keller et al. We followed up on a subset of 137 candidates with a range of magnitudes (V ∼ 14-20) using the Faulkes Telescope (FT) database and confirmed 57 candidates as real RRLs. A cross-match between SEKBO RRL survey and Sloan Digital Sky Survey Data Release-7 (SDSS DR-7) revealed 272 RRL candidates in common. Applying the color selection criteria proposed by Ivezić et al. resulted in 193 likely RRLs. The completeness as a function of magnitude was calculated empirically from the combined set of SEKBO RRL candidates from current FT data, SDSS cross-matched data, and the Prior et al. catalog. This resulted in a spatial density distribution characterized by two power laws with a break radius R within a range between 45 kpc and 50 kpc, similar to the results of Keller et al. We find the power-law slopes for the inner halo as ninner = –2.78 ± 0.02 and for the outer halo as nouter = –5.0 ± 0.2. [ABSTRACT FROM AUTHOR]

Sixty percent of gamma-ray bursts (GRBs) reveal strong Mg II absorbing systems, which is a factor of ∼2 times the rate seen along lines of sight to quasars. Previous studies argue that the discrepancy in the strong Mg II covering factor is most likely to be the result of either quasars being obscured due to dust or the consequence of many GRBs being strongly gravitationally lensed. We analyze observations of quasars that show strong foreground Mg II absorption. We find that GRB lines of sight pass closer to bright galaxies than would be expected for random lines of sight within the impact parameter expected for strong Mg II absorption. While this cannot be explained by obscuration in the GRB sample, it is a natural consequence of gravitational lensing. Upon examining the particular configurations of galaxies near a sample of GRBs with strong Mg II absorption, we find several intriguing lensing candidates. Our results suggest that lensing provides a viable contribution to the observed enhancement of strong Mg II absorption along lines of sight to GRBs, and we outline the future observations required to test this hypothesis conclusively. [ABSTRACT FROM AUTHOR]