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Register a new userA progenitor binary and an ejected mass donor remnant of faint type Ia supernovae
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Type Ia supernovae (SN Ia) are the most important standard candles for measuring the expansion history of the universe. The thermonuclear explosion of a white dwarf can explain their observed properties, but neither the progenitor systems nor any stellar remnants have been conclusively identified. Underluminous SN Ia have been proposed to originate from a so-called double-detonation of a white dwarf. After a critical amount of helium is deposited on the surface through accretion from a close companion, the helium is ignited causing a detonation wave that triggers the explosion of the white dwarf itself. We have discovered both shallow transits and eclipses in the tight binary system CD-30 11223 composed of a carbon/oxygen white dwarf and a hot helium star, allowing us to determine its component masses and fundamental parameters. In the future the system will transfer mass from the helium star to the white dwarf. Modelling this process we find that the detonation in the accreted helium layer is sufficiently strong to trigger the explosion of the core. The helium star will then be ejected at so large a velocity that it will escape the Galaxy. The predicted properties of this remnant are an excellent match to the so-called hypervelocity star US 708, a hot, helium-rich star moving at more than 750 km/s, sufficient to leave the Galaxy. The identification of both progenitor and remnant provides a consistent picture of the formation and evolution of underluminous type Ia supernovae.
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There is compelling evidence that the peak brightness of a Type Ia supernova is affected by the electron fraction Ye at the time of the explosion. The electron fraction is set by the aboriginal composition of the white dwarf and the reactions that occur during the pre explosive convective burning. To date, determining the makeup of the white dwarf progenitor has relied on indirect proxies, such as the average metallicity of the host stellar population. In this paper, we present analytical calculations supporting the idea that the electron fraction of the progenitor systematically influences the nucleosynthesis of silicon group ejecta in Type Ia supernovae. In particular, we suggest the abundances generated in quasi nuclear statistical equilibrium are preserved during the subsequent freezeout. This allows one to potential recovery of Ye at explosion from the abundances recovered from an observed spectra. We show that measurement of 28Si, 32S, 40Ca, and 54Fe abundances can be used to construct Ye in the silicon rich regions of the supernovae. If these four abundances are determined exactly, they are sufficient to recover Ye to 6 percent. This is because these isotopes dominate the composition of silicon-rich material and iron rich material in quasi nuclear statistical equilibrium. Analytical analysis shows that the 28Si abundance is insensitive to Ye, the 32S abundance has a nearly linear trend with Ye, and the 40Ca abundance has a nearly quadratic trend with Ye. We verify these trends with post-processing of 1D models and show that these trends are reflected in model synthetic spectra.
Close double degenerate binaries are one of the favoured progenitor channels for type Ia supernovae, but it is unclear how many suitable systems there are in the Galaxy. We report results of a large radial velocity survey for double degenerate (DD) binaries using the UVES spectrograph at the ESO VLT (ESO SN Ia Progenitor surveY - SPY). Exposures taken at different epochs are checked for radial velocity shifts indicating close binary systems. We observed 689 targets classified as DA (displaying hydrogen-rich atmospheres), of which 46 turned out to possess a cool companion. We measured radial velocities (RV) of the remaining 643 DA white dwarfs. We managed to secure observations at two or more epochs for 625 targets, supplemented by eleven objects meeting our selection criteria from literature. The data reduction and analysis methods applied to the survey data are described in detail. The sample contains 39 double degenerate binaries, only four of which were previously known. 20 are double-lined systems, in which features from both components are visible, the other 19 are single-lined binaries. We provide absolute RVs transformed to the heliocentric system suitable for kinematic studies. Our sample is large enough to sub-divide by mass: 16 out of 44 low mass targets (<= 0.45 Msun) are detected as DDs, while just 23 of the remaining 567 with multiple spectra and mass >0.45 Msun are double. Although the detected fraction amongst the low mass objects (36.4 +/- 7.3%) is significantly higher than for the higher-mass, carbon/oxygen-core dominated part of the sample (3.9 +/- 0.8%), it is lower than the detection efficiency based upon companion star masses >= 0.05 Msun. This suggests either companion stars of mass < 0.05 Msun, or that some of the low mass white dwarfs are single.
The ultimate understanding of Type Ia Supernovae diversity is one of the most urgent issues to exploit thermonuclear explosions of accreted White Dwarfs (WDs) as cosmological yardsticks. In particular, we investigate the impact of the progenitor system metallicity on the physical and chemical properties of the WD at the explosion epoch. We analyze the evolution of CO WDs through the accretion and simmering phases by using evolutionary models based on time-dependent convective mixing and an extended nuclear network including the most important electron captures, beta decays and URCA processes. We find that, due to URCA processes and electron-captures, the neutron excess and density at which the thermal runaway occurs are substantially larger than previously claimed. Moreover, we find that the higher the progenitor metallicity, the larger the neutron excess variation during the accretion and simmering phases and the higher the central density and the convective velocity at the explosion. Hence, the simmering phase acts as an amplifier of the differences existing in SNe Ia progenitors. When applying our results to the neutron excess estimated for the Tycho and Kepler young Supernova remnants, we derive that the metallicity of the progenitors should be in the range Z=0.030-0.032, close to the average metallicity value of the thin disk of the Milky Way. As the amount of ${^{56}}$Ni produced in the explosion depends on the neutron excess and central density at the thermal runaway, our results suggest that the light curve properties depend on the progenitor metallicity.
The ejected mass distribution of type Ia supernovae directly probes progenitor evolutionary history and explosion mechanisms, with implications for their use as cosmological probes. Although the Chandrasekhar mass is a natural mass scale for the explosion of white dwarfs as type Ia supernovae, models allowing type Ia supernovae to explode at other masses have attracted much recent attention. Using an empirical relation between the ejected mass and the light curve width, we derive ejected masses $M_mathrm{ej}$ and $^{56}$Ni masses $M_mathrm{Ni}$ for a sample of 337 type Ia supernovae with redshifts $z < 0.7$ used in recent cosmological analyses. We use hierarchical Bayesian inference to reconstruct the joint $M_mathrm{ej}$-$M_mathrm{Ni}$ distribution, accounting for measurement errors. The inferred marginal distribution of $M_mathrm{ej}$ has a long tail towards sub-Chandrasekhar masses, but cuts off sharply above 1.4 $M_odot$. Our results imply that 25%-50% of normal type Ia supernovae are inconsistent with Chandrasekhar-mass explosions, with almost all of these being sub-Chandrasekhar-mass; super-Chandrasekhar-mass explosions make up no more than 1% of all spectroscopically normal type Ia supernovae. We interpret the type Ia supernova width-luminosity relation as an underlying relation between $M_mathrm{ej}$ and $M_mathrm{Ni}$, and show that the inferred relation is not naturally explained by the predictions of any single known explosion mechanism.
Type Ia supernovae (SNe Ia) are thermonuclear explosions of carbon-oxygen white dwarfs (WDs) that accrete mass from a binary companion, which can be either a non-degenerate star (a main-sequence star or a giant) or an other WD in a binary merger (single- and double-degenerate scenario, respectively). In population-synthesis studies of SNe Ia the contribution of asymptotic giant branch (AGB) stars to either scenario is marginal. However, most of these studies adopt simplified assumptions to compute the effects of wind mass loss and accretion in binary systems. This work investigates the impact of wind mass transfer on a population of binary stars and discusses the role of AGB stars as progenitors of SNe Ia.
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