The single degenerate channel for the progenitors of type Ia supernovae

We have carried out a detailed study of one of the most popular evolutionary channels for the production of Type Ia supernova (SN Ia) progenitors, the semi-degenerate channel (CO+MS), where a carbon/oxygen (CO) white dwarf (WD) accretes matter from an unevolved or slightly evolved non-degenerate star until it reaches the Chandrasekhar mass limit. Employing Eggleton's stellar evolution code and adopting the prescription of Hachisu et al. (1999) for the accretion efficiency, we have carried out full binary evolution calculations for about 2300 close WD binary systems and mapped out the initial parameters in the orbital period -- secondary mass ($P$--$M_2$) plane (for a range of WD masses) which lead to a successful Type Ia supernova. We obtained accurate, analytical fitting formulae to describe this parameter range which can be used for binary population synthesis (BPS) studies. The contours in the $P$--$M_2$ plane differ from those obtained by Hachisu et al. (1999) for low-mass CO WDs, which are more common than massive CO WDs. We show that white dwarfs with a mass as low as $0.67M_\odot$ can accrete efficiently and reach the Chandrasekhar limit. We have implemented these results in a BPS study to obtain the birthrates for SNe Ia and the evolution of birthrates with time of SNe Ia for both a constant star formation rate and a single star burst. The birthrates are somewhat lower than (but comparable to) those inferred observationally.Comment: 10 pages, 14 figures, submitted to MNRA

The Slow Merger of Massive Stars

We study the complete merger of two massive stars inside a common envelope and the subsequent evolution of the merger product, a rapidly rotating massive supergiant. Three qualitatively different types of mergers have been identified and investigated in detail, and the post-merger evolution has been followed to the immediate presupernova stage. The ``quiet merger'' case does not lead to significant changes in composition, and the star remains a red supergiant. In the case of a ``moderate merger'', the star may become a blue supergiant and end its evolution as a blue supergiant, depending on the core to total mass ratio (as may be appropriate for the progenitor of SN 1987A). In the case of the most effective ``explosive merger'', the merger product stays a red giant. In last two cases, the He abundance in the envelope is increased drastically, but significant s-processing is mainly expected in the ``explosive merger'' case.Comment: 4 pages, 1 figure, proc. ESO/MPA/MPE Workshop "From Twilight to Highlight: The Physics of Supernovae

The birth rate of SNe Ia from hybrid CONe white dwarfs

Considering the uncertainties of the C-burning rate (CBR) and the treatment of convective boundaries, Chen et al. (2014) found that there is a regime where it is possible to form hybrid CONe white dwarfs (WDs), i.e. ONe WDs with carbon-rich cores. As these hybrid WDs can be as massive as 1.30 $M_{\odot}$, not much mass needs to be accreted for these objects to reach the Chandrasekhar limit and to explode as Type Ia supernovae (SNe Ia). We have investigated their contribution to the overall SN Ia birth rate and found that such SNe Ia tend to be relatively young with typical time delays between 0.1 and 1 Gyr, where some may be as young as 30 Myr. SNe Ia from hybrid CONe WDs may contribute several percent to all SNe Ia, depending on the common-envelope ejection efficiency and the CBR. We suggest that these SNe Ia may produce part of the 2002cx-like SN Ia class.Comment: 4 figures, accepted for publication for ApJ Lette

A common-envelope wind model for Type Ia supernovae (I): binary evolution and birth rate

The single-degenerate (SD) model is one of the principal models for the progenitors of type Ia supernovae (SNe Ia), but some of the predictions in the most widely studied version of the SD model, i.e. the optically thick wind (OTW) model, have not been confirmed by observations. Here, we propose a new version of the SD model in which a common envelope (CE) is assumed to form when the mass-transfer rate between a carbon-oxygen white dwarf (CO WD) and its companion exceeds a critical accretion rate. The WD may gradually increase its mass at the base of the CE. Due to the large nuclear luminosity for stable hydrogen burning, the CE may expand to giant dimensions and will lose mass from the surface of the CE by a CE wind (CEW). Because of the low CE density, the binary system will avoid a fast spiral-in phase and finally re-emerge from the CE phase. Our model may share the virtues of the OTW model but avoid some of its shortcomings. We performed binary stellar evolution calculations for more than 1100 close WD + MS binaries. Compared with the OTW model, the parameter space for SNe Ia from our CEW model extends to more massive companions and less massive WDs. Correspondingly, the Galactic birth rate from the CEW model is higher than that from the OTW model by $\sim$30\%. Finally, we discuss the uncertainties of the CEW model and the differences between our CEW model and the OTW model.Comment: 28 pages, 24 figures, accepted for publication in MNRA

The metallicity dependence of the long-duration GRB rate from host galaxy luminosities

We investigate the difference between the host galaxy properties of core-collapse supernovae and long-duration gamma-ray bursts (LGRBs), and quantify a possible metallicity dependence of the efficiency of producing LGRBs. We use a sample of 16 CC SNe and 16 LGRBs from Fruchter et al. (2006) which have similar redshift distributions to eliminate galaxy evolution biases. We make a forward prediction of their host galaxy luminosity distributions from the overall cosmic metallicity distribution of star formation. This appoach is supported by the finding that LGRB hosts follow the L-Z relations of star-forming galaxies. We then compare predictions for metallicity-dependent event efficiencies with the observed host data. We find that UV-based SFR estimates predict the hosts distribution of CC SNe perfectly well in a metallicity-independent form. In contrast, LGRB hosts are fainter on average by one magnitude, almost as faint as the Large Magellanic Cloud. Assuming this is a metallicity effect, the present data are insufficient to discriminate between a sharp metallicity cutoff and a soft decrease in efficiency towards higher metallicity. For a sharp cut-off, however, we find a best value for the cutoff metallicity, as reflected in the oxygen abundance, 12+log (O/H)_lim ~ 8.7+/-0.3 at 95% confidence including systematic uncertainties on the calibration of Kobulnicky & Kewley (2004). This value is somewhat lower than the traditionally quoted value for the Sun, but is comparable to the revised solar oxygen abundance (Asplund, Grevesse & Sauval 2005). LGRB models that require sharp metallicity cutoffs well below ~1/2 the revised solar metallicity appear to be effectively ruled out (abridged).Comment: MNRAS resubmitte

Constraining the spin-down timescale of the white-dwarf progenitors of Type Ia supernovae

Justham (2011) and DiStefano et al.\ (2011) proposed that the white-dwarf progenitor of a Type Ia supernova (SN Ia) may have to spin down before it can explode. As the white dwarf spin-down timescale is not well known theoretically, we here try to constrain it empirically (within the framework of this spin-down model) for progenitor systems that contain a giant donor and for which circumbinary material has been detected after the explosion: we obtain an upper limit of a few $10^{\rm 7} {\rm yr}$. Based on the study of Di Stefano & Kilic (2012) this means that it is too early to rule out the existence of a surviving companion in SNR 0509-67.5.Comment: 4 figures, accepted for publication in ApJ lette

Fast orbital shrinkage of black hole X-ray binaries driven by circumbinary disks

Recently, the black hole X-ray binary (BHXB) Nova Muscae 1991 has been reported to be experiencing an extremely rapid orbital decay. So far, three BHXBs have anomalously high orbital period derivatives, which can not be interpreted by the standard stellar evolution theory. In this work, we investigate whether the resonant interaction between the binary and a surrounding circumbinary (CB) disk could produce the observed orbital period derivatives. Analytical calculations indicate that the observed orbital period derivatives of XTE J1118+480 and A0620-00 can originate from the tidal torque between the binary and a CB disk with a mass of $10^{-9}~\rm M_{\odot}$, which is approximately in agreement with the dust disk mass detected in these two sources. However, Nova Muscae 1991 was probably surrounded by a heavy CB disk with a mass of $10^{-7}~\rm M_{\odot}$. Based on the CB disk model and the anomalous magnetic braking theory, we simulate the evolution of the three BHXBs with intermediate-mass donor stars by using the MESA code. Our simulated results are approximately consistent with the observed donor star masses, orbital periods, and orbital-period derivatives. However, the calculated effective temperatures of the donor stars are higher than indicated by the observed spectral types of two sources.Comment: 6 pages, 5 figures, accepted for publication by ApJ

Common-Envelope Evolution: the Nucleosynthesis in Mergers of Massive Stars

We study the merging of massive stars inside a common envelope for binary systems consisting of a red supergiant with a mass of 15-20 Msun and a main-sequence companion of 1-5 Msun. We are particularly interested in the stage when the secondary, having overfilled its Roche lobe inside the common envelope, starts to transfer mass to the core of the primary at a very high mass-transfer rate and the subsequent nucleo-synthesis in the core-impact region. Using a parametrized model for the structure of the envelope at this stage, we perform 2-dimensional hydrodynamical calculations with the Munich Prometheus code to calculate the dynamics of the stream emanating from the secondary and its impact on the core of the primary. We find that, for the lower end of the estimated mass-transfer rate, low-entropy, hydrogen-rich material can penetrate deep into the primary core where nucleosynthesis through the hot CNO cycle can take place and that the associated neutron exposure may be sufficiently high for significant s-processing. For mass-transfer rates at the high end of our estimated range and higher densities in the stream, the stream impact can lead to the dredge-up of helium, but the neutron production is too low for significant s-processing.Comment: 5 pages, 2 figures, to appear in the proceeding of ``Binary and Multiple Star Systems'' (Bormio (Italy), June 2000

He-accreting carbon-oxygen white dwarfs and type Ia supernovae

He accretion onto carbon-oxygen white dwarfs (CO WDs) plays a fundamental role when studying the formation of type Ia supernovae (SNe Ia). Employing the MESA stellar evolution code, we calculated the long-term evolution of He-accreting CO WDs. Previous studies usually supposed that a WD can grow in mass to the Chandrasekhar limit in the stable He burning region and finally produce a SN Ia. However, in this study we find that off-centre carbon ignition occurs in the stable He burning region if the accretion rate is above a critical value (~2.05*10^{-6}{M$_\odot$}{yr}^-1), resulting in accretion-induced collapse rather than a SN Ia. If the accretion rate is below the critical value, explosive carbon ignition will eventually happen in the centre producing a SN Ia. Taking into account the possibility of off-centre carbon ignition, we have re-determined the initial parameter space that produces SNe Ia in the He star donor channel, one of the promising channels to produce SNe Ia in young populations. Since this parameter space is smaller than was found in the previous study of Wang et al. (2009), the SN Ia rates are also correspondingly smaller. We also determined the chemical abundance profile of the He-accreting WDs at the moment of explosive carbon ignition, which can be used as initial input for SN Ia explosion models.Comment: 14 pages, 9 figures, accepted for publication in MNRA