Is the X-ray pulsating companion of HD 49798 a possible type Ia supernova progenitor?

HD 49798(a hydrogen depleted subdwarf O6 star) with its massive white dwarf(WD) companion has been suggested to be a progenitor candidate of a type Ia supernova(SN Ia). However, it is still uncertain whether the companion of HD 49798 is a carbon-oxygen(CO) WD or an oxygen-neon(ONe) WD. A CO WD will explode as an SN Ia when its mass grows and approaches the Chandrasekhar limit, but the outcome of an accreting ONe WD is likely to be a neutron star. We generated a series of Monte Carlo calculations that incorperate binary population synthesis to simulate the formation of ONe WD + He star systems. We found that there is almost no orbital period as large as HD 49798 with its WD companion in these ONe WD + He star systems based on our simulations, which means that the companion of HD 49798 might not be an ONe WD. We suggest that the companion of HD 49798 is most likely a CO WD, which can be expected to increase its mass to the Chandrasekhar limit by accreting He-rich material from HD 49798. Thus, HD 49798 and its companion may produce an SN Ia as a result of its future evolution.     

WD + He star systems as the progenitors of Type Ia supernovae and their surviving companion stars

Employing Eggleton’s stellar evolution code with the optically thick wind assumption, we have systematically studied the WD + He star channel of Type Ia supernovae (SNe Ia), in which a carbon–oxygen WD accretes material from a He main-sequence star or a He subgiant to increase its mass to the Chandrasekhar mass. We mapped out the parameter spaces for producing SNe Ia. According to a detailed binary population synthesis approach, we find that the Galactic SN Ia birthrate from this channel is ～0.3×10^?3 yr^?1, and that this channel can produce SNe Ia with short delay times (～45–140 Myr). We also find that the surviving companion stars in this channel have a high spatial velocity (>400 km/s) after the SN explosion, which could be an alternative origin for hypervelocity stars (HVSs), especially for HVSs such as US 708.     

WD+MS systems as the progenitor of SNe Ia

The single-degenerate (SD) channel for the progenitors of type Ia supernovae (SNe Ia) is one of the most popular channels, in which a carbon–oxygen white dwarf (CO WD) accretes hydrogen-rich material from its companion, increases its mass to the Chandrasekhar mass limit, and then explodes as a SN Ia. We show the initial and final parameter space for SNe Ia in a ( $\log P^{\mathrm{i}},M_{2}^{\mathrm{i}}$ ) plane and find that the positions of some famous recurrent novae, as well as a supersoft X-ray source (SSS), RX J0513.9-6951, are well explained by our model. The model can also explain the space velocity and mass of Tycho G, which is now suggested to be the companion star of Tycho’s supernova. Our study indicates that the SSS, V Sge, might be the potential progenitor of supernovae like SN 2002ic if the delayed dynamical-instability model due to Han and Podsiadlowski (Mon. Not. R. Astron. Soc. 368:1095, 2006) is appropriate. Following the work of Meng et al. (Mon. Not. R. Astron. Soc. 395:2103, 2009a), we found that the SD model (WD+MS) with an optically thick wind can explain the birth rate of supernovae like SN 2006X and reproduce the distribution of the color excess of SNe Ia. The model also predicts that at least 75% of all SNe Ia may show a polarization signal in their spectra.     

Progenitors of type Ia supernovae

Type Ia supernovae (SNe Ia) play an important role in astrophysics and are crucial for the studies of stellar evolution, galaxy evolution and cosmology. They are generally thought to be thermonuclear explosions of accreting carbon–oxygen white dwarfs (CO WDs) in close binaries, however, the nature of the mass donor star is still unclear. In this article, we review various progenitor models proposed in the past years and summarize many observational results that can be used to put constraints on the nature of their progenitors. We also discuss the origin of SN Ia diversity and the impacts of SN Ia progenitors on some fields. The currently favourable progenitor model is the single-degenerate (SD) model, in which the WD accretes material from a non-degenerate companion star. This model may explain the similarities of most SNe Ia. It has long been argued that the double-degenerate (DD) model, which involves the merger of two CO WDs, may lead to an accretion-induced collapse rather than a thermonuclear explosion. However, recent observations of a few SNe Ia seem to support the DD model, and this model can produce normal SN Ia explosion under certain conditions. Additionally, the sub-luminous SNe Ia may be explained by the sub-Chandrasekhar mass model. At present, it seems likely that more than one progenitor model, including some variants of the SD and DD models, may be required to explain the observed diversity of SNe Ia.     

Radial velocities of pulsating subdwarf B stars: KPD 2109+4401 and PB 8783

High-speed spectroscopy of two pulsating subdwarf B stars, KPD 2109+4401 and PB 8783, is presented. Radial motions are detected with the same frequencies as reported from photometric observations and with amplitudes of ∼2 km s⁻¹ in two or more independent modes. These represent the first direct observations of surface motion arising from multimode non-radial oscillations in subdwarf B stars. In the case of the sdB+F binary PB 8783, the velocities of both components are resolved; high-frequency oscillations are found only in the sdB star and not the F star. There also appears to be evidence for mutual motion of the binary components. If confirmed, it implies that the F-type companion is ≳1.2 times more massive than the sdB star, while the amplitude of the F-star acceleration over 4 h would constrain the orbital period to lie between 0.5 and 3.2 d.     

Brief history of metal accumulation in the intracluster medium

We use models of the rates of Type Ia supernovae (SNe Ia) and core-collapsed supernovae, built in such a way that both are consistent with recent observational constraints at   z ≲ 1.6  and can reproduce the measured cosmic star formation rate, to recover the history of metal accumulation in the intracluster medium. We show that these SN rates, in unit of SN number per comoving volume and rest-frame year, provide on average a total amount of iron that is marginally consistent with the value measured in galaxy clusters in the redshift range 0–1, and a relative evolution with redshift that is in agreement with the observational constraints up to   z ≈ 1.2  . Moreover, we verify that the predicted metals-to-iron ratios reproduce the measurements obtained in nearby clusters through X-ray analysis, implying that (1) about half of the iron mass and ≳75 per cent of the nickel mass observed locally are produced by SN Ia ejecta, (2) the SN Ia contribution to the metal budget decreases steeply with redshift and by   z ≈ 1  is already less than half of the local amount, and (3) a transition in the abundance ratios relative to iron is present between redshifts ∼0.5 and 1.4, with core-collapsed SN products becoming dominant at higher redshifts.     

EXOTIME: searching for planets around pulsating subdwarf B stars

In 2007, a companion with planetary mass was found around the pulsating subdwarf B star V391 Pegasi with the timing method, indicating that a previously undiscovered population of substellar companions to apparently single subdwarf B stars might exist. Following this serendipitous discovery, the EXOTIME (http://www.na.astro.it/~silvotti/exotime/) monitoring program has been set up to follow the pulsations of a number of selected rapidly pulsating subdwarf B stars on time scales of several years with two immediate observational goals:

1. determine $\dot{P}$ of the pulsational periods P
2. search for signatures of substellar companions in O–C residuals due to periodic light travel time variations, which would be tracking the central star’s companion-induced wobble around the centre of mass

These sets of data should therefore, at the same time, on the one hand be useful to provide extra constraints for classical asteroseismological exercises from the $\dot{P}$ (comparison with “local” evolutionary models), and on the other hand allow one to investigate the preceding evolution of a target in terms of possible “binary” evolution by extending the otherwise unsuccessful search for companions to potentially very low masses. While timing pulsations may be an observationally expensive method to search for companions, it samples a different range of orbital parameters, inaccessible through orbital photometric effects or the radial velocity method: the latter favours massive close-in companions, whereas the timing method becomes increasingly more sensitive toward wider separations. In this paper we report on the status of the on-going observations and coherence analysis for two of the currently five targets, revealing very well-behaved pulsational characteristics in HS?0444+0458, while showing HS?0702+6043 to be more complex than previously thought.     

UV properties of type Ia supernova and their host galaxies

Type Ia Supernova (SN Ia) are a powerful, albeit not completely understood, tool for cosmology. Gaps in our understanding of their progenitors and detailed physics can lead to systematic errors in the cosmological distances they measure. We use UV data in two context to help further our understanding of SN Ia progenitors and physics. We analyze a set of nearly 700 light curves, and find no signature of the shock heating of a red giant companion, predicted by Kasen (Astrophys. J. 708:1025, 2010), casting doubt as to frequency of this SN Ia channel. We also use UV imaging of high redshift host galaxies of SN Ia to better understand the environments which SN Ia occur. We show that some high-z elliptical galaxies have current star formation, hindering efforts to use them as low-extinction environments. We show cosmological scatter of SN distances at large effective radii in their hosts is significantly reduced, and argue this is due to the smaller amounts of dust affecting the SN Ia. Finally, we find a two component dependence of SN distance measurements as a function of their host galaxy’s FUV-V color. This indicates that both the age and metallicity/mass of the host galaxy maybe important ingredients in measuring SN Ia distances.     

Hot subdwarfs in binary systems and the nature of their unseen companions

A large fraction of the sdB stars reside in short period binaries. It is therefore clear that binary evolution plays an important role in the still unsolved problem of hot subdwarf formation. Here we present new results from different projects devoted to the analysis of sdBs in close binaries. The nature and masses of the unseen companions of 31 sdBs have been constrained by an analysis of high resolution spectra. In the course of this study candidate systems with massive compact companions have been discovered. The HYPERMUCHFUSS project aims at finding such systems making use of the huge spectral database of SDSS. A multi-site follow-up campaign of promising radial velocity variable sdBs started in 2009 and preliminary results are shown here. The most recent discovery of a substellar companion to the bright sdB HD?149382 may provide new evidence for the decisive role of low mass companions for sdB formation in general. A mysterious IR-excess has been detected, which may be caused by this otherwise invisible companion. Another low mass companion has been found to orbit the sdB star EGB?5 within 16.5 days. The space mission CoRoT is performing wide field and high precision photometry. First preliminary results from a spectroscopic survey of the COROT fields are also reported.     

Spectroscopic analysis of sdB stars from the ESO Supernova Ia Progenitor Survey

We report on the analysis of high-resolution optical spectra for 77 subdwarf B (sdB) stars from the ESO Supernova Ia Progenitor Survey. Effective temperature, surface gravity, and photospheric helium abundance are determined simultaneously by spectral line profile fitting of hydrogen and helium lines, and are found to be in agreement with previous studies of sdB stars. Twenty four objects show spectral signs of a cool companion, being either companion absorption lines or a flux contribution at Hα. Five stars with relatively high luminosity show peculiar Hα profiles, possibly indicating stellar winds. Our results are compared to recent theoretical simulations by Han et al. [MNRAS, 341, 669] for the distribution in effective temperature and surface gravity, and are found to agree very well with these calculations. Finally, we present a binary system consisting of two helium-rich hot subdwarfs.     

Accreting He-rich material onto carbon-oxygen white dwarfs until explosive carbon ignition

Type Ia supernovae(SNe Ia) play an important role in studies of cosmology and galactic chemical evolution.They are believed to be thermonuclear explosions of carbon-oxygen white dwarfs(CO WDs)when their masses approach the Chandrasekar(Ch) mass limit.However,it is still not completely understood how a CO WD increases its mass to the Ch-mass limit in the classical single-degenerate(SD) model.In this paper,we studied the mass accretion process in the SD model to examine whether the WD can explode as an SN Ia.Employing the stellar evolution code called modules for experiments in stellar astrophysics(MESA),we simulated the He accretion process onto CO WDs.We found that the WD can increase its mass to the Ch-mass limit through the SD model and explosive carbon ignition finally occurs in its center,which will lead to an SN Ia explosion.Our results imply that SNe Ia can be produced from the SD model through steady helium accretion.Moreover,this work can provide initial input parameters for explosion models of SNe Ia.     

Photometric study of type Ia supernova SN 2002hu

Optical UBVRI photometry of the type Ia supernova (SN Ia) SN 2002hu covering the period from −2 to +73 d since B maximum is presented. The supernova reached at maximum brightness in B on  JD 245 2591.78 ± 0.5  with an apparent magnitude of  16.83 ± 0.02 mag  and a relatively blue colour  ( B − V ) =−0.08 ± 0.04 mag  . The luminosity decline rate of  Δ m ₁₅( B ) = 1.00 ± 0.05  indicates an absolute B magnitude at maximum of   M ^max _B =−19.38 ± 0.3  . The estimated absolute B magnitude, together with the photometric evolution, indicate SN 2002hu was slightly overluminous compared to the average SNe Ia. The distance modulus to the parent galaxy is estimated to be  μ= 36.04 ± 0.20  .     

Evolution of neutron stars in high-mass X-ray binaries

We consider the evolution of neutron stars during the X-ray phase of high-mass binaries. Calculations are performed assuming a crustal origin of the magnetic field. A strong wind from the companion can significantly influence the magnetic and spin behaviour of a neutron star even during the main-sequence life of the companion. In the course of evolution, the neutron star passes through four evolutionary phases ('isolated pulsar', propeller, wind accretion, and Roche lobe overflow). The model considered can naturally account for the observed magnetic fields and spin periods of neutron stars, as well as the existence of pulsating and non-pulsating X-ray sources in high-mass binaries. Calculations also predict the existence of a particular sort of high-mass binary with a secondary that fills its Roche lobe and a neutron star that does not accrete the overflowing matter because of fast spin.     

Abundance analysis of the γ Doradus–δ Scuti hybrid metallic line (Am) star HD 8801

Low frequency oscillation, typical for γ Doradus g‐mode type stellar core sensitive pulsation, as well as higher frequency δ Scuti type pulsation typical for p ‐modes, sensitive to the envelope, make HD 8801 a remarkable hybrid pulsator with the potential to probe a stellar structure over a wide range of radius. In addition HD 8801 is a rare pulsating metallic line (Am) star. We determined the astro‐physical fundamental parameters to locate HD 8801 in the H‐R diagram. We analyzed the element abundances, paying close attention to the errors involved, and confirm the nature of HD 8801 as a metallic line (Am) star. We also determined an upper limit on the magnetic field strength. Our abundance analysis is based on classical techniques, but uses for the final step a model atmosphere calculated with the abundances determined by us. We also discuss spectropolarimetric observations obtained for HD 8801. This object is remarkable in several respects. It is a nonmagnetic metallic line (Am) star, pulsating simultaneously in p‐ and g‐modes, but also shows oscillations with periods in between these two domains, whose excitation requires explanation. Overall, the pulsational incidence in unevolved classical Am stars is believed to be quite low; HD 8801 does not conform to this picture. Finally, about 75 % of Am stars are located in short‐period binaries, but there is no evidence that HD 8801 has a companion. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical and infrared observations of SN 2002dj: some possible common properties of fast-expanding Type Ia supernovae

As part of the European Supernova Collaboration, we obtained extensive photometry and spectroscopy of the Type Ia supernova (SN Ia) SN 2002dj covering epochs from 11 d before to nearly two years after maximum. Detailed optical and near-infrared observations show that this object belongs to the class of the high-velocity gradient events as indicated by Si, S and Ca lines. The light curve shape and velocity evolution of SN 2002dj appear to be nearly identical to SN 2002bo. The only significant difference is observed in the optical to near-infrared colours and a reduced spectral emission beyond 6500 Å. For high-velocity gradient SNe Ia, we tentatively identify a faster rise to maximum, a more pronounced inflection in the V and R light curves after maximum and a brighter, slower declining late-time B light curve as common photometric properties of this class of objects. They also seem to be characterized by a different colour and colour evolution with respect to 'normal' SNe Ia. The usual light curve shape parameters do not distinguish these events. Stronger, more blueshifted absorption features of intermediate-mass elements and lower temperatures are the most prominent spectroscopic features of SNe Ia displaying high-velocity gradients. It appears that these events burn more intermediate-mass elements in the outer layers. Possible connections to the metallicity of the progenitor star are explored.     

Resolving the pulsations of subdwarf B stars: PG 0154+182, HS 1824+5745 and HS 2151+0857

We continue our programme of extended single-site observations of pulsating subdwarf B (sdB) stars and present the results of extensive time-series photometry to resolve the pulsation spectra for use in asteroseismological analyses. PG 0154+182, HS 1824+5745 and HS2151+0857 were observed at the MDM Observatory during 2004 and 2005. Our observations are sufficient to resolve the pulsations of all three target stars. We extend the number of known frequencies for PG 0154+182 from one to six, confirm that HS 1824+5745 is a monoperiodic pulsator and extend the number of known frequencies to five for HS 2151+0857. We perform standard tests to search for multiplet structure, measure amplitude variations as pertains to stochastic excitation and examine the mode density to constrain the mode degree ℓ.     

Mass-accreting white dwarfs and type Ia supernovae

Type Ia supernovae(SNe Ia) play a prominent role in understanding the evolution of the Universe. They are thought to be thermonuclear explosions of mass-accreting carbon-oxygen white dwarfs(CO WDs) in binaries, although the mass donors of the accreting WDs are still not well determined. In this article, I review recent studies on mass-accreting WDs, including H-and He-accreting WDs. I also review currently most studied progenitor models of SNe Ia, i.e., the single-degenerate model(including the WD+MS channel, the WD+RG channel and the WD+He star channel), the doubledegenerate model(including the violent merger scenario) and the sub-Chandrasekhar mass model.Recent progress on these progenitor models is discussed, including the initial parameter space for producing SNe Ia, the binary evolutionary paths to SNe Ia, the progenitor candidates for SNe Ia, the possible surviving companion stars of SNe Ia, some observational constraints, etc. Some other potential progenitor models of SNe Ia are also summarized, including the hybrid CONe WD model, the core-degenerate model, the double WD collision model, the spin-up/spin-down model and the model of WDs near black holes. To date, it seems that two or more progenitor models are needed to explain the observed diversity among SNe Ia.