Gijs Nelemans

Reconstructing the evolution of white dwarf binaries: further evidence for an alternative algorithm for the outcome of the common-envelope phase in close binaries

We determine the possible masses and radii of the progenitors of white dwarfs in binaries from fits to detailed stellar evolution models and use these to reconstruct the mass-transfer phase in which the white dwarf was formed. We confirm the earlier finding that in the first phase of mass transfer in the binary evolution leading to a close pair of white dwarfs, the standard common-envelope formalism (the alpha-formalism) equating the energy balance in the system (implicitly assuming angular momentum conservation) does not work. An algorithm equating the angular momentum balance (implicitly assuming energy conservation) can explain the observations. This conclusion is now based on 10 observed systems rather than three. With the latter algorithm (the gamma-algorithm) the separation does not change much for approximately equal-mass binaries. Assuming constant efficiency in the standard alpha-formalism and a constant value of gamma, we investigate the effect of both methods on the change in separation in general and conclude that when there is observational evidence for strong shrinkage of the orbit, the gamma-algorithm also leads to this. We then extend our analysis to all close binaries with at least one white dwarf component and reconstruct the mass-transfer phases that lead to these binaries. In this way we find all possible values of the efficiency of the standard alpha-formalism and of gamma that can explain the observed binaries for different progenitor and companion masses. We find that all observations can be explained with a single value of gamma, making the gamma-algorithm a useful tool to predict the outcome of common-envelope evolution. We discuss the consequences of our findings for different binary populations in the Galaxy, including massive binaries, for which the reconstruction method cannot be used.

Short-period AM CVn systems as optical, X-ray and gravitational-wave sources

We model the population of AM CVn systems in the Galaxy and discuss the detectability of these systems with optical, X-ray and gravitational-wave detectors. We concentrate on the short-period (P < 1500 s) systems, some of which are expected to be in a phase of direct-impact accretion. Using a self-consistent model for the star-formation history and radial distribution of stars in the Galaxy plus simple models for the emission of optical and X-ray radiation from the AM CVn systems and interstellar absorption, we derive the sample of short-period AM CVn systems that can be detected in the optical and/or X-ray bands. At the shortest periods, the detectable systems are all X-ray sources, some with periods as short as three minutes. At periods above 10 min, most detectable systems are optical sources. About one-third of the X-ray sources are also detectable in the optical band. We also calculate the gravitational-wave signal of the short-period AM CVn systems. We find that potentially several thousand AM CVn systems can be resolved by the gravitational-wave detector Laser Interferometer Space Antenna (LISA), comparable to the expected number of detached double white dwarfs that can be resolved. We estimate that several hundreds of the AM CVn systems resolvable by LISA are also detectable in the optical and/or X-ray bands.

Close binary EHB stars from SPY

We present the results of a radial velocity (RV) survey of 46 subdwarf B (sdB) and 23 helium-rich subdwarf O (He-sdO) stars. We detected 18 (39%) new sdB binary systems, but only one (4%) He-sdO binary. Orbital parameters of nine sdB and sdO binaries, derived from follow-up spectroscopy, are presented. Our results are compared with evolutionary scenarios and previous observational investigations.

Optical spectra of the carbon–oxygen accretion discs in the ultra‐compact X‐ray binaries 4U 0614+09, 4U 1543−624 and 2S 0918−549

We present optical spectra in the range 4600‐8600 ˚ A for three low-mass X-ray binaries which have been suggested to belong to the class of ultra-compact X-ray binaries based on their X-ray spectra. Our spectra show no evidence for hydrogen or helium emission lines, as are seen in classical X-ray binaries. The spectrum of 4U 0614+09 does show emission lines, which we identify with carbon and oxygen lines of C II ,C III ,O II and O III. While the spectra of 4U 1543−624 and 2S 0918−549 have a lower signal-to-noise ratio, and thus are more difficult to interpret, some of the characteristic features of 4U 0614+09 are present in these spectra too, although sometimes they are clearly weaker. We conclude that the optical spectra give further evidence for the ultra-compact nature of these X-ray binaries and for their donor stars being carbon‐oxygen white dwarfs.

Binaries discovered by the SPY project. IV. Five single-lined DA double white dwarfs

We present results from our ongoing follow-up observations of double white dwarf binaries detected in the ESO SN Ia Progenitor SurveY (SPY). We discuss our observing strategy and data analysis and present the orbital solutions of five close double white dwarf binaries: HE0320−1917, HE1511−0448, WD0326−273, WD1013−010 and WD1210+140. Their periods range from 0.44 to 3.22 days. In none of these systems we find any spectral lines originating from the companion. This rules out main sequence companions and indicates that the companion white dwarfs are significantly older and cooler than the bright component. Infrared photometry suggests the presence of a cool, helium-rich white dwarf companion in the binary WD 0326−273. We briefly discuss the consequences of our findings for our understanding of the formation and evolution of double white dwarfs.

Search for progenitors of supernovae type Ia with SPY

Institute of Astronomy of the Russian Academy of Sciences, 48 Pyatnitskaya Str., 109017 Moscow, RussiaReceived date; accepted dateAbstract. We have started a large survey for double degenerate (DD) binaries as potential progenitors of type Ia supernovaewith the UVES spectrograph at the ESO VLT (ESO SNIa Progenitor surveY – SPY). About 400 white dwarfs were checkedfor radial velocity variations during the first 15 months of t his project, twice the number of white dwarfs investigated duringthe last 20 years. We give an overview of the SPY project and present first results Fifty four new DDs have been discovered,seven of them double lined (only 18 and 6 objects of these groups were known before, respectively). The final sample isexpected to contain 150 to 200 DDs. Eight new pre-cataclysmic binaries were also detected. SPY is the first DD surveywhich encompasses also non-DA white dwarfs. SPY produces animmense, unique sample of very high resolution whitedwarf spectra, which provides a lot of spin-off opportunities. We describe our projects to exploit the SPY sample for thedetermination of basic parameters, kinematics, and rotational velocities of white dwarfs. A catalogue with a first subs et ofour white dwarf data has already been published by Koester et al. (2001).

Optical spectroscopy of (candidate) ultracompact X‐ray binaries: constraints on the composition of the donor stars

We present optical spectroscopy of several (candidate) ultracompact X-ray binaries (UCXBs) obtained with the European Southern Observatory Very Large Telescope and Gemini-North telescopes. In only one of five observed UCXB candidates did we find evidence for H in its spectrum (4U 1556−60). For XB 1905+00 the optical counterpart is not detected. For the known UCXBs 4U 1626−67 and XB 1916−05 we find spectra consistent with a C/O and a He/N accretion disc, respectively, the latter is the first optical spectrum of a He-rich donor in an UCXB. Interestingly, the C/O spectrum of 4U 1626−67 shows both similarities as well as marked differences from the optical C/O spectrum of 4U 0614+09. We obtained phase-resolved spectroscopy of 4U 0614+09 and the 44-min transient XTE J0929−314. In neither object were we able to detect clear orbital periodicities, highlighting the difficulties of period determinations in UCXBs. We reanalysed the spectra of XTE J0929−314 that were taken close to the peak of its 2003 X-ray outburst and do not confirm the detection of Hα emission as was claimed in the literature. The peak spectra do show strong C or N emission around 4640 A, as has also been detected in other UCXBs. We discuss the implications of our findings for our understanding of the formation of UCXBs and the Galactic population of UCXBs. At the moment all studied systems are consistent with having white dwarf donors, the majority being C/O rich.

Six detached white-dwarf close binaries

We determine the orbits of four double-degenerate systems (DDs), composed of two white dwarfs and two white-dwarf-M-dwarf binaries. The four DDs, WD1022+050, WD1428+373, WD1824+040 and WD2032+188, show orbital periods of 1.157155(5), 1.15674(2), 6.26602(6) and 5.0846(3) d, respectively. These periods combined with estimates for the masses of the brighter component, based on their effective temperatures, allow us to constrain the masses of the unseen companions. We estimate that the upper limit for the contribution of the unseen companions to the total luminosity in the four DDs ranges between 10 and 20 per cent. In the case of the two white-dwarf-M-dwarf binaries, WD 1042-690 and WD2009+622, we calculate the orbital parameters by fitting simultaneously the absorption line from the white dwarf and the emission core from the M dwarf. Their orbital periods are 0.337083(1) and 0.741 226(2) d, respectively. We find signatures of irradiation on the inner face of the companion to WD2009+622. We calculate the masses of both components from the gravitational redshift and the mass-radius relationship for white dwarfs and find masses of 0.75-0.78 and 0.61-0.64 M ○. for WD 1042-690 and WD2009+622, respectively. This indicates that the stars probably reached the asymptotic giant branch in their evolution before entering a common envelope phase. These two white-dwarf-M-dwarf binaries will become cataclysmic variables, although not within a Hubble time, with orbital periods below the period gap.

The origin and fate of short-period low-mass black-hole binaries

We present results of a population synthesis study for semidetached short orbital period binaries which contain low-mass ( 4 M� ) compact accretors, which in our model represent black holes. Evolution of these binaries is determined by nuclear evolution of the donors and/or orbital angular momentum loss due to magnetic braking by the stellar wind of the donors and gravitational wave radiation. According to our model the estimated total number of this type of black-hole binaries in the Galaxy is >10 000. If the magnetic braking removing angular momentum in semidetached black-hole binaries is described by the Verbunt & Zwaan formula, the model predicts that among them around 3000 systems with periods >2 h would be transient. In addition one finds several hundreds of luminous and stable systems with periods between 3 and 8 h. Several dozens of these bright systems should be observed above the RXTE ASM sensitivity limit. The absence of such systems implies that upon Roche-lobe overflow by the low-mass donor angular momentum losses are reduced by a factor more than 2 with respect to the Verbunt & Zwaan prescription. In such a case short period black-hole soft X-ray transients (SXT) may have donors that overflow the Roche lobe in the core hydrogen-burning stage. We show that it is unlikely that the transient behaviour of black-hole short-period X-ray binaries is explained by the evolved nature of the stellar companion. On the other hand a substantial fraction of black-hole binaries with periods >3 h could be faint with truncated, stable cold accretion discs as proposed by Menou et al. Most of the semidetached black-hole binaries are expected to have periods shorter than ∼2 h. Properties of such, still to be observed, very small mass-ratio (q = M2/M1 < 0.02) binaries are different from those of their longer period cousins.

The Faint Sky Variability Survey — I. Goals and data reduction process

The Faint Sky Variability Survey is aimed at finding photometric and/or astrometric variable objects in the brightness range between ∼16th and ∼24th mag on time-scales between tens of minutes and years with photometric precisions ranging from 3 millimag for the brightest to 0.2 mag for the faintest objects. An area of ∼23 deg 2 , located at mid and high Galactic latitudes, has been covered using the Wide Field Camera on the 2.5-m Isaac Newton Telescope on La Palma. Here we describe the main goals of the Faint Sky Variability Survey and the data reduction process.