Companions of Post-Common Envelope sdB Binaries

Preliminary results are presented from two ongoing complementary surveys intended to investigate the nature and characteristics of the optically invisible secondaries in post-common envelope subdwarf B (sdB) binary stars. We obtain precise radial velocities to derive periods and minimum companion masses for bright field sdB stars. These data are combined with light curves to search for eclipses, reflection effects, or ellipsoidal variations. We emphasize the importance of using complete unbiased samples, without which it will not be possible to understand the details of the multiple processes that produce these stars. It remains true that all known secondary companions in short-period sdB binaries are nearly invisible, thus they must be either low mass main sequence (MS) stars or compact objects, e.g., white dwarfs. In our small, nearly-complete sample, white dwarf secondaries outnumber MS secondaries by about a factor of five. Known MS masses in short-period sdB binaries are all surprisingly low, indicating a possible bimodal mass distribution for all MS secondaries in sdB binaries.     

Photometry of four binary subdwarf B stars and the nature of their unseen companion stars

We present light curves of four binary subdwarf B stars (sdB), Ton 245, Feige 11, PG 1432+159 and PG 1017−086. We also present new spectroscopic data for PG 1017−086 from which we derive its orbital period,   P =0.073 d  , and the mass function,   f _m=0.0010±0.0002 M_⊙.  This is the shortest period for an sdB binary measured to date. The values of P and f _m for the other sdB binaries have been published elsewhere. We are able to exclude the possibility that the unseen companion stars to Ton 245, Feige 11 and PG 1432+159 are main-sequence stars or subgiant stars from the absence of a sinusoidal signal, which would be caused by the irradiation of such a companion star, i.e. they show no reflection effect. The unseen companion stars in these binaries are likely to be white dwarf stars. In contrast, the reflection effect in PG 1017−086 is clearly seen. The lack of eclipses in this binary combined with other data suggests that the companion is a low-mass M-dwarf or, perhaps, a brown dwarf.     

Orbital periods of the binary sdB stars PG 0940+068 and PG 1247+554

We have used the radial velocity variations of two sdB stars previously reported to be binaries to establish their orbital periods. They are PG 0940+068 ( P =8.33 d) and PG 1247+554 ( P =0.599 d). The minimum masses of the unseen companions, assuming a mass of 0.5 M_⊙ for the sdB stars, are 0.090±0.003 M. for PG 1247+554 and 0.63±0.02 M_⊙ for PG 0940+068. The nature of the companions is not constrained further by our data.     

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.     

The Progenitors of sdB Binaries: Confronting Theory with Observations

We have re-examined the main binary evolution channels that produce sdB stars (by stable and unstable mass transfer, the merging of two helium white dwarfs). We find that all three main channels are of comparable importance, but produce slightly different mass distributions and dominate at different times. We show how the short-period sdB binaries can be used to constrain the efficiency of common-envelope ejection and discuss various observational tests that will allow further testing and constrain the binary evolution parameters that go into binary population synthesis simulations.     

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.     

The HYPER-MUCHFUSS project—target selection and analysis

The HYPER-MUCHFUSS project targets a population of high velocity subluminous B stars to discover either close binaries with massive unseen companions or hyper-velocity stars. Our starting point is the enormous database of SDSS. We preselected sdO/B candidates by colour and classified them by visual inspection of their spectra. We measured the radial velocity from the coadded SDSS spectra, which serves as first epoch measurement. Stars with high Galactic rest-frame velocities were selected and second epoch observations were obtained starting in 2007 at several sites. For the brighter targets we also included the SDSS individual spectra as additional information. In the course of our survey we observed 88 out of 265 stars from our target list. We discovered 39 HVS candidates as well as 49 close binaries. In addition we analysed all single spectra of sdBs from SDSS and found 120 close binaries. For the targets with constant RVs we performed a proper motion analysis with the highest possible accuracy from the available digitised photographic plates. Together with the analysed spectra and the calculation of the spectroscopic distance, we calculated complete trajectories and deduced the origins of these stars. Targets with high RV variability on short timescales were selected for follow-up. Numerical simulations based on the period and companion mass distribution of the known sdB binary sample were carried out to optimise the target selection and single out candidate binaries with massive companions. The follow-up campaign using WHT/ISIS and CAHA-3.5m/TWIN started in 2009.     

Companions to sdB binaries−degenerate or non-degenerate?

We report the results of an on-going programme to collect lightcurves for EHB binaries with orbital periods P ? 1 d. Degenerate and non-degenerate companions are clearly distinguished by the presence or absence of a reflection effect in these binaries. The amplitude of the reflection effect can be combined with other data to provide an estimate of the mass of the companion star. We find that the fraction of sdB binaries in our sample which have non-degenerate companions is 0.08±0.06 (1?σ error). These non-degenerate companions have very low masses (? 0.1M⊙). This property is not predicted by existing population synthesis models.     

Binary population synthesis and sdBs at different metallicities

One of the ways by which subdwarf B stars are thought to form is through binary star interactions. The metallicity of the sdB progenitor stars in such binary systems should not seem to be a major factor in the formation of sdB stars. However, given the different environments in which sdB stars are found, binary population synthesis simulations have been conducted in order to examine how metallicity might be a subtle factor in the formation of sdB stars in such environments. This is then applied to clusters of stars and to the UV Upturn phenomenon.     

The Coma Morphology Due to an Extended Active Region and the Implications for the Spin State of Comet Hale-Bopp

We have compared the kinematics and metallicity of the main-sequence binary and single uvby F stars from the Hipparcos catalog to see if the populations of these stars originate from the same statistical ensemble. The velocity dispersions of the known unresolved binary F stars have been found to be dramatically smaller than those of the single F stars. This suggests that the population of these binaries is, in fact, younger than that of the single stars, which is further supported by the difference in metal abundance: the binaries turn out to be, on average, more metal rich than the single stars. So, we conclude that the population of these binaries is indeed younger than that of the single F stars. Comparison of the single F stars with the C binaries (binary candidates identified in Suchkov & McMaster) has shown, on the other hand, that the latter stars are, on average, older than the single F stars. We suggest that the age difference between the single F stars, known unresolved binaries, and C binaries is associated with the fact that stellar evolution in a binary systems depends on the binary components' mass ratio and separation, with these parameters being statistically very different for the known binaries and C binaries (e.g., mostly substellar secondaries in C binaries vs. stellar secondaries in known binaries). In general we conclude that the populations of known binaries, C binaries, and single F stars do not belong to the same statistical ensemble. The implications of the discovered age difference between these populations along with the corresponding differences in kinematics and metallicity should be important not only for understanding the evolution of stars but also for the history of star formation and the evolution of the local Galactic disk.     

Atmospheric Parameters and Abundances of sdB Stars

We summarize recent results of quantitative spectral analyses using NLTE and metal line-blanketed LTE model atmospheres. Temperatures and gravities derived for hundreds of sdB stars are now available and allow us to investigate systematic uncertainties of T _e ff, log g scales and to test the theory of stellar evolution and pulsations. Surface abundance patterns of about two dozen sdB stars are surprisingly homogenous. In particular the iron abundance is almost solar for most sdBs. We highlight one iron-deficient and three super metal-rich sdBs, a challenge to diffusion theory. sdB stars are slowly rotating stars unless they are in close binary systems, which is hard to understand if the sdB stars were formed in merger events. The only exception is the pulsator PG 1605+072 rotating at vsin i= 39 km/s. Signatures of stellar winds from sdB stars may have been found.     

Neutron star retention and millisecond pulsar production in globular clusters

We investigate the conditions by which neutron star retention in globular clusters is favoured. We find that neutron stars formed in massive binaries are far more likely to be retained. Such binaries are likely to then evolve into contact before encountering other stars, possibly producing a single neutron star after a common envelope phase. A large fraction of the single neutron stars in globular clusters are then likely to exchange into binaries containing moderate-mass main-sequence stars, replacing the lower-mass components of the original systems. These binaries will become intermediate-mass X-ray binaries (IMXBs), once the moderate-mass star evolves off the main sequence, as mass is transferred on to the neutron star, possibly spinning it up in the process. Such systems may be responsible for the population of millisecond pulsars (MSPs) that has been observed in globular clusters. Additionally, the period of mass-transfer (and thus X-ray visibility) in the vast majority of such systems will have occurred 5–10 Gyr ago, thus explaining the observed relative paucity of X-ray binaries today, given the MSP population.     

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.     

Close encounters in young stellar clusters: implications for planetary systems in the solar neighbourhood

The stars that populate the solar neighbourhood were formed in stellar clusters. Through N -body simulations of these clusters, we measure the rate of close encounters between stars. By monitoring the interaction histories of each star, we investigate the singleton fraction in the solar neighbourhood. A singleton is a star which formed as a single star, has never experienced any close encounters with other stars or binaries, or undergone an exchange encounter with a binary. We find that, of the stars which formed as single stars, a significant fraction is not singletons once the clusters have dispersed. If some of these stars had planetary systems, with properties similar to those of the Solar System, the planets' orbits may have been perturbed by the effects of close encounters with other stars or the effects of a companion star within a binary. Such perturbations can lead to strong planet–planet interactions which eject several planets, leaving the remaining planets on eccentric orbits. Some of the single stars exchange into binaries. Most of these binaries are broken up via subsequent interactions within the cluster, but some remain intact beyond the lifetime of the cluster. The properties of these binaries are similar to those of the observed binary systems containing extrasolar planets. Thus, dynamical processes in young stellar clusters will alter significantly any population of Solar System-like planetary systems. In addition, beginning with a population of planetary systems exactly resembling the Solar System around single stars, dynamical encounters in young stellar clusters may produce at least some of the extrasolar planetary systems observed in the solar neighbourhood.     

Evolution of massive binary stars in the LMC and its implications for radio pulsar population

The Hertzsprung-Russell diagram of the Large Magellanic Cloud compiled recently by Fitzpatrick & Garmany (1990) shows that there are a number of supergiant stars immediately redward of the main sequence although theoretical models of massive stars with normal hydrogen abundance predict that the region 4.5 ≤ logT _eff ≤ 4.3 should be un-populated (“gap”). Supergiants having surface enrichment of helium acquired for example from a previous phase of accretion from a binary companion, however, evolve in a way so that the evolved models and observed data are consistent — an observation first made by Tuchman & Wheeler (1990). We compare the available optical data on OB supergiants with computed evolutionary tracks of massive stars of metallicity relevant to the LMC with and without helium-enriched envelopes and conclude that a large fraction (≈ 60 per cent) of supergiant stars may occur in binaries. As these less evolved binaries will later evolve into massive X-ray binaries, the observed number and orbital period distribution of the latter can constrain the evolutionary scenarios of the supergiant binaries. The distributions of post main sequence binaries and closely related systems like WR + O stars are bimodal-consisting of close and wide binaries in which the latter type is numerically dominating. When the primary star explodes as a supernova leaving behind a neutron star, the system receives a kick and in some cases can lead to runaway O-stars. We calculate the expected space velocity distribution for these systems. After the second supernova explosion, the binaries in most cases, will be disrupted leading to two runaway neutron stars. In between the two explosions, the first born neutron star’s spin evolution will be affected by accretion of mass from the companion star. We determine the steady-state spin and radio luminosity distributions of single pulsars born from the massive stars under some simple assumptions. Due to their great distance, only the brightest radio pulsars may be detected in a flux-limited survey of the LMC. A small but significant number of observable single radio pulsars arising out of the disrupted massive binaries may appear in the short spin period range. Most pulsars will have a low velocity of ejection and therefore may cluster around the OB associations in the LMC.     

Progress on the Front of Pulsating Subdwarf B Stars

We briefly review the recent advances that have been made on the front of pulsating subdwarf B (sdB) stars. The first family of sdB pulsators, the EC 14026 stars, was discovered a few years ago and consists of short-period (～100?200 s) p-mode variables. The second type of pulsating sdB’s consists of the PG 1716+426 stars, a group of variables showing long-period (～1 h) g-mode pulsations. The existence of the latter was first reported less than a year ago. While the two types of sdB pulsators differ markedly in their observational characteristics, we recently found a unifying property in the sense that the observed modes in these objects are excited through the same driving process, a classic kappa mechanism associated with the radiative levitation of iron in the stellar envelope.     

Discovery of A New Class of Pulsating Stars: Gravity-mode Pulsators among Subdwarf B Stars

During the course of an ongoing CCD monitoring program to investigate low-level light variations in subdwarf B (sdB) stars, weserendipitously discovered a new class of multimode pulsators withperiods of the order of an hour. These periods are a factor of tenlonger than those of previously known multimode sdB pulsators (EC14026 stars), implying the new pulsations are due to gravity modes rather than pressure modes. The iron opacity instability that drives the short period EC 14026 stars is effective in hot sdB's. Thelong period pulsators are found only among cooler sdB stars, wherethey are surprisingly common. The mechanism responsible for excitingthe deeper g-modes in cool sdB's is currently unknown, but thetemperature and gravity range in which these stars occur must be animportant clue. We present the first observational results for thisnew class of pulsating sdB stars, and discuss some possible implications.     

A stochastic Monte Carlo approach to model real star cluster evolution – II. Self-consistent models and primordial binaries

The new approach outlined in Paper I to follow the individual formation and evolution of binaries in an evolving, equal point-mass star cluster is extended for the self-consistent treatment of relaxation and close three- and four-body encounters for many binaries (typically a few per cent of the initial number of stars in the cluster mass). The distribution of single stars is treated as a conducting gas sphere with a standard anisotropic gaseous model. A Monte Carlo technique is used to model the motion of binaries, their formation and subsequent hardening by close encounters, and their relaxation (dynamical friction) with single stars and other binaries. The results are a further approach towards a realistic model of globular clusters with primordial binaries without using special hardware. We present, as our main result, the self-consistent evolution of a cluster consisting of 300 000 equal point-mass stars, plus 30 000 equal-mass binaries over several hundred half-mass relaxation times, well into the phase where most of the binaries have been dissolved and evacuated from the core. The cluster evolution is about three times slower than found by Gao et al. Other features are rather comparable. At every moment we are able to show the individual distribution of binaries in the cluster.     

The massive star binary fraction in young open clusters – I. NGC 6231 revisited

We present the results of a long-term high-resolution spectroscopy campaign on the O-type stars in NGC 6231. We revise the spectral classification and multiplicity of these objects and we constrain the fundamental properties of the O-star population. Almost three quarters of the O-type stars in the cluster are members of a binary system. The minimum binary fraction is 0.63, with half the O-type binaries having an orbital period of the order of a few days. The eccentricities of all the short-period binaries are revised downward, and henceforth match a normal period–eccentricity distribution. The mass ratio distribution shows a large preference for O + OB binaries, ruling out the possibility that, in NGC 6231, the companion of an O-type star is randomly drawn from a standard initial mass function. Obtained from a complete and homogeneous population of O-type stars, our conclusions provide interesting observational constraints to be confronted with the formation and early evolution theories of O-stars.     

Magnetic interaction in ultra-compact binary systems

This article reviews the current works on ultra-compact double-degenerate binaries in the presence of magnetic interaction, in particular, unipolar induction. The orbital dynamics and evolution of compact white-dwarf pairs are discussed in detail. Models and predictions of electron cyclotron masers from unipolar-inductor compact binaries and unipolar-inductor white-dwarf planetary systems are presented. Einstein-Laub effects in compact binaries are briefly discussed.