Phases of Mass Transfer from Hot Subdwarfs to White Dwarf Companions and Their Photometric Properties

Binary systems of a hot subdwarf B (sdB) star + a white dwarf (WD) with orbital periods less than 2–3 hr can come into contact due to gravitational waves and transfer mass from the sdB star to the WD before the sdB star ceases nuclear burning and contracts to become a WD. Motivated by the growing class of observed systems in this category, we study the phases of mass transfer in these systems. We find that because the residual outer hydrogen envelope accounts for a large fraction of an sdB star’s radius, sdB stars can spend a significant amount of time (∼tens of megayears) transferring this small amount of material at low rates (∼10−10–10−9 M ⊙ yr−1) before transitioning to a phase where the bulk of their He transfers at much faster rates ( ≳10−8 M ⊙ yr−1). These systems therefore spend a surprising amount of time with Roche-filling sdB donors at orbital periods longer than the range associated with He star models without an envelope. We predict that the envelope transfer phase should be detectable by searching for ellipsoidal modulation of Roche-filling objects with P orb = 30–100 minutes and T eff = 20,000–30,000 K, and that many (≥10) such systems may be found in the Galactic plane after accounting for reddening. We also argue that many of these systems may go through a phase of He transfer that matches the signatures of AM CVn systems, and that some AM CVn systems associated with young stellar populations likely descend from this channel.

Hot subdwarfs from the surviving companions of the white dwarf + main-sequence channel of Type Ia supernovae

ABSTRACT Some surviving companions of Type Ia supernovae (SNe Ia) from the white dwarf + main-sequence (WD+MS) channel may evolve to hot subdwarfs. In this paper, we perform stellar evolution calculations for the surviving companions of close WD+MS systems in the spin-up/spin-down model and the canonical non-rotating model. This enables us to map out the initial parameter spaces in the orbital period–secondary-mass plane in which the surviving companions can evolve to hot subdwarfs. Based on these results, we carry out a series of binary population synthesis calculations to obtain the Galactic birth rate of hot subdwarfs from the WD+MS channel, which is $2.3{-}6\times 10^{\rm -4}\, {\rm yr}^{\rm -1}$ for the spin-up/spin-down model and $0.7{-}3\times 10^{\rm -4}\, {\rm yr}^{\rm -1}$ for the canonical non-rotating model. We also show the distributions of some integral properties of the hot subdwarfs, for example the mass and space velocity, for different models. In addition, by comparing our results with observations of intermediate helium-rich (iHe-rich) hot subdwarfs, we find that the hot subdwarfs from the WD+MS channel may explain some observational features of the iHe-rich hot subdwarfs, especially those from the spin-up/spin-down model. Although we expect that the SN Ia channel will contribute only a small fraction of the iHe-rich hot subdwarf population, some of these may help to explain cases with unusual kinematics.

Asteroseismic Observations of Hot Subdwarfs

There are a number of reasons for studying hot subdwarf pulsation; the most obvious being that these stars remain a poorly understood late-stage of stellar evolution and knowledge of their interior structure, which pulsation studies reveal, constrains evolution models. Of particular interest are the red giant progenitors as in looking at a hot subdwarf we are seeing a stripped-down red giant as it would have been just before the Helium Flash. Moreover, hot subdwarfs may have formed through the merger of two helium white dwarfs and their study gives insight into how such a merger may have happened. A less obvious reason for studying pulsation in hot subdwarfs is that they provide a critical test of stellar envelope opacities and the atomic physics upon which they depend.

Search and Studies of the First Byurakan Survey Blue Stellar Objects

The First Byurakan Survey (FBS) 2nd Part was devoted to search and studies of Blue Stellar Objects (BSOs) and Late-type Stars. Eleven lists of 1103 BSOs were published in Astrophysics in 1990-1996, found in FBS low-dispersion spectroscopic plates. The selection was carried out in the region with +33◦ > δ > +45◦ and δ >+61◦ with a surface of ∼4000 deg2. As a result, the catalogue of the FBS BSOs was compiled. Its preliminary version has been available at CDS since 1999. We revised and updated the FBS BSOs catalogue with the new data from recently published optical and multiwavelength catalogues to give access to all available data and make further comparative studies of the properties of these objects possible. We made cross-correlations of the FBS BSOs catalogue with the MAPS, USNO-B1.0, SDSS, and 2MASS, as well as ROSAT, IRAS, NVSS, and FIRST catalogs, added updated SIMBAD and NED data for the objects, and provided accurate DSS1 and DSS2 positions and revised photometry. We also checked the objects for proper motion and variability. A refined classification for the low-dispersion spectra in the Digitized First Byurakan Survey (DFBS) was carried out. The revised and updated catalogue of FBS blue stellar objects contains 1101 objects. The FBS blue stellar objects catalogue can be used to study a complete sample of white dwarfs, hot subdwarfs, HBB stars, cataclysmic variables, bright AGN, and to investigate individual interesting objects.

The SALT survey of helium-rich hot subdwarfs: methods, classification, and coarse analysis

ABSTRACT A medium- and high-resolution spectroscopic survey of helium-rich hot subdwarfs is being carried out using the Southern African Large Telescope (SALT). Objectives include the discovery of exotic hot subdwarfs and of sequences connecting chemically peculiar subdwarfs of different types. The first phase consists of medium-resolution spectroscopy of over 100 stars selected from low-resolution surveys. This paper describes the selection criteria, and the observing, classification, and analysis methods. It presents 107 spectral classifications on the MK-like Drilling system and 106 coarse analyses (${T_{\rm eff}}, \log g, \log y$) based on a hybrid grid of zero-metal non-LTE and line-blanketed LTE model atmospheres. For 75 stars, atmospheric parameters have been derived for the first time. The sample may be divided into six distinct groups including the classical ‘helium-rich’ sdO stars with spectral types (Sp) sdO6.5–sdB1 (74) comprising carbon-rich (35) and carbon-weak (39) stars, very hot He-sdO’s with Sp ≲ sdO6 (13), extreme helium stars with luminosity class ≲5 (5), intermediate helium-rich subdwarfs with helium class 25–35 (8), and intermediate helium-rich subdwarfs with helium class 10–25 (6). The last covers a narrow spectral range (sdB0–sdB1) including two known and four candidate heavy-metal subdwarfs. Within other groups are several stars of individual interest, including an extremely metal-poor helium star, candidate double-helium subdwarf binaries, and a candidate low-gravity He-sdO star.

Heavy-metal enrichment of intermediate He-sdOB stars: the pulsators Feige 46 and LS IV–14°116 revisited

Hot subdwarf stars of spectral types O and B represent a poorly understood phase in the evolution of low-mass stars, in particular of close compact binaries. A variety of phenomena are observed, which make them important tools for several astronomical disciplines. For instance, the richness of oscillations of many subdwarfs are important for asteroseismology. Furthermore, hot subdwarfs are among the most chemically peculiar stars known. Two intermediate He-rich hot subdwarf stars, LS IV–14°116 and Feige 46, are particularly interesting, because they show extreme enrichments of heavy elements such as Ge, Sr, Y, and Zr, which are strikingly similar in both stars. In addition, both stars show light oscillations at periods incompatible with standard pulsation theory and form the class of V366 Aqr variables. We investigated whether the similar chemical compositions extend to more complete abundance patterns in both stars and validate the pulsations in Feige 46 using its recent TESS light curve. High-resolution optical and near-ultraviolet spectroscopy are combined with non-local thermodynamical-equilibrium model atmospheres and synthetic spectra calculated with TLUSTY and SYNSPEC to consistently determine detailed metal abundance patterns in both stars. Many previously unidentified lines were identified for the first time with transitions originating from Ga III, Ge III-IV, Se III, Kr III, Sr II-III, Y III, Zr III-IV, and Sn IV, most of which have not yet been observed in any star. The abundance patterns of 19 metals in both stars are almost identical, light metals being only slightly more abundant in Feige 46, while Zr, Sn, and Pb are slightly less enhanced compared to LS IV–14°116. Both abundance patterns are distinctively different from those of normal He-poor hot subdwarfs of a similar temperature. The extreme enrichment in heavy metals of more than 4 dex compared to the Sun is likely the result of strong atmospheric diffusion processes that operate similarly in both stars while their similar patterns of C, N, O, and Ne abundances might provide clues to their as yet unclear evolutionary history. Finally, we find that the periods of the pulsation modes in Feige 46 are stable to better than Ṗ ≲ 10−8 s s−1. This is not compatible with Ṗ predicted for pulsations driven by the ɛ-mechanism and excited by helium-shell flashes in a star that is evolving, for example, onto the extended horizontal branch.

PHL 417: a zirconium-rich pulsating hot subdwarf (V366 Aquarid) discovered in K2 data

ABSTRACT The Kepler spacecraft observed the hot subdwarf star PHL 417 during its extended K2 mission, and the high-precision photometric light curve reveals the presence of 17 pulsation modes with periods between 38 and 105 min. From follow-up ground-based spectroscopy, we find that the object has a relatively high temperature of 35 600 K, a surface gravity of $\log g / {\rm cm\, s^{-2}}\, =\, 5.75$ and a supersolar helium abundance. Remarkably, it also shows strong zirconium lines corresponding to an apparent +3.9 dex overabundance compared with the Sun. These properties clearly identify this object as the third member of the rare group of pulsating heavy-metal stars, the V366-Aquarii pulsators. These stars are intriguing in that the pulsations are inconsistent with the standard models for pulsations in hot subdwarfs, which predicts that they should display short-period pulsations rather than the observed longer periods. We perform a stability analysis of the pulsation modes based on data from two campaigns with K2. The highest amplitude mode is found to be stable with a period drift, $\dot{P}$, of less than 1.1 × 10−9 s s−1. This result rules out pulsations driven during the rapid stages of helium flash ignition.

Alone but not lonely: Observational evidence that binary interaction is always required to form hot subdwarf stars

Context. Hot subdwarfs are core-helium burning stars that show lower masses and higher temperatures than canonical horizontal branch stars. They are believed to be formed when a red giant suffers an extreme mass-loss episode. Binary interaction is suggested to be the main formation channel, but the high fraction of apparently single hot subdwarfs (up to 30%) has prompted single star formation scenarios to be proposed. Aims. We investigate the possibility that hot subdwarfs could form without interaction by studying wide binary systems. If single formation scenarios were possible, there should be hot subdwarfs in wide binaries that have undergone no interaction. Methods. Angular momentum accretion during interaction is predicted to cause the hot subdwarf companion to spin up to the critical velocity. The effect of this should still be observable given the timescales of the hot subdwarf phase. To study the rotation rates of companions, we have analysed light curves from the Transiting Exoplanet Survey Satellite for all known hot subdwarfs showing composite spectral energy distributions indicating the presence of a main sequence wide binary companion. If formation without interaction were possible, that would also imply the existence of hot subdwarfs in very wide binaries that are not predicted to interact. To identify such systems, we have searched for common proper motion companions with projected orbital distances of up to 0.1 pc to all known spectroscopically confirmed hot subdwarfs using Gaia DR2 astrometry. Results. We find that the companions in composite hot subdwarfs show short rotation periods when compared to field main sequence stars. They display a triangular-shaped distribution with a peak around 2.5 days, similar to what is observed for young open clusters. We also report a shortage of hot subdwarfs with candidate common proper motion companions. We identify only 16 candidates after probing 2938 hot subdwarfs with good astrometry. Out of those, at least six seem to be hierarchical triple systems, in which the hot subdwarf is part of an inner binary. Conclusions. The observed distribution of rotation rates for the companions in known wide hot subdwarf binaries provides evidence of previous interaction causing spin-up. Additionally, there is a shortage of hot subdwarfs in common proper motion pairs, considering the frequency of such systems among progenitors. These results suggest that binary interaction is always required for the formation of hot subdwarfs.

Opacity modelling of heavy-metal hot subdwarfs: photoionization of Sr0, Y+, and Zr2 +

ABSTRACT Heavy-metal hot subdwarfs (sdB and sdO) represent a small group of stars with unusually high concentrations of trans-iron elements in their atmospheres, having abundances ${\sim } 10\, 000$ times solar. One example is LS IV-14 116, where a number of heavy-metal absorption lines of Sr ii, Y iii, and Zr iv have been observed in the optical band $4000\!-\!5000\, {\mathring{\rm A}}$. We use a fully relativistic Dirac Atomic R-Matrix Code (darc) to calculate photoionization cross-sections of Sr0, Y+, and Zr2 + from their ground state to the 20th excited level. We use the cross-sections and the oscillator strengths to simulate the spectrum of a hot subdwarf. We obtain complete sets of photoionization cross-sections for the three ions under study. We use these data to calculate the opacity of the stellar atmospheres of hot subdwarf stars, and show that for overabundances observed in some heavy-metal subdwarves, photoexcitation from zirconium, in particular, does contribute some back warming in the model.

TESS photometry of helium-rich hot subdwarfs: r modes in BD+37°442 and BD+37°1977

ABSTRACT Transiting Exoplanet Survey Satellite photometry of the extremely helium-rich hot subdwarfs BD+37°442 and BD+37°1977 demonstrates multiperiodic low-amplitude variability with principal periods of 0.56 and 1.14 d, respectively, and with both first and second harmonics present. The light curves are not perfectly regular, implying additional periodic and/or non-periodic content. Possible causes are examined, including the binary hypothesis originally introduced to explain X-ray observations, differentially rotating surface inhomogeneities, and pulsations. If the principal photometric periods correspond to the rotation periods, the stars are rotating at approximately 0.7 and 0.3 × break-up, respectively. Surface Rossby waves (r modes) therefore provide the most likely solution.