We present simultaneous, multi-colour optical light curves of the companion star to the black-widow pulsar PSR J2051-0827, obtained approximately 10 years apart using ULTRACAM and HiPERCAM, ...respectively. The ULTRACAM light curves confirm the previously reported asymmetry in which the leading hemisphere of the companion star appears to be brighter than the trailing hemisphere. The HiPERCAM light curves, however, do not show this asymmetry, demonstrating that whatever mechanism is responsible for it varies on timescales of a decade or less. We fit the symmetrical HiPERCAM light curves with a direct-heating model to derive the system parameters, finding an orbital inclination of $55.9^{+4.8}_{-4.1}$ degrees, in good agreement with radio-eclipse constraints. We find that approximately half of the pulsar's spin-down energy is converted to optical luminosity, resulting in temperatures ranging from approximately $5150^{+190}_{-190}$ K on the day side to $2750^{+130}_{-150}$ K on the night side of the companion star. The companion star is close to filling its Roche lobe ($f_{\rm RL} =0.88^{+0.02}_{-0.02}$) and has a mass of $0.039^{+0.010}_{-0.011}$ M$_{\odot}$, giving a mean density of $20.24^{+0.59}_{-0.44}$ g cm$^{-3}$ and an apsidal motion constant in the range $0.0036 < k_2 < 0.0047$. The companion mass and mean density values are consistent with those of brown dwarfs, but the apsidal motion constant implies a significantly more centrally-condensed internal structure than is typical for such objects.
ABSTRACT We present a spectroscopic survey of 230 white dwarf candidates within 40 pc of the Sun from the William Herschel Telescope and Gran Telescopio Canarias. All candidates were selected from ...Gaia Data Release 2 (DR2) and in almost all cases, had no prior spectroscopic classifications. We find a total of 191 confirmed white dwarfs and 39 main-sequence star contaminants. The majority of stellar remnants in the sample are relatively cool (〈Teff〉 = 6200 K), showing either hydrogen Balmer lines or a featureless spectrum, corresponding to 89 DA and 76 DC white dwarfs, respectively. We also recover two DBA white dwarfs and 9–10 magnetic remnants. We find two carbon-bearing DQ stars and 14 new metal-rich white dwarfs. This includes the possible detection of the first ultra-cool white dwarf with metal lines. We describe three DZ stars for which we find at least four different metal species, including one that is strongly Fe- and Ni-rich, indicative of the accretion of a planetesimal with core-Earth composition. We find one extremely massive (1.31 ± 0.01 M⊙) DA white dwarf showing weak Balmer lines, possibly indicating stellar magnetism. Another white dwarf shows strong Balmer line emission but no infrared excess, suggesting a low-mass sub-stellar companion. A high spectroscopic completeness (>99 per cent) has now been reached for Gaia DR2 sources within 40-pc sample, in the Northern hemisphere (δ > 0°) and located on the white dwarf cooling track in the Hertzsprung–Russell diagram. A statistical study of the full northern sample is presented in a companion paper.
Hot subdwarfs in close binaries with either M dwarf, brown dwarf or white dwarf companions show unique light variations. In hot subdwarf binaries with M dwarf or brown dwarf companions we can observe ...the so-called reflection effect and in hot subdwarfs with close white dwarf companions ellipsoidal modulation and/or Doppler beaming. Aims. The analysis of these light variations can be used to derive the mass and radius of the companion and hence determine its nature. Thereby we assume the most probable sdB mass and the radius of the sdB derived by the fit of the spectral energy distribution and the Gaia parallax. In the high signal-to-noise space-based light curves from the Transiting Exoplanet Survey Satellite and the K2 space mission, several reflection effect binaries and ellipsoidal modulation binaries have been observed with much better quality than possible for ground-based observations. The high quality of the light curves allowed us to analyse a large sample of sdB binaries with M dwarf or white dwarf companions using lcurve. For the first time we can constrain the absolute parameters of 19 companions of reflection effect systems covering periods from 2.5 to 19 hours and companion masses from the hydrogen burning limit to early M dwarfs. Moreover, we could determine the mass of eight white dwarf companion to hot subdwarf binaries showing ellipsoidal modulations, covering a so far unexplored period range from 7 to 19 hours. The derived masses of the white dwarf companions show that all but two of the white dwarf companions are most likely helium-core white dwarfs. Combining our results with previously measured rotation velocities allowed us to derive the rotation period of seven sdBs in short-period binaries. In four of those systems the rotation period of the sdB agrees with a tidally locked orbit, in the other three systems the sdB rotates significantly slower.
About a third of the hot subdwarfs of spectral type B, which are mostly core-helium burning objects on the extreme horizontal branch, are found in close binaries with cool, low-mass stellar, ...substellar, or white dwarf companions. They can show light variations due to different phenomena. We used light curves from the Transiting Exoplanet Survey Satellite and the \textit{K2} space mission to look for more sdB binaries. Their light curves can be used to study the hot subdwarf primaries and their companions and get orbital, atmospheric, and absolute parameters for those systems. By classifying the light variations and combining this with the fit of the spectral energy distribution, the distance derived by the parallaxes obtained by \textit{Gaia} and the atmospheric parameters, we could derive the nature of the primary and secondary in 122 (75\%) of the known sdB binaries and 82 newly found reflection effect systems. We derive absolute masses, radii, and luminosities for a total of 39 hot subdwarfs with cool, low-mass companions, as well 29 known and newly found sdBs with white dwarf companions. The mass distribution of hot subdwarfs with cool, low-mass stellar and substellar companions differs from those with white dwarf companions, implying they come from different populations. By comparing the period and minimum companion mass distributions, we find that there are several different populations of hot subdwarfs with white dwarf binaries. We also derive the first orbital period distribution for hot subdwarfs with cool, low-mass stellar or substellar systems selected from light variations instead of radial velocity variations. It shows a period distribution from 1.5 hours to 35 hours compared to the distribution of hot subdwarfs with white dwarfs, which ranges from 1 hour to 30 days. These period distributions can be used to constrain the previous common envelope phase.
We report the discovery of the first hot subdwarf B (sdB) star with a massive compact companion in a wide (\(P=892.5\pm60.2\,{\rm d}\)) binary system. It was discovered based on an astrometric binary ...solution provided by the Gaia mission Data Release 3. We performed detailed analyses of the spectral energy distribution (SED) as well as spectroscopic follow-up observations and confirm the nature of the visible component as a sdB star. The companion is invisible despite of its high mass of \(M_{\rm comp}=1.50_{-0.45}^{+0.37}\,M_{\rm \odot}\). A main sequence star of this mass would significantly contribute to the SED and can be excluded. The companion must be a compact object, either a massive white dwarf or a neutron star. Stable Roche lobe overflow to the companion likely led to the stripping of a red giant and the formation of the sdB, the hot and exposed helium core of the giant. Based on very preliminary data, we estimate that \(\sim9\%\) of the sdBs might be formed through this new channel. This binary might also be the prototype for a new progenitor class of supernovae type Ia, which has been predicted by theory.
Hot subdwarf stars represent a late and peculiar stage in the evolution of low-mass stars, because they are likely formed by close binary interactions. Here we performed a radial velocity (RV) ...variability study of a sample of 646 hot subdwarfs with multi-epoch radial velocities from Sloan Digital Sky Survey (SDSS) and Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) spectra ... As diagnostics we used the fraction of RV-variable stars and the distribution of the maximum RV variations \(\Delta RV_{\rm max}\). Both indicators turned out to be quite inhomogeneous across the studied parameter ranges. A striking feature is the completely different behaviour of He-poor and He-rich hot subdwarfs. While the former have a high fraction of close binaries, almost no significant RV variations could be detected for the latter. This led us to the conclusion that there likely is no evolutionary connection between these subtypes. Intermediate He-rich- and extreme He-rich sdOB/Os on the other hand are likely related. We conclude further that the vast majority of this population is formed via one or several binary merger channels. Hot subdwarfs with temperatures cooler than \(\sim24\,000\,{\rm K}\) tend to show less and smaller RV-variations. These objects might constitute a new subpopulation of binaries with longer periods and late-type or compact companions. The RV-variability properties of the extreme horizontal branch (EHB) and corresponding post-EHB populations of the He-poor hot subdwarfs match and confirm the predicted evolutionary connection between them. Stars found below the canonical EHB at somewhat higher surface gravities show large RV-variations and a high RV-variability fraction, which is consistent with most of them being low-mass EHB stars or progenitors of low-mass helium white dwarfs in close binaries.
The spectroscopic class of subdwarf A-type (sdA) stars has come into focus in recent years because of their possible link to extremely low-mass white dwarfs, a rare class of objects resulting from ...binary evolution. Although most sdA stars are consistent with metal-poor halo main-sequence stars, the formation and evolution of a fraction of these stars are still matters of debate. The identification of photometric variability can help to put further constraints on the evolutionary status of sdA stars, in particular through the analysis of pulsations. Moreover, the binary ratio, which can be deduced from eclipsing binaries and ellipsoidal variables, is important as input for stellar models. In order to search for variability due to either binarity or pulsations in objects of the spectroscopic sdA class, we have extracted all available high precision light curves from the Kepler K2 mission. We have performed a thorough time series analysis on all available light curves, employing three different methods. Frequencies with a signal-to-noise ratio higher than four have been used for further analysis. From the 25 targets, 13 turned out to be variables of different kinds (i.e. classical pulsating stars, ellipsoidal and cataclysmic variables, eclipsing binaries, and rotationally induced variables). For the remaining 12 objects, a variability threshold was determined.
Helium-rich hot subdwarf stars of spectral type O (He-sdO) are considered prime candidates for stellar merger remnants. Such events should lead to the generation of strong magnetic fields. However, ...no magnetic He-sdO has yet been unambiguously discovered despite the high magnetic rate (20%) among white dwarf stars, the progeny of hot subdwarfs. Here we present the discovery of a strong magnetic field (B = 353 \(\pm\) 10 kG) from Zeeman-split hydrogen, helium, and metal lines in the optical X-SHOOTER spectrum of an He-sdO and present the first spectroscopic analysis of any magnetic hot subdwarf. For this we used line-blanketed Tlusty non-local thermodynamic equilibrium models and assumed a simple homogeneous magnetic field. The derived atmospheric parameters \(T_\mathrm{eff}\) = 44900 \(\pm\) 1000 K and log g = 5.93 \(\pm\) 0.15 are typical for He-sdO stars, while the star is less hydrogen-poor than most He-sdOs at log n(He)/n(H) = +0.28 \(\pm\) 0.10. The star is a slow rotator (\(v_\mathrm{rot}\sin i\) < 40 km s\(^{-1}\)). Its chemical composition is N-rich and C- and O-poor, and the Si and S abundances are close to solar. Combining the atmospheric parameters with Gaia parallax and photometry, the stellar radius and luminosity are found to be typical for He-sdOs and place the star on the helium main sequence in the Hertzsprung-Russell diagram. Its mass of \(0.93^{+0.44}_{-0.30}\) \(M_\odot\), although uncertain, appears to be remarkably high. The strong magnetic field along with the atmospheric parameters and metal abundances provide overwhelming evidence for the double-degenerate merger scenario.
Hot subdwarfs (sdO/B) are the stripped helium cores of red giants formed by binary interactions. Close hot subdwarf binaries with massive white dwarf companions have been proposed as possible ...progenitors of thermonuclear supernovae type Ia (SN Ia). If the supernova is triggered by stable mass transfer from the helium star, the companion should survive the explosion and should be accelerated to high velocities. The hypervelocity star US 708 is regarded as the prototype for such an ejected companion. To find more of those objects we conducted an extensive spectroscopic survey. Candidates for such fast stars have been selected from the spectroscopic database of the Sloan Digital Sky Survey (SDSS) and several ground-based proper motion surveys. Follow-up spectroscopy has been obtained with several 4m- to 10m-class telescopes. Combining the results from quantitative spectroscopic analyses with space-based astrometry from \textit{Gaia} Early Data Release 3 (EDR3) we determined the atmospheric and kinematic parameters of 53 fast hot subdwarf stars. None of these stars is unbound to the Galaxy, although some have Galactic restframe velocities close to the Galactic escape velocity. 21 stars are apparently single objects, which crossed the Galactic disc within their lifetimes in the sdO/B stage and could be regarded as potential candidates for the SN Ia ejection scenario. However, the properties of the full sample are more consistent with a pure old Galactic halo population. We therefore conclude that the fast sdO/B stars we found are likely to be extreme halo stars.
PDF