TY - JOUR A1 - Schaffenroth, Veronika A1 - Pelisoli, Ingrid A1 - Barlow, Brad N. A1 - Geier, Stephan A1 - Kupfer, Thomas T1 - Hot subdwarfs in close binaries observed from space I. BT - orbital, atmospheric, and absolute parameters and the nature of their companions JF - Astronomy and astrophysics : an international weekly journal N2 - Context: About a third of the hot subdwarfs of spectral type B (sdBs), 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 di fferent phenomena. Aims: Many hot subdwarfs now have space-based light curves with a high signal-to-noise ratio available. We used light curves from the Transiting Exoplanet Survey Satellite and the 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 obtained orbital, atmospheric, and absolute parameters for those systems, when combined with other analysis methods. Methods: By classifying the light variations and combining these with the fit of the spectral energy distribution, the distance derived by the parallaxes obtained by Gaia, and the atmospheric parameters, mainly from the literature, we could derive the nature of the primaries and secondaries in 122 (75%) of the known sdB binaries and 82 newly found reflection e ffect systems. We derived 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. Results: The mass distribution of hot subdwarfs with cool, low-mass stellar and substellar companions, di ffers from those with white dwarf companions, implying they come from di fferent populations. By comparing the period and minimum companion mass distributions, we find that the reflection e ffect systems all have M dwarf or brown dwarf companions, and that there seem to be several di fferent populations of hot subdwarfs with white dwarf binaries - one with white dwarf minimum masses around 0.4 M-circle dot, one with longer periods and minimum companion masses up to 0.6 M-circle dot, and at the shortest period, another with white dwarf minimum masses around 0.8 M-circle dot. 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 narrower period distribution, from 1.5 h to 35 h, compared to the distribution of hot subdwarfs with white dwarfs, which ranges from 1 h to 30 days. These period distributions can be used to constrain the previous common-envelope phase. KW - binaries: close KW - subdwarfs KW - white dwarfs KW - stars: late-type KW - stars: KW - horizontal-branch KW - stars: fundamental parameters Y1 - 2022 U6 - https://doi.org/10.1051/0004-6361/202244214 SN - 0004-6361 SN - 1432-0746 VL - 666 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Geier, Stephan A1 - Dorsch, Matti A1 - Pelisoli, Ingrid A1 - Reindl, Nicole A1 - Heber, Ulrich A1 - Irrgang, Andreas T1 - Radial velocity variability and the evolution of hot subdwarf stars JF - Astronomy and astrophysics : an international weekly journal N2 - Hot subdwarf stars represent a late and peculiar stage in the evolution of low-mass stars, since they are likely formed by close binary interactions. In this work, we perform a radial velocity (RV) variability study of a sample of 646 hot subdwarfs with multi-epoch radial velocities based on spectra from Sloan Digital Sky Survey (SDSS) and Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST). The atmospheric parameters and RVs were taken from the literature. For stars with archival spectra but without literature values, we determined the parameters by fitting model atmospheres. In addition, we redetermined the atmospheric parameters and RVs for all the He-enriched sdO/Bs. This broad sample allowed us to study RV-variability as a function of the location in the T-eff - log g- and T-eff - log n(He)/n(H) diagrams in a statistically significant way. We used the fraction of RV-variable stars and the distribution of the maximum RV variations Delta RVmax as diagnostics. Both indicators turned out to be quite inhomogeneous across the studied parameter ranges. A striking feature is the completely dissimilar 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 has led us to the conclusion that there is likely no evolutionary connection between these subtypes. On the other hand, intermediate He-rich- and extreme He-rich sdOB/Os are more likely to be related. Furthermore, we conclude that the vast majority of this population is formed via one or several binary merger channels. Hot subdwarfs with temperatures cooler than similar to 24 000 K tend to show fewer 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. These properties are consistent with most of them being low-mass EHB stars or progenitors of low-mass helium white dwarfs in close binaries. KW - subdwarfs KW - binaries: spectroscopic KW - stars: horizontal-branch Y1 - 2022 U6 - https://doi.org/10.1051/0004-6361/202143022 SN - 0004-6361 SN - 1432-0746 VL - 661 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Culpan, Richard A1 - Geier, Stephan A1 - Reindl, Nicole A1 - Pelisoli, Ingrid A1 - Gentile Fusillo, Nicola Pietro A1 - Vorontseva, Alina T1 - The population of hot subdwarf stars studied with Gaia BT - IV. catalogues of hot subluminous stars based on Gaia EDR3 JF - Astronomy and astrophysics : an international weekly journal N2 - In light of substantial new discoveries of hot subdwarfs by ongoing spectroscopic surveys and the availability of the Gaia mission Early Data Release 3 (EDR3), we compiled new releases of two catalogues of hot subluminous stars: the data release 3 (DR3) catalogue of the known hot subdwarf stars contains 6616 unique sources and provides multi-band photometry, and astrometry from Gaia EDR3 as well as classifications based on spectroscopy and colours. This is an increase of 742 objects over the DR2 catalogue. This new catalogue provides atmospheric parameters for 3087 stars and radial velocities for 2791 stars from the literature. In addition, we have updated the Gaia Data Release 2 (DR2) catalogue of hot subluminous stars using the improved accuracy of the Gaia EDR3 data set together with updated quality and selection criteria to produce the Gaia EDR3 catalogue of 61 585 hot subluminous stars, representing an increase of 21 785 objects. The improvements in Gaia EDR3 astrometry and photometry compared to Gaia DR2 have enabled us to define more sophisticated selection functions. In particular, we improved hot subluminous star detection in the crowded regions of the Galactic plane as well as in the direction of the Magellanic Clouds by including sources with close apparent neighbours but with flux levels that dominate the neighbourhood. KW - subdwarfs KW - Hertzsprung-Russell and C-M diagrams KW - binaries: general KW - stars: horizontal-branch KW - catalogs Y1 - 2022 U6 - https://doi.org/10.1051/0004-6361/202243337 SN - 1432-0746 VL - 662 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Hillwig, Todd C. A1 - Reindl, Nicole A1 - Rotter, Hannah M. A1 - Rengstorf, Adam W. A1 - Heber, Ulrich A1 - Irrgang, Andreas T1 - Two evolved close binary stars: GALEX J015054.4+310745 and the central star of the planetary nebula Hen 2-84 JF - Monthly notices of the Royal Astronomical Society N2 - As part of a survey to find close binary systems among central stars of planetary nebula, we present two newly discovered binary systems. GALEX J015054.4+310745 is identified as the central star of the possible planetary nebula Fr 2-22. We find it to be a single-lined spectroscopic binary with an orbital period of 0.2554435(10) d. We support the previous identification of GALEX J015054.4+310745 as an sdB star and provide physical parameters for the star from spectral modelling. We identify its undetected companion as a likely He white dwarf. Based on this information, we find it unlikely that Fr 2-22 is a true planetary nebula. In addition, the central star of the true planetary nebula Hen 2-84 is found to be a photometric variable, likely due to the irradiation of a cool companion. The system has an orbital period of 0.485645(30) d. We discuss limits on binary parameters based on the available light-curve data. Hen 2-84 is a strongly shaped bipolar planetary nebula, which we now add to the growing list of axially or point-symmetric planetary nebulae with a close binary central star. KW - binaries: close KW - stars: individual: GALEX J015054.4+310745 KW - subdwarfs KW - planetary nebulae: individual: Hen 2-84 Y1 - 2022 U6 - https://doi.org/10.1093/mnras/stac226 SN - 0035-8711 SN - 1365-2966 VL - 511 IS - 2 SP - 2033 EP - 2039 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Werner, Klaus A1 - Reindl, Nicole A1 - Dorsch, Matti A1 - Geier, Stephan A1 - Munari, Ulisse A1 - Raddi, Roberto T1 - Non-local thermodynamic equilibrium spectral analysis of five hot, hydrogen-deficient pre-white dwarfs JF - Astronomy and Astrophysics N2 - Hot, compact, hydrogen-deficient pre-white dwarfs (pre-WDs) with effective temperatures of Teff > 70 000 K and a surface gravity of 5.0 < logg < 7.0 are rather rare objects despite recent and ongoing surveys. It is believed that they are the outcome of either single star evolution (late helium-shell flash or late helium-core flash) or binary star evolution (double WD merger). Their study is interesting because the surface elemental abundances reflect the physics of thermonuclear flashes and merger events. Spectroscopically they are divided in three different classes, namely PG1159, O(He), or He-sdO. We present a spectroscopic analysis of five such stars that turned out to have atmospheric parameters in the range Teff = 70 000-80 000 K and logg = 5.2-6.3. The three investigated He-sdOs have a relatively high hydrogen mass fraction (10%) that is unexplained by both single (He core flash) and binary evolution (He-WD merger) scenarios. The O(He) star JL 9 is probably a binary helium-WD merger, but its hydrogen content (6%) is also at odds with merger models. We found that RL 104 is the 'coolest' (Teff = 80 000 K) member of the PG1159 class in a pre-WD stage. Its optical spectrum is remarkable because it exhibits C※ IV lines involving Rydberg states with principal quantum numbers up to n = 22. Its rather low mass (0.48-0.02+0.03 M·) is difficult to reconcile with the common evolutionary scenario for PG1159 stars due to it being the outcome of a (very) late He-shell flash. The same mass-problem faces a merger model of a close He-sdO plus CO WD binary that predicts PG1159-like abundances. Perhaps RL 104 originates from a very late He-shell flash in a CO/He WD formed by a merger of two low-mass He-WDs. KW - stars: atmospheres KW - stars: abundances KW - stars: evolution KW - subdwarfs KW - white dwarfs Y1 - 2022 U6 - https://doi.org/10.1051/0004-6361/202142397 SN - 0004-6361 SN - 1432-0746 VL - 658 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Pelisoli, Ingrid A1 - Dorsch, Matti A1 - Heber, Ulrich A1 - Gänsicke, Boris A1 - Geier, Stephan A1 - Kupfer, Thomas A1 - Nemeth, Peter A1 - Scaringi, Simone A1 - Schaffenroth, Veronika T1 - Discovery and analysis of three magnetic hot subdwarf stars BT - evidence for merger-induced magnetic fields JF - Monthly notices of the Royal Astronomical Society N2 - Magnetic fields can play an important role in stellar evolution. Among white dwarfs, the most common stellar remnant, the fraction of magnetic systems is more than 20 per cent. The origin of magnetic fields in white dwarfs, which show strengths ranging from 40 kG to hundreds of MG, is still a topic of debate. In contrast, only one magnetic hot subdwarf star has been identified out of thousands of known systems. Hot subdwarfs are formed from binary interaction, a process often associated with the generation of magnetic fields, and will evolve to become white dwarfs, which makes the lack of detected magnetic hot subdwarfs a puzzling phenomenon. Here we report the discovery of three new magnetic hot subdwarfs with field strengths in the range 300-500 kG. Like the only previously known system, they are all helium-rich O-type stars (He-sdOs). We analysed multiple archival spectra of the three systems and derived their stellar properties. We find that they all lack radial velocity variability, suggesting formation via a merger channel. However, we derive higher than typical hydrogen abundances for their spectral type, which are in disagreement with current model predictions. Our findings suggest a lower limit to the magnetic fraction of hot subdwarfs of 0.147(+0.143)(-0.047) per cent, and provide evidence for merger-induced magnetic fields which could explain white dwarfs with field strengths of 50-150 MG, assuming magnetic flux conservation. KW - stars: magnetic field KW - subdwarfs Y1 - 2022 U6 - https://doi.org/10.1093/mnras/stac1069 SN - 0035-8711 SN - 1365-2966 VL - 515 IS - 2 SP - 2496 EP - 2510 PB - Oxford University Press CY - Oxford ER - TY - JOUR A1 - Neunteufel, Patrick A1 - Preece, H. A1 - Kruckow, Matthias U. A1 - Geier, Stephan A1 - Hamers, Adrian S. A1 - Justham, S. A1 - Podsiadlowski, Philipp T1 - Properties and applications of a predicted population of runaway He-sdO/B stars ejected from single degenerate He-donor SNe JF - Astronomy and astrophysics : an international weekly journal N2 - Context. Thermonuclear supernovae (SNe), a subset of which are the highly important SNe of Type Ia and Iax, are relatively poorly understood phenomena. One of the more promising scenarios leading up to the creation of a thermonuclear SN involves accretion of helium-rich material from a binary companion. Following the SN, the binary companion is then ejected from the location of the progenitor binary at velocities possibly large enough to unbind it from the gravitational potential of the Galaxy. Ejected companion stars should form a detectable population, if their production mechanism is not exceedingly rare. Aims. This study builds on previous works, producing the most extensive prediction of the properties of such a hypothetical population to date, taking both Chandrasekhar and non-Chandrasekhar mass events into account. These results are then used to define criteria for membership of this population and characterise putative subpopulations. Methods. This study contains 6 x 10(6) individual ejection trajectories out of the Galactic plane calculated with the stellar kinematics framework SHyRT, which are analysed with regard to their bulk observational properties. These are then put into context with the only previously identified population member US 708 and applied to a number of other possible candidate objects. Results. We find that two additional previously observed objects possess properties to warrant a designation as candidate objects. Characterisation of these object with respect to the predicted population finds all of them to be extreme in at least one astrometric observable. Higher mass ( >0 :7 M-circle dot) objects should be over-represented in the observationally accessible volume, with the ratio of bound to unbound objects being an accessible observable for the determination of the dominant terminal accretor mass. We find that current observations of runaway candidates within 10 kpc support a Galactic SN rate of the order of similar to 3 x 10(-7) yr(-1) to similar to 2 x 10(-6) yr(-1), three orders of magnitude below the inferred Galactic SN Ia rate and two orders of magnitude below the formation rate of predicted He-donor progenitors. Conclusions. The number of currently observed population members suggests that the He-donor scenario, as suspected before, is not a dominant contributor to the number of observed SNe Ia. However, even at the low event rate suggested, we find that the majority of possibly detectable population members is still undetected. The extreme nature of current population members suggests that a still larger number of objects has simply evaded detection up to this point, hinting at a higher contribution than is currently supported by observation. KW - binaries: close KW - stars: kinematics and dynamics KW - stars: distances KW - supernovae: general KW - subdwarfs KW - white dwarfs Y1 - 2022 U6 - https://doi.org/10.1051/0004-6361/202142864 SN - 0004-6361 SN - 1432-0746 VL - 663 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Dorsch, Matti A1 - Jeffery, C. Simon A1 - Irrgang, Andreas A1 - Woolf, Vincent A1 - Heber, Ulrich T1 - EC 22536-5304 BT - a lead-rich and metal-poor long-period binary JF - Astronomy and astrophysics : an international weekly journal N2 - Helium-burning hot subdwarf stars of spectral types O and B (sdO/B) are thought to be produced through various types of binary interactions. The helium-rich hot subdwarf star EC 22536-5304 was recently found to be extremely enriched in lead. Here, we show that EC 22536-5304 is a binary star with a metal-poor subdwarf F-type (sdF) companion. We performed a detailed analysis of high-resolution SALT/HRS and VLT/UVES spectra, deriving metal abundances for the hot subdwarf, as well as atmospheric parameters for both components. Because we consider the contribution of the sdF star, the derived lead abundance for the sdOB, + 6.3 +/- 0.3 dex relative to solar, is even higher than previously thought. We derive T-eff = 6210 +/- 70 K, log g = 4.64 +/- 0.10, [FE/H] = - 1.95 +/- 0.04, and [alpha/Fe] = + 0.40 +/- 0.04 for the sdF component. Radial velocity variations, although poorly sampled at present, indicate that the binary system has a long orbital period of about 457 days. This suggests that the system was likely formed through stable Roche lobe overflow (RLOF). A kinematic analysis shows that EC 22536-5304 is on an eccentric orbit around the Galactic centre. This, as well as the low metallicity and strong alpha enhancement of the sdF-type companion, indicate that EC 22536-5304 is part of the Galactic halo or metal-weak thick disc. As the first long-period hot subdwarf binary at [FE/H] less than or similar to- 1, EC 22536-5304 may help to constrain the RLOF mechanism for mass transfer from low-mass, low-metallicity red giant branch (RGB) stars to main-sequence companions. KW - stars: abundances KW - stars: chemically peculiar KW - subdwarfs KW - stars: individual: EC 22536-5304 KW - binaries: spectroscopic Y1 - 2021 U6 - https://doi.org/10.1051/0004-6361/202141381 SN - 1432-0746 VL - 653 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Mösenlechner, Gerald A1 - Paunzen, Ernst A1 - Pelisoli, Ingrid D. A1 - Seelig, Joseph A1 - Stidl, Sarah A1 - Maitzen, Hans Michael T1 - A Kepler K2 view of subdwarf A-type stars JF - Astronomy and astrophysics : an international weekly journal N2 - Context. 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. Aims. 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. Methods. 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. Results. 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. KW - subdwarfs KW - white dwarfs KW - binaries: general KW - stars: evolution KW - stars: variables: general Y1 - 2022 U6 - https://doi.org/10.1051/0004-6361/202037789 SN - 0004-6361 SN - 1432-0746 VL - 657 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Neunteufel, Patrick A1 - Kruckow U., Matthias A1 - Geier, Stephan A1 - Hamers, Adrian S. T1 - Predicted spatial and velocity distributions of ejected companion stars of helium accretion-induced thermonuclear supernovae JF - Astronomy and astrophysics : an international weekly journal N2 - Context Thermonuclear supernovae (SNe), a subset of which are the highly important SNe Type Ia, remain one of the more poorly understood phenomena known to modern astrophysics. In recent years, the single degenerate helium (He) donor channel, where a white dwarf star accretes He-rich matter from a hydrogen-depleted companion, has emerged as a promising candidate progenitor scenario for these events. An unresolved question in this scenario is the fate of the companion star, which would be evident as a runaway hot subdwarf O/B stars (He sdO/B) in the aftermath of the SN event. Aims Previous studies have shown that the kinematic properties of an ejected companion provide an opportunity to closer examine the properties of an SN progenitor system. However, with the number of observed objects not matching predictions by theory, the viability of this mechanism is called into question. In this study, we first synthesize a population of companion stars ejected by the aforementioned mechanism, taking into account predicted ejection velocities, the inferred population density in the Galactic mass distribution, and subsequent kinematics in the Galactic potential. We then discuss the astrometric properties of this population. Methods We present 10(6) individual ejection trajectories, which were numerically computed with a newly developed, lightweight simulation framework. Initial conditions were randomly generated, but weighted according to the Galactic mass density and ejection velocity data. We then discuss the bulk properties (Galactic distribution and observational parameters) of our sample. Results Our synthetic population reflects the Galactic mass distribution. A peak in the density distribution for close objects is expected in the direction of the Galactic centre. Higher mass runaways should outnumber lower mass ones. If the entire considered mass range is realised, the radial velocity distribution should show a peak at 500 km s(-1). If only close US 708 analogues are considered, there should be a peak at (similar to 750-850) km s(-1). In either case, US 708 should be a member of the high-velocity tail of the distribution. Conclusions We show that the puzzling lack of confirmed surviving companion stars of thermonuclear SNe, though possibly an observation-related selection effect, may indicate a selection against high mass donors in the SD He donor channel. KW - stars: kinematics and dynamics KW - binaries: close KW - supernovae: general KW - subdwarfs Y1 - 2021 U6 - https://doi.org/10.1051/0004-6361/202040022 SN - 0004-6361 SN - 1432-0746 VL - 646 PB - EDP Sciences CY - Les Ulis ER -