arXiv subjects

Ulrich Heber

Publications and source records attributed to Ulrich Heber.

At least 19 recordsLinked to original sources

arXiv2026

Intermediate-mass runaway and hypervelocity star candidates in DESI DR1

Runaway stars ejected from the Galactic disc span a range of velocities, with two extreme regimes of particular interest. Hyper-runaway stars reach velocities approaching the local Galactic escape speed while remaining gravitationally bound to the Galaxy, while hypervelocity stars (HVS) have speeds which exceed the escape speed, leaving them formally unbound. Studying both populations places stringent constraints on star cluster formation, supernova explosions in massive binaries, and the shape of the stellar halo. Here we present a spectroscopic search for such extreme-velocity stars using data from the first data release of the Dark Energy Spectroscopic Instrument (DESI DR1). Because these objects are typically located at distances beyond the range where \textit{gaia} parallaxes are reliable, we derive stellar parameters through a combined analysis of spectroscopy and spectral energy distributions (SEDs). These parameters are then compared with stellar evolutionary tracks to infer distances, which are subsequently used to compute the kinematics of the candidates. We identify a sub-sample of eight stars that appear unbound to the Galaxy, likely of extragalactic origin. These stars exhibit projected rotational velocities exceeding $50$ km s$^{-1}$, consistent with a main-sequence nature. An additional eleven stars are identified as potential HVS candidates under the assumption that they are main-sequence stars. This interpretation is not supported by their rotational velocities, making their unbound status less secure. Finally, $26$ stars show evidence of being main-sequence runaway stars ejected from the Galactic disc. The fastest object in the sample is a metal-poor HVS candidate with a Galactocentric velocity upper-limit of $934 \pm 110$ km s$^{-1}$. The star with the largest measured radial velocity of $513$ km s$^{-1}$ is most likely a slowly pulsating B-type runaway star.

astro-ph.SR↗

arXiv2026

Studying hot evolved stars with ultraviolet spectroscopy

Hot evolved stars are key objects to reconstruct the various evolutionary pathways of Sun-like stars, to probe binary interactions and the physics of supernovae. They serve as powerful observational constraints to test diffusion, mixing, and mass loss in hot stellar atmospheres. Furthermore, hot stars serve as laboratories to test and derive atomic data for highly ionised trans-iron group elements and to investigate different nucleosynthesis models. Hot evolved stars emit most of their flux in the ultraviolet (UV) and a lot of progress has been made in characterizing their UV-spectra both on the observational and on the modelling side. The unique capabilities of HST to obtain high- and medium-resolution UV-spectra played a crucial role and are needed to further advance this field also in preparation for HWO.

astro-ph.IM↗

arXiv2026

Hot subdwarf stars from the Hamburg Quasar Survey

Hot subluminous stars (sdO/B) are evolved low mass stars originating from red giants that lost their envelope almost entirely. The multitude of observed phenomena imply that several pathways may form hot subdwarfs, most involving close binary channels. The Hamburg Quasar Survey (HQS) led to the discovery of many faint blue stars including hot subdwarf. Many of the HQS-sdB stars have been studied in detail, but analyses of the helium-rich sdOB and sdO stars are lacking. The recent development of hybrid LTE/non-LTE model spectra 2nd generation Bamberg model grids enables us to improve the spectroscopic analyses of the sdB stars as well as of the previously unstudied sdO stars allowing precise atmospheric parameters to be derived, while consistently accounting for parameter correlations and systematic uncertainties. ... We use spectral energy distributions to identify composite-colour sdB binaries and present the result of detailed spectroscopic analyses of 122 non-composite subdwarfs from the HQS to identify potential evolutionary pathways. ...Their derived mass distribution and median mass of 0.45 Msun is consistent with the canonical EHB mass. ... The helium-rich sdOB and sdO stars, are found near the helium main-sequence (He-MS). The derived mass distribution of the extremely He-rich subdwarfs is broader (0.48 to 1.05 Msun) and peaks at a median of 0.70 Msun, significantly larger than those of the hydrogen-rich stars. Intermediate He-rich subdwarfs are also He-MS stars, but of lower mass (0.55 Msun) than the extremely He-rich. This strongly supports the merger scenario for the origin of He-rich sdO stars, in which two helium white dwarfs merge following orbital decay driven by gravitational-wave emission, producing a He-rich sdO or sdOB star. From comparison to the results of similar studies we speculate that older populations produce more massive He-WD mergers.(abbreviated)

astro-ph.SR↗

arXiv2026

Discovery of a double white dwarf in the Galactic globular cluster NGC 6397

Binaries in the cores of globular clusters are known to prevent the gravitational collapse of the cluster, and simulations predict that the core of NGC 6397 contains a large number of white dwarfs (WDs), of which many are expected to be part of a binary system. In this work, we report the discovery of a compact binary system consisting of two WDs in the centre of the Galactic globular cluster NGC 6397. The system, known in the literature as NF1, was observed as part of a MUSE radial-velocity survey aiming at characterizing the binary population in the centre of NGC 6397. The spectral analysis of NF1 provides an effective temperature of 16000 K and a surface gravity (log g) of 5.72 (cgs), which is consistent with an extremely low-mass He-core WD nature. This is further supported by the mass of 0.23 +/- 0.03 Msun obtained from fitting the star's spectral energy distribution using its HST magnitude in various filters. The system has a circular orbit with a period of 0.54 days. The radial velocities show a large semi-amplitude of 200 km/s, implying a minimum mass of 0.78 Msun for the invisible companion, which is likely another WD, or a neutron star if the inclination of the system is smaller than about 50 deg. Some significant residuals in radial velocity remain with our best orbital solution and we tested whether a model with a third body can explain these deviations. While this possibility seems promising, additional measurements are needed to confirm whether the star is actually part of a triple system.

astro-ph.SR↗

arXiv2025

A slowly pulsating run-away B star at high Galactic latitude ejected from a spiral arm

We report the discovery of the young B6V run-away star LAMOST J083323.18+430825.4, 2.5\,kpc above the Galactic plane. Its atmospheric parameters and chemical composition are determined from LAMOST spectra, indicating normal composition. Effective temperature (Teff=14,500) and gravity (log g=3.79) suggest that the star is close to terminating hydrogen burning. An analysis of the spectral energy distribution allowed us to determine the angular diameter as well as the interstellar reddening. Using evolutionary models from the MIST database we derived the stellar mass (4.75Msun) and age (104^+11_-13 Myr). The spectroscopic distance (4.17 kpc), the radius (4.5 Rsun), and the luminosity (log(L/Lsun)=2.89) then result from the atmospheric parameters. Using Gaia proper motions, the trajectory is traced back to the Galactic disk to identify the place of birth in a spiral arm. The ejection velocity of 92 km s^{-1} is typical for runaway stars in the halo. The age of the star is larger than its time of flight (78+-4 Myr), which favors a binary supernova event as the likely ejection mechanism. The TESS light curve shows variations with a period of 3.58 days from which we conclude that it is a slowly pulsating B-star, one of very few run-away B-stars known to pulsate.

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arXiv2025

Testing the mass-radius relation of white dwarfs in common proper motion pairs I.Hydrogen-dominated atmospheres

The main goal of this work was to measure the masses and radii of white dwarfs that belong to widely separated, common proper motion binaries with non-degenerate companions. These can be assessed, independently from theoretical mass-radius relations, through measurements of gravitational redshifts and photometric radii. We studied 50 white dwarfs with hydrogen-dominated atmospheres, performing a detailed analysis of high-resolution (R ~ 18,500) spectra by means of state-of-the-art grids of synthetic models and specialized software. Hence, we measured accurate radial velocities from the H-alpha and H-beta line-cores, thus obtaining the white dwarf gravitational redshifts. Jointly with a photometric analysis that is formalized by a Bayesian inference method, we measured precise white dwarf radii that allowed us to directly measure the white dwarf masses from their gravitational redshifts. The distributions of measured masses and radii agree within 6% (at the 1-sigma level) from the theoretical mass-radius relation, thus delivering a much smaller scatter in comparison with previous analyses that used gravitational redshift measurements from low-resolution spectra. A comparison against model-dependent spectroscopic estimates produces a larger scatter of 15% on the mass determinations. We find an agreement within ~10% from previous model-based, photometric mass estimates from the literature. Combining gravitational redshift measurements and photometric analysis of white dwarfs delivers precise and accurate, empirical estimates of their masses and radii. This work confirms the reliability of the theoretical mass-radius relation from the lightest to the heaviest white dwarfs in our sample (0.38-1.3 Msun). [abridged]

astro-ph.SR↗

arXiv2024

Multiwavelength study of 1eRASS J085039.9-421151 with eROSITA NuSTAR and X-shooter

The eROSITA instrument on board Spectrum-Roentgen-Gamma has completed four scans of the X-ray sky, leading to the detection of almost one million X-ray sources in eRASS1 only, including multiple new X-ray binary candidates. We report on analysis of the X-ray binary 1eRASS J085039.9-421151, using a ~55\,ks long NuSTAR observation, following its detection in each eROSITA scan. Analysis of the eROSITA and NuSTAR X-ray spectra in combination with X-shooter data of the optical counterpart provide evidence of an X-ray binary with a red supergiant (RSG) companion, confirming previous results, although we determine a cooler spectral type of M2-3, owing to the presence of TiO bands in the optical and near infrared spectra. The X-ray spectrum is well-described by an absorbed power law with a high energy cutoff typically applied for accreting high mass X-ray binaries. In addition, we detect a strong fluorescent neutral iron line with an equivalent width of ~700\,eV and an absorption edge, the latter indicating strong absorption by a partial covering component. It is unclear if the partial absorber is ionised. There is no significant evidence of a cyclotron resonant scattering feature. We do not detect any pulsations in the NuSTAR lightcurves, possibly on account of a large spin period that goes undetected due to insufficient statistics at low frequencies or potentially large absorption that causes pulsations to be smeared out. Even so, the low persistent luminosity, the spectral parameters observed (photon index, $\Gamma<1.0$), and the minuscule likelihood of detection of RSG-black hole systems, suggest that the compact object is a neutron star.

astro-ph.HE↗

arXiv2024

Hot Subdwarf Stars

Hot subdwarf (SD) stars are the stripped cores of red giant stars in transition to the white dwarf sequence. The B-type subdwarfs (sdB) are powered by helium fusion in the core, more evolved ones (sdO) by shell burning. Low mass SDs may evolve through this stage without any support by nuclear fusion. Because the loss of the giants' envelopes is likely caused by mass transfer in binaries, hot SDs are test beds for close-binary evolution through stable and unstable Roche lobe overflow, common envelope formation and ejection as well as mergers. Many classes of hot SDs can be identified according to surface composition, binarity, magnetism, pulsation characteristics and population membership, including members of globular clusters. Observed binaries show a wide spread of orbital periods from 20 minutes to more than 1,000 days with white dwarf or main sequence companions. The closest systems qualify as type Ia supernova progenitors and LISA detectable gravitational wave sources. High-precision light curves from Kepler and TESS combined with radial velocity curves are used to derive masses, while asteroseismology adds information on the internal structure, slow rotation, and synchronization. Gaia's parallax measurements now allow us to place the stars in the Hertzsprung-Russell diagram and derive stellar parameters by combining them with multi-band photometry. The stellar radius can be determined to high precision this way. Newton's law can then be used to derive masses if accurate surface gravities are available. Large-scale spectroscopic surveys will provide atmospheric parameters for large samples of stars, allowing the mass distributions for the diverse subtypes to be established. These are crucial for testing binary synthesis models and constraining poorly known parameters such as the common envelope efficiency as well as the critical threshold mass-ratio for mass transfer stability.

astro-ph.SR↗

arXiv2024

Archives of Photographic PLates for Astronomical USE (APPLAUSE) Digitisation of astronomical plates and their integration into the International Virtual Observatory

The Archives of Photographic PLates for Astronomical USE (APPLAUSE) project is aimed at digitising astronomical photographic plates from three major German plate collections, making them accessible through integration into the International Virtual Observatory (IVO). Photographic plates and related materials (logbooks, envelopes, etc.) were scanned with commercial flatbed scanners. Astrometric and photometric calibrations were carried out with the developed PyPlate software, using Gaia EDR3 data as a reference. The APPLAUSE data publication complies with IVO standards. The latest data release contains images and metadata from 27 plate collections from the partner institutes in Hamburg, Bamberg, and Potsdam, along with digitised archives provided by Tautenburg, Tartu, and Vatican observatories. Altogether, over two billion calibrated measurements extracted from about 70,000 direct photographic plates can readily be used to create long-term light curves. For instance, we constructed the historic light curve of the enigmatic dipping star KIC 8462852. We found no evidence of previously assumed variations on timescales of decades in our light curve. Potential uses of APPLAUSE images for transient sources can be appreciated by following the development of the nova shell of GK Per (1901) over time and the change in brightness of two extragalactic supernovae. The database holds about 10,000 spectral plates. We made use of objective prism plates to follow the temporal changes of Nova DN Gem through 1912 and 1913, highlighting an outburst in early 1913.

astro-ph.IM↗

arXiv2024

Probing the inner Galactic Halo with blue horizontal branch stars: Gaia DR3 based catalogue with atmospheric and stellar parameters

Context. Stars that are found on the blue horizontal-branch (BHB) have evolved from low-mass stars that have completed their core hydrogen burning main sequence stage and have undergone the helium flash at the end of their red-giant phase. The fact that their luminosity is virtually constant at all effective temperatures also makes them good standard candles. Aims. We provide a catalogue of BHB stars with stellar parameters that have been calculated from spectral energy distributions (SED), as constructed from multiple large-scale photometric surveys. In addition, we update our previous, Gaia Early Data Release 3 catalogue of BHB stars with parallax errors less than 20% by using the SED results to define the selection criteria. Methods. We selected a large dataset of Gaia Data Release 3 (DR3) objects based only on their position in the colour magnitude diagram, tangential velocity and parallax errors. Spectral energy distributions were then used to evaluate contamination levels in the dataset and derive optimised data quality acceptance constraints. This allowed us to extend the Gaia DR3 colour and absolute magnitude criteria further towards the extreme horizontal-branch. The level of contamination found using SED analysis was confirmed by acquiring spectra using the Ondrejov Echelle spectrograph attached to the Perek 2m telescope at the Astronomical Institute of the Czech Academy of Sciences. Results. We present a catalogue of 9,172 Galactic Halo BHB candidate stars with atmospheric and stellar parameters calculated from synthetic SEDs. We also present an extended Gaia DR3 based catalogue of 22,335 BHB candidate stars with a wider range of effective temperatures and Gaia DR3 parallax errors of less than 20%. This represents an increase of 33% compared to the our 2021 catalogue, with a contamination level of 10%.

astro-ph.SR↗

arXiv2023

The fastest hot subdwarfs revisited

Hyper-velocity stars (HVS) are enigmatic objects because they are travelling so fast that they escape from the Galaxy. Among hot subdwarfs, only one such star is known, the He-sdO US 708. The Hyper-MUCHFUSS collaboration provided additional HVS candidates. Here we revisit the fastest candidates including US 708 by analysing optical spectra and spectral energy distributions using a new grid of tailored model atmospheres and report preliminary results. The sample is dominated by H-rich subdwarfs and their distribution in the Kiel diagram appears to be bimodal for the sdB stars but otherwise fits canonical evolutionary models well. Gaia proper motion measurements allowed a precise kinematic investigation to be made. It turns out that all previously proposed HVS candidates are actually bound to the Galaxy, except US 708. The original candidate sample turns out to belong to an extreme halo population. The scarcity of available observations of US 708 calls for space-based UV and IR photometry as well as high precision radial velocity measurements.

astro-ph.SR↗

arXiv2022

The origin of early-type runaway stars from open clusters

Runaway stars are ejected from their place of birth in the Galactic disk, with some young B-type runaways found several tens of kiloparsecs from the plane traveling at speeds beyond the escape velocity. Young open clusters are a likely place of origin, and ejection may be either through N-body interactions or in binary supernova explosions. The excellent quality of Gaia astrometry opens up the path to study the kinematics of young runaway stars to such a high precision that the place of origin in open stellar clusters can be identified uniquely. We developed an efficient minimization method to calculate whether two or more objects may come from the same place, which we tested against samples of Orion runaways. Our fitting procedure was then used to calculate trajectories for known runaway stars where we used Gaia data and updated radial velocities. We found that only half of the sample could be classified as runaways while the others were walkaway stars. Most of the latter stars turned out to be binaries. We identified parent clusters for runaways based on their trajectories and then used cluster age and flight time of the stars to investigate whether the ejection was likely due to a binary supernova or due to a dynamical ejection. In particular, we show that the classical runaways AE Aurigae and $\mu$ Columbae might not have originated together, with $\mu$ Columbae having an earlier ejection from Collinder 69, a cluster near the ONC. The second sample investigated comprises a set of distant runaway B stars in the halo which have been studied carefully by quantitative spectral analyses. We are able to identify candidate parent clusters for at least four stars including the hyper-runaway candidate HIP 60350. The ejection events had to be very violent, ejecting stars at velocities as large as 150 to 400 km/s.

astro-ph.SR↗

arXiv2021

Discovery of a double detonation thermonuclear supernova progenitor

We present the discovery of a new double detonation progenitor system consisting of a hot subdwarf B (sdB) binary with a white dwarf companion with an P=76.34179(2) min orbital period. Spectroscopic observations are consistent with an sdB star during helium core burning residing on the extreme horizontal branch. Chimera light curves are dominated by ellipsoidal deformation of the sdB star and a weak eclipse of the companion white dwarf. Combining spectroscopic and light curve fits we find a low mass sdB star, $M_{\rm sdB}=0.383\pm0.028$ M$_\odot$ with a massive white dwarf companion, $M_{\rm WD}=0.725\pm0.026$ M$_\odot$. From the eclipses we find a blackbody temperature for the white dwarf of 26,800 K resulting in a cooling age of $\approx$25 Myrs whereas our MESA model predicts an sdB age of $\approx$170 Myrs. We conclude that the sdB formed first through stable mass transfer followed by a common envelope which led to the formation of the white dwarf companion $\approx$25 Myrs ago. Using the MESA stellar evolutionary code we find that the sdB star will start mass transfer in $\approx$6 Myrs and in $\approx$60 Myrs the white dwarf will reach a total mass of $0.92$ M$_\odot$ with a thick helium layer of $0.17$ M$_\odot$. This will lead to a detonation that will likely destroy the white dwarf in a peculiar thermonuclear supernova. PTF1 2238+7430 is only the second confirmed candidate for a double detonation thermonuclear supernova. Using both systems we estimate that at least $\approx$1% of white dwarf thermonuclear supernovae originate from sdB+WD binaries with thick helium layers, consistent with the small number of observed peculiar thermonuclear explosions.

astro-ph.SR↗

arXiv2021

Blue extreme disk-runaway stars with Gaia EDR3

Since the discovery of hypervelocity stars in 2005, it has been widely believed that only the disruption of a binary system by a supermassive black hole at the Galactic center (GC), that is, the so-called Hills mechanism, is capable of accelerating stars to beyond the Galactic escape velocity. In the meantime, however, driven by the Gaia space mission, there is mounting evidence that many of the most extreme high-velocity early-type stars at high Galactic latitudes do originate in the Galactic disk and not in the GC. Moreover, the ejection velocities of these extreme disk-runaway stars exceed the predicted limits of the classical scenarios for the production of runaway stars. Based on proper motions from the Gaia early data release 3 and on recent and new spectrophotometric distances, we studied the kinematics of 30 such extreme disk-runaway stars, allowing us to deduce their spatial origins in and their ejection velocities from the Galactic disk with unprecedented precision. Only three stars in the sample have past trajectories that are consistent with an origin in the GC, most notably S5-HVS1, which is the most extreme object in the sample by far. All other program stars are shown to be disk runaways with ejection velocities that sharply contrast at least with classical ejection scenarios. They include HVS5 and HVS6, which are both gravitationally unbound to the Milky Way. While most stars originate from within a galactocentric radius of 15kpc, which corresponds to the observed extent of the spiral arms, a group of five stars stems from radii of about 21-29kpc. This indicates a possible link to outer Galactic rings and a potential origin from infalling satellite galaxies.

astro-ph.GA↗

arXiv2020

Runaway blue main-sequence stars at high Galactic latitudes. Target selection with Gaia and spectroscopic identification

Motivated by the historical identification of runaway main-sequence (MS) stars of early spectral type at high Galactic latitudes, we test the capability of Gaia at identifying new such stars. We have selected ~2300 sources with Gaia magnitudes of GBP - GRP < 0.05, compatible with the colors of low-extinction MS stars earlier than mid-A spectral type, and obtained low-resolution optical spectroscopy for 48 such stars. By performing detailed photometric and spectroscopic analyses, we derive their atmospheric and physical parameters (effective temperature, surface gravity, radial velocity, interstellar reddening, spectrophotometric distance, mass, radius, luminosity, and age). The comparison between spectrophotometric and parallax-based distances enables us to disentangle the MS candidates from older blue horizontal branch (BHB) candidates. We identify 12 runaway MS candidates, with masses between 2 and 6 Msun. Their trajectories are traced back to the Galactic disc to identify their most recent Galactic plane crossings and the corresponding flight times. All 12 candidates are ejected from the Galactic disc within 2 to 16.5 kpc from the Galactic center and possess flight times that are shorter than their evolutionary ages, compatible with a runaway hypothesis. Three MS candidates have ejection velocities exceeding 450 km/s, thus, appear to challenge the canonical ejection scenarios for late B-type stars. The fastest star of our sample also has a non-negligible Galactic escape probability if its MS nature can be confirmed. We identify 27 BHB candidates, and the two hottest stars in our sample are rare late O and early B type stars of low mass evolving towards the white dwarf cooling sequence.

astro-ph.SR↗

arXiv2020

EVR-CB-004: An Inflated Hot Subdwarf O star + Unseen WD Companion in a Compact Binary Discovered with the Evryscope

We present the discovery of EVR-CB-004, a close binary with a remnant stellar core and an unseen white dwarf companion. The analysis in this work reveals the primary is potentially an inflated hot subdwarf (sdO) and more likely is a rarer post-blue horizontal branch (post-BHB) star. Post-BHBs are the short-lived shell-burning final stage of a blue horizontal star or hot subdwarf before transitioning to a WD. This object was discovered using Evryscope photometric data in a southern-all-sky hot subdwarf variability survey. The photometric light curve for EVR-CB-004 shows multi-component variability from ellipsoidal deformation of the primary and from Doppler boosting as well as gravitational limb darkening. EVR-CB-004 is one of just a handful of known systems, and has a long period (6.08426 hours) and large amplitude ellipsoidal modulation (16.0 $\%$ change in brightness from maximum to minimum) for these extremely close binary systems, while the properties of the primary make it a truly unique system. EVR-CB-004 also shows a peculiar low-amplitude (less than $1\%$) sinusoidal light curve variation with a period that is a 1/3 resonance of the binary period. We tentatively identify this additional variation source as a tidally-induced resonant pulsation, and we suggest followup observations that could verify this interpretation. From the evolutionary state of the system, its components, and its mass fraction, EVR-CB-004 is a strong merger candidate to form a single high-mass ($\approx1.2M_{\odot}$) WD. EVR-CB-004 offers a glimpse into a brief phase of a remnant core evolution and secondary variation, not seen before in a compact binary.

astro-ph.SR↗

arXiv2020

A new class of Roche lobe-filling hot subdwarf binaries

We present the discovery of the second binary with a Roche lobe-filling hot subdwarf transferring mass to a white dwarf (WD) companion. This 56 minute binary was discovered using data from the Zwicky Transient Facility. Spectroscopic observations reveal an He-sdOB star with an effective temperature of $T_{\rm eff}=33,700\pm1000$ K and a surface gravity of $log(g)=5.54\pm0.11$. The GTC+HiPERCAM light curve is dominated by the ellipsoidal deformation of the He-sdOB star and shows an eclipse of the He-sdOB by an accretion disk as well as a weak eclipse of the WD. We infer a He-sdOB mass of $M_{\rm sdOB}=0.41\pm0.04$ M$_\odot$ and a WD mass of $M_{\rm WD}=0.68\pm0.05$ M$_\odot$. The weak eclipses imply a WD black-body temperature of $63,000\pm10,000$ K and a radius $R_{\rm WD}=0.0148\pm0.0020$ M$_\odot$ as expected for a WD of such high temperature. The He-sdOB star is likely undergoing hydrogen shell burning and will continue transferring mass for $\approx1$ Myrs at a rate of $10^{-9} M_\odot {\rm yr}^{-1}$ which is consistent with the high WD temperature. The hot subdwarf will then turn into a WD and the system will merge in $\approx30$ Myrs. We suggest that Galactic reddening could bias discoveries towards preferentially finding Roche lobe-filling systems during the short-lived shell burning phase. Studies using reddening corrected samples should reveal a large population of helium core-burning hot subdwarfs with $T_{\rm eff}\approx25,000$ K in binaries of 60-90 minutes with WDs. Though not yet in contact, these binaries would eventually come into contact through gravitational wave emission and explode as a sub-luminous thermonuclear supernova or evolve into a massive single WD.

astro-ph.SR↗

arXiv2020

A proto-helium white dwarf stripped by a substellar companion via common-envelope ejection -- Uncovering the true nature of a candidate hypervelocity B-type star

In the past, SDSS J160429.12+100002.2 was spectroscopically classified as a blue horizontal branch (BHB) star. Assuming a luminosity that is characteristic of BHB stars, the object's radial velocity and proper motions from Gaia Early Data Release 3 would imply that its Galactic rest-frame velocity exceeds its local escape velocity. Consequently, the object would be considered a hypervelocity star, which would prove particularly interesting because its Galactic trajectory points in our direction. However, based on the spectroscopic analysis of follow-up observations, we show that the object is actually a short-period ($P \approx 3.4$ h) single-lined spectroscopic binary system with a visible B-type star (effective temperature $T_{\mathrm{eff}} = 15\,840\pm160$ K and surface gravity $\log(g) = 4.86\pm0.04$) that is less luminous than typical BHB stars. Accordingly, the distance of the system is lower than originally thought, which renders its Galactic orbit bound to the Galaxy. Nevertheless, it is still an extreme halo object on a highly retrograde orbit. The abundances of He, C, N, O, Ne, Mg, Al, Si, S, and Ca are subsolar by factors from 3 to more than 100, while Fe is enriched by a factor of about 6. This peculiar chemical composition pattern is most likely caused by atomic diffusion processes. Combining constraints from astrometry, orbital motion, photometry, and spectroscopy, we conclude that the visible component is an unevolved proto-helium white dwarf with a thin hydrogen envelope that was stripped by a substellar companion through common-envelope ejection. Its unique configuration renders the binary system an interesting test bed for stellar binary evolution in general and common-envelope evolution in particular.

astro-ph.SR↗