@article{SchaffenrothPelisoliBarlowetal.2022, author = {Schaffenroth, Veronika and Pelisoli, Ingrid and Barlow, Brad N. and Geier, Stephan and Kupfer, Thomas}, title = {Hot subdwarfs in close binaries observed from space I.}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {666}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {0004-6361}, doi = {10.1051/0004-6361/202244214}, pages = {19}, year = {2022}, abstract = {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.}, language = {en} } @article{WernerReindlDorschetal.2022, author = {Werner, Klaus and Reindl, Nicole and Dorsch, Matti and Geier, Stephan and Munari, Ulisse and Raddi, Roberto}, title = {Non-local thermodynamic equilibrium spectral analysis of five hot, hydrogen-deficient pre-white dwarfs}, series = {Astronomy and Astrophysics}, volume = {658}, journal = {Astronomy and Astrophysics}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {0004-6361}, doi = {10.1051/0004-6361/202142397}, pages = {15}, year = {2022}, abstract = {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{\^a}€» 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.}, language = {en} } @article{NeunteufelPreeceKruckowetal.2022, author = {Neunteufel, Patrick and Preece, H. and Kruckow, Matthias U. and Geier, Stephan and Hamers, Adrian S. and Justham, S. and Podsiadlowski, Philipp}, title = {Properties and applications of a predicted population of runaway He-sdO/B stars ejected from single degenerate He-donor SNe}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {663}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {0004-6361}, doi = {10.1051/0004-6361/202142864}, pages = {26}, year = {2022}, abstract = {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.}, language = {en} } @article{GriggioBedinRaddietal.2022, author = {Griggio, Massimo and Bedin, Luigi R. and Raddi, Roberto and Reindl, Nicole and Tomasella, Lina and Scalco, M. and Salaris, M. and Cassisi, S. and Ochner, P. and Ciroi, S. and Rosati, P. and Nardiello, Domenico and Anderson, J. and Libralato, Mattia and Bellini, A. and Vallenari, A. and Spina, L. and Pedani, M.}, title = {Astro-photometric study of M37 with Gaia and wide-field ugi-imaging}, series = {Monthly notices of the Royal Astronomical Society}, volume = {515}, journal = {Monthly notices of the Royal Astronomical Society}, number = {2}, publisher = {Oxford University Press}, address = {Oxford}, issn = {0035-8711}, doi = {10.1093/mnras/stac1920}, pages = {1841 -- 1853}, year = {2022}, abstract = {We present an astrometric and photometric wide-field study of the Galactic open star cluster M37 (NGC 2099). The studied field was observed with ground-based images covering a region of about four square degrees in the Sloan-like filters ugi. We exploited the Gaia catalogue to calibrate the geometric distortion of the large field mosaics, developing software routines that can be also applied to other wide-field instruments. The data are used to identify the hottest white dwarf (WD) member candidates of M37. Thanks to the Gaia EDR3 exquisite astrometry we identified seven such WD candidates, one of which, besides being a high-probability astrometric member, is the putative central star of a planetary nebula. To our knowledge, this is a unique object in an open cluster, and we have obtained follow-up low-resolution spectra that are used for a qualitative characterization of this young WD. Finally, we publicly release a three-colour atlas and a catalogue of the sources in the field of view, which represents a complement of existing material.}, language = {en} } @article{ReindlSchaffenrothFilizetal.2021, author = {Reindl, Nicole and Schaffenroth, Veronika and Filiz, Semih and Geier, Stephan and Pelisoli, Ingrid and Kepler, Souza Oliveira}, title = {Mysterious, variable, and extremely hot}, series = {Astronomy and astrophysics : an international weekly journal / European Southern Observatory (ESO)}, volume = {647}, journal = {Astronomy and astrophysics : an international weekly journal / European Southern Observatory (ESO)}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/202140289}, pages = {22}, year = {2021}, abstract = {Context. About 10\% of all stars exhibit absorption lines of ultra-highly excited (UHE) metals (e.g., O VIII) in their optical spectra when entering the white dwarf cooling sequence. This is something that has never been observed in any other astrophysical object, and poses a decades-long mystery in our understanding of the late stages of stellar evolution. The recent discovery of a UHE white dwarf that is both spectroscopically and photometrically variable led to the speculation that the UHE lines might be created in a shock-heated circumstellar magnetosphere. Aims. We aim to gain a better understanding of these mysterious objects by studying the photometric variability of the whole population of UHE white dwarfs, and white dwarfs showing only the He II line problem, as both phenomena are believed to be connected. Methods. We investigate (multi-band) light curves from several ground- and space-based surveys of all 16 currently known UHE white dwarfs (including one newly discovered) and eight white dwarfs that show only the He II line problem. Results. We find that 75(-13)(+8) \% of the UHE white dwarfs, and 75(-19)(+9)\% of the He II line problem white dwarfs are significantly photometrically variable, with periods ranging from 0.22 d to 2.93 d and amplitudes from a few tenths to a few hundredths of a magnitude. The high variability rate is in stark contrast to the variability rate amongst normal hot white dwarfs (we find 9(2)(+4)\%), marking UHE and He II line problem white dwarfs as a new class of variable stars. The period distribution of our sample agrees with both the orbital period distribution of post-common-envelope binaries and the rotational period distribution of magnetic white dwarfs if we assume that the objects in our sample will spin-up as a consequence of further contraction. Conclusions. We find further evidence that UHE and He II line problem white dwarfs are indeed related, as concluded from their overlap in the Gaia HRD, similar photometric variability rates, light-curve shapes and amplitudes, and period distributions. The lack of increasing photometric amplitudes towards longer wavelengths, as well as the nondetection of optical emission lines arising from the highly irradiated face of a hypothetical secondary in the optical spectra of our stars, makes it seem unlikely that an irradiated late-type companion is the origin of the photometric variability. Instead, we believe that spots on the surfaces of these stars and/or geometrical effects of circumstellar material might be responsible.}, language = {en} } @article{MoesenlechnerPaunzenPelisolietal.2021, author = {M{\"o}senlechner, Gerald and Paunzen, Ernst and Pelisoli, Ingrid D. and Seelig, Joseph and Stidl, Sarah and Maitzen, Hans Michael}, title = {A Kepler K2 view of subdwarf A-type stars}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {657}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {0004-6361}, doi = {10.1051/0004-6361/202037789}, pages = {11}, year = {2021}, abstract = {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.}, language = {en} } @article{PelisoliVos2019, author = {Pelisoli, Ingrid Domingos and Vos, Joris}, title = {Gaia Data Release 2 catalogue of extremely low-mass white dwarf candidates}, series = {Monthly notices of the Royal Astronomical Society}, volume = {488}, journal = {Monthly notices of the Royal Astronomical Society}, number = {2}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0035-8711}, doi = {10.1093/mnras/stz1876}, pages = {2892 -- 2903}, year = {2019}, abstract = {Extremely low-mass white dwarf stars (ELMs) are M < 0.3 M-circle dot helium-core white dwarfs born either as a result of a common-envelope phase or after a stable Roche lobe overflow episode in a multiple system. The Universe is not old enough for ELMs to have formed through single-star evolution channels. As remnants of binary evolution, ELMs can shed light onto the poorly understood phase of common-envelope evolution and provide constraints to the physics of mass accretion. Most known ELMs will merge in less than a Hubble time, providing an important contribution to the signal to be detected by upcoming space-based gravitational wave detectors. There are currently less than 150 known ELMs; most were selected by colour, focusing on hot objects, in a magnitude-limited survey of the Northern hemisphere only. Recent theoretical models have predicted a much larger space density for ELMs than estimated observationally based on this limited sample. In order to perform meaningful comparisons with theoretical models and test their predictions, a larger well-defined sample is required. In this work, we present a catalogue of ELM candidates selected from the second data release of Gaia (DR2). We have used predictions from theoretical models and analysed the properties of the known sample to map the space spanned by ELMs in the Gaia Hertzsprung-Russell diagram. Defining a set of colour cuts and quality flags, we have obtained a final sample of 5762 ELM candidates down to T-eff approximate to 5000 K.}, language = {en} } @article{RaddiHollandsKoesteretal.2019, author = {Raddi, Roberto and Hollands, M. A. and Koester, D. and Hermes, J. J. and Gansicke, B. T. and Heber, Ulrich and Shen, Ken J. and Townsley, D. M. and Pala, Anna Francesca and Reding, J. S. and Toloza, O. F. and Pelisoli, Ingrid Domingos and Geier, Stephan and Fusillo, Nicola Pietro Gentile and Munari, Ullisse and Strader, J.}, title = {Partly burnt runaway stellar remnants from peculiar thermonuclear supernovae}, series = {Monthly notices of the Royal Astronomical Society}, volume = {489}, journal = {Monthly notices of the Royal Astronomical Society}, number = {2}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0035-8711}, doi = {10.1093/mnras/stz1618}, pages = {1489 -- 1508}, year = {2019}, abstract = {We report the discovery of three stars that, along with the prototype LP 40-365, form a distinct class of chemically peculiar runaway stars that are the survivors of thermonuclear explosions. Spectroscopy of the four confirmed LP 40-365 stars finds ONe-dominated atmospheres enriched with remarkably similar amounts of nuclear ashes of partial O- and Si-burning. Kinematic evidence is consistent with ejection from a binary supernova progenitor; at least two stars have rest-frame velocities indicating they are unbound to the Galaxy. With masses and radii ranging between 0.20 and 0.28M(circle dot) and between 0.16 and 0.60 R-circle dot, respectively, we speculate these inflated white dwarfs are the partly burnt remnants of either peculiar Type Iax or electron-capture supernovae. Adopting supernova rates from the literature, we estimate that similar to 20 LP 40-365 stars brighter than 19 mag should be detectable within 2 kpc from the Sun at the end of the Gaia mission. We suggest that as they cool, these stars will evolve in their spectroscopic appearance, and eventually become peculiar O-rich white dwarfs. Finally, we stress that the discovery of new LP 40-365 stars will be useful to further constrain their evolution, supplying key boundary conditions to the modelling of explosion mechanisms, supernova rates, and nucleosynthetic yields of peculiar thermonuclear explosions.}, language = {en} } @article{ReindlGeierOstensen2018, author = {Reindl, Nicole and Geier, Stephan and Ostensen, R. H.}, title = {Discovery of two bright DO-type white dwarfs}, series = {Monthly notices of the Royal Astronomical Society}, volume = {480}, journal = {Monthly notices of the Royal Astronomical Society}, number = {1}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0035-8711}, doi = {10.1093/mnras/sty1875}, pages = {1211 -- 1217}, year = {2018}, abstract = {We discovered two bright DO-type white dwarfs, GALEXJ053628.3+544854 (J0536+5448) and GALEXJ231128.0+292935(J2311+2929), which rank among the eight brightest DO-type white dwarfs known. Our non-LTE model atmosphere analysis reveals effective temperatures and surface gravities of T-eff = 80000 +/- 4600K and log g = 8.25 +/- 0.15 for J0536+5448 and T-eff = 69400 +/- 900K and log g = 7.80 +/- 0.06 for J2311+2929. The latter shows a significant amount of carbon in its atmosphere (C = 0.003(-0.002)(+0.005), by mass), while for J0536+5448 we could derive only an upper limit of C < 0.003. Furthermore, we calculated spectroscopic distances for the two stars and found a good agreement with the distances derived from the Gaia parallaxes.}, language = {en} } @article{FusilloTremblayGaensickeetal.2018, author = {Fusillo, Nicola Pietro Gentile and Tremblay, Pier-Emmanuel and G{\"a}nsicke, Boris T. and Manser, Christopher J. and Cunningham, Tim and Cukanovaite, Elena and Hollands, Mark and Marsh, Thomas and Raddi, Roberto and Jordan, Stefan and Toonen, Silvia and Geier, Stephan and Barstow, Martin and Cummings, Jeffrey D.}, title = {A Gaia Data Release 2 catalogue of white dwarfs and a comparison with SDSS}, series = {Monthly notices of the Royal Astronomical Society}, volume = {482}, journal = {Monthly notices of the Royal Astronomical Society}, number = {4}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0035-8711}, doi = {10.1093/mnras/sty3016}, pages = {4570 -- 4591}, year = {2018}, abstract = {We present a catalogue of white dwarf candidates selected from the second data release of Gaia (DR2). We used a sample of spectroscopically confirmed white dwarfs from the Sloan Digital Sky Survey (SDSS) to map the entire space spanned by these objects in the Gaia Hertzsprung-Russell diagram. We then defined a set of cuts in absolute magnitude, colour, and a number of Gaia quality flags to remove the majority of contaminating objects. Finally, we adopt a method analogous to the one presented in our earlier SDSS photometric catalogues to calculate a probability of being a white dwarf (PWD) for all Gaia sources that passed the initial selection. The final catalogue is composed of 486641 stars with calculated PWD from which it is possible to select a sample of ≃260000 high-confidence white dwarf candidates in the magnitude range 8 < G < 21. By comparing this catalogue with a sample of SDSS white dwarf candidates, we estimate an upper limit in completeness of 85 per cent for white dwarfs with G ≤ 20 mag and Teff >7000 K, at high Galactic latitudes (|b| > 20°). However, the completeness drops at low Galactic latitudes, and the magnitude limit of the catalogue varies significantly across the sky as a function of Gaia's scanning law. We also provide the list of objects within our sample with available SDSS spectroscopy. We use this spectroscopic sample to characterize the observed structure of the white dwarf distribution in the H-R diagram.}, language = {en} }