TY - JOUR A1 - Reindl, Nicole A1 - Geier, Stephan A1 - Ostensen, R. H. T1 - Discovery of two bright DO-type white dwarfs JF - Monthly notices of the Royal Astronomical Society N2 - 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. KW - stars: abundances KW - stars: atmospheres KW - white dwarfs Y1 - 2018 U6 - https://doi.org/10.1093/mnras/sty1875 SN - 0035-8711 SN - 1365-2966 VL - 480 IS - 1 SP - 1211 EP - 1217 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Latour, Marilyn A1 - Randall, Suzanna K. A1 - Calamida, Annalisa A1 - Geier, Stephan A1 - Moehler, Sabine T1 - The ultimate spectroscopic census of extreme horizontal branch stars in omega Centauri JF - Astronomy and astrophysics : an international weekly journal N2 - The presence of extreme horizontal branch (EHB) and blue hook stars in some Galactic globular clusters (GGCs) constitutes one of the remaining mysteries of stellar evolution. While several evolutionary scenarios have been proposed to explain the characteristics of this peculiar population of evolved stars, their observational verification has been limited by the availability of spectroscopic data for a statistically significant sample of such objects in any single GGC. We recently launched the SHOTGLAS project with the aim of providing a comprehensive picture of this intriguing stellar population in terms of spectroscopic properties for all readily accessible GGCs hosting an EHB. In this first paper, we focus on omega Cen, a peculiar, massive GGC that hosts multiple stellar populations. We use non-LTE model atmospheres to derive atmospheric parameters (Te ff, log g and N(He) / N(H)) and spectroscopic masses for 152 EHB stars in the cluster. This constitutes the largest spectroscopic sample of EHB stars ever analyzed in a GGC and represents similar to 20% of the EHB population of omega Cen. We also search for close binaries among these stars based on radial velocity variations. Our results show that the EHB population of omega Cen is divided into three spectroscopic groups that are very distinct in the Te ff helium abundance plane. The coolest sdB-type stars (Te ff. 30 000 K) have a hydrogen-rich atmosphere, populate the theoretical EHB region in the Te ff log g plane, and form 26% of our sample. The hottest sdO-type stars (Te ff & 42 000 K) make up 10% of the sample, have a hydrogen-rich atmosphere and are thought to be in a post-EHB evolutionary phase. The majority of our sample is found at intermediate temperatures and consists of sdOB stars that have roughly solar or super-solar atmospheric helium abundances. It is these objects that constitute the blue hook at V > 18 : 5 mag in the omega Cen color-magnitude diagram. Interestingly, the helium-enriched sdOBs do not have a significant counterpart population in the Galactic field, indicating that their formation is dependent on the particular environment found in omega Cen and other select GGCs. Another major di ff erence between the EHB stars in omega Cen and the field is the fraction of close binaries. From our radial velocity survey we identify two binary candidates, however no orbital solutions could be determined. We estimate an EHB close binary fraction of similar to 5% in omega Cen. This low fraction is in line with findings for other GGCs, but in sharp contrast to the situation in the field, where around 50% of the sdB stars reside in close binaries. Finally, the mass distribution derived is very similar for all three spectroscopic groups, however the average mass (0.38 M fi) is lower than that expected from stellar evolution theory. While this mass conundrum has previously been noted for EHB stars in omega Cen, it so far appears to be unique to that cluster. KW - stars: atmospheres KW - stars: horizontal-branch KW - subdwarfs KW - stars: fundamental parameters KW - binaries: close KW - globular clusters: individual: NGC5139 Y1 - 2018 U6 - https://doi.org/10.1051/0004-6361/201833129 SN - 1432-0746 VL - 618 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Geier, Stephan A1 - Raddi, Roberto A1 - Fusillo, Nicola Pietro Gentile A1 - Marsh, T. R. T1 - The population of hot subdwarf stars studied with Gaia BT - II. The Gaia DR2 catalogue of hot subluminous stars JF - Astronomy and astrophysics : an international weekly journal N2 - Based on data from the ESA Gaia Data Release 2 (DR2) and several ground-based, multi-band photometry surveys we have compiled an all-sky catalogue of 39 800 hot subluminous star candidates selected in Gaia DR2 by means of colour, absolute magnitude, and reduced proper motion cuts. We expect the majority of the candidates to be hot subdwarf stars of spectral type B and O, followed by blue horizontal branch stars of late B-type (HBB), hot post-AGB stars, and central stars of planetary nebulae. The contamination by cooler stars should be about 10%. The catalogue is magnitude limited to Gaia G < 19 mag and covers the whole sky. Except within the Galactic plane and LMC/SMC regions, we expect the catalogue to be almost complete up to about 1.5 kpc. The main purpose of this catalogue is to serve as input target list for the large-scale photometric and spectroscopic surveys which are ongoing or scheduled to start in the coming years. In the long run, securing a statistically significant sample of spectroscopically confirmed hot subluminous stars is key to advance towards a more detailed understanding of the latest stages of stellar evolution for single and binary stars. KW - subdwarfs KW - stars: horizontal-branch KW - catalogs Y1 - 2019 U6 - https://doi.org/10.1051/0004-6361/201834236 SN - 0004-6361 SN - 1432-0746 VL - 621 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Fusillo, Nicola Pietro Gentile A1 - Tremblay, Pier-Emmanuel A1 - Gänsicke, Boris T. A1 - Manser, Christopher J. A1 - Cunningham, Tim A1 - Cukanovaite, Elena A1 - Hollands, Mark A1 - Marsh, Thomas A1 - Raddi, Roberto A1 - Jordan, Stefan A1 - Toonen, Silvia A1 - Geier, Stephan A1 - Barstow, Martin A1 - Cummings, Jeffrey D. T1 - A Gaia Data Release 2 catalogue of white dwarfs and a comparison with SDSS JF - Monthly notices of the Royal Astronomical Society N2 - 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. KW - catalogues KW - surveys KW - white dwarfs Y1 - 2018 U6 - https://doi.org/10.1093/mnras/sty3016 SN - 0035-8711 SN - 1365-2966 VL - 482 IS - 4 SP - 4570 EP - 4591 PB - Oxford Univ. Press CY - Oxford ER -