A spectroscopic and kinematic survey of fast hot subdwarfs

¹ Institut für Physik und Astronomie, Universität Potsdam, Haus 28, Karl-Liebknecht-Str. 24/25, 14476 Potsdam-Golm, Germany
² Dr. Karl Remeis-Observatory & ECAP, Astronomical Institute, Friedrich-Alexander University Erlangen-Nuremberg, Sternwartstr. 7, 96049 Bamberg, Germany
³ Max Planck Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85748 Garching bei München, Germany
⁴ Hamburger Sternwarte, University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁵ Texas Tech University, Department of Physics & Astronomy, Box 41051 79409 Lubbock, TX, USA
⁶ Kavli Institute for Theoretical Physics, University of California, Santa Barbara, CA 93106, USA
⁷ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010 Nijmegen 6500 GL, The Netherlands
⁸ Department of Physics, University of Warwick, Conventry CV4 7AL, UK
⁹ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
¹⁰ Department of Physics, High Point University, One University Parkway, High Point, NC 27268, USA
¹¹ Departament de Física, Universitat Politecnica de Catalunya, c/Esteve Terrades 5, 08860 Castelldefels, Spain
¹² Gran Telescopio Canarias (GRANTECAN), Cuesta de San Jose s/n, 38712 Brena Baja, La Palma, Spain
¹³ Instituto de Astrofisica de Canarias, Via Lactea s/n, 38205 La Laguna, Tenerife, Spain
¹⁴ Landessternwarte Heidelberg, Zentrum für Astronomie, Ruprecht-Karls-Universität, Königstuhl 12, 69117 Heidelberg, Germany
¹⁵ Institute for Astronomy and Astrophysics, Kepler Center for Astro and Particle Physics, Eberhard Karls University, Sand 1, 72076 Tübingen, Germany
¹⁶ Astronomical Institute of the Czech Academy of Sciences, CZ 251 65 Ondřejov, Czech Republic
¹⁷ Astroserver.org, Fő tér 1, 8533 Malomsok, Hungary

Received: 18 May 2024
Accepted: 5 July 2024

Abstract

Hot subdwarfs (sdO/B) are the stripped helium cores of red giants formed via 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 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, that 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.