The bright blue side of the night sky: Spectroscopic survey of bright and hot (pre-) white dwarfs

¹ Institut für Physik und Astronomie, Universität Potsdam, Haus 28, Karl-Liebknecht-Str. 24/25, 14476 Potsdam-Golm, Germany
e-mail: nreindl885@gmail.com
² Institut für Astronomie und Astrophysik, Kepler Center for Astro and Particle Physics, Eberhard Karls Universität, Sand 1, 72076 Tübingen, Germany
³ Instituto de Fisica, Universidade Federal do Rio Grande do Sul, 91501-970 Porto-Alegre, RS, Brazil
⁴ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
⁵ Department of Physics, University of Warwick, Coventry, UK
⁶ Instituto de Física y Astronomía, Universidad de Valparaíso, Gran Bretaña 1111, Playa Ancha, Valparaíso 2360102, Chile
⁷ European Southern Observatory, Alonso de Cordova 3107, Santiago, Chile
⁸ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University in Toruń, Grudziądzka 5, 87-100 Toruń, Poland
⁹ Department of Physics and Astronomy Newark, University of Delaware, DE 19716, USA
¹⁰ Delaware Asteroseismic Research Center, Mt. Cuba Observatory, Greenville, DE 19807, USA
¹¹ Anton Pannekoek Institute for Astronomy and GRAPPA, University of Amsterdam, 1090 GE Amsterdam, The Netherlands
¹² School of Astronomy & Space Science, University of the Chinese Academy of Sciences, Beijing 100012, PR China
¹³ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, PR China
¹⁴ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85748 Garching, Germany

Received: 10 May 2023
Accepted: 22 June 2023

Abstract

We report on the spectroscopic confirmation of 68 new bright (G = 13.5–17.2 mag) and blue (pre-)white dwarfs (WDs). This finding has allowed us to almost double the number of the hottest (T_eff ≥ 60 kK) known WDs brighter than G = 16 mag. We increased the number of known ultra-high excitation (UHE) WDs by 20%, found one unambiguous close binary system consisting of one DA WD with an irradiated low-mass companion, one DAO, and one DOA WD that are likely in their transformation phase of becoming pure DA WDs, one rare, naked O(H) star, two DA and two DAO WDs with T_eff possibly in excess of 100 kK, three new DOZ WDs, and three of our targets are central stars of (possible) planetary nebulae. Using non-local thermodynamic equilibrium models, we derived the atmospheric parameters of these stars and by fitting their spectral energy distribution we derived their radii, luminosities, and gravity masses. In addition, we derived their masses in the Kiel and Hertzsprung-Russell diagram (HRD). We find that Kiel, HRD, and gravity mass agree only in half of the cases. This is not unexpected and we attribute this to the neglect of metal opacities, possibly stratified atmospheres, as well as possible uncertainties of the parallax zero point determination. Furthermore, we carried out a search for photometric variability in our targets using archival data, finding that 26% of our targets are variable. This includes 15 new variable stars, with only one of them being clearly an irradiation effect system. Strikingly, the majority of the variable stars exhibit non-sinusoidal light-curve shapes, which are unlikely explained in terms of close binary systems. We propose that a significant fraction of all (not just UHE) WDs develop spots when entering the WD cooling phase. We suggest that this could be related to the on-set of weak magnetic fields and possibly diffusion.