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First discovery of an ultra-cool white dwarf benchmark in common proper motion with an M dwarf

Lam, MC, Hambly, NC, Lodieu, N, Blouin, S, Harvey, EJ, Smith, RJ, Galvez-Ortiz, MC and Zhang, ZH (2020) First discovery of an ultra-cool white dwarf benchmark in common proper motion with an M dwarf. Monthly Notices of the Royal Astronomical Society, 493 (4). pp. 6001-6010. ISSN 0035-8711

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Ultra-cool white dwarfs are among the oldest stellar remnants in the Universe. Their efficient gravitational settling and low effective temperatures are responsible for the smooth spectra they exhibit. For that reason, it is not possible to derive their radial velocities or to find the chemistry of the progenitors. The best that can be done is to infer such properties from associated sources, which are coeval. The simplest form of such a system is a common proper motion pair where one star is an evolved degenerate and the other a main-sequence star. In this work, we present the discovery of the first of such a system, the M dwarf LHS 6328 and the ultra-cool white dwarf PSO J1801+625, from the Pan-STARRS 1 3π survey and the Gaia Data Release 2. Follow-up spectra were collected covering a usable wavelength range of 3750–24 500 Å. Their spectra show that the white dwarf has an effective temperature of 3550 K and surface gravity of log g = 7.45 ± 0.13 or log g = 7.49 ± 0.13 for a CO or He core, respectively, when compared against synthetic spectra of ultra-cool white dwarf atmosphere models. The system has slightly subsolar metallicity with −0.25 < [Fe/H] < 0.0, and a spatial velocity of (U, V, W) = (−114.26 ± 0.24, 222.94 ± 0.60, 10.25 ± 0.34) km s−1, the first radial velocity and metallicity measurements of an ultra-cool white dwarf. This makes it the first and only benchmark of its kind to date.

Item Type: Article
Additional Information: This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2020 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Uncontrolled Keywords: Science & Technology; Physical Sciences; Astronomy & Astrophysics; binaries: visual; stars: low-mass; white dwarfs; solar neighbourhood; SUBDWARF-B-STARS; LOW-MASS STARS; STELLAR EVOLUTION; BINARY-SYSTEMS; METALLICITY; CLUSTER; MODELS; DIFFUSION; NGC-6791; SPECTRA; astro-ph.SR; astro-ph.SR; astro-ph.EP; astro-ph.GA; Astronomy & Astrophysics; 0201 Astronomical and Space Sciences
Subjects: Q Science > QB Astronomy
Q Science > QC Physics
Divisions: Astrophysics Research Institute
Publisher: Oxford University Press
SWORD Depositor: A Symplectic
Date Deposited: 04 Nov 2022 11:26
Last Modified: 04 Nov 2022 11:30
DOI or ID number: 10.1093/mnras/staa584
URI: https://researchonline.ljmu.ac.uk/id/eprint/18018
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