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

Abstract

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 to 24500Å. 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 sub-solar 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.

Automated SpectroPhotometric Image REDuction (ASPIRED) -- A Python-based spectral data reduction toolkit

“…The default option is to use the LOWESS fit because it is the least sensitive to noise, cosmic ray contamination and detector artefacts. (Lam et al 2020b). Middle: The extracted spectra in electron counts using the three extraction methods are very similar.…”

“…Figure 4. Top: An LT/SPRAT spectrum of a featureless ultra-cool white dwarf PSO J1801+625(Lam et al 2020b). Middle: The extracted spectra in electron counts using the three extraction methods are very similar.…”

$^{22}$Ne Phase Separation As A Solution To The Ultramassive White Dwarf Cooling Anomaly

“…They have been used to determine the ages of individual stellar populations (Hansen et al 2007) and to reconstruct the star formation history of the Milky Way (Tremblay et al 2014;Kilic et al 2017;Fantin et al 2019). When paired with a stellar companion, they also become useful benchmarks for harder-to-model objects (e.g., M, L, and T dwarfs, Lam et al 2020;Meisner et al 2020). They could even prove useful to track the galactic evolution of lithium (Kaiser et al 2021).…”

Constraining the solar neighbourhood age-metallicity relation from white dwarf-main sequence binaries

“…WDs are the typical end stage of the vast majority of main sequence (MS) stars (see the review of Althaus et al 2010), and because their evolution follows a relatively simple and well understood cooling process, they can be used as reliable observational measures of stellar age (e.g. Fouesneau et al 2019;Qiu et al 2020;Lam et al 2020). In order to determine the total age of a WD, defined as the sum of its cooling age and its MS progenitor lifetime, two processes are required.…”