An accurate mass and radius measurement for an ultracool white dwarf

Abstract

Studies of cool white dwarfs in the solar neighbourhood have placed a limit on the age of the Galactic disk of 8-9 billion years. However, determining their cooling ages requires the knowledge of their effective temperatures, masses, radii, and atmospheric composition. So far, these parameters could only be inferred for a small number of ultracool white dwarfs for which an accurate distance is known, by fitting their spectral energy distributions (SEDs) in conjunction with a theoretical mass-radius relation. However, the mass-radius relation remains largely untested, and the derived cooling ages are hence model-dependent. Here we report direct measurements of the mass and radius of an ultracool white dwarf in the double-lined eclipsing binary SDSS J013851.54-001621.6. We find M(WD)=0.529+/-0.010Msol and R(WD)=0.0131+/-0.0003Rsol. Our measurements are consistent with the mass-radius relation and we determine a robust cooling age of 9.5 billion years for the 3570K white dwarf. We find that the mass and radius of the low mass companion star, M(sec)=0.132+/-0.003Msol and R(sec)=0.165+/-0.001Rsol, are in agreement with evolutionary models. We also find evidence that this >9.5 Gyr old M5 star is still active, far beyond the activity lifetime for a star of its spectral type. This is likely caused by the high tidally-enforced rotation rate of the star. The companion star is close to filling its Roche lobe and the system will evolve into a cataclysmic variable in only 70 Myr. Our direct measurements demonstrate that this system can be used to calibrate ultracool white dwarf atmospheric models.Comment: 9 pages, 9 figures and 4 tables. Accepted for publication in MNRA

The crowded magnetosphere of the post-common-envelope binary QS Virginis

“…This is consistent with the results from other white dwarfs in PCEBs, such as NN Ser and GK Vir, which both possesses hydrogen envelope masses of MH/MWD = 10 −4 (Parsons et al 2010a(Parsons et al , 2012a, although several other eclipsing white dwarfs have thinner hydrogen envelopes e.g. SDSS J0138-0016 (MH/MWD = 10 −5 , Parsons et al 2012b) and SDSS J1212-0123 (MH/MWD = 10 −6 , Parsons et al 2012a). …”

“…An alternative method would be to use the amplitude of the ellipsoidal modulation, which is related to the mass ratio (which we have already spectroscopically constrained) and the scaled radius of the M star (e.g. Parsons et al 2012b). However, the effects of starspots make this approach unreliable, even with the Roche tomography results.…”

“…Indeed, it appears as though virtually every PCEB hosts an active main-sequence star, regardless of its spectral type (Rebassa-Mansergas et al 2013b). Even extremely old systems still show signs of activity (Parsons et al 2012b), manifesting as starspots and flaring. These features allow us to constrain the configuration of the underlying magnetic field of the main-sequence star, crucial for understanding the evolution of these binaries, since the magnetic field is able to remove angular momentum from the system, driving the two stars closer together (Rappaport et al 1983;Kawaler 1988).…”

An M Dwarf Companion to an F-Type Star in a Young Main-Sequence Binary

“…Using different stellar models for the characterization of the primary star did not lead to significant changes in the mass-radius relation of either of the stars. (Ségransan et al 2003;Bouchy et al 2005;Pont et al 2005Pont et al , 2006Hebb et al 2006;Beatty et al 2007;Maxted et al 2007;Blake et al 2008;Fernandez et al 2009;Morales et al 2009;Vida et al 2009;Dimitrov & Kjurkchieva 2010;Parsons et al 2010Parsons et al , 2012Hartman et al 2011;Carter et al 2011;Irwin et al 2011;Pyrzas et al 2012;Nefs et al 2013;Tal-Or et al 2013;Gómez Maqueo Chew et al 2014;Zhou et al 2014) and spectroscopic characterized single low mass stars (gray triangles) (Mann et al 2015). The best fit to data from Mann et al (2015) is given by the green dashed line.…”

“…For these stars, stellar models also show systematic discrepancies in the observed mass-radius relations, but on a larger scale. Over 30 eclipsing very low-mass stars (VLMSs) with masses below 0.3 M e and radii known to better than 10% have been observed so far (e.g., Parsons et al 2012;Pyrzas et al 2012;Nefs et al 2013;Gómez Maqueo Chew et al 2014;Zhou et al 2014;Kraus et al 2015;David et al 2016). However, only eight have radii known to a precision better than 2%.…”