Letter to the Editor

A new extremely low-mass white dwarf in the NLTT catalogue^*

Astronomický ústav, Akademie věd České republiky, Fričova 298, 251 65 Ondřejov, Czech Republic e-mail: [kawka;vennes]@sunstel.asu.cas.cz

Received: 22 July 2009
Accepted: 14 September 2009

Abstract

We report on the discovery of the extremely low-mass, hydrogen-rich white dwarf, NLTT 11748. Based on measurements of the effective temperature (8540 ± 50 K) and surface gravity (log g = 6.20 ± 0.15) obtained by fitting the observed Balmer line profiles with synthetic spectra, we derive a mass of 0.167 ± 0.005 . This object is one of only a handful of white dwarfs with masses below 0.2 that are believed to be the product of close binary evolution with an episode of Roche lobe overflow onto a degenerate companion (neutron star or white dwarf). Assuming membership in the halo population, as suggested by the kinematics and adopting a cooling age of  Gyr for the white dwarf, we infer a progenitor mass of 0.87-0.93 . The likely companion has yet to be identified, but a search for radial velocity variations may help constrain its nature.