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Issue A&A Volume 506, Number 2, November I 2009 Page(s) L25 - L28 Section Letters DOI 10.1051/0004-6361/200912954 Published online 15 September 2009

A&A 506, L25-L28 (2009)
DOI: 10.1051/0004-6361/200912954

Letter

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

A. Kawka and S. Vennes

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 4.0-6.3 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.

Key words: white dwarfs -- stars: individual: NLTT 11748 -- stars: atmospheres

© ESO 2009

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