The cool magnetic DAZ white dwarf NLTT 10480⋆
Astronomický ústav, Akademie věd České republiky, Fričova 298, 251 65 Ondřejov, Czech Republic
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Received: 14 April 2011
Accepted: 1 June 2011
We have identified a new cool magnetic white dwarf in the New Luyten Two-Tenths (NLTT) catalogue. The high proper-motion star NLTT 10480 (μ = 0.5″ yr-1) shows weak Zeeman-split lines of calcium as well as characteristic Hα and β Zeeman triplets. Using VLT X-shooter spectra, we measured a surface-averaged magnetic field BS ~ 0.5 MG. The relative intensity of the π and σ components of the calcium and hydrogen lines imply a high inclination (i ≳ 60°). The optical-to-infrared V − J colour index and the Ca I/Ca II ionization balance indicate a temperature between 4900 and 5200 K, while the Balmer line profiles favour a higher temperature of 5400 K. The discrepancy is potentially resolved by increasing the metallicity to 0.03 × solar, hence increasing the electron pressure. However, the measured calcium abundance and abundance upper limits for other elements (Na, Al, Si, and Fe) imply a low photospheric metallicity ≲ 10-4 solar. Assuming diffusion steady-state, a calcium accretion rate of log Ṁ(g s-1) = 5.6 ± 0.3 is required to sustain a calcium abundance of log n(Ca)/n(H) = −10.30 ± 0.30 in the white dwarf atmosphere. We examine the implications of this discovery for the incidence of planetary debris and weak magnetic fields in cool white dwarf stars.
Key words: white dwarfs / stars: individual: NLTT 10480 / stars: atmospheres / stars: abundances
© ESO, 2011