The cool magnetic DAZ white dwarf NLTT 10480

0367844 - ASÚ 2012 RIV FR eng J - Journal Article
Kawka, Adela - Vennes, Stephane
The cool magnetic DAZ white dwarf NLTT 10480.
Astronomy & Astrophysics. Roč. 532, August (2011), A7/1-A7/8. ISSN 0004-6361. E-ISSN 1432-0746
R&D Projects: GA AV ČR(CZ) IAA300030908; GA AV ČR IAA301630901; GA ČR GAP209/10/0967; GA MŠMT(CZ) LC06014
Institutional research plan: CEZ:AV0Z10030501
Keywords : white dwarfs * individual star NLTT 10480
Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics
Impact factor: 4.587, year: 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 shows weak Zeeman-split lines of calcium as well as characteristic Halpha and beta Zeeman triplets. Using VLT X-shooter spectra, we measured a surface-averaged magnetic field BS about 0.5 MG. The relative intensity of the pi and sigma components of the calcium and hydrogen lines imply a high inclination. 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. Assuming diffusion steady-state, a calcium accretion rate of log M(g s-1) = 5.6 plus/minus 0.3 is required to sustain a calcium abundance of log n(Ca)/n(H) = -10.30 plus/minus 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. Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile under programme ID 080.D-0521, 082.D-0750 and 086.D-0562.
Permanent Link: http://hdl.handle.net/11104/0202377