2012MNRAS.425.1394K

We conducted a spectropolarimetic survey of 58 high proper-motion white dwarfs which achieved uncertainties of >rsim 2kG in the Hα line and >rsim 5kG in the upper Balmer line series. The survey aimed at detecting low magnetic fields ( ≲ 100kG) and helped identify the new magnetic white dwarfs NLTT2219, with a longitudinal field B_l= -97kG, and NLTT10480 (B_l= -212kG). Furthermore, we report the possible identification of a very low-field white dwarf with B_l= -4.6kG. The observations show that ≈ 5 per cent of white dwarfs harbour low fields ( ∼ 10 to ∼ 10²kG) and that increased survey sensitivity may help uncover several new magnetic white dwarfs with fields below ∼ 1kG. A series of observations of the high-field white dwarf NLTT12758 revealed changes in polarity occurring within an hour possibly associated with an inclined, fast rotating dipole. Also, the relative strength of the π and σ components in NLTT12758 possibly revealed the effect of a field concentration (`spot'), or, most likely, the presence of a non-magnetic white dwarf companion. Similar observations of NLTT13015 also showed possible polarity variations, but without a clear indication of the time-scale. The survey data also proved useful in constraining the chemical composition, age and kinematics of a sample of cool white dwarfs as well as in constraining the incidence of double degenerates.

Based on observations made with ESO telescopes at the La Silla Paranal Observatory under programme IDs 80.D-0521, 82.D-0521, 83.D-0750 and 84.D-0862.