A study of high proper-motion white dwarfs - I. Spectropolarimetry of a cool hydrogen-rich sample

We conducted a spectropolarimetic survey of 58 high proper-motion white dwarfs which achieved uncertainties of >rsim 2?kG in the Ha line and >rsim 5?kG in the upper Balmer line series. The survey aimed at detecting low magnetic fields (? 100?kG) and helped identify the new magnetic white dwarfs NLTT?2219, with a longitudinal field Bl = -97?kG, and NLTT?10480 (Bl = -212?kG). Furthermore, we report the possible identification of a very low-field white dwarf with Bl = -4.6?kG. The observations show that 5 per cent of white dwarfs harbour low fields (similar to 10 to similar to 102?kG) and that increased survey sensitivity may help uncover several new magnetic white dwarfs with fields below similar to 1?kG. A series of observations of the high-field white dwarf NLTT?12758 revealed changes in polarity occurring within an hour possibly associated with an inclined, fast rotating dipole. Also, the relative strength of the p and s components in NLTT?12758 possibly revealed the effect of a field concentration (spot), or, most likely, the presence of a non-magnetic white dwarf companion. Similar observations of NLTT?13015 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.