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

TL;DR

This spectropolarimetric survey of 58 high proper-motion white dwarfs detected new low and high magnetic fields, revealing that about 5% have low magnetic fields and providing insights into their magnetic, chemical, and kinematic properties.

Contribution

First comprehensive spectropolarimetric survey targeting high proper-motion white dwarfs to identify low magnetic fields and analyze their properties and variability.

Findings

Detected two new magnetic white dwarfs with fields of -97 kG and -212 kG.

Found approximately 5% of white dwarfs have low magnetic fields (~10 to 100 kG).

Observed polarity changes suggesting rapid rotation or magnetic spots.

Abstract

We conducted a spectropolarimetic survey of 58 high proper-motion white dwarfs which achieved uncertainties of >2 kG in the Halpha line and >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 B_l = -97 kG, and NLTT 10480 (B_l=-212 kG). Also, we report the possible identification of a very low-field white dwarf with B_l = -4.6 kG. The observations show that ~5% of white dwarfs harbour low fields (~10 to ~10^2 kG) and that increased survey sensitivity may help uncover several new magnetic white dwarfs with fields below ~1 kG. A series of observations of the high field white dwarf NLTT 12758 revealed changes in polarity occurring within an hour possibly associated to an inclined, fast rotating dipole. Also, the relative strength of the pi and sigma 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 timescale. 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.