Optical linear polarization of 74 white dwarfs with the RoboPol polarimeter

Abstract

We present the first linear polarimetric survey of white dwarfs (WDs). Our sample consists of WDs of DA and DC spectral types in the SDSS r magnitude range from 13 to 17. We performed polarimetric observations with the RoboPol polarimeter attached to the 1.3-m telescope at the Skinakas Observatory. We have 74 WDs in our sample, of which almost all are low polarized WDs with polarization degree (PD) smaller than 1%, while only 2 have PD higher than 1%. There is an evidence that on average the isolated WDs of DC type have higher PD (with median PD of 0.78%) than the isolated DA type WDs (with median PD of 0.36%). On the other hand, the median PD of isolated DA type WDs is almost the same, i.e. 0.36% as the median PD of DA type white dwarfs in binary systems with red dwarfs (dM type), i.e. 0.33%. This shows, as expected, that there is no contribution to the PD from the companion if the WD companion is the red dwarf, which is the most common situation for WDs binary systems. We do not find differences in the polarization degree between magnetic and non-magnetic WDs. Because 97% of WDs in our sample have PD lower than 1%, they can be used as faint zero-polarized standard star in the magnitude range from 13 up to 17 of SDSS r filter. They cover the Northern sky between 13 hour to 23 hour in right ascension and from −11 • to 78 • in declination. Additionally, we found that for low extinction values (< 0.04) the best model that describes the dependence of PD on E(B-V) is given by the equation: PD max,ISM [%] = 0.65 E(B − V) 0.12 .

Calibration of EFOSC2 Broadband Linear Imaging Polarimetry

“…All sources, with exception of WD 1344+106, are confirmed zero-polarisation calibrators in the thorough study of Fossati et al (2007). WD 1344+106 has been studied by Żejmo et al (2017), who confirm its suitability as zero-polarisation calibrator. Presence of bright Moon, cloud cover, and the general paucity of faint southern-hemisphere calibrators limited the choice of sources, and the range of parallactic angles at which we could obtain measurements.…”

Polarization due to rotational distortion in the bright star Regulus

“…In this case the same calculation as above shows that the 425SP band is the one most likely to be effected; and an intrinsic contribution from the companion of ~2500 ppm would be required to change our measurement by 1 ppm. A recent R-band survey of non-magnetic white dwarfs 50 failed to make any 3-sigma detections, with a detection limit of typically ~5000 ppm. This gives us confidence that a significant intrinsic contribution from the companion is unlikely.…”

Polarization measurements of the polluted white dwarf G29-38

“…West 1989). When linear polarization is seen in blind surveys of white dwarfs, it is attributed to magnetism ( Żejmo et al 2016;Słowikowska et al 2018). However, G29-38 has a magnetic field less than ∼100 G (Liebert, Saffer & Pilachowski 1989), so it is assumed that a measured polarization will be due to its dust.…”