Optical linear polarization of 74 white dwarfs with the RoboPol polarimeter

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 degrees to 78 degrees 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 x E(B-V)^{0.12}.