Magnetism, X-rays and accretion rates in WD 1145+017 and other polluted white dwarf systems

Abstract

This paper reports circular spectropolarimetry and X-ray observations of\nseveral polluted white dwarfs including WD 1145+017, with the aim to constrain\nthe behavior of disk material and instantaneous accretion rates in these\nevolved planetary systems. Two stars with previously observed Zeeman splitting,\nWD 0322-019 and WD 2105-820, are detected above 5 sigma and <Bz> > 1 kG, while\nWD 1145+017, WD 1929+011, and WD 2326+049 yield (null) detections below this\nminimum level of confidence. For these latter three stars, high-resolution\nspectra and atmospheric modeling are used to obtain limits on magnetic field\nstrengths via the absence of Zeeman splitting, finding B* < 20 kG based on data\nwith resolving power R near 40 000. An analytical framework is presented for\nbulk Earth composition material falling onto the magnetic polar regions of\nwhite dwarfs, where X-rays and cyclotron radiation may contribute to accretion\nluminosity. This analysis is applied to X-ray data for WD 1145+017, WD\n1729+371, and WD 2326+049, and the upper bound count rates are modeled with\nspectra for a range of plasma kT = 1 - 10 keV in both the magnetic and\nnon-magnetic accretion regimes. The results for all three stars are consistent\nwith a typical dusty white dwarf in a steady-state at 1e8 - 1e9 g/s. In\nparticular, the non-magnetic limits for WD 1145+017 are found to be well below\nprevious estimates of up to 1e12 g/s, and likely below 1e10 g/s, thus\nsuggesting the star-disk system may be average in its evolutionary state, and\nonly special in viewing geometry.\n