Magnetism, X-rays, and Accretion Rates in WD 1145+017 and other Polluted White Dwarf Systems

TL;DR

This study combines spectropolarimetry and X-ray observations to investigate magnetic fields and accretion rates in polluted white dwarfs, revealing typical accretion levels and constraining magnetic field strengths.

Contribution

It introduces an analytical framework for accretion onto magnetic white dwarfs and applies it to observational data, providing new limits on magnetic fields and accretion rates.

Findings

Magnetic fields in some white dwarfs are constrained to B* < 20 kG.

Accretion rates are consistent with steady-state dusty white dwarfs at 1e8 - 1e9 g/s.

WD 1145+017 has a lower accretion rate than previous estimates.

Abstract

This paper reports circular spectropolarimetry and X-ray observations of several polluted white dwarfs including WD 1145+017, with the aim to constrain the behavior of disk material and instantaneous accretion rates in these evolved planetary systems. Two stars with previously observed Zeeman splitting, WD 0322-019 and WD 2105-820, are detected above 5 sigma and <Bz> > 1 kG, while WD 1145+017, WD 1929+011, and WD 2326+049 yield (null) detections below this minimum level of confidence. For these latter three stars, high-resolution spectra and atmospheric modeling are used to obtain limits on magnetic field strengths via the absence of Zeeman splitting, finding B* < 20 kG based on data with resolving power R near 40 000. An analytical framework is presented for bulk Earth composition material falling onto the magnetic polar regions of white dwarfs, where X-rays and cyclotron radiation may contribute to accretion luminosity. This analysis is applied to X-ray data for WD 1145+017, WD 1729+371, and WD 2326+049, and the upper bound count rates are modeled with spectra for a range of plasma kT = 1 - 10 keV in both the magnetic and non-magnetic accretion regimes. The results for all three stars are consistent with a typical dusty white dwarf in a steady-state at 1e8 - 1e9 g/s. In particular, the non-magnetic limits for WD 1145+017 are found to be well below previous estimates of up to 1e12 g/s, and likely below 1e10 g/s, thus suggesting the star-disk system may be average in its evolutionary state, and only special in viewing geometry.