VLT/X-shooter observations and the chemical composition of cool white dwarfs

TL;DR

This study uses VLT/X-shooter spectra to analyze the atmospheres and chemical compositions of cool hydrogen-rich white dwarfs, revealing heavy elements, magnetic fields, and contributing to understanding accretion processes in DAZ white dwarfs.

Contribution

It provides detailed atmospheric parameters, heavy element abundances, and magnetic field limits for cool white dwarfs, enhancing knowledge of their composition and accretion phenomena.

Findings

Detection of heavy elements in three white dwarfs.

High iron to calcium ratios in two objects.

Upper limits on magnetic fields in three DAZ white dwarfs.

Abstract

We present a model atmosphere analysis of cool hydrogen-rich white dwarfs observed at the Very Large Telescope (VLT) with the X-shooter spectrograph. The intermediate-dispersion and high signal-to-noise ratio of the spectra allowed us to conduct a detailed analysis of hydrogen and heavy element line profiles. In particular, we tested various prescriptions for hydrogen Balmer line broadening parameters and determined the effective temperature and surface gravity of each star. Three objects (NLTT 1675, 6390 and 11393) show the presence of heavy elements (Mg, Al, Ca, or Fe). Our abundance analysis revealed a relatively high iron to calcium ratio in NLTT 1675 and NLTT 6390. We also present an analysis of spectropolarimetric data obtained at the VLT using the focal reducer and low dispersion spectrograph (FORS) and we established strict upper limits on the magnetic field strengths in three of the DAZ white dwarfs and determined the longitudinal magnetic field strength in the DAZ NLTT 10480. The class of DAZ white dwarfs comprises objects that are possibly accreting material from their immediate circumstellar environment and the present study contributes in establishing class properties.