Line Profiles of the Calcium I Resonance Line in Cool Metal-polluted White Dwarfs

Abstract

Metal-polluted white dwarfs (DZ stars) are characterized by a helium-rich atmosphere contaminated by heavy elements traces originating from accreted rocky planetesimals. As a detailed spectroscopic analysis of those objects can reveal the composition of the accreted debris, there is a great interest in developing accurate DZ atmosphere models. However, the coolest DZ white dwarfs are challenging to model due to the fluidlike density of their atmospheres. Under such extreme conditions, spectral absorption lines are heavily broadened by interactions with neutral helium and it is no longer justified to use the conventional Lorentzian profiles. In this work, we determine the theoretical profiles of the Ca I resonance line (the most prominent spectral line for the coolest DZ white dwarfs) in the dense atmospheres of cool DZ white dwarfs. To do so, we use a unified theory of collisional line profiles and accurate ab initio potential energies and transition dipole moments for the CaHe molecule. We present the resulting profiles for the full range of temperatures and helium densities relevant for the modeling of cool, metal-polluted white dwarfs (from 3000 to 6000 K and from $10^{21}$ to $10^{23}{\mathrm{cm}}^{-3}$). We also implement these new profiles in our atmosphere models and show that they lead to improved fits to the Ca I resonance line of the coolest DZ white dwarfs.

6 pages, 9 figures, 1 table. Accepted for publication in The Astrophysical Journal

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• Alkali metals in white dwarf atmospheres as tracers of ancient planetary crusts
• A blue depression in the optical spectra of M dwarfs
• Lithium pollution of a white dwarf records the accretion of an extrasolar planetesimal
• Spectral analysis of ultra-cool white dwarfs polluted by planetary debris
• Magnesium abundances in cool metal-polluted white dwarfs
• Computational Tools for the Spectroscopic Analysis of White Dwarfs
• Spectral analysis of cool white dwarfs accreting from planetary systems: from the UV to the optical
• The nearby magnetic cool DZ white dwarf PM J08186-3110