Abstract
Abstract Between 30% and 50% of white dwarfs (WDs) show heavy elements in their atmospheres. This pollution is thought to arise from the accretion of planetesimals perturbed by outer planet(s) to within the WD’s tidal disruption radius. A small fraction of these WDs show either emission or absorption from circumstellar (C-S) gas. The abundances of metals in the photospheres of WDs with C-S gas are mostly similar to the bulk composition of the Earth. The C-S component arises from gas produced through collisions and/or the sublimation of disintegrating planetesimals. High-resolution spectroscopic observations of WD 1124−293 reveal photospheric and C-S absorption of Ca in multiple transitions. Here, we present high signal-to-noise ratio spectra, an updated WD atmosphere analysis, and a self-consistent model of its C-S gas. We constrain the abundances of Ca, Mg, and Fe in the photosphere of WD 1124−293, and find agreement with the abundances of these three species in the C-S gas. We find the location of the C-S gas is ∼100 white dwarf radii, the C-S and photospheric compositions are thus far consistent, the gas is not isothermal, and the amount of C-S Ca has not changed in two decades. We also demonstrate how to use Cloudy to model C-S gas viewed in absorption around polluted WDs. Modeling the abundances of gas around polluted WDs with Cloudy provides a new method to measure the composition of exoplanetesimals and will allow a direct comparison to the composition of rocky bodies in the solar system.
Highlights
The spectra of white dwarfs (WDs) should show only pressure-broadened hydrogen and/or helium absorption lines, yet at least 27% of young WDs with temperatures less than ∼27,000 K have photospheres polluted by elements heavier than helium (Koester et al 2014)
We find the location of the C-S gas is about a hundred white dwarf radii, the C-S and photospheric compositions are far consistent, the gas is not isothermal, and the amount of C-S Ca has not changed in two decades
We demonstrate how to use Cloudy to model C-S gas viewed in absorption around polluted WDs
Summary
The spectra of white dwarfs (WDs) should show only pressure-broadened hydrogen and/or helium absorption lines, yet at least 27% of young WDs with temperatures less than ∼27,000 K have photospheres polluted by elements heavier than helium (Koester et al 2014). These metals should settle out of the atmospheres of WDs on timescales of days to Myr depending on the WD temperature, surface gravity, and main atmospheric composition (H vs He) (Koester 2009). We investigate the gas toward WD 1124-293 to explore the conditions necessary to produce the observed absorption.
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