Missing metals in DQ stars: A simple explanation

Abstract

Classical DQ stars are white dwarfs whose atmospheres contain detectable traces of carbon brought up to the surface by a convective dredge-up process. Intriguingly, unlike other white dwarf spectral classes, DQ stars virtually never show signs of external pollution by elements heavier than carbon. In this Letter, we solve this long-standing problem by showing that the absence of detectable external pollution in DQ stars is naturally explained by the impact of metal accretion on the atmospheric structure of the white dwarf. A DQ star that accretes heavy elements sees its atmospheric density decrease, which leads to a sharp drop in the molecular carbon abundance and a strong suppression of the C${}_2$ Swan bands. We show that a typical DQ star that accretes heavy elements from planetary material generally transforms directly into a DZ star.

5 pages, 5 figures (+12 pages of supplementary figures). Accepted for publication as a Letter in A&A

References in corpus (15)

• The Chemical Composition of an Extrasolar Minor Planet
• On the Spectral Evolution of Cool, Helium-Atmosphere White Dwarfs: Detailed Spectroscopic and Photometric Analysis of DZ Stars
• On the Measurement of Fundamental Parameters of White Dwarfs in the Gaia Era
• The frequency and infrared brightness of circumstellar discs at white dwarfs
• Oxygen fugacities of extrasolar rocks: Evidence for an Earth-like geochemistry of exoplanets
• $Gaia$ white dwarfs within 40 pc II: the volume-limited northern hemisphere sample
• The unbiased frequency of planetary signatures around single and binary white dwarfs using $Spitzer$ and $Hubble$
• A New Generation of Cool White Dwarf Atmosphere Models. IV. Revisiting the Spectral Evolution of Cool White Dwarfs
• The frequency of gaseous debris discs around white dwarfs
• Recurring Planetary Debris Transits and Circumstellar Gas around White Dwarf ZTF J0328$-$1219
• A white dwarf accreting planetary material determined from X-ray observations
• Bayesian constraints on the origin and geology of exo-planetary material using a population of externally polluted white dwarfs
• On the Spectral Evolution of Hot White Dwarf Stars. III. The PG 1159$-$DO$-$DB$-$DQ Evolutionary Channel Revisited
• The evolution of carbon-polluted white dwarfs at low effective temperatures
• Spectral analysis of cool white dwarfs accreting from planetary systems: from the UV to the optical

Cited by in corpus (2)

• $\text{Gaia}$ white dwarfs within 40 pc III: spectroscopic observations of new candidates in the southern hemisphere
• Carbon dredge-up required to explain the Gaia white dwarf colour-magnitude bifurcation