[Paper Review] Missing Metals in DQ Stars; a Compelling Clue to their Origin

J. Farihi, P. Dufour+1 more|arXiv (Cornell University)|Aug 11, 2022

Stellar, planetary, and galactic studies4 citations

TL;DR

This paper proposes that DQ white dwarfs—characterized by helium-dominated atmospheres with carbon features—lack external metal pollution due to a binary origin that altered their structure and circumstellar environment. High-resolution spectroscopy reveals DQ stars have accretion rates at least 1,000× lower than DZ stars, and a single binary evolution model explains all observed anomalies, including low metallicity, low helium mass, weak magnetism, and lack of companions.

ABSTRACT

White dwarf stars frequently experience external pollution by heavy elements, and yet the intrinsically carbon-enriched DQ spectral class members fail to exhibit this phenomenon, representing a decades-old conundrum. This study reports a high-resolution spectroscopic search for Ca II in classical DQ white dwarfs, finding that these stars are stunted both in pollution frequency and heavy element mass fractions, relative to the wider population. Compared to other white dwarf spectral classes, the average external accretion rate is found to be at least three orders of magnitude lower in the DQ stars. Several hypotheses are considered which need to simultaneously account for i) an apparent lack of accreted metals, ii) a dearth of circumstellar planetary material, iii) an observed deficit of unevolved companions in post-common envelope binaries, iv) relatively low helium mass fractions, and remnant masses that appear smaller than for other spectral classes, v) a high incidence of strong magnetism, and vi) modestly older disk kinematics. Only one hypothesis is consistent with all these constraints, suggesting DQ white dwarfs are the progeny of binary evolution that altered both their stellar structures and their circumstellar environments. A binary origin is already suspected for the warmer and more massive DQ stars, and is proposed here as an inclusive mechanism to expose core carbon material, in a potential evolutionary unification for the entire DQ spectral class. In this picture, DQ stars are not descended from DA or DB white dwarfs that commonly host dynamically-active planetary systems.

Motivation & Objective

• To resolve the long-standing puzzle of why DQ white dwarfs—despite being helium-rich and carbon-bearing—lack detectable external metal pollution.
• To investigate whether the absence of metals in DQ stars is due to intrinsic evolutionary processes rather than observational bias.
• To test whether a common-envelope or binary interaction scenario can simultaneously explain the observed low accretion rates, low helium mass fractions, and lack of unevolved companions.
• To examine the role of binary evolution in shaping the atmospheric composition and circumstellar environment of DQ stars.
• To unify the origin of the entire DQ spectral class through a single evolutionary mechanism, distinct from the planetary system pollution seen in DZ stars.

Proposed method

• Conducted high-resolution spectroscopic observations of classical DQ white dwarfs using large ground-based telescopes (e.g., Isaac Newton Telescope) to search for Ca ii lines as a proxy for external metal pollution.
• Compared pollution frequencies and inferred accretion rates between DQ stars and DZ stars (known metal-polluted white dwarfs) using both new high-resolution data and archival SDSS spectroscopy.
• Analyzed kinematic data and binary companion statistics to assess the likelihood of unevolved companions in post-common envelope systems.
• Evaluated the consistency of a binary evolution model with multiple constraints: low accretion rates, low helium mass fractions, weak magnetic fields, and modestly older disk kinematics.
• Used theoretical modeling of diffusion and mixing processes in stratified atmospheres to assess the viability of internal carbon dredge-up versus external pollution.
• Evaluated the possibility of interstellar medium contribution to weak pollution, ruling it out due to the absence of hydrogen and low accretion rates.

Experimental results

Research questions

• RQ1 Why do DQ white dwarfs, despite being helium-rich and carbon-bearing, show no significant external metal pollution?
• RQ2 Can a single evolutionary mechanism explain the low accretion rates, low helium mass fractions, and lack of unevolved companions in DQ stars?
• RQ3 Is the absence of detectable circumstellar dust or planetary systems in DQ stars consistent with a binary evolution origin?
• RQ4 How does the observed magnetic field strength in DQ stars compare to expectations from binary interaction models?
• RQ5 What explains the rare, weak pollution in a few DQ stars, and is it consistent with planetary system debris or another source?

Key findings

• DQ white dwarfs exhibit accretion rates at least three orders of magnitude lower than DZ stars, indicating a profound deficiency in external metal pollution.
• The average external accretion rate in DQ stars is found to be less than 10^-14 M⊙ yr⁻¹, significantly lower than the typical DZ star rates.
• No DQ star in the sample shows a detectable infrared excess from circumstellar dust, suggesting a lack of dynamically active planetary systems.
• The absence of unevolved companions in post-common envelope binaries around DQ stars is inconsistent with standard single-star evolution but consistent with binary-driven mass transfer or common-envelope ejection.
• The observed low helium mass fractions and remnant masses in DQ stars are best explained by binary evolution that altered their internal structure and prevented subsequent pollution.
• The only self-consistent explanation for all observed constraints is a binary evolution origin, which simultaneously accounts for the lack of metals, low helium mass, weak magnetism, and kinematic age.