Spectroscopic Analysis of the White Dwarf KUV 02196+2816: A New Unresolved DA+DB Degenerate Binary

TL;DR

This paper presents spectroscopic evidence that the white dwarf KUV 02196+2816 is an unresolved binary system composed of a DA and a DB white dwarf, with implications for stellar evolution and spectral classification.

Contribution

It provides the first spectroscopic confirmation of an unresolved DA+DB binary, challenging previous homogeneous models and offering insights into double degenerate evolution.

Findings

KUV 02196+2816 is an unresolved DA+DB binary.

The system's total mass is close to the Chandrasekhar limit.

The DB component lies within the DB gap, rare in bright DB stars.

Abstract

A spectroscopic analysis of the DBA (or DAB) white dwarf KUV 02196+2816 is presented. The observed hydrogen and helium line profiles are shown to be incompatible with model spectra calculated under the assumption of a homogeneous hydrogen and helium chemical composition. In contrast, an excellent fit to the optical spectrum of KUV 02196+2816 can be achieved if the object is interpreted as an unresolved double degenerate composed of a hydrogen-line DA star and a helium-line DB star. The atmospheric parameters obtained from the best fit are Teff=27,170 K and log g=8.09 for the DA star, Teff=36,340 K and log g=8.09 for the DB star, which implies that the total mass of the system (M~1.4 Msol) is very close to the Chandrasekhar limit. Moreover, the effective temperature of the DB stars lies well within the so-called DB gap where very few bright DB stars are found. The implications of this discovery with respect to the DAB and DBA spectral classes and to the evolution of double degenerate binaries are discussed.