The 1.17-day orbit of the double-degenerate (DA+DQ) NLTT 16249

TL;DR

This paper reports the discovery of the first close double degenerate system consisting of a DA and DQ white dwarf with a 1.17-day orbit, providing insights into binary evolution and stellar atmospheres.

Contribution

It presents the first detailed characterization of a DA+DQ double degenerate system with a short orbital period and unique atmospheric composition, including nitrogen dredge-up evidence.

Findings

System has a 1.17-day orbit with a total mass between 1.47 and 2.04 solar masses.

First DA+DQ close double degenerate system identified.

DQ component shows nitrogen dredged-up from the core.

Abstract

New spectroscopic observations show that the double degenerate system NLTT 16249 is in a close orbit (a = 5.6+/-0.3 R_sun) with a period of 1.17 d. The total mass of the system is estimated between 1.47 and 2.04 M_sun but it is not expected to merge within a Hubble time-scale (t_merge ~ 10^11 yr). Vennes & Kawka (2012, ApJ, 745, L12) originally identified the system because of the peculiar composite hydrogen (DA class) and molecular (C_2--DQ class--and CN) spectra and the new observations establish this system as the first DA plus DQ close double degenerate. Also, the DQ component was the first of its class to show nitrogen dredged-up from the core in its atmosphere. The star may be viewed as the first known DQ descendant of the born-again PG1159 stars. Alternatively, the presence of nitrogen may be the result of past interactions and truncated evolution in a close binary system.