PG 1115+166 – a long-period DA+DB binary

P F L Maxted,M R Burleigh,N P Bannister,T R Marsh

doi:10.1046/j.1365-8711.2002.05545.x

Abstract

We present spectra of the DAB white dwarf PG 1115+166. Radial velocity measurements of the Balmer lines and the He i 6678 A line show that this is a binary white dwarf with a period of 30.09 d in which the Balmer lines move in anti-phase to the He i line, i.e. PG 1115+166 is a DA+DB binary. The minimum masses of the stars are MDA= 0.43 ± 0.15 M⊙ and MDB= 0.52 ± 0.12 M⊙. The separation of the stars is about 45 R⊙, which is much smaller than a typical asymptotic giant branch (AGB) progenitor of a white dwarf, implying that there has been at least one common envelope phase in this binary. Indeed, it is possible that this binary may have suffered up to three mass transfer episodes – two associated with the red giant phase prior to the formation of each white dwarf, and a third associated with the ‘born-again’ red giant phase of the DB white dwarf. PG 1115+166 has the longest orbital period of any post common envelope white dwarf–white dwarf binary found to date. Published models for the formation of white dwarf–white dwarf binaries do not predict any white dwarfs with the combination of a long orbital period and high mass found in PG 1115+166. We conclude that PG 1115+166 is a key object for testing models of binary star evolution, and it may also be a key object for our understanding of the formation of DB white dwarfs. We outline the observational tests which can be applied to scenarios for the formation of PG 1115+166 and apply them to the simplest case of a single common-envelope phase. This suggests that some part of the internal energy stored in the envelope of the AGB star, e.g. as ionized hydrogen, may have contributed to the ejection of the common envelope, but there are several unanswered questions concerning this simple scenario.

Highlights

The chemical composition of the atmosphere of a white dwarf star reflects a balance between gravitational settling, which tends to produce a pure hydrogen atmosphere, and processes which “pollute” the atmosphere such as radiative levitation, convection and accretion (Fontaine & Wesemael 1987)
This balance will change during the lifetime of a white dwarf as it cools and this is seen as a change in the relative numbers of hydrogen-rich (DA) and helium rich white dwarfs as a function of effective temperature
Favoured explanation for the DB gap is that as the DO white dwarf cools to 45 000K, small amounts of hydrogen hidden in the atmosphere rise to the surface, eventually masking the underlying helium

Summary

INTRODUCTION

The chemical composition of the atmosphere of a white dwarf star reflects a balance between gravitational settling, which tends to produce a pure hydrogen atmosphere, and processes which “pollute” the atmosphere such as radiative levitation, convection and accretion (Fontaine & Wesemael 1987). Those white dwarfs with thicker hydrogen layers remain as DA stars in this temperature range In this scenario, ∼ 20 percent of DA white dwarfs within the DB gap have thin hydrogen layers which are sufficient to mask the underlying helium. Bergeron & Liebert (2002) have analysed the optical spectrum of PG 1115+166 and found an excellent fit to the spectrum by assuming it is an unresolved DA+DB double degenerate binary Their fit to the observed spectrum used a DA star with Teff = 22090K, log g = 8.12 combined with a helium-line DB star with Teff = 16210K, log g = 8.19. Those DAB stars which are binaries may be interesting as they may be examples of stars which have undergone two mass transfer episodes, e.g., a common envelope phase (Iben & Livio 1993) The number of such close white dwarf pairs has increased rapidly in recent years.

OBSERVATIONS AND REDUCTIONS

ANALYSIS

The orbital period

Spectroscopic orbit

Findings

CONCLUSION