KOI 1224, a Fourth Bloated Hot White Dwarf Companion Found With Kepler

TL;DR

This paper analyzes KOI 1224, a binary system with a bloated hot white dwarf and an F-star, using Kepler data and evolutionary models, revealing its likely origin from an Algol-type binary.

Contribution

It introduces a new modeling code Icarus for detailed analysis of irradiated eclipsing binaries and applies it to characterize KOI 1224's components and evolutionary history.

Findings

KOI 1224 contains a white dwarf with Teff = 14400 K and mass = 0.20 Msun.

The system has an orbital period of 2.69802 days.

The white dwarf is a likely descendant of an Algol-type binary.

Abstract

We present an analysis and interpretation of the Kepler binary system KOI 1224. This is the fourth binary found with Kepler that consists of a thermally bloated, hot white dwarf in a close orbit with a more or less normal star of spectral class A or F. As we show, KOI 1224 contains a white dwarf with Teff = 14400 +/- 1100 K, mass = 0.20 +/- 0.02 Msun, and radius = 0.103 +/- 0.004 Rsun, and an F-star companion of mass = 1.59 +/- 0.07 Msun that is somewhat beyond its terminal-age main sequence. The orbital period is quite short at 2.69802 days. The ingredients that are used in the analysis are the Kepler binary light curve, including the detection of the Doppler boosting effect; the NUV and FUV fluxes from the Galex images of this object; an estimate of the spectral type of the F-star companion; and evolutionary models of the companion designed to match its effective temperature and mean density. The light curve is modelled with a new code named Icarus which we describe in detail. Its features include the full treatment of orbital phase-resolved spectroscopy, Doppler boosting, irradiation effects and transits/eclipses, which are particularly suited to irradiated eclipsing binaries. We interpret the KOI 1224 system in terms of its likely evolutionary history. We infer that this type of system, containing a bloated hot white dwarf, is the direct descendant of an Algol-type binary. In spite of this basic understanding of the origin of KOI 1224, we discuss a number of problems associated with producing this type of system with this short of an short orbital period.