Observations of Doppler Boosting in Kepler Lightcurves

TL;DR

This paper re-analyzes Kepler lightcurves of KOI 74 and KOI 81, revealing the companions are likely white dwarfs and demonstrating the first photometric measurement of a radial velocity curve via Doppler boosting.

Contribution

It provides the first photometric radial velocity measurement using Doppler boosting and clarifies the nature of the companions as white dwarfs in these systems.

Findings

Companions are likely white dwarfs.

Doppler boosting causes measurable brightness modulation.

Radial velocity amplitude inferred photometrically.

Abstract

Among the initial results from Kepler were two striking lightcurves, for KOI 74 and KOI 81, in which the relative depths of the primary and secondary eclipses showed that the more compact, less luminous object was hotter than its stellar host. That result became particularly intriguing because a substellar mass had been derived for the secondary in KOI 74, which would make the high temperature challenging to explain; in KOI 81, the mass range for the companion was also reported to be consistent with a substellar object. We re-analyze the Kepler data and demonstrate that both companions are likely to be white dwarfs. We also find that the photometric data for KOI 74 show a modulation in brightness as the more luminous star orbits, due to Doppler boosting. The magnitude of the effect is sufficiently large that we can use it to infer a radial velocity amplitude accurate to 1 km/s. As far as we are aware, this is the first time a radial-velocity curve has been measured photometrically. Combining our velocity amplitude with the inclination and primary mass derived from the eclipses and primary spectral type, we infer a secondary mass of 0.22+/-0.03 Msun. We use our estimates to consider the likely evolutionary paths and mass-transfer episodes of these binary systems.