Global Analysis of KOI-977: Spectroscopy, Asteroseismology, and Phase-curve Analysis

TL;DR

This study conducts a comprehensive analysis of KOI-977, combining spectroscopy, asteroseismology, and phase-curve analysis, revealing that the observed transit signal is a false positive caused by an eclipsing binary unrelated to the red giant host.

Contribution

It provides a detailed methodology for distinguishing false positives in Kepler data using combined spectroscopic, asteroseismic, and light-curve analyses.

Findings

KOI-977 is confirmed as a red giant in the red clump.

The transit signal is caused by an unrelated eclipsing binary, not a planet.

The eclipsing binary likely consists of a solar-type star and an M dwarf.

Abstract

We present a global analysis of KOI-977, one of the planet host candidates detected by {\it Kepler}. Kepler Input Catalog (KIC) reports that KOI-977 is a red giant, for which few close-in planets have been discovered. Our global analysis involves spectroscopic and asteroseismic determinations of stellar parameters (e.g., mass and radius) and radial velocity (RV) measurements. Our analyses reveal that KOI-977 is indeed a red giant in the red clump, but its estimated radius ($\gtrsim 20R_\odot=0.093$ AU) is much larger than KOI-977.01's orbital distance ($\sim 0.027$ AU) estimated from its period ($P_\mathrm{orb}\sim 1.35$ days) and host star's mass. RV measurements show a small variation, which also contradicts the amplitude of ellipsoidal variations seen in the light-curve folded with KOI-977.01's period. Therefore, we conclude that KOI-977.01 is a false positive, meaning that the red giant, for which we measured the radius and RVs, is different from the object that produces the transit-like signal (i.e., an eclipsing binary). On the basis of this assumption, we also perform a light-curve analysis including the modeling of transits/eclipses and phase-curve variations, adopting various values for the dilution factor $D$, which is defined as the flux ratio between the red giant and eclipsing binary. Fitting the whole folded light-curve as well as individual transits in the short cadence data simultaneously, we find that the estimated mass and radius ratios of the eclipsing binary are consistent with those of a solar-type star and a late-type star (e.g., an M dwarf) for $D\gtrsim 20$.