The Apache Point Observatory Galactic Evolution Experiment ( APOGEE ) has observed \sim 600 transiting exoplanets and exoplanet candidates from Kepler ( Kepler Objects of Interest , KOIs ) , most with \geq 18 epochs .
The combined multi-epoch spectra are of high signal-to-noise ( typically \geq 100 ) and yield precise stellar parameters and chemical abundances .
We first confirm the ability of the APOGEE abundance pipeline , ASPCAP , to derive reliable [ Fe/H ] and effective temperatures for FGK dwarf stars — the primary Kepler host stellar type — by comparing the ASPCAP-derived stellar parameters to those from independent high-resolution spectroscopic characterizations for 221 dwarf stars in the literature .
With a sample of 282 close-in ( P < 100 days ) KOIs observed in the APOGEE KOI goal program , we find a correlation between orbital period and host star [ Fe/H ] characterized by a critical period , P _ { \mathrm { crit } } = 8.3 ^ { +0.1 } _ { -4.1 } days , below which small exoplanets orbit statistically more metal-enriched host stars .
This effect may trace a metallicity dependence of the protoplanetary disk inner-radius at the time of planet formation or may be a result of rocky planet ingestion driven by inward planetary migration .
We also consider that this may trace a metallicity dependence of the dust sublimation radius , but find no statistically significant correlation with host T _ { \mathrm { eff } } and orbital period to support such a claim .