The Metallicity Distribution and Hot Jupiter Rate of the Kepler Field: Hectochelle High-resolution Spectroscopy for 776 Kepler Target Stars

Abstract

The occurrence rate of hot Jupiters from the Kepler transit survey is roughly half that of radial velocity surveys targeting solar neighborhood stars. One hypothesis to explain this difference is that the two surveys target stars with different stellar metallicity distributions. To test this hypothesis, we measure the metallicity distribution of the Kepler targets using the Hectochelle multi-fiber, high-resolution spectrograph. Limiting our spectroscopic analysis to 610 dwarf stars in our sample with g log >3.5, we measure a metallicity distribution characterized by a mean of , and the samples come from different parent populations according to a Kolmogorov-Smirnov test. We refit the exponential relation between the fraction of stars hosting a close-in giant planet and the host star metallicity using a sample of dwarf stars from the California Planet Search with updated metallicities. The best-fit relation tells us that the difference in metallicity between the two samples is insufficient to explain the discrepant hot Jupiter occurrence rates; the metallicity difference would need to be ;0.2-0.3 dex for perfect agreement. We also show that (sub)giant contamination in the Kepler sample cannot reconcile the two occurrence calculations. We conclude that other factors, such as binary contamination and imperfect stellar properties, must also be at play.

Investigating the planet–metallicity correlation for hot Jupiters

“…While all of the above results are from radial velocity surveys, there have also been previous attempts to calculate β from transit surveys, in particular from the Kepler survey (Borucki et al 2010). Guo et al (2017) and Petigura et al (2018) both evaluate β for the population of 14 hot Jupiters in the Kepler data. Guo et al (2017) find a value of β = 2.1 ± 0.7, consistent with radial velocity results, while Petigura et al (2018) find a value of β = 3.4 +0.9 −0.8 , which is higher than previous studies.…”

The California-Kepler Survey. IV. Metal-rich Stars Host a Greater Diversity of Planets

“…Dong et al (2014), through analyzing the LAMOST Data Releases 1 and 2, found that the mean metallicity of 12,000 Kepler field stars was −0.04dex, much closer to solar than to the value of −0.14dex assumed in the construction of the KIC. Guo et al (2017) also found a near-solar mean metallicity of −0.04dex in an analysis of 610 Kepler field stars observed by the Hectochelle R=34,000 spectrometer at the MMT. Thus, a metallicity offset cannot explain differences in the hot Jupiter rates.…”

“…It is worth recalling that both studies included long period planets (P 1 year > ), which constituted the bulk of the sample. Guo et al (2017) repeated the Johnson et al (2010) analysis, but restricted the study to hot Jupiters and found β=2.1±0.7. While this result differs with our measurement of β by about 2σ, it is clear that hot Jupiters in the solar neighborhood and in the Kepler field are both strongly associated with metallicity.…”

Long-term Photometric Variability in Kepler Full-frame Images: Magnetic Cycles of Sun–like Stars

“…While spectroscopic surveys of the Kepler field are underway (De Cat et al 2015;Guo et al 2017), these surveys generally target the bright stars in the field. Photometric surveys are more complete (Brown et al 2011;Huber et al 2014), but provide larger uncertainties on the physical parameters of each individual star.…”