M Dwarf Metallicities and Giant Planet Occurrence: Ironing Out Uncertainties and Systematics

Comparisons between the planet populations around solar-type stars and those orbiting M dwarfs shed light on the possible dependence of planet formation and evolution on stellar mass. However, such analyses must control for other factors, i.e. metallicity, a stellar parameter which strongly influences the occurrence of gas giant planets. We obtained infrared spectra of 121 M dwarfs stars monitored by the California Planet Search (CPS) and determined metallicities with an accuracy of 0.08 dex. The mean and standard deviation of the sample is -0.05 and 0.20 dex, respectively. We parameterized the metallicity dependence of the occurrence of giant planets on orbits with period less than 2 yr around solar-type stars and applied this to our M dwarf sample to estimate the expected number of giant planets. The number of detected planets (3) is lower than the predicted number (6.4) but the difference is not very significant (12% probability of finding as many or fewer planets). The three M dwarf planet hosts are not especially metal rich and the most likely value of the power-law index relating planet occurrence to metallicity is 1.06 dex per dex for M dwarfs compared to 1.80 for solar-type stars; this difference, however, is comparable to uncertainties. Giant planet occurrence around both types of stars allows, but does not necessarily require, mass dependence of $\sim 1$ dex per dex. The actual planet-mass-metallicity relation may be complex and elucidating it will require larger surveys like those to be conducted by ground-based infrared spectrographs and the Gaia space astrometry mission.Comment: Accepted to The Astrophysical Journa

A Survey of 10-Micron Silicate Emission from Dust around Young Sun-Like Stars

We obtained low resolution (R = 100) mid-infrared (8-13 micron wavelengths) spectra of 8 nearby young main sequence stars with the Keck 1 telescope and Long-Wavelength Spectrometer (LWS) to search for 10 micron silicate (Si-O stretch) emission from circumstellar dust. No stars exhibited readily apparent emission: Spectra were then analyzed by least-squares fitting of a template based on a spectrum of Comet Hale-Bopp. Using this technique, we were able to constrain the level of silicate emission to a threshold ten times below what was previously possible from space. We found one star, HD 17925, with a spectrum statistically different from its calibrator and consistent with a silicate emission peak of 7% of the photosphere at a wavelength of 10 microns. Excess emission at 60 microns from this star has already been reported.Comment: 19 total pages, 5 Postscript figures, 2 tables, Late