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Register a new userOn the Metallicities of Kepler Stars
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Subo Dong
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We use 12000 stars from Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) spectroscopic data to show that the metallicities of Kepler field stars as given in the Kepler Input Catalog (KIC) systematically underestimate both the true metallicity and the dynamic range of the Kepler sample. Specifically, to the first order approximation, we find [Fe/H]_KIC = -0.20 + 0.43 [Fe/H]_LAMOST, with a scatter of ~0.25 dex, due almost entirely to errors in KIC. This relation is most secure for -0.3<[Fe/H]_LAMOST<+0.4 where we have >200 comparison stars per 0.1 dex bin and good consistency is shown between metallicities determined by LAMOST and high-resolution spectra. It remains approximately valid in a slightly broader range. When the relation is inverted, the error in true metallicity as derived from KIC is (0.25 dex)/0.43~0.6 dex. We thereby quantitatively confirm the cautionary note by Brown et al. (2011) that KIC estimates of [Fe/H] should not be used by anyone with a particular interest in stellar metallicities. Fortunately, many more LAMOST spectroscopic metallicities will be available in the near future.
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We present a variability analysis of the early-release first quarter of data publicly released by the Kepler project. Using the stellar parameters from the Kepler Input Catalog, we have separated the sample into 129,000 dwarfs and 17,000 giants, and further sub-divided the luminosity classes into temperature bins corresponding approximately to the spectral classes A, F, G, K, and M. Utilizing the inherent sampling and time baseline of the public dataset (30 minute sampling and 33.5 day baseline), we have explored the variability of the stellar sample. The overall variability rate of the dwarfs is 25% for the entire sample, but can reach 100% for the brightest groups of stars in the sample. G-dwarfs are found to be the most stable with a dispersion floor of $sigma sim 0.04$ mmag. At the precision of Kepler, $>95$% of the giant stars are variable with a noise floor of $sim 0.1$ mmag, 0.3 mmag, and 10 mmag for the G-giants, K-giants, and M-giants, respectively. The photometric dispersion of the giants is consistent with acoustic variations of the photosphere; the photometrically-derived predicted radial velocity distribution for the K-giants is in agreement with the measured radial velocity distribution. We have also briefly explored the variability fraction as a function of dataset baseline (1 - 33 days), at the native 30-minute sampling of the public Kepler data. To within the limitations of the data, we find that the overall variability fractions increase as the dataset baseline is increased from 1 day to 33 days, in particular for the most variable stars. The lower mass M-dwarf, K-dwarf, G-dwarf stars increase their variability more significantly than the higher mass F-dwarf and A-dwarf stars as the time-baseline is increased, indicating that the variability of the lower mass stars is mostly characterized by timescales of weeks whi...astroph will not allow longer abstract!
Kepler-78b is a transiting Earth-mass planet in an 8.5 hr orbit discovered by the Kepler Space Mission. We performed an analysis of the published radial velocity measurements for Kepler-78 in order to derive a refined measurement for the planet mass. Kepler-78 is an active star and radial velocity variations due to activity were removed using a Floating Chunk Offset (FCO) method where an orbital solution was made to the data by allowing the velocity offsets of individual nights to vary. We show that if we had no a priori knowledge of the transit period the FCO method used as a periodogram would still have detected Kepler-78b in the radial velocity data. It can thus be effective at finding unknown short-period signals in the presence of significant activity noise. Using the FCO method while keeping the ephemeris and orbital phase fixed to the photometric values and using only data from nights where 6-10 measurements were taken results in a K-amplitude of 1.34 +/- 0.25 m/s. a planet mass of 1.31 +/- 0.24 M_Earth, and a planet density of rho = 4.5 (-2.0/+2.2) g/cm^3. Allowing the orbital phase to be a free parameter reproduces the transit phase to within the uncertainty. The corresponding density implies that Kepler-78b may have a structure that is deficient in iron and is thus more like the Moon. Although the various approaches that were used to filter out the activity of Kepler 78 produce consistent radial velocity amplitudes to within the errors, these are still too large to constrain the structure of this planet. The uncertainty in the mass for Kepler-78b is large enough to encompass models with structures ranging from Mercury-like (iron enriched) to Moon-like (iron deficient). A more accurate K-amplitude as well as a better determination of the planet radius are needed to distinguish between these models.
It has been known for a decade that hot stars with hot Jupiters tend to have high obliquities. Less is known about the degree of spin-orbit alignment for hot stars with other kinds of planets. Here, we re-assess the obliquities of hot Kepler stars with transiting planets smaller than Neptune, based on spectroscopic measurements of their projected rotation velocities (vsini). The basis of the method is that a lower obliquity -- all other things being equal -- causes sini to be closer to unity and increases the value of vsini. We sought evidence for this effect using a sample of 150 Kepler stars with effective temperatures between 5950 and 6550K and a control sample of 101 stars with matching spectroscopic properties and random orientations. The planet hosts have systematically higher values of vsini than the control stars, but not by enough to be compatible with perfect spin-orbit alignment. The mean value of sini is 0.856 +/- 0.036, which is 4-sigma away from unity (perfect alignment), and 2-sigma away from pi/4 (random orientations). There is also evidence that the hottest stars have a broader obliquity distribution: when modeled separately, the stars cooler than 6250K have <sini> = 0.928 +/- 0.042, while the hotter stars are consistent with random orientations. This is similar to the pattern previously noted for stars with hot Jupiters. Based on these results, obliquity excitation for early-G and late-F stars appears to be a general outcome of star and planet formation, rather than being exclusively linked to hot Jupiter formation.
(abridged) Kepler-278 and Kepler-391 are two of the three evolved stars known to date on the RGB to host multiple short-period transiting planets. Moreover, these planets are among the smallest discovered around RGB stars. Here we present a detailed stellar and planetary characterization of these remarkable systems. Based on high-quality spectra from Gemini-GRACES for Kepler-278 and Kepler-391, we obtained refined stellar parameters and precise chemical abundances for 25 elements. Also, combining our new stellar parameters with a photodynamical analysis of the Kepler light curves, we determined accurate planetary properties of both systems. The precise spectroscopic parameters of Kepler-278 and Kepler-391, along with their high $^{12}mathrm{C}/^{13}mathrm{C}$ ratios, show that both stars are just starting their ascent on the RGB. The planets Kepler-278b, Kepler-278c, and Kepler-391c are warm sub-Neptunes, whilst Kepler-391b is a hot sub-Neptune that falls in the hot super-Earth desert and, therefore, it might be undergoing photoevaporation of its outer envelope. The high-precision obtained in the transit times allowed us not only to confirm Kepler-278cs TTV signal, but also to find evidence of a previously undetected TTV signal for the inner planet Kepler-278b. From the presence of gravitational interaction between these bodies we constrain, for the first time, the mass of Kepler-278b ($M_{mathrm{p}}$ = 56 $substack{+37-13}$ $M_{mathrm{oplus}}$) and Kepler-278c ($M_{mathrm{p}}$ = 35 $substack{+9.9 -21} $ $M_{mathrm{oplus}}$). Finally, our photodynamical analysis also shows that the orbits of both planets around Kepler-278 are highly eccentric ($e sim$ 0.7) and, surprisingly, coplanar. Further observations of this system are needed to confirm the eccentricity values presented here.
We present calculations of the occurrence rate of small close-in planets around low mass dwarf stars using the known planet populations from the $Kepler$ and $K2$ missions. Applying completeness corrections clearly reveals the radius valley in the maximum a-posteriori occurrence rates as a function of orbital separation and planet radius. We measure the slope of the valley to be $r_{p,text{valley}} propto F^{-0.060pm 0.025}$ which bears the opposite sign from that measured around Sun-like stars thus suggesting that thermally driven atmospheric mass loss may not dominate the evolution of planets in the low stellar mass regime or that we are witnessing the emergence of a separate channel of planet formation. The latter notion is supported by the relative occurrence of rocky to non-rocky planets increasing from $0.5pm 0.1$ around mid-K dwarfs to $8.5pm 4.6$ around mid-M dwarfs. Furthermore, the center of the radius valley at $1.54pm 0.16$ R$_{oplus}$ is shown to shift to smaller sizes with decreasing stellar mass in agreement with physical models of photoevaporation, core-powered mass loss, and gas-poor formation. Although current measurements are insufficient to robustly identify the dominant formation pathway of the radius valley, such inferences may be obtained by $TESS$ with $mathcal{O}(85,000)$ mid-to-late M dwarfs observed with 2-minute cadence. The measurements presented herein also precisely designate the subset of planetary orbital periods and radii that should be targeted in radial velocity surveys to resolve the rocky to non-rocky transition around low mass stars.
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