CHARACTERIZING THE VARIABILITY OF STARS WITH EARLY-RELEASEKEPLERDATA

Abstract

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 σ ∼ 0.04 mmag. At the precision of Kepler, > 95% of the giant stars are variable with a noise floor of ∼ 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 photometricallyderived 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 while the variability of the higher mass stars is mostly characterized by timescales of days. A study of the distribution of the variability as a function of galactic latitude suggests sources closer to the galactic plane are more variable. This may be the result of sampling differing populations (i.e., ages) as a function of latitude or may be the result of higher background contamination that is inflating the variability fractions at lower latitudes. A comparison of the M dwarf statistics to the variability of 29 known bright M dwarfs indicates that the M dwarfs are primarily variable on timescales of weeks or longer presumably dominated by spots and binarity. But on shorter timescales of hours which are relevant for planetary transit detection, the stars are significantly less variable, with ∼ 80% having 12-hour dispersions of 0.5 mmag or less.

An HST search for planets in the lower main sequence of the globular cluster NGC 6397

“…The fraction of stars f with σ jit > 2 mmag is f 2%. This value should be compared with the results found by Ciardi et al (2011) examining the first quarter of Kepler photometry on 2182 field M dwarfs: these data cover an interval of 33 days with an average cadence of 30 min, which is quite similar to our cadence and time scale. They found a fraction f 20% of stars with σ > 2 mmag, that is at least an order of magnitude larger fraction than that we measured in NGC 6397.…”

Statistics of Stellar Variability in Kepler Data with ARC Systematics Removal

“…Using the ARC data we revisit and confirm many of the relationships presented by Basri et al (2010), Basri et al (2011) and Ciardi et al (2011) and extend that work to a more thorough study of the periodic and stochastic nature of the variability. We divide the targets into high-and low-variability groups by comparison to the active Sun (top lines in Fig.…”

Gran Telescopio Canarias OSIRIS transiting exoplanet atmospheric survey: detection of potassium in XO-2b from narrowband spectrophotometry

“…The final TrES-2 combined transit light curve was successfully fitted down to the 20 ppm level, which is nearly one order of magnitude below our required levels for XO2b (∼100 ppm). Hat-P7 and Kepler-#4, #5, #6, #7, and #8 also show a low photometric variability around the ∼0.2 mmag level with removed transits when calculating the light curve statistics (Ciardi et al 2010), and all have similar activity indices to XO-2 .…”

“…For the highly active star HD 189733, f λ ∼ 2−3% in the visible (Henry & Winn 2008), while for the non-active star TrES-2, f λ was found to have an amazingly low value of (5.18 ± 0.19) × 10 −6 from the Kepler data (Kipping & Bakos 2010). Ciardi et al (2010) studied the stellar photometric variability for stars in the Kepler sample, finding a bi-model distribution between active and non-active stars. K-dwarfs with a low activity (2/3 of the sample) were found to typically display a stochastic photometric variability of less than 1 mmag, while active stars were often found to be periodic on timescales of days to weeks.…”