Knowledge of the chemical composition and absolute masses of Capella are key to understanding the evolutionary state of this benchmark binary system comprising two giant stars. Previous efforts, including our own 2009 study, have largely failed to re
ach an acceptable agreement between the observations and current stellar evolution models, preventing us from assessing the status of the primary. Here we report a revision of the physical properties of the components incorporating recently published high-precision radial velocity measurements, and a new detailed chemical analysis providing abundances for more than 20 elements in both stars. We obtain highly precise (to about 0.3%) masses of 2.5687 +/- 0.0074 and 2.4828 +/- 0.0067 solar masses, radii of 11.98 +/- 0.57 and 8.83 +/- 0.33 solar radii, effective temperatures of 4970 +/- 50 K and 5730 +/- 60 K, and independently measured luminosities based on the orbital parallax (78.7 +/- 4.2 and 72.7 +/- 3.6 solar luminosities). We find an excellent match to stellar evolution models at the measured composition of [Fe/H] = -0.04 +/- 0.06. Three different sets of models place the primary star firmly at the end of the core helium-burning phase (clump), while the secondary is known to be evolving rapidly across the Hertzprung gap. The measured lithium abundance, the C/N ratio, and the 12C/13C isotopic carbon abundance ratio, which change rapidly in the giant phase, are broadly in agreement with expectations from models. Predictions from tidal theory for the spin rates, spin-orbit alignment, and other properties do not fare as well, requiring a 40-fold increase in the efficiency of the dissipation mechanisms in order to match the observations.
We present an investigation of twelve candidate transiting planets from Kepler with orbital periods ranging from 34 to 207 days, selected from initial indications that they are small and potentially in the habitable zone (HZ) of their parent stars. F
ew of these objects are known. The expected Doppler signals are too small to confirm them by demonstrating that their masses are in the planetary regime. Here we verify their planetary nature by validating them statistically using the BLENDER technique, which simulates large numbers of false positives and compares the resulting light curves with the Kepler photometry. This analysis was supplemented with new follow-up observations (high-resolution optical and near-infrared spectroscopy, adaptive optics imaging, and speckle interferometry), as well as an analysis of the flux centroids. For eleven of them (KOI-0571.05, 1422.04, 1422.05, 2529.02, 3255.01, 3284.01, 4005.01, 4087.01, 4622.01, 4742.01, and 4745.01) we show that the likelihood they are true planets is far greater than that of a false positive, to a confidence level of 99.73% (3 sigma) or higher. For KOI-4427.01 the confidence level is about 99.2% (2.6 sigma). With our accurate characterization of the GKM host stars, the derived planetary radii range from 1.1 to 2.7 R_Earth. All twelve objects are confirmed to be in the HZ, and nine are small enough to be rocky. Excluding three of them that have been previously validated by others, our study doubles the number of known rocky planets in the HZ. KOI-3284.01 (Kepler-438b) and KOI-4742.01 (Kepler-442b) are the planets most similar to the Earth discovered to date when considering their size and incident flux jointly.
As part of the BANANA project (Binaries Are Not Always Neatly Aligned), we have found that the eclipsing binary CV Velorum has misaligned rotation axes. Based on our analysis of the Rossiter-McLaughlin effect, we find sky-projected spin-orbit angles
of $beta_{rm p} = -52pm6^{circ}$ and $beta_{rm s}= 3pm7^{circ}$ for the primary and secondary stars (B2.5V $+$ B2.5V, $P=6.9$ d). We combine this information with several measurements of changing projected stellar rotation speeds ($v sin i_{star}$) over the last $30$ years, leading to a model in which the primary stars obliquity is $approx65^{circ}$, and its spin axis precesses around the total angular momentum vector with a period of about $140$ years. The geometry of the secondary star is less clear, although a significant obliquity is also implicated by the observed time variations in the $v sin i_{star}$. By integrating the secular tidal evolution equations backward in time, we find that the system could have evolved from a state of even stronger misalignment similar to DI Herculis, a younger but otherwise comparable binary.
We report differential photometric observations and radial-velocity measurements of the detached, 1.69-day period, double-lined eclipsing binary AQ Ser. Accurate masses and radii for the components are determined to better than 1.8% and 1.1%, respect
ively, and are M1 = 1.417 +/- 0.021 MSun, M2 = 1.346 +/- 0.024 MSun, R1 = 2.451 +/- 0.027 RSun, and R2 = 2.281 +/- 0.014 RSun. The temperatures are 6340 +/- 100 K (spectral type F6) and 6430 +/- 100 K (F5), respectively. Both stars are considerably evolved, such that predictions from stellar evolution theory are particularly sensitive to the degree of extra mixing above the convective core (overshoot). The component masses are different enough to exclude a location in the H-R diagram past the point of central hydrogen exhaustion, which implies the need for extra mixing. Moreover, we find that current main-sequence models are unable to match the observed properties at a single age even when allowing the unknown metallicity, mixing length parameter, and convective overshooting parameter to vary freely and independently for the two components. The age of the more massive star appears systematically younger. AQ Ser and other similarly evolved eclipsing binaries showing the same discrepancy highlight an outstanding and largely overlooked problem with the description of overshooting in current stellar theory.
We report the discovery that the pre-main sequence object LkCa3 in the Taurus-Auriga star-forming region is a hierarchical quadruple system of M stars. It was previously known to be a close (~0.5 arc sec) visual pair, with one component being a moder
ately eccentric 12.94-day single-lined spectroscopic binary. A re-analysis of archival optical spectra complemented with new near-infrared spectroscopy shows both visual components to be double-lined, the second one having a period of 4.06 days and a circular orbit. In addition to the orbital elements, we determine optical and near-infrared flux ratios, effective temperatures, and projected rotational velocities for all four stars. Using existing photometric monitoring observations of the system that had previously revealed the rotational period of the primary in the longer-period binary, we detect also the rotational signal of the primary in the 4.06-day binary, which is synchronized with the orbital motion. With only the assumption of coevality, a comparison of all of these constraints with current stellar evolution models from the Dartmouth series points to an age of 1.4 Myr and a distance of 133 pc, consistent with previous estimates for the region and suggesting the system is on the near side of the Taurus complex. Similar comparisons of the properties of LkCa3 and of the well-known quadruple pre-main sequence system GG Tau with the widely used models from the Lyon series for a mixing length parameter of alpha_ML = 1.0 strongly favor the Dartmouth models.
With observations of the EP Cru system, we continue our series of measurements of spin-orbit angles in eclipsing binary star systems, the BANANA project (Binaries Are Not Always Neatly Aligned). We find a close alignment between the sky projections o
f the rotational and orbital angular momentum vectors for both stars (beta_p = -1.8+-1.6 deg and |beta_s|<17 deg). We also derive precise absolute dimensions and stellar ages for this system. The EP Cru and DI Her systems provide an interesting comparison: they have similar stellar types and orbital properties, but DI Her is younger and has major spin-orbit misalignments, raising the question of whether EP Cru also had a large misalignment at an earlier phase of evolution. We show that tidal dissipation is an unlikely explanation for the good alignment observed today, because realignment happens on the same timescale as spin-orbit synchronization, and the stars in EP Cru are far from syncrhonization (they are spinning 9 times too quickly). Therefore it seems that some binaries form with aligned axes, while other superficially similar binaries are formed with misaligned axes.
We report homogeneous spectroscopic determinations of the effective temperature, metallicity, and projected rotational velocity for the host stars of 56 transiting planets. Our analysis is based primarily on the Stellar Parameter Classification (SPC)
technique. We investigate systematic errors by examining subsets of the data with two other methods that have often been used in previous studies (SME and MOOG). The SPC and SME results, both based on comparisons between synthetic spectra and actual spectra, show strong correlations between temperature, [Fe/H], and log g when solving for all three quantities simultaneously. In contrast the MOOG results, based on a more traditional curve-of-growth approach, show no such correlations. To combat the correlations and improve the accuracy of the temperatures and metallicities, we repeat the SPC analysis with a constraint on log g based on the mean stellar density that can be derived from the analysis of the transit light curves. Previous studies that have not taken advantage of this constraint have been subject to systematic errors in the stellar masses and radii of up to 20% and 10%, respectively, which can be larger than other observational uncertainties, and which also cause systematic errors in the planetary mass and radius.
Since the discovery of the first extrasolar giant planets around Sun-like stars, evolving observational capabilities have brought us closer to the detection of true Earth analogues. The size of an exoplanet can be determined when it periodically pass
es in front of (transits) its parent star, causing a decrease in starlight proportional to its radius. The smallest exoplanet hitherto discovered has a radius 1.42 times that of the Earths radius (R Earth), and hence has 2.9 times its volume. Here we report the discovery of two planets, one Earth-sized (1.03R Earth) and the other smaller than the Earth (0.87R Earth), orbiting the star Kepler-20, which is already known to host three other, larger, transiting planets. The gravitational pull of the new planets on the parent star is too small to measure with current instrumentation. We apply a statistical method to show that the likelihood of the planetary interpretation of the transit signals is more than three orders of magnitude larger than that of the alternative hypothesis that the signals result from an eclipsing binary star. Theoretical considerations imply that these planets are rocky, with a composition of iron and silicate. The outer planet could have developed a thick water vapour atmosphere.
We report the detection of three transiting planets around a Sunlike star, which we designate Kepler-18. The transit signals were detected in photometric data from the Kepler satellite, and were confirmed to arise from planets using a combination of
large transit-timing variations, radial-velocity variations, Warm-Spitzer observations, and statistical analysis of false-positive probabilities. The Kepler-18 star has a mass of 0.97M_sun, radius 1.1R_sun, effective temperature 5345K, and iron abundance [Fe/H]= +0.19. The planets have orbital periods of approximately 3.5, 7.6 and 14.9 days. The innermost planet b is a super-Earth with mass 6.9 pm 3.4M_earth, radius 2.00 pm 0.10R_earth, and mean density 4.9 pm 2.4 g cm^-3. The two outer planets c and d are both low-density Neptune-mass planets. Kepler-18c has a mass of 17.3 pm 1.9M_earth, radius 5.49 pm 0.26R_earth, and mean density 0.59 pm 0.07 g cm^-3, while Kepler-18d has a mass of 16.4 pm 1.4M_earth, radius 6.98 pm 0.33R_earth, and mean density 0.27 pm 0.03 g cm^-3. Kepler-18c and Kepler-18d have orbital periods near a 2:1 mean-motion resonance, leading to large and readily detected transit timing variations.
