Objects in Kepler's mirror may be larger than they appear: BIAS and selection effects in transiting planet surveys

Statistical analyses of large surveys for transiting planets such as the Kepler mission must account for systematic errors and biases. Transit detection depends not only on the planet's radius and orbital period, but also on host star properties. Thus, a sample of stars with transiting planets may not accurately represent the target population. Moreover, targets are selected using criteria such as a limiting apparent magnitude. These selection effects, combined with uncertainties in stellar radius, lead to biases in the properties of transiting planets and their host stars. We quantify possible biases in the Kepler survey. First, Eddington bias produced by a steep planet radius distribution and uncertainties in stellar radius results in a 15%-20% overestimate of planet occurrence. Second, the magnitude limit of the Kepler target catalog induces Malmquist bias toward large, more luminous stars and underestimation of the radii of about one-third of candidate planets, especially those larger than Neptune. Third, because metal-poor stars are smaller, stars with detected planets will be very slightly (<0.02 dex) more metal-poor than the target average. Fourth, uncertainties in stellar radii produce correlated errors in planet radius and stellar irradiation. A previous finding, that highly irradiated giants are more likely to have "inflated" radii, remains significant, even accounting for this effect. In contrast, transit depth is negatively correlated with stellar metallicity even in the absence of any intrinsic correlation, and a previous claim of a negative correlation between giant planet transit depth and stellar metallicity is probably an artifact

X-ray-optical classification of cluster mergers and the evolution of the cluster merger fraction

We present the results of a simple but robust morphological classification of a statistically complete sample of 108 of the most X-ray-luminous clusters at 0.15 ≤z≤ 0.7 observed with Chandra. Our aims are to (a) identify the most disturbed massive clusters to be used as gravitational lenses for studies of the distant Universe and as probes of particle acceleration mechanisms resulting in non-thermal radio emission; (b) find cluster mergers featuring subcluster trajectories that make them suitable for quantitative analyses of cluster collisions; and (c) constrain the evolution with redshift of the cluster merger fraction. Finally, (d) this paper represents the third public release of clusters from the MAssive Cluster Survey sample, adding 24 clusters to the 46 published previously. To classify clusters by the degree of relaxation, we use the projected offset of the brightest cluster galaxy from the peak (or the global centroid) of the X-ray emission as a measure of the segregation between the intracluster gas and dark matter, and also perform a visual assessment of the optical and X-ray morphology on all scales. Regarding (a), we identify 10 complex systems likely to have undergone multiple merger events in the recent past. Regarding (b), we identify 11 systems likely to be post-collision, binary, head-on mergers (BHOMs), as well as another six mergers that are possible BHOMs but probably harder to interpret because of non-negligible impact parameters and merger axes closer to our line of sight. Regarding (c), we find a highly significant increase with redshift in the fraction of morphologically disturbed clusters (and thus a clear decrease in the number of fully relaxed systems) starting at z∼ 0.4, in spite of a detection bias in our sample against very disturbed systems at high redshift. Since our morphological diagnostics are all based on imaging data and thus sensitive to projection effects, the measured merger fractions should be considered lower limits and our list of mergers incomplete, as we are likely to miss systems forming along axes close to our line of sight. A larger sample of clusters with high-quality X-ray data in particular at high redshift will be needed to trace the evolutionary history of cluster growth and relaxation closer to the primary epoch of cluster formation z∼ 1

THE ENIGMATIC and EPHEMERAL M DWARF SYSTEM KOI 6705: CHESHIRE CAT or WILD GOOSE?

We confirm a 0.995 day periodic planetary transit-like signal, KOI 6705.01, in the Kepler light curve of the star KIC 6423922. Optical and infrared spectra show that this star is a mid M-type dwarf with an effective temperature K, metallicity [Fe/H] = -0.08 ±0.10, radius Ro, and mass = 0.28 ±0.05Mo. The star is pc away and its space motion, rotation period, and lack of Hα emission indicate it is an older member of the "thin disk" population. On the other hand, the star exhibits excess infrared emission suggesting a dust disk more typical of a very young star. If the KOI 6705.01 signal is produced by a planet, the transit depth of 60 ppm means its radius is only R⊕, or about the size of the Moon. However, the duration ( hr) and time variation of KOI 6705.01 are anomalous: the signal was undetected in the first two years of the mission and increased through the latter two years. These characteristics require implausible orbits and material properties for any planet and rule out such an explanation, although a dust cloud is possible. We excluded several false positive scenarios including background stars, scattered light from stars that are nearby on the sky, and electronic cross-talk between detector readout channels. We find the most likely explanation to be that KOI 6705.01 is a false positive created by charge transfer inefficiency in a detector column on which KIC 6423922 and a 1.99 day eclipsing binary both happened to fall

They might be giants: Luminosity class, planet occurrence, and planetmetallicity relation of the coolest kepler target stars

We estimate the stellar parameters of late K- and early M-type Kepler target stars. We obtain medium-resolution visible spectra of 382 stars with KP - J &gt; 2 (≃K5 and later spectral type). We determine luminosity class by comparing the strength of gravity-sensitive indices (CaH, K I, Ca II, and Na I) to their strength in a sample of stars of known luminosity class. We find that giants constitute 96% ± 1% of the bright (K P &lt; 14) Kepler target stars, and 7% ± 3% of dim (K P &gt; 14) stars, significantly higher than fractions based on the stellar parameters quoted in the Kepler Input Catalog (KIC). The KIC effective temperatures are systematically (110+15 - 35K) higher than temperatures we determine from fitting our spectra to PHOENIX stellar models. Through Monte Carlo simulations of the Kepler exoplanet candidate population, we find a planet occurrence of 0.36 ± 0.08 when giant stars are properly removed, somewhat higher than when a KIC log g &gt; 4 criterion is used (0.27 ± 0.05). Last, we show that there is no significant difference in g - r color (a probe of metallicity) between late-type Kepler stars with transiting Earth-to-Neptune-size exoplanet candidates and dwarf stars with no detected transits. We show that a previous claimed offset between these two populations is most likely an artifact of including a large number of misidentified giants

A Search for Exoplanets in Open Clusters and Young Associations based on TESS Objects of Interest

We report the results of our search for planet candidates in open clusters and young stellar associations based on the Transiting Exoplanet Survey Satellite (TESS) Objects of Interest Catalog. We find one confirmed planet, one promising candidate, one brown dwarf and three unverified planet candidates in a sample of 1229 open clusters from the second Gaia data release. We discuss individual planet-star systems based on their basic parameters, membership probability and the observation notes from the ExoFOP-TESS website. We also find ten planet candidates (P > 95%) in young stellar associations by using the BANYAN ς Multivariate Bayesian Algorithm. Among the ten candidates, five are known planetary systems. We estimate the rotation periods of the host stars using the TESS light curves and estimate their ages based on gyrochronology. Two candidates with periodic variations are likely to be young planets, but their exact memberships to young stellar associations remain unknown

Zodiacal Exoplanets in Time (ZEIT). V. A Uniform Search for Transiting Planets in Young Clusters Observed by K2

Detection of transiting exoplanets around young stars is more difficult than for older systems owing to increased stellar variability. Nine young open cluster planets have been found in the K2 data, but no single analysis pipeline identified all planets. We have developed a transit search pipeline for young stars that uses a transit-shaped notch and quadratic continuum in a 12 or 24 hr window to fit both the stellar variability and the presence of a transit. In addition, for the most rapid rotators ( days) we model the variability using a linear combination of observed rotations of each star. To maximally exploit our new pipeline, we update the membership for four stellar populations observed by K2 (Upper Scorpius, Pleiades, Hyades, Praesepe) and conduct a uniform search of the members. We identify all known transiting exoplanets in the clusters, 17 eclipsing binaries, one transiting planet candidate orbiting a potential Pleiades member, and three orbiting unlikely members of the young clusters. Limited injection recovery testing on the known planet hosts indicates that for the older Praesepe systems we are sensitive to additional exoplanets as small as 1-2 R ⊕, and for the larger Upper Scorpius planet host (K2-33) our pipeline is sensitive to ∼4 R ⊕ transiting planets. The lack of detected multiple systems in the young clusters is consistent with the expected frequency from the original Kepler sample, within our detection limits. With a robust pipeline that detects all known planets in the young clusters, occurrence rate testing at young ages is now possible

Prospecting in late-type dwarfs: A calibration of infrared and visible spectroscopic metallicities of late K and M dwarfs spanning 1.5 dex

Knowledge of late K and M dwarf metallicities can be used to guide planet searches and constrain planet formation models. However, the determination of metallicities of late-type stars is difficult because visible wavelength spectra of their cool atmospheres contain many overlapping absorption lines, preventing the measurement of equivalent widths. We present new methods, and improved calibrations of existing methods, to determine metallicities of late K and M dwarfs from moderate resolution (1300 -0.5, but are less useful for more metal-poor stars

Convergence of chiral effective field theory

We formulate the expansion for the mass of the nucleon as a function of pion mass within chiral perturbation theory using a number of different ultra-violet regularisation schemes; including dimensional regularisation and various finite-ranged regulators. Leading and next-to-leading order non-analytic contributions are included through the standard one-loop Feynman graphs. In addition to the physical nucleon mass, the expansion is constrained by recent, extremely accurate, lattice QCD data obtained with two flavors of dynamical quarks. The extent to which different regulators can describe the chiral expansion is examined, while varying the range of quark mass over which the expansions are matched. Renormalised chiral expansion parameters are recovered from each regularisation prescription and compared. We find that the finite-range regulators produce consistent, model-independent results over a wide range of quark mass sufficient to solve the chiral extrapolation problem in lattice QCD.Comment: 13 pages, 13 figures; To appear in Progress in Particle and Nuclear Physics; presented at Erice School on Quarks in Hadrons and Nuclei, September 200

SOAR TESS Survey. II. The Impact of Stellar Companions on Planetary Populations

We present the results of the second year of exoplanet candidate host speckle observations from the SOAR TESS survey. We find 89 of the 589 newly observed TESS planet candidate hosts have companions within 3″, resulting in light-curve dilution, that, if not accounted for, leads to underestimated planetary radii. We combined these observations with those from Paper I to search for evidence of the impact binary stars have on planetary systems. Removing the one-quarter of the targets observed identified as false-positive planet detections, we find that transiting planets are suppressed by nearly a factor of seven in close solar-type binaries, nearly twice the suppression previously reported. The result on planet occurrence rates that are based on magnitude-limited surveys is an overestimation by a factor of two if binary suppression is not taken into account. We also find tentative evidence for similar close binary suppression of planets in M-dwarf systems. Last, we find that the high rates of widely separated companions to hot Jupiter hosts previously reported was likely a result of false-positive contamination in our sample

The μ Tau association: A 60 Myr old coeval group at 150 pc from the Sun

We present an analysis of the newly identified μTau Association (MUTA) of young stars at ≈150 pc from the Sun that is part of the large Cas-Tau structure, coeval and comoving with the α Persei cluster. This association is also located in the vicinity of the Taurus-Auriga star-forming region and the Pleiades association, although it is unrelated to them. We identify more than 500 candidate members of MUTA using Gaia DR2 data and the BANYAN Σ tool, and we determine an age of 62 ± 7 Myr for its population based on an empirical comparison of its color-magnitude diagram sequence with those of other nearby young associations. The MUTA association is related to the Theia 160 group of Kounkel & Covey and corresponds to the e Tau group of Liu et al. It is also part of the Cas-Tau group of Blaauw. As part of this analysis, we introduce an iterative method based on spectral templates to perform an accurate correction of interstellar extinction of Gaia DR2 photometry, needed because of its wide photometric bandpasses. We show that the members of MUTA display an expected increased rate of stellar activity and faster rotation rates compared with older stars, and that literature measurements of the lithium equivalent width of nine G0- to K3-type members are consistent with our age determination. We show that the present-day mass function of MUTA is consistent with other known nearby young associations. We identify WD 0340+103 as a hot, massive white dwarf remnant of a B2 member that left its planetary nebula phase only 270,000 yr ago, posing an independent age constraint of - 60+8-6 Myr for MUTA, consistent with our isochrone age. This relatively large collection of comoving young stars near the Sun indicates that more work is required to unveil the full kinematic structure of the complex of young stars surrounding α Persei and Cas-Tau