One bottleneck for the exploitation of data from the Kepler mission for stellar astrophysics and exoplanet research has been the lack of precise radii and evolutionary states for most of the observed stars. We report revised radii of 177,911 Kepler stars derived by combining parallaxes from Gaia Data Release 2 with the DR25 Kepler Stellar Properties Catalog. The median radius precision is ≈ 8%, a typical improvement by a factor of 4-5 over previous estimates for typical Kepler stars. We find that ≈ 67% (≈ 120,000) of all Kepler targets are main-sequence stars, ≈ 21% (≈ 37,000) are subgiants, and ≈ 12% (≈ 21,000) are red giants, demonstrating that subgiant contamination is less severe than some previous estimates and that Kepler targets are mostly main-sequence stars. Using the revised stellar radii, we recalculate the radii for 2123 confirmed and 1922 candidate exoplanets. We confirm the presence of a gap in the radius distribution of small, close-in planets, but find that the gap is mostly limited to incident fluxes > 200 F ⊕ and its location may be at a slightly larger radius (closer to ≈ 2 R ⊕ ) when compared to previous results. Further, we find several confirmed exoplanets occupying a previously-described "hot super-Earth desert" at high irradiance, show the relation between gas-giant planet radius and incident flux, and establish a bona-fide sample of eight confirmed planets and 30 planet candidates with R p < 2 R ⊕ in circumstellar "habitable zones" (incident fluxes between 0.25-1.50 F ⊕ ). The results presented here demonstrate the potential for transformative characterization of stellar and exoplanet populations using Gaia data.

Kepler ultra-high precision photometry of long and continuous observations provides a unique dataset in which surface rotation and variability can be studied for thousands of stars. Because many of these old field stars also have independently measured asteroseismic ages, measurements of rotation and activity are particularly interesting in the context of age-rotation-activity relations. In particular, age-rotation relations generally lack good calibrators at old ages, a problem that this Kepler sample of old-field stars is uniquely suited to address. We study the surface rotation and photometric magnetic activity of a subset of 540 solar-like stars on the mainsequence and the subgiant branch for which stellar pulsations have been measured. The rotation period was determined by comparing the results from two different analysis methods: i) the projection onto the frequency domain of the time-period analysis, and ii) the autocorrelation function of the light curves. Reliable surface rotation rates were then extracted by comparing the results from two different sets of calibrated data and from the two complementary analyses. General photometric levels of magnetic activity in this sample of stars were also extracted by using a photometric activity index, which takes into account the rotation period of the stars. We report rotation periods for 310 out of 540 targets (excluding known binaries and candidate planet-host stars); our measurements span a range of 1 to 100 days. The photometric magnetic activity levels of these stars were computed, and for 61.5% of the dwarfs, this level is similar to the range, from minimum to maximum, of the solar magnetic activity. We demonstrate that hot dwarfs, cool dwarfs, and subgiants have very different rotation-age relationships, highlighting the importance of separating out distinct populations when interpreting stellar rotation periods. Our sample of cool dwarf stars with age and metallicity data of the highest quality is consistent with gyrochronology relations reported in the literature.

A knowledge of stellar ages is crucial for our understanding of many astrophysical phenomena, and yet ages can be difficult to determine. As they become older, stars lose mass and angular momentum, resulting in an observed slowdown in surface rotation 1 . The technique of 'gyrochronology' uses the rotation period of a star to calculate its age 2,3 . However, stars of known age must be used for calibration, and, until recently, the approach was untested for old stars (older than 1 gigayear, Gyr). Rotation periods are now known for stars in an open cluster of intermediate age 4 (NGC 6819; 2.5 Gyr old), and for old field stars whose ages have been determined with asteroseismology 5,6 . The data for the cluster agree with previous period-age relations 4 , but these relations fail to describe the asteroseismic sample 7 . Here we report stellar evolutionary modelling 5, 6, 8, 9, 10 , and confirm the presence of unexpectedly rapid rotation in stars that are more evolved than the Sun.We demonstrate that models that incorporate dramatically weakened magnetic braking for old stars can-unlike existing models-reproduce both the asteroseismic and the cluster data. Our findings might suggest a fundamental change in the nature of ageing stellar dynamos, with the Sun being close to the critical transition to much weaker magnetized winds. This weakened braking limits the diagnostic power of gyrochronology for those stars that are more than halfway through their main-sequence lifetimes.There are two approaches to the calibration and testing of gyrochronology. The first is a purely empirical approach, which utilizes a sample of stars with independently measured ages and rotation periods to construct a period-age relationship. These relationships are generally simple power-laws in age, period, and some mass-dependent quantity, and have seen wide usage 1,2,4,5,7 . The second, model-based approach uses stellar models and a prescription for magnetic braking to account for the functional dependence on all relevant stellar quantities, but relies on calibrators to determine the magnitude of the angular momentum loss. For this reason, it is well-suited to calibrating samples that only sparsely cover parameter space. It furthermore provides a method to attach physical meaning to observed braking behavior.Magnetic braking prescriptions are typically scaled from the solar case; the Skumanich relation 1 yields angular momentum loss of the form dJ/dt ∝ ω 3 , where ω is the angular rotation velocity 11 . These relations often use the dimensionless Rossby number, defined as the ratio of the period to the convective overturn timescale, Ro = P/τcz to characterize departures from this simple power law. Rossby number thresholds and scalings are routinely invoked to parameterize the magnetic field strength 12,13 , the mass and composition dependence to the spin-down 2,14 , observed saturation of the magnetic braking in rapid rotators, and the sharp transition from slow to rapid rotation in hot stars (>6250 K ) due to thinning convective envelopes 14 ...

An accurate and precise Kepler Stellar Properties Catalog is essential for the interpretation of the Kepler exoplanet survey results. Previous Kepler Stellar Properties Catalogs have focused on reporting the best-available parameters for each star, but this has required combining data from a variety of heterogeneous sources. We present the Gaia–Kepler Stellar Properties Catalog, a set of stellar properties of 186,301 Kepler stars, homogeneously derived from isochrones and broadband photometry, Gaia Data Release 2 parallaxes, and spectroscopic metallicities, where available. Our photometric effective temperatures, derived from colors, are calibrated on stars with interferometric angular diameters. Median catalog uncertainties are 112 K for , 0.05 dex for , 4% for , 7% for , 13% for , 10% for , and 56% for stellar age. These precise constraints on stellar properties for this sample of stars will allow unprecedented investigations into trends in stellar and exoplanet properties as a function of stellar mass and age. In addition, our homogeneous parameter determinations will permit more accurate calculations of planet occurrence and trends with stellar properties.

The Apache Point Observatory Galactic Evolution Experiment (APOGEE) is a high-resolution infrared spectroscopic survey spanning all Galactic environments (i.e., bulge, disk, and halo), with the principal goal of constraining dynamical and chemical evolution models of the Milky Way. APOGEE takes advantage of the reduced effects of extinction at infrared wavelengths to observe the inner Galaxy and bulge at an unprecedented level of detail. The survey's broad spatial and wavelength coverage enables users of APOGEE data to address numerous Galactic structure and stellar populations issues. In this paper we describe the APOGEE targeting scheme and document its various target classes to provide the necessary background and reference information to analyze samples of APOGEE data with awareness of the imposed selection criteria and resulting sample properties. APOGEE's primary sample consists of ∼10 5 red giant stars, selected to minimize observational biases in age and metallicity. We present the methodology and considerations that drive the selection of this sample and evaluate the accuracy, efficiency, and caveats of the selection and sampling algorithms. We also describe additional target classes that contribute to the APOGEE sample, including numerous ancillary science programs, and we outline the targeting data that will be included in the public data releases.

SDSS-V will be an all-sky, multi-epoch spectroscopic survey of over six million objects. It is designed to decode the history of the Milky Way Galaxy (MW), trace the emergence of the chemical elements, reveal the inner workings of stars, and investigate the origin of planets. It will also create an integral-field spectroscopic map of the interstellar gas in the Galaxy and the Local Group that is 1,000 times larger than the current state of the art and at high enough spatial resolution to reveal the self-regulation mechanisms of galactic ecosystems. SDSS-V will pioneer systematic, spectroscopic monitoring across the whole sky, revealing changes on timescales from 20 minutes to 20 years. The survey will thus track the flickers, flares, and radical transformations of the most luminous persistent objects in the universe: massive black holes growing at the centers of galaxies.The scope and flexibility of SDSS-V will be unique among both extant and anticipated spectroscopic surveys: it is all-sky, with matched survey infrastructures in both hemispheres; it provides near-infrared and optical multi-object fiber spectroscopy that is rapidly reconfigurable to serve high target densities, targets of opportunity, and time-domain monitoring; and it provides optical, ultrawide-field integral field spectroscopy. SDSS-V, with its programs anticipated to start in 2020, will be perfectly timed to multiply the scientific output from major space missions (e.g., TESS, Gaia, Spektr-RG-eROSITA) and ground-based projects. SDSS-V builds on the 25-year heritage of SDSS's advances in data analysis, collaboration spirit and infrastructure, and product deliverables in astronomy. The project is now refining its science scope, optimizing the survey strategies, and developing new hardware that builds on the SDSS-IV infrastructure. We present here an overview of the current state of these developments. SDSS-V is actively seeking to build its consortium of institutional and individual members for a worldwide, partner-driven collaboration.

We present the results from a detailed analysis of photometric and spectrophotometric data on five Seyfert 1 galaxies observed as a part of a recent reverberation mapping program. The data were collected at several observatories over a 140-day span beginning in 2010 August and ending in 2011 January. We obtained high sampling-rate light curves for Mrk 335, Mrk 1501, 3C 120, Mrk 6, and PG 2130+099, from which we have measured the time lag between variations in the 5100Å continuum and the Hβ broad emission line. We then used these measurements to calculate the mass of the supermassive black hole at the center of each of these galaxies. Our new measurements substantially improve previous measurements of M BH and the size of the broad line-emitting region for four sources and add a measurement for one new object. Our new measurements are consistent with photoionization physics regulating the location of the broad line region in active galactic nuclei.

We present the discovery of KELT-1b, the first transiting low-mass companion from the wide-field Kilodegree Extremely Little Telescope-North (KELT-North) transit survey, which surveys ∼ 40% of the northern sky to search for transiting planets around bright stars. The initial transit signal was robustly identified in the KELT-North survey data, and the low-mass nature of the occultor was confirmed via a combination of followup photometry, high-resolution spectroscopy, and radial velocity measurements. False positives are disfavored by the achromaticity of the primary transits in several bands, a lack of evidence for a secondary eclipse, and insignificant bisector variations. A joint analysis of the spectroscopic, radial velocity, and photometric data indicates that the V = 10.7 primary is a mildly evolved mid-F star with T eff = 6518 ± 50 K, log g * = 4.229 +0.012 −0.019