We present the Kepler Object of Interest (KOI) catalog of transiting exoplanets based on searching 4 yr of Kepler time series photometry (Data Release 25, Q1–Q17). The catalog contains 8054 KOIs, of which 4034 are planet candidates with periods between 0.25 and 632 days. Of these candidates, 219 are new, including two in multiplanet systems (KOI-82.06 and KOI-2926.05) and 10 high-reliability, terrestrial-size, habitable zone candidates. This catalog was created using a tool called the Robovetter, which automatically vets the DR25 threshold crossing events (TCEs). The Robovetter also vetted simulated data sets and measured how well it was able to separate TCEs caused by noise from those caused by low signal-to-noise transits. We discuss the Robovetter and the metrics it uses to sort TCEs. For orbital periods less than 100 days the Robovetter completeness (the fraction of simulated transits that are determined to be planet candidates) across all observed stars is greater than 85%. For the same period range, the catalog reliability (the fraction of candidates that are not due to instrumental or stellar noise) is greater than 98%. However, for low signal-to-noise candidates between 200 and 500 days around FGK-dwarf stars, the Robovetter is 76.7% complete and the catalog is 50.5% reliable. The KOI catalog, the transit fits, and all of the simulated data used to characterize this catalog are available at the NASA Exoplanet Archive.

We present results from high-resolution, optical to near-IR imaging of host stars of Kepler Objects of Interest (KOIs), identified in the original Kepler field. Part of the data were obtained under the Kepler imaging follow-up observation program over six years (2009–2015). Almost 90% of stars that are hosts to planet candidates or confirmed planets were observed. We combine measurements of companions to KOI host stars from different bands to create a comprehensive catalog of projected separations, position angles, and magnitude differences for all detected companion stars (some of which may not be bound). Our compilation includes 2297 companions around 1903 primary stars. From high-resolution imaging, we find that ∼10% (∼30%) of the observed stars have at least one companion detected within 1″ (4″). The true fraction of systems with close (≲4″) companions is larger than the observed one due to the limited sensitivities of the imaging data. We derive correction factors for planet radii caused by the dilution of the transit depth: assuming that planets orbit the primary stars or the brightest companion stars, the average correction factors are 1.06 and 3.09, respectively. The true effect of transit dilution lies in between these two cases and varies with each system. Applying these factors to planet radii decreases the number of KOI planets with radii smaller than 2 by ∼2%–23% and thus affects planet occurrence rates. This effect will also be important for the yield of small planets from future transit missions such as TESS.

We present the occurrence rates for rocky planets in the habitable zones (HZs) of main-sequence dwarf stars based on the Kepler DR25 planet candidate catalog and Gaia-based stellar properties. We provide the first analysis in terms of star-dependent instellation flux, which allows us to track HZ planets. We define η _⊕ as the HZ occurrence of planets with radii between 0.5 and 1.5 R _⊕ orbiting stars with effective temperatures between 4800 and 6300 K. We find that η _⊕ for the conservative HZ is between (errors reflect 68% credible intervals) and planets per star, while the optimistic HZ occurrence is between and planets per star. These bounds reflect two extreme assumptions about the extrapolation of completeness beyond orbital periods where DR25 completeness data are available. The large uncertainties are due to the small number of detected small HZ planets. We find similar occurrence rates between using Poisson likelihood Bayesian analysis and using Approximate Bayesian Computation. Our results are corrected for catalog completeness and reliability. Both completeness and the planet occurrence rate are dependent on stellar effective temperature. We also present occurrence rates for various stellar populations and planet size ranges. We estimate with 95% confidence that, on average, the nearest HZ planet around G and K dwarfs is ∼6 pc away and there are ∼4 HZ rocky planets around G and K dwarfs within 10 pc of the Sun.

Magnetohydrodynamic (MHD) and photoevaporative winds are thought to play an important role in the evolution and dispersal of planet-forming disks. We report the first high-resolution (Δv ∼ 6 km s⁻¹) analysis of [S ii] λ4068, [O i] λ5577, and [O i] λ6300 lines from a sample of 48 T Tauri stars. Following Simon et al. we decompose them into three kinematic components: a high-velocity component (HVC) associated with jets, and low-velocity narrow (LVC-NC) and broad (LVC-BC) components. We confirm previous findings that many LVCs are blueshifted by more than 1.5 km s⁻¹ and thus most likely trace a slow disk wind. We further show that the profiles of individual components are similar in the three lines. We find that most LVC-NC and LVC-BC line ratios are explained by thermally excited gas with temperatures between 5000 and 10,000 K and electron densities of ∼10⁷–10⁸ cm⁻³. The HVC ratios are better reproduced by shock models with a pre-shock H number density of ∼10⁶–10⁷ cm⁻³. Using these physical properties, we estimate for the LVC and for the HVC. In agreement with previous work, the mass carried out in jets is modest compared to the accretion rate. With the likely assumption that the LVC-NC wind height is larger than the LVC-BC, the LVC-BC is found to be higher than the LVC-NC. These results suggest that most of the mass loss occurs close to the central star, within a few au, through an MHD-driven wind. Depending on the wind height, MHD winds might play a major role in the evolution of the disk mass.

We present a spectroscopic analysis of main sequence (MS) stars in the young globular cluster NGC 1818 (age ∼40 Myr) in the Large Magellanic Cloud. Our photometric survey of Magellanic Clouds clusters has revealed that NGC 1818, similar to other young objects with ages ≲600 Myr, displays not only an extended MS turnoff (eMSTO), as observed in intermediate-age clusters (age ∼1–2 Gyr), but also a split MS. The most straightforward interpretation of the double MS is the presence of two stellar populations: a sequence of slowly rotating stars lying on the blue-MS (bMS) and a sequence of fast rotators, with rotation close to the breaking speed, defining a red-MS (rMS). We report the first direct spectroscopic measurements of projected rotational velocities for the double MS, eMSTO, and Be stars of a young cluster. The analysis of line profiles includes non-local thermodynamic equilibrium effects, required for correctly deriving values. Our results suggest that: (i) the mean rotation for bMS and rMS stars is =71 ± 10 km s⁻¹ (σ = 37 km s⁻¹) and = 202 ± 23 km s⁻¹ (σ = 91 km s⁻¹), respectively; (ii) eMSTO stars have different , which are generally lower than those inferred for rMS stars, and (iii) as expected, Be stars display the highest values. This analysis supports the idea that distinct rotational velocities play an important role in the appearance of multiple stellar populations in the color–magnitude diagrams of young clusters, and poses new constraints on the current scenarios.

A survey of 28 stars (22 active binary systems, plus six single stars or wide binaries for comparison) using extreme ultraviolet spectra has been conducted to establish the structure of stellar coronae in active binary systems from the emission measure distribution (EMD), electron densities, and scale sizes. Observations obtained by the Extreme Ultraviolet Explorer satellite (EUVE) during 9 years of operation are included for the stars in the sample. EUVE data allow a continuous EMD to be constructed in the range log Te(K) ~ 5.6-7.4, using iron emission lines. These data are complemented with IUE observations to model the lower temperature range [log Te(K) ~ 4.0-5.6]. Inspection of the EMD shows an outstanding narrow enhancement, or bump peaking around log Te(K) ~ 6.9 in 25 of the stars, defining a fundamental coronal structure. The emission measure per unit stellar area decreases with increasing orbital (or photometric) periods of the target stars; stars in binaries generally have more material at coronal temperatures than slowly rotating single stars. High electron densities (Ne 1012 cm-3) are derived at ~log Te(K) ~ 7.0 for some targets, implying small emitting volumes. The observations suggest the magnetic stellar coronae of these stars are consistent with two basic classes of magnetic loops: solar-like loops with maximum temperature around log Te(K) ~ 6.3 and lower electron densities (Ne 109-1010.5 cm-3), and hotter loops peaking around log Te(K) ~ 6.9 with higher electron densities (Ne 1012 cm-3). For the most active stars, material exists at much higher temperatures [log Te(K) ≥ 6.9] as well. However, current ab initio stellar loop models cannot reproduce such a configuration. Analysis of the light curves of these systems reveals signatures of rotation of coronal material, as well as apparent seasonal (i.e., year-to-year) changes in the activity levels.