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All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

DETECTION OF SOLAR-LIKE OSCILLATIONS, OBSERVATIONAL CONSTRAINTS, AND STELLAR MODELS FOR θ CYG, THE BRIGHTEST STAR OBSERVED BY THE KEPLER MISSION

θ Cygni is an F3 spectral type magnitude V = 4.48 main-sequence star that was the brightest star observed by the original Kepler spacecraft mission. Short-cadence (58.8 s) photometric data using a custom aperture were first obtained during Quarter 6 (2010 June–September) and subsequently in Quarters 8 and 12–17. We present analyses of solar-like oscillations based on Q6 and Q8 data, identifying angular degree l = 0, 1, and 2 modes with frequencies of 1000–2700 μ Hz, a large frequency separation of 83.9 ± 0.4 μ Hz, and maximum oscillation amplitude at frequency ν {sub max} = 1829 ± 54 μ Hz. We also present analyses of new ground-based spectroscopic observations, which, combined with interferometric angular diameter measurements, give T {sub eff} = 6697 ± 78 K, radius 1.49 ± 0.03 R {sub ⊙}, [Fe/H] = -0.02 ± 0.06 dex, and log g = 4.23 ± 0.03. We calculate stellar models matching these constraints using the Yale Rotating Evolution Code and the Asteroseismic Modeling Portal. The best-fit models have masses of 1.35–1.39 M {sub ⊙} and ages of 1.0–1.6 Gyr. θ Cyg’s T {sub eff} and log g place it cooler than the red edge of the γ Doradus instability region established from pre- Kepler ground-based observations, but just at the red edge derived from pulsation modeling. The pulsation models show γ Dor gravity modes driven by the convective blocking mechanism, with frequencies of 1–3 cycles per day (11 to 33 μ Hz). However, gravity modes were not seen in Kepler data; one signal at 1.776 cycles per day (20.56 μ Hz) may be attributable to a faint, possibly background, binary.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Obliquity Variations of Habitable Zone Planets Kepler-62f and Kepler-186f

Obliquity variability could play an important role in the climate and habitability of a planet. Orbital modulations caused by planetary companions and the planet’s spin axis precession due to the torque from the host star may lead to resonant interactions and cause large-amplitude obliquity variability. Here we consider the spin axis dynamics of Kepler-62f and Kepler-186f, both of which reside in the habitable zone around their host stars. Using N-body simulations and secular numerical integrations, we describe their obliquity evolution for particular realizations of the planetary systems. We then use a generalized analytic framework to characterize regions in parameter space where the obliquity is variable with large amplitude. We find that the locations of variability are fine-tuned over the planetary properties and system architecture in the lower-obliquity regimes (≲40°). As an example, assuming a rotation period of 24 hr, the obliquities of both Kepler-62f and Kepler-186f are stable below ∼40°, whereas the high-obliquity regions (60°–90°) allow moderate variabilities. However, for some other rotation periods of Kepler-62f or Kepler-186f, the lower-obliquity regions could become more variable owing to resonant interactions. Even small deviations from coplanarity (e.g., mutual inclinations ∼3°) could stir peak-to-peak obliquity variations up to ∼20°. Undetected planetary companions and/or the existence of a satellite could also destabilize the low-obliquity regions. In all cases, the high-obliquity region allows for moderate variations, and all obliquities corresponding to retrograde motion (i.e., >90°) are stable.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

Transiting circumbinary planets Kepler-34 b and Kepler-35 b

Most Sun-like stars in the Galaxy reside in gravitationally-bound pairs of stars called 'binary stars'. While long anticipated, the existence of a 'circumbinary planet' orbiting such a pair of normal stars was not definitively established until the discovery of Kepler-16. Incontrovertible evidence was provided by the miniature eclipses ('transits') of the stars by the planet. However, questions remain about the prevalence of circumbinary planets and their range of orbital and physical properties. Here we present two additional transiting circumbinary planets, Kepler-34 and Kepler-35. Each is a low-density gas giant planet on an orbit closely aligned with that of its parent stars. Kepler-34 orbits two Sun-like stars every 289 days, while Kepler-35 orbits a pair of smaller stars (89% and 81% of the Sun's mass) every 131 days. Due to the orbital motion of the stars, the planets experience large multi-periodic variations in incident stellar radiation. The observed rate of circumbinary planets implies > ~1% of close binary stars have giant planets in nearly coplanar orbits, yielding a Galactic population of at least several million.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

A SEMI-ANALYTICAL MODEL OF VISIBLE-WAVELENGTH PHASE CURVES OF EXOPLANETS AND APPLICATIONS TO KEPLER- 7 B AND KEPLER- 10 B

Kepler has detected numerous exoplanet transits by measuring stellar light in a single visible-wavelength band. In addition to detection, the precise photometry provides phase curves of exoplanets, which can be used to study the dynamic processes on these planets. However, the interpretation of these observations can be complicated by the fact that visible-wavelength phase curves can represent both thermal emission and scattering from the planets. Here we present a semi-analytical model framework that can be applied to study Kepler and future visible-wavelength phase curve observations of exoplanets. The model efficiently computes reflection and thermal emission components for both rocky and gaseous planets, considering both homogeneous and inhomogeneous surfaces or atmospheres. We analyze the phase curves of the gaseous planet Kepler- 7 b and the rocky planet Kepler- 10 b using the model. In general, we find that a hot exoplanet’s visible-wavelength phase curve having a significant phase offset can usually be explained by two classes of solutions: one class requires a thermal hot spot shifted to one side of the substellar point, and the other class requires reflective clouds concentrated on the same side of the substellar point. Particularly for Kepler- 7 b, reflective clouds located on the west side of the substellar point can best explain its phase curve. The reflectivity of the clear part of the atmosphere should be less than 7% and that of the cloudy part should be greater than 80%, and the cloud boundary should be located at 11° ± 3° to the west of the substellar point. We suggest single-band photometry surveys could yield valuable information on exoplanet atmospheres and surfaces.

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DOE OSTI2015

Precise radial velocity measurements for Kepler giants hosting planetary candidates: Kepler-91 and KOI-1894

We present results of radial-velocity follow-up observations for the two Kepler evolved stars Kepler-91 (KOI-2133) and KOI-1894, which had been announced as candidates to host transiting giant planets, with the Subaru 8.2 m telescope and the High Dispersion Spectrograph (HDS). By global modeling of the high-precision radial-velocity data taken with Subaru/HDS and photometric data taken by the Kepler mission accounting for orbital brightness modulations (ellipsoidal variations, reflected/emitted light, etc.) of the host stars, we independently confirmed that Kepler-91 hosts a transiting planet with a mass of 0.66 M{sub Jup}(Kepler-91b), and newly detected an offset of ∼20 m s{sup −1} between the radial velocities taken at ∼1 yr interval, suggesting the existence of an additional companion in the system. As for KOI-1894, we detected possible phased variations in the radial velocities and light curves with 2–3σ confidence level, which could be explained as a reflex motion and ellipsoidal variation of the star caused by a transiting sub-Saturn-mass (∼0.18 M{sub Jup}) planet.

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DOE OSTI2017

ROBO-AO KEPLER PLANETARY CANDIDATE SURVEY. III. ADAPTIVE OPTICS IMAGING OF 1629 KEPLER EXOPLANET CANDIDATE HOST STARS

The Robo-AO Kepler Planetary Candidate Survey is observing every Kepler planet candidate host star with laser adaptive optics imaging to search for blended nearby stars, which may be physically associated companions and/or responsible for transit false positives. In this paper, we present the results of our search for stars nearby 1629 Kepler planet candidate hosts. With survey sensitivity to objects as close as ∼0.″15, and magnitude differences Δm ⩽6, we find 223 stars in the vicinity of 206 target KOIs; 209 of these nearby stars have not been previously imaged in high resolution. We measure an overall nearby-star probability for Kepler planet candidates of 12.6%±0.9% at separations between 0.″15 and 4.″0. Particularly interesting KOI systems are discussed, including 26 stars with detected companions that host rocky, habitable zone candidates and five new candidate planet-hosting quadruple star systems. We explore the broad correlations between planetary systems and stellar binarity, using the combined data set of Baranec et al. and this paper. Our previous 2σ result of a low detected nearby star fraction of KOIs hosting close-in giant planets is less apparent in this larger data set. We also find a significant correlation between detected nearby star fraction and KOI number, suggesting possible variation between early and late Kepler data releases.

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DOE OSTI2020

The Gaia–Kepler Stellar Properties Catalog. I. Homogeneous Fundamental Properties for 186,301 Kepler Stars

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 g to K{sub s} colors, are calibrated on stars with interferometric angular diameters. Median catalog uncertainties are 112 K for T{sub eff}, 0.05 dex for log g, 4% for R{sub ⋆}, 7% for M{sub ⋆}, 13% for ρ{sub ⋆}, 10% for L{sub ⋆}, 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.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

KEPLER 453 b—THE 10th KEPLER TRANSITING CIRCUMBINARY PLANET

We present the discovery of Kepler-453 b, a 6.2 R{sub ⊕} planet in a low-eccentricity, 240.5 day orbit about an eclipsing binary. The binary itself consists of a 0.94 and 0.195 M{sub ⊙} pair of stars with an orbital period of 27.32 days. The plane of the planet's orbit is rapidly precessing, and its inclination only becomes sufficiently aligned with the primary star in the latter portion of the Kepler data. Thus three transits are present in the second half of the light curve, but none of the three conjunctions that occurred during the first half of the light curve produced observable transits. The precession period is ∼103 years, and during that cycle, transits are visible only ∼8.9% of the time. This has the important implication that for every system like Kepler-453 that we detect, there are ∼11.5 circumbinary systems that exist but are not currently exhibiting transits. The planet's mass is too small to noticeably perturb the binary, and consequently its mass is not measurable with these data; however, our photodynamical model places a 1σ upper limit of 16 M{sub ⊕}. With a period 8.8 times that of the binary, the planet is well outside the dynamical instability zone. It does, however, lie within the habitable zone of the binary, making it the third of 10 Kepler circumbinary planets to do so.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

STELLAR MAGNETIC CYCLES IN THE SOLAR-LIKE STARS KEPLER-17 AND KEPLER-63

The stellar magnetic field plays a crucial role in the star internal mechanisms, as in the interactions with its environment. The study of starspots provides information about the stellar magnetic field and can characterize the cycle. Moreover, the analysis of solar-type stars is also useful to shed light onto the origin of the solar magnetic field. The objective of this work is to characterize the magnetic activity of stars. Here, we studied two solar-type stars, Kepler-17 and Kepler-63, using two methods to estimate the magnetic cycle length. The first one characterizes the spots (radius, intensity, and location) by fitting the small variations in the light curve of a star caused by the occultation of a spot during a planetary transit. This approach yields the number of spots present in the stellar surface and the flux deficit subtracted from the star by their presence during each transit. The second method estimates the activity from the excess in the residuals of the transit light curves. This excess is obtained by subtracting a spotless model transit from the light curve and then integrating all the residuals during the transit. The presence of long-term periodicity is estimated in both time series. With the first method, we obtained P {sub cycle} = 1.12 ± 0.16 year (Kepler-17) and P {sub cycle} = 1.27 ± 0.16 year (Kepler-63), and for the second approach the values are 1.35 ± 0.27 year and 1.27 ± 0.12 year, respectively. The results of both methods agree with each other and confirm their robustness.

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DOE OSTI2017

The California-Kepler Survey. I. High-resolution Spectroscopy of 1305 Stars Hosting Kepler Transiting Planets

The California-Kepler Survey (CKS) is an observational program developed to improve our knowledge of the properties of stars found to host transiting planets by NASA’s Kepler Mission. The improvement stems from new high-resolution optical spectra obtained using HIRES at the W. M. Keck Observatory. The CKS stellar sample comprises 1305 stars classified as Kepler objects of interest, hosting a total of 2075 transiting planets. The primary sample is magnitude-limited (Kp<14.2) and contains 960 stars with 1385 planets. The sample was extended to include some fainter stars that host multiple planets, ultra-short period planets, or habitable zone planets. The spectroscopic parameters were determined with two different codes, one based on template matching and the other on direct spectral synthesis using radiative transfer. We demonstrate a precision of 60 K in T{sub eff}, 0.10 dex in logg, 0.04 dex in [Fe/H], and 1.0 km s{sup −1} in Vsin i. In this paper, we describe the CKS project and present a uniform catalog of spectroscopic parameters. Subsequent papers in this series present catalogs of derived stellar properties such as mass, radius, and age; revised planet properties; and statistical explorations of the ensemble. CKS is the largest survey to determine the properties of Kepler stars using a uniform set of high-resolution, high signal-to-noise ratio spectra. The HIRES spectra are available to the community for independent analyses.

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DOE OSTI2020

Kepler-1661 b: A Neptune-sized Kepler Transiting Circumbinary Planet around a Grazing Eclipsing Binary

We report the discovery of a Neptune-sized (R{sub p}=3.87±0.06R{sub ⊕}) transiting circumbinary planet, Kepler-1661 b, found in the Kepler photometry. The planet has a period of ∼175 days and its orbit precesses with a period of only 35 yr. The precession causes the alignment of the orbital planes to vary, and the planet is in a transiting configuration only ∼7% of the time as seen from Earth. As with several other Kepler circumbinary planets, Kepler-1661 b orbits close to the stability radius, and is near the (hot) edge of the habitable zone. The planet orbits a single-lined, grazing eclipsing binary, containing a 0.84 M{sub ⊙} and 0.26 M{sub ⊙} pair of stars in a mildly eccentric (e = 0.11), 28.2 day orbit. The system is fairly young, with an estimated age of ∼1–3 Gyr, and exhibits significant starspot modulations. The grazing eclipse configuration means the system is very sensitive to changes in the binary inclination, which manifests itself as a change in the eclipse depth. The starspots contaminate the eclipse photometry, but not in the usual way of inducing spurious eclipse timing variations. Rather, the starspots alter the normalization of the light curve, and hence the eclipse depths. This can lead to spurious eclipse depth variations, which are then incorrectly ascribed to binary orbital precession.

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DOE OSTI2017

The Metallicity Distribution and Hot Jupiter Rate of the Kepler Field: Hectochelle High-resolution Spectroscopy for 776 Kepler Target Stars

The occurrence rate of hot Jupiters from the Kepler transit survey is roughly half that of radial velocity surveys targeting solar neighborhood stars. One hypothesis to explain this difference is that the two surveys target stars with different stellar metallicity distributions. To test this hypothesis, we measure the metallicity distribution of the Kepler targets using the Hectochelle multi-fiber, high-resolution spectrograph. Limiting our spectroscopic analysis to 610 dwarf stars in our sample with logg > 3.5, we measure a metallicity distribution characterized by a mean of [M/H]{sub mean}=−0.045±0.009, in agreement with previous studies of the Kepler field target stars. In comparison, the metallicity distribution of the California Planet Search radial velocity sample has a mean of [M/H]{sub CPS,mean}=−0.005±0.006, and the samples come from different parent populations according to a Kolmogorov–Smirnov test. We refit the exponential relation between the fraction of stars hosting a close-in giant planet and the host star metallicity using a sample of dwarf stars from the California Planet Search with updated metallicities. The best-fit relation tells us that the difference in metallicity between the two samples is insufficient to explain the discrepant hot Jupiter occurrence rates; the metallicity difference would need to be ≃0.2–0.3 dex for perfect agreement. We also show that (sub)giant contamination in the Kepler sample cannot reconcile the two occurrence calculations. We conclude that other factors, such as binary contamination and imperfect stellar properties, must also be at play.

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NASA NTRS2010

The Kepler End-to-End Model: Creating High-Fidelity Simulations to Test Kepler Ground Processing

The Kepler mission is designed to detect the transit of Earth-like planets around Sun-like stars by observing 100,000 stellar targets. Developing and testing the Kepler ground-segment processing system, in particular the data analysis pipeline, requires high-fidelity simulated data. This simulated data is provided by the Kepler End-to-End Model (ETEM). ETEM simulates the astrophysics of planetary transits and other phenomena, properties of the Kepler spacecraft and the format of the downlinked data. Major challenges addressed by ETEM include the rapid production of large amounts of simulated data, extensibility and maintainability.

Bryson, Stephen T.↗
DOE OSTI2012

KEPLER ECLIPSING BINARY STARS. III. CLASSIFICATION OF KEPLER ECLIPSING BINARY LIGHT CURVES WITH LOCALLY LINEAR EMBEDDING

We present an automated classification of 2165 Kepler eclipsing binary (EB) light curves that accompanied the second Kepler data release. The light curves are classified using locally linear embedding, a general nonlinear dimensionality reduction tool, into morphology types (detached, semi-detached, overcontact, ellipsoidal). The method, related to a more widely used principal component analysis, produces a lower-dimensional representation of the input data while preserving local geometry and, consequently, the similarity between neighboring data points. We use this property to reduce the dimensionality in a series of steps to a one-dimensional manifold and classify light curves with a single parameter that is a measure of 'detachedness' of the system. This fully automated classification correlates well with the manual determination of morphology from the data release, and also efficiently highlights any misclassified objects. Once a lower-dimensional projection space is defined, the classification of additional light curves runs in a negligible time and the method can therefore be used as a fully automated classifier in pipeline structures. The classifier forms a tier of the Kepler EB pipeline that pre-processes light curves for the artificial intelligence based parameter estimator.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

ASTEROSEISMIC DETERMINATION OF OBLIQUITIES OF THE EXOPLANET SYSTEMS KEPLER-50 AND KEPLER-65

Results on the obliquity of exoplanet host stars-the angle between the stellar spin axis and the planetary orbital axis-provide important diagnostic information for theories describing planetary formation. Here we present the first application of asteroseismology to the problem of stellar obliquity determination in systems with transiting planets and Sun-like host stars. We consider two systems observed by the NASA Kepler mission which have multiple transiting small (super-Earth sized) planets: the previously reported Kepler-50 and a new system, Kepler-65, whose planets we validate in this paper. Both stars show rich spectra of solar-like oscillations. From the asteroseismic analysis we find that each host has its rotation axis nearly perpendicular to the line of sight with the sines of the angles constrained at the 1{sigma} level to lie above 0.97 and 0.91, respectively. We use statistical arguments to show that coplanar orbits are favored in both systems, and that the orientations of the planetary orbits and the stellar rotation axis are correlated.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Experimental limits on primordial black hole dark matter from the first 2 yr of Kepler data

We present our analysis on new limits of the dark matter (DM) halo consisting of primordial black holes (PBHs) or massive compact halo objects. We present a search of the first two yr of publicly available Kepler mission data for potential signatures of gravitational microlensing caused by these objects as well as an extensive analysis of the astrophysical sources of background error. These include variable stars, flare events, and comets or asteroids that are moving through the Kepler field. We discuss the potential of detecting comets using the Kepler light curves, presenting measurements of two known comets and one unidentified object, most likely an asteroid or comet. After removing the background events with statistical cuts, we find no microlensing candidates. We therefore present our Monte Carlo efficiency calculation in order to constrain the PBH DM with masses in the range of 2 × 10{sup –9} M {sub ☉} to 10{sup –7} M {sub ☉}. We find that PBHs in this mass range cannot make up the entirety of the DM, thus closing a full order of magnitude in the allowed mass range for PBH DM.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Architecture of Kepler's multi-transiting systems. II. New investigations with twice as many candidates

We report on the orbital architectures of Kepler systems having multiple-planet candidates identified in the analysis of data from the first six quarters of Kepler data and reported by Batalha et al. (2013). These data show 899 transiting planet candidates in 365 multiple-planet systems and provide a powerful means to study the statistical properties of planetary systems. Using a generic mass-radius relationship, we find that only two pairs of planets in these candidate systems (out of 761 pairs total) appear to be on Hill-unstable orbits, indicating ∼96% of the candidate planetary systems are correctly interpreted as true systems. We find that planet pairs show little statistical preference to be near mean-motion resonances. We identify an asymmetry in the distribution of period ratios near first-order resonances (e.g., 2:1, 3:2), with an excess of planet pairs lying wide of resonance and relatively few lying narrow of resonance. Finally, based upon the transit duration ratios of adjacent planets in each system, we find that the interior planet tends to have a smaller transit impact parameter than the exterior planet does. This finding suggests that the mode of the mutual inclinations of planetary orbital planes is in the range 1.°0-2.°2, for the packed systems of small planets probed by these observations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

CHARACTERIZING THE COOL KEPLER OBJECTS OF INTERESTS. NEW EFFECTIVE TEMPERATURES, METALLICITIES, MASSES, AND RADII OF LOW-MASS KEPLER PLANET-CANDIDATE HOST STARS

We report stellar parameters for late-K and M-type planet-candidate host stars announced by the Kepler Mission. We obtained medium-resolution, K-band spectra of 84 cool (T{sub eff} {approx}< 4400 K) Kepler Objects of Interest (KOIs) from Borucki et al. We identified one object as a giant (KOI 977); for the remaining dwarfs, we measured effective temperatures (T{sub eff}) and metallicities [M/H] using the K-band spectral indices of Rojas-Ayala et al. We determine the masses (M{sub *}) and radii (R{sub *}) of the cool KOIs by interpolation onto the Dartmouth evolutionary isochrones. The resultant stellar radii are significantly less than the values reported in the Kepler Input Catalog and, by construction, correlate better with T{sub eff}. Applying the published KOI transit parameters to our stellar radius measurements, we report new physical radii for the planet candidates. Recalculating the equilibrium temperatures of the planet-candidates assuming Earth's albedo and re-radiation fraction, we find that three of the planet-candidates are terrestrial sized with orbital semimajor axes that lie within the habitable zones of their host stars (KOI 463.01, KOI 812.03, and KOI 854.01). The stellar parameters presented in this Letter serve as a resource for prioritization of future follow-up efforts to validate and characterize the cool KOI planet candidates.

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