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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 829 records · Page 46
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DOE OSTI2012

THE PHOTOECCENTRIC EFFECT AND PROTO-HOT JUPITERS. II. KOI-1474.01, A CANDIDATE ECCENTRIC PLANET PERTURBED BY AN UNSEEN COMPANION

The exoplanets known as hot Jupiters-Jupiter-sized planets with periods of less than 10 days-likely are relics of dynamical processes that shape all planetary system architectures. Socrates et al. argued that high eccentricity migration (HEM) mechanisms proposed for situating these close-in planets should produce an observable population of highly eccentric proto-hot Jupiters that have not yet tidally circularized. HEM should also create failed-hot Jupiters, with periapses just beyond the influence of fast circularization. Using the technique we previously presented for measuring eccentricities from photometry (the ''photoeccentric effect''), we are distilling a collection of eccentric proto- and failed-hot Jupiters from the Kepler Objects of Interest (KOI). Here, we present the first, KOI-1474.01, which has a long orbital period (69.7340 days) and a large eccentricity e 0.81{sup +0.10}{sub -0.07}, skirting the proto-hot Jupiter boundary. Combining Kepler photometry, ground-based spectroscopy, and stellar evolution models, we characterize host KOI-1474 as a rapidly rotating F star. Statistical arguments reveal that the transiting candidate has a low false-positive probability of 3.1%. KOI-1474.01 also exhibits transit-timing variations of the order of an hour. We explore characteristics of the third-body perturber, which is possibly the ''smoking-gun'' cause of KOI-1474.01's large eccentricity. We use the host star's period, radius, and projected rotational velocity to measure the inclination of the stellar spin. Comparing KOI 1474.01's inclination, we find that its orbit is marginally consistent with being aligned with the stellar spin axis, although a reanalysis is warranted with future additional data. Finally, we discuss how the number and existence of proto-hot Jupiters will not only demonstrate that hot Jupiters migrate via HEM, but also shed light on the typical timescale for the mechanism.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

ON THE SURVIVABILITY AND METAMORPHISM OF TIDALLY DISRUPTED GIANT PLANETS: THE ROLE OF DENSE CORES

A large population of planetary candidates in short-period orbits have been found recently through transit searches, mostly with the Kepler mission. Radial velocity surveys have also revealed several Jupiter-mass planets with highly eccentric orbits. Measurements of the Rossiter-McLaughlin effect indicate that the orbital angular momentum vector of some planets is inclined relative to the spin axis of their host stars. This diversity could be induced by post-formation dynamical processes such as planet-planet scattering, the Kozai effect, or secular chaos which brings planets to the vicinity of their host stars. In this work, we propose a novel mechanism to form close-in super-Earths and Neptune-like planets through the tidal disruption of gas giant planets as a consequence of these dynamical processes. We model the core-envelope structure of gas giant planets with composite polytropes which characterize the distinct chemical composition of the core and envelope. Using three-dimensional hydrodynamical simulations of close encounters between Jupiter-like planets and their host stars, we find that the presence of a core with a mass more than 10 times that of the Earth can significantly increase the fraction of envelope which remains bound to it. After the encounter, planets with cores are more likely to be retained by their host stars in contrast with previous studies which suggested that coreless planets are often ejected. As a substantial fraction of their gaseous envelopes is preferentially lost while the dense incompressible cores retain most of their original mass, the resulting metallicity of the surviving planets is increased. Our results suggest that some gas giant planets can be effectively transformed into either super-Earths or Neptune-like planets after multiple close stellar passages. Finally, we analyze the orbits and structure of known planets and Kepler candidates and find that our model is capable of producing some of the shortest-period objects.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

ON THE HABITABLE ZONES OF CIRCUMBINARY PLANETARY SYSTEMS

The effect of the stellar flux on exoplanetary systems is becoming an increasingly important property as more planets are discovered in the habitable zone (HZ). The Kepler mission has recently uncovered circumbinary planets with relatively complex HZs due to the combined flux from the binary host stars. Here, we derive HZ boundaries for circumbinary systems and show their dependence on the stellar masses, separation, and time while accounting for binary orbital motion and the orbit of the planet. We include stability regimes for planetary orbits in binary systems with respect to the HZ. These methods are applied to several of the known circumbinary planetary systems such as Kepler-16, 34, 35, and 47. We also quantitatively show the circumstances under which single-star approximations break down for HZ calculations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

THE TRIPLY ECLIPSING HIERARCHICAL TRIPLE STAR KIC002856960

In a recent study, Armstrong et al. presented an eclipsing binary star of about 6.2 hr period with transit-like tertiary signals occurring every 204.2 days in the Kepler public data of KIC002856960 and proposed three possible hierarchical structures: (AB)b, (AB)C, and A(BC). We analyzed the Kepler light curve by including a third light source and one starspot on each binary component. The results represent that the close eclipsing pair is in a low-mass eccentric-orbit, detached configuration. Based on 123 eclipse timings calculated from the Wilson-Devinney binary model, a period study of the close binary reveals that the orbital period has experienced a sinusoidal variation with a period and a semi-amplitude of 205 {+-} 2 days and 0.0021 {+-} 0.0002 days, respectively. The period variation would be produced by the light-travel-time effect due to a gravitationally bound third body with a minimum mass of M {sub 3}sin i {sub 3} = 0.76 M {sub Sun} in an eccentric orbit of e {sub 3} = 0.61. This is consistent with the presence of third light found in our light curve solution and the tertiary signal of 204.2 day period most likely arises from the K-type star crossed by the close eclipsing binary. Then, KIC002856960 is a triply eclipsing hierarchical system, A(BC), consisting of a close binary with two M-type dwarfs and a more massive K-type component. The presence of the third star may have played an important role in the formation and evolution of the close pair, which may ultimately evolve into a contact system by angular momentum loss.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

IMPROVED THEORETICAL PREDICTIONS OF MICROLENSING RATES FOR THE DETECTION OF PRIMORDIAL BLACK HOLE DARK MATTER

Primordial black holes (PBHs) remain a dark matter (DM) candidate of the Standard Model of Particle Physics. Previously, we proposed a new method of constraining the remaining PBH DM mass range using microlensing of stars monitored by NASA's Kepler mission. We improve this analysis using a more accurate treatment of the population of the Kepler source stars, their variability, and limb darkening. We extend the theoretically detectable PBH DM mass range down to 2 Multiplication-Sign 10{sup -10} M{sub Sun }, two orders of magnitude below current limits and one-third order of magnitude below our previous estimate. We address how to extract the DM properties, such as mass and spatial distribution, if PBH microlensing events were detected. We correct an error in a well-known finite-source limb-darkening microlensing formula and also examine the effects of varying the light curve cadence on PBH DM detectability. We also introduce an approximation for estimating the predicted rate of detection per star as a function of the star's properties, thus allowing for selection of source stars in future missions, and extend our analysis to planned surveys, such as the Wide-Field Infrared Survey Telescope.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

THE MASS OF KOI-94d AND A RELATION FOR PLANET RADIUS, MASS, AND INCIDENT FLUX

We measure the mass of a modestly irradiated giant planet, KOI-94d. We wish to determine whether this planet, which is in a 22 day orbit and receives 2700 times as much incident flux as Jupiter, is as dense as Jupiter or rarefied like inflated hot Jupiters. KOI-94 also hosts at least three smaller transiting planets, all of which were detected by the Kepler mission. With 26 radial velocities of KOI-94 from the W. M. Keck Observatory and a simultaneous fit to the Kepler light curve, we measure the mass of the giant planet and determine that it is not inflated. Support for the planetary interpretation of the other three candidates comes from gravitational interactions through transit timing variations, the statistical robustness of multi-planet systems against false positives, and several lines of evidence that no other star resides within the photometric aperture. We report the properties of KOI-94b (M{sub P} = 10.5 {+-} 4.6 M{sub Circled-Plus }, R{sub P} = 1.71 {+-} 0.16 R{sub Circled-Plus }, P = 3.74 days), KOI-94c (M{sub P} = 15.6{sup +5.7}{sub -15.6} M{sub Circled-Plus }, R{sub P} = 4.32 {+-} 0.41 R{sub Circled-Plus }, P = 10.4 days), KOI-94d (M{sub P} = 106 {+-} 11 M{sub Circled-Plus }, R{sub P} = 11.27 {+-} 1.06 R{sub Circled-Plus }, P = 22.3 days), and KOI-94e (M{sub P} = 35{sup +18}{sub -28} M{sub Circled-Plus }, R{sub P} = 6.56 {+-} 0.62 R{sub Circled-Plus }, P = 54.3 days). The radial velocity analyses of KOI-94b and KOI-94e offer marginal (>2{sigma}) mass detections, whereas the observations of KOI-94c offer only an upper limit to its mass. Using the KOI-94 system and other planets with published values for both mass and radius (138 exoplanets total, including 35 with M{sub P} < 150 M{sub Circled-Plus }), we establish two fundamental planes for exoplanets that relate their mass, incident flux, and radius from a few Earth masses up to 13 Jupiter masses: (R{sub P}/R{sub Circled-Plus }) = 1.78(M{sub P}/M{sub Circled-Plus }){sup 0.53}(F/erg s{sup -1} cm{sup -2}){sup -0.03} for M{sub P} < 150 M{sub Circled-Plus }, and R{sub P}/R{sub Circled-Plus} = 2.45(M{sub P}/M{sub Circled-Plus }){sup -0.039}(F/erg s{sup -1} cm{sup -2}){sup 0.094} for M{sub P} > 150 M{sub Circled-Plus }. These equations can be used to predict the radius or mass of a planet.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

OGLE-2011-BLG-0417: A RADIAL VELOCITY TESTBED FOR MICROLENSING

Microlensing experiments are returning increasingly detailed information about the planetary and binary systems that are being detected, far beyond what was originally expected. In several cases the lens mass and distance are measured, and a few very special cases have yielded complete eight-parameter Kepler solutions, i.e., the masses of both components, five Kepler invariants, and the phase. We identify one such case that is suitable for a precision test that could be carried out by comparing Doppler radial velocity (RV) measurements with the predictions from the microlensing solution. The lens primary is reasonably bright (I = 16.3, V = 18.2) and is expected to have a relatively large RV semi-amplitude (K {approx} 6.35 km s{sup -1}).

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

STARS DO NOT EAT THEIR YOUNG MIGRATING PLANETS: EMPIRICAL CONSTRAINTS ON PLANET MIGRATION HALTING MECHANISMS

The discovery of ''hot Jupiters'' very close to their parent stars confirmed that Jovian planets migrate inward via several potential mechanisms. We present empirical constraints on planet migration halting mechanisms. We compute model density functions of close-in exoplanets in the orbital semi-major axis-stellar mass plane to represent planet migration that is halted via several mechanisms, including the interior 1:2 resonance with the magnetospheric disk truncation radius, the interior 1:2 resonance with the dust sublimation radius, and several scenarios for tidal halting. The models differ in the predicted power-law dependence of the exoplanet orbital semi-major axis as a function of stellar mass, and thus we also include a power-law model with the exponent as a free parameter. We use a Bayesian analysis to assess the model success in reproducing empirical distributions of confirmed exoplanets and Kepler candidates that orbit interior to 0.1 AU. Our results confirm a correlation of the halting distance with stellar mass. Tidal halting provides the best fit to the empirical distribution of confirmed Jovian exoplanets at a statistically robust level, consistent with the Kozai mechanism and the spin-orbit misalignment of a substantial fraction of hot Jupiters. We can rule out migration halting at the interior 1:2 resonances with the magnetospheric disk truncation radius and the interior 1:2 resonance with the dust disk sublimation radius, a uniform random distribution, and a distribution with no dependence on stellar mass. Note that our results do not rule out Type-II migration, but rather eliminate the role of a circumstellar disk in stopping exoplanet migration. For Kepler candidates, which have a more restricted range in stellar mass compared to confirmed planets, we are unable to discern between the tidal dissipation and magnetospheric disk truncation braking mechanisms at a statistically significant level. The power-law model favors exponents in the range of 0.38-0.9. This is larger than that predicted for tidal halting (0.23-0.33), which suggests that additional physics may be missing in the tidal halting theory.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

LOW STELLAR OBLIQUITIES IN COMPACT MULTIPLANET SYSTEMS

We measure the sky-projected stellar obliquities ({lambda}) in the multiple-transiting planetary systems KOI-94 and Kepler-25, using the Rossiter-McLaughlin effect. In both cases, the host stars are well aligned with the orbital planes of the planets. For KOI-94 we find {lambda} = -11 Degree-Sign {+-} 11 Degree-Sign , confirming a recent result by Hirano and coworkers. Kepler-25 was a more challenging case, because the transit depth is unusually small (0.13%). To obtain the obliquity, it was necessary to use prior knowledge of the star's projected rotation rate and apply two different analysis methods to independent wavelength regions of the spectra. The two methods gave consistent results, {lambda} = 7 Degree-Sign {+-} 8 Degree-Sign and -0. Degree-Sign 5 {+-} 5. Degree-Sign 7. There are now a total of five obliquity measurements for host stars of systems of multiple-transiting planets, all of which are consistent with spin-orbit alignment. This alignment is unlikely to be the result of tidal interactions because of the relatively large orbital distances and low planetary masses in the systems. In this respect, the multiplanet host stars differ from hot-Jupiter host stars, which commonly have large spin-orbit misalignments whenever tidal interactions are weak. In particular, the weak-tide subset of hot-Jupiter hosts has obliquities consistent with an isotropic distribution (p = 0.6), but the multiplanet hosts are incompatible with such a distribution (p {approx} 10{sup -6}). This suggests that high obliquities are confined to hot-Jupiter systems, and provides further evidence that hot-Jupiter formation involves processes that tilt the planetary orbit.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

TRANSITS AND OCCULTATIONS OF AN EARTH-SIZED PLANET IN AN 8.5 hr ORBIT

We report the discovery of an Earth-sized planet (1.16 {+-} 0.19 R{sub Circled-Plus }) in an 8.5 hr orbit around a late G-type star (KIC 8435766, Kepler-xx). The object was identified in a search for short-period planets in the Kepler database and confirmed to be a transiting planet (as opposed to an eclipsing stellar system) through the absence of ellipsoidal light variations or substantial radial-velocity variations. The unusually short orbital period and the relative brightness of the host star (m{sub Kep} = 11.5) enable robust detections of the changing illumination of the visible hemisphere of the planet, as well as the occultations of the planet by the star. We interpret these signals as representing a combination of reflected and reprocessed light, with the highest planet dayside temperature in the range of 2300 K-3100 K. Follow-up spectroscopy combined with finer sampling photometric observations will further pin down the system parameters and may even yield the mass of the planet.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Inside-out planet formation

The compact multi-transiting planet systems discovered by Kepler challenge planet formation theories. Formation in situ from disks with radial mass surface density, Σ, profiles similar to the minimum mass solar nebula but boosted in normalization by factors ≳ 10 has been suggested. We propose that a more natural way to create these planets in the inner disk is formation sequentially from the inside-out via creation of successive gravitationally unstable rings fed from a continuous stream of small (∼cm-m size) 'pebbles', drifting inward via gas drag. Pebbles collect at the pressure maximum associated with the transition from a magnetorotational instability (MRI)-inactive ('dead zone') region to an inner MRI-active zone. A pebble ring builds up until it either becomes gravitationally unstable to form an ∼1 M {sub ⊕} planet directly or induces gradual planet formation via core accretion. The planet may undergo Type I migration into the active region, allowing a new pebble ring and planet to form behind it. Alternatively, if migration is inefficient, the planet may continue to accrete from the disk until it becomes massive enough to isolate itself from the accretion flow. A variety of densities may result depending on the relative importance of residual gas accretion as the planet approaches its isolation mass. The process can repeat with a new pebble ring gathering at the new pressure maximum associated with the retreating dead-zone boundary. Our simple analytical model for this scenario of inside-out planet formation yields planetary masses, relative mass scalings with orbital radius, and minimum orbital separations consistent with those seen by Kepler. It provides an explanation of how massive planets can form with tightly packed and well-aligned system architectures, starting from typical protoplanetary disk properties.

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DOE OSTI2014

Habitable zone dependence on stellar parameter uncertainties

An important property of exoplanetary systems is the extent of the Habitable Zone (HZ), defined as that region where water can exist in a liquid state on the surface of a planet with sufficient atmospheric pressure. Both ground- and space-based observations have revealed a plethora of confirmed exoplanets and exoplanetary candidates, most notably from the Kepler mission using the transit detection technique. Many of these detected planets lie within the predicted HZ of their host star. However, as is the case with the derived properties of the planets themselves, the HZ boundaries depend on how well we understand the host star. Here we quantify the uncertainties of HZ boundaries on the parameter uncertainties of the host star. We examine the distribution of stellar parameter uncertainties from confirmed exoplanet hosts and Kepler candidate hosts and translate these into HZ boundary uncertainties. We apply this to several known systems with an HZ planet to determine the uncertainty in their HZ status.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

KOI-2700b—a planet candidate with dusty effluents on a 22 hr orbit

Kepler planet candidate KOI-2700b (KIC 8639908b), with an orbital period of 21.84 hr, exhibits a distinctly asymmetric transit profile, likely indicative of the emission of dusty effluents, and reminiscent of KIC 1255b. The host star has T {sub eff} = 4435 K, M ≅ 0.63 M {sub ☉}, and R ≅ 0.57 R {sub ☉}, comparable to the parameters ascribed to KIC 12557548. The transit egress can be followed for ∼25% of the orbital period and, if interpreted as extinction from a dusty comet-like tail, indicates a long lifetime for the dust grains of more than a day. We present a semiphysical model for the dust tail attenuation and fit for the physical parameters contained in that expression. The transit is not sufficiently deep to allow for a study of the transit-to-transit variations, as is the case for KIC 1255b; however, it is clear that the transit depth is slowly monotonically decreasing by a factor of ∼2 over the duration of the Kepler mission. We infer a mass-loss rate in dust from the planet of ∼2 lunar masses per Gyr. The existence of a second star hosting a planet with a dusty comet-like tail would help to show that such objects may be more common and less exotic than originally thought. According to current models, only quite small planets with M{sub p} ≲ 0.03 M {sub ⊕} are likely to release a detectable quantity of dust. Thus, any 'normal-looking' transit that is inferred to arise from a rocky planet of radius greater than ∼1/2 R {sub ⊕} should not exhibit any hint of a dusty tail. Conversely, if one detects an asymmetric transit due to a dusty tail, then it will be very difficult to detect the hard body of the planet within the transit because, by necessity, the planet must be quite small (i.e., ≲ 0.3 R {sub ⊕}).

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Surface activity and oscillation amplitudes of red giants in eclipsing binaries

Among the 19 red-giant stars belonging to eclipsing binary systems that have been identified in Kepler data, 15 display solar-like oscillations. We study whether the absence of mode detection in the remaining 4 is an observational bias or possibly evidence of mode damping that originates from tidal interactions. A careful analysis of the corresponding Kepler light curves shows that modes with amplitudes that are usually observed in red giants would have been detected if they were present. We observe that mode depletion is strongly associated with short-period systems, in which stellar radii account for 16%-24% of the semi-major axis, and where red-giant surface activity is detected. We suggest that when the rotational and orbital periods synchronize in close binaries, the red-giant component is spun up, so that a dynamo mechanism starts and generates a magnetic field, leading to observable stellar activity. Pressure modes would then be damped as acoustic waves dissipate in these fields.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Strömgren survey for asteroseismology and galactic archaeology: Let the saga begin

Asteroseismology has the capability of precisely determining stellar properties that would otherwise be inaccessible, such as radii, masses, and thus ages of stars. When coupling this information with classical determinations of stellar parameters, such as metallicities, effective temperatures, and angular diameters, powerful new diagnostics for Galactic studies can be obtained. The ongoing Strömgren survey for Asteroseismology and Galactic Archaeology has the goal of transforming the Kepler field into a new benchmark for Galactic studies, similar to the solar neighborhood. Here we present the first results from a stripe centered at a Galactic longitude of 74° and covering latitude from about 8° to 20°, which includes almost 1000 K giants with seismic information and the benchmark open cluster NGC 6819. We describe the coupling of classical and seismic parameters, the accuracy as well as the caveats of the derived effective temperatures, metallicities, distances, surface gravities, masses, and radii. Confidence in the achieved precision is corroborated by the detection of the first and secondary clumps in a population of field stars with a ratio of 2 to 1 and by the negligible scatter in the seismic distances among NGC 6819 member stars. An assessment of the reliability of stellar parameters in the Kepler Input Catalog is also performed, and the impact of our results for population studies in the Milky Way is discussed, along with the importance of an all-sky Strömgren survey.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Stellar rotation-planetary orbit period commensurability in the HAT-P-11 system

A number of planet host stars have been observed to rotate with a period equal to an integer multiple of the orbital period of their close planet. We expand this list by analyzing Kepler data of HAT-P-11 and finding a period ratio of 6:1. In particular, we present evidence for a long-lived spot on the stellar surface that is eclipsed by the planet in the same position four times, every sixth transit. We also identify minima in the out-of-transit light curve and confirm that their phase with respect to the stellar rotation is mostly stationary for the 48 month time frame of the observations, confirming the proposed rotation period. For comparison, we apply our methods to Kepler-17 and confirm the findings of Bonomo and Lanza that the period ratio is not exactly 8:1 in that system. Finally, we provide a hypothesis on how interactions between a star and its planet could possibly result in an observed commensurability for systems where the stellar differential rotation profile happens to include a period at some latitude that is commensurable to the planetary orbit.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Angular momentum transport within evolved low-mass stars

Asteroseismology of 1.0-2.0 M {sub ☉} red giants by the Kepler satellite has enabled the first definitive measurements of interior rotation in both first ascent red giant branch (RGB) stars and those on the helium burning clump. The inferred rotation rates are 10-30 days for the ≈0.2 M {sub ☉} He degenerate cores on the RGB and 30-100 days for the He burning core in a clump star. Using the Modules for Experiments in Stellar Evolution code, we calculate state-of-the-art stellar evolution models of low mass rotating stars from the zero-age main sequence to the cooling white dwarf (WD) stage. We include transport of angular momentum due to rotationally induced instabilities and circulations, as well as magnetic fields in radiative zones (generated by the Tayler-Spruit dynamo). We find that all models fail to predict core rotation as slow as observed on the RGB and during core He burning, implying that an unmodeled angular momentum transport process must be operating on the early RGB of low mass stars. Later evolution of the star from the He burning clump to the cooling WD phase appears to be at nearly constant core angular momentum. We also incorporate the adiabatic pulsation code, ADIPLS, to explicitly highlight this shortfall when applied to a specific Kepler asteroseismic target, KIC8366239.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Asteroseismic estimate of helium abundance of a solar analog binary system

16 Cyg A and B are among the brightest stars observed by Kepler. What makes these stars more interesting is that they are solar analogs. 16 Cyg A and B exhibit solar-like oscillations. In this work we use oscillation frequencies obtained using 2.5 yr of Kepler data to determine the current helium abundance of these stars. For this we use the fact that the helium ionization zone leaves a signature on the oscillation frequencies and that this signature can be calibrated to determine the helium abundance of that layer. By calibrating the signature of the helium ionization zone against models of known helium abundance, the helium abundance in the envelope of 16 Cyg A is found to lie in the range of 0.231 to 0.251 and that of 16 Cyg B lies in the range of 0.218 to 0.266.

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