3,828 research outputs found
An Understanding of the Shoulder of Giants: Jovian Planets around Late K Dwarf Stars and the Trend with Stellar Mass
Analyses of exoplanet statistics suggest a trend of giant planet occurrence
with host star mass, a clue to how planets like Jupiter form. One missing piece
of the puzzle is the occurrence around late K dwarf stars (masses of
0.5-0.75Msun and effective temperatures of 3900-4800K). We analyzed four years
of Doppler radial velocities data of 110 late K dwarfs, one of which hosts two
previously reported giant planets. We estimate that 4.0+/-2.3% of these stars
have Saturn-mass or larger planets with orbital periods <245d, depending on the
planet mass distribution and RV variability of stars without giant planets. We
also estimate that 0.7+/-0.5% of similar stars observed by Kepler have giant
planets. This Kepler rate is significantly (99% confidence) lower than that
derived from our Doppler survey, but the difference vanishes if only the single
Doppler system (HIP 57274) with completely resolved orbits is considered. The
difference could also be explained by the exclusion of close binaries (without
giant planets) from the Doppler but not Kepler surveys, the effect of
long-period companions and stellar noise on the Doppler data, or an intrinsic
difference between the two populations. Our estimates for late K dwarfs bridge
those for solar-type stars and M dwarfs and support a positive trend with
stellar mass. Small sample size precludes statements about finer structure,
e.g. a "shoulder" in the distribution of giant planets with stellar mass.
Future surveys such as the Next Generation Transit Survey and the Transiting
Exoplanet Satellite Survey will ameliorate this deficiency.Comment: Accepted to The Astrophysical Journa
Astrophysical Insights into Radial Velocity Jitter from an Analysis of 600 Planet-search Stars
Radial velocity (RV) detection of planets is hampered by astrophysical processes on the surfaces of stars that induce a stochastic signal, or "jitter," which can drown out or even mimic planetary signals. Here, we empirically and carefully measure the RV jitter of more than 600 stars from the California Planet Search sample on a star by star basis. As part of this process, we explore the activityâRV correlation of stellar cycles and include appendices listing every ostensibly companion-induced signal we removed and every activity cycle we noted. We then use precise stellar properties from Brewer et al. to separate the sample into bins of stellar mass and examine trends with activity and with evolutionary state. We find that RV jitter tracks stellar evolution and that in general, stars evolve through different stages of RV jitter: the jitter in younger stars is driven by magnetic activity, while the jitter in older stars is convectively driven and dominated by granulation and oscillations. We identify the "jitter minimum"âwhere activity-driven and convectively driven jitter have similar amplitudesâfor stars between 0.7 and 1.7 Mâ and find that more-massive stars reach this jitter minimum later in their lifetime, in the subgiant or even giant phases. Finally, we comment on how these results can inform future RV efforts, from prioritization of follow-up targets from transit surveys like TESS to target selection of future RV surveys
Some Bright Stars with Smooth Continua for Calibrating the Response of High Resolution Spectrographs
When characterizing a high resolution echelle spectrograph, for instance for
precise Doppler work, it is useful to observe featureless sources such as
quartz lamps or hot stars to determine the response of the instrument. Such
sources provide a way to determine the blaze function of the orders,
pixel-to-pixel variations in the detector, fringing in the system, and other
important characteristics. In practice, however, many B or early A stars do not
provide a smooth continuum, whether because they are not rotating rapidly
enough or for some other reason. In fact, we have found that published
rotational velocities and temperatures are not a specific and sensitive guide
to whether a star's continuum will be smooth. A useful resource for observers,
therefore, is a list of "good" hot stars: bright, blue stars known empirically
to have no lines or other spectral features beyond the Balmer series with
minima below 95% of the continuum.
We have compiled a list of such stars visible from Northern Hemisphere
telescopes. This list includes all stars listed in the Yale Bright Star Catalog
(Hoffleit & Jaschek 1991) as being single with V 175 km/s, and
declination > -30, and many other hot stars that we have found useful for
calibration purposes.
The list here of "bad" stars may also be of interest in studies of hot,
slowly rotating stars
Prevalence of Earth-size planets orbiting Sun-like stars
Determining whether Earth-like planets are common or rare looms as a
touchstone in the question of life in the universe. We searched for Earth-size
planets that cross in front of their host stars by examining the brightness
measurements of 42,000 stars from National Aeronautics and Space
Administration's Kepler mission. We found 603 planets, including 10 that are
Earth size (1-2 Earth-radii) and receive comparable levels of stellar energy to
that of Earth (within a factor of four). We account for Kepler's imperfect
detectability of such planets by injecting synthetic planet-caused dimmings
into the Kepler brightness measurements and recording the fraction detected. We
find that of Sun-like stars harbor an Earth-size planet receiving
between one and four times the stellar intensity as Earth. We also find that
the occurrence of Earth-size planets is constant with increasing orbital period
(P), within equal intervals of logP up to d. Extrapolating, one finds
of Sun-like stars harbor an Earth-size planet with orbital
periods of 200-400 d.Comment: Main text: 6 pages, 5 figures, 1 table. Supporting information: 54
pages, 17 pages, 3 tables. Published in the Proceedings of the National
Academy of Sciences available at
http://www.pnas.org/cgi/doi/10.1073/pnas.131990911
Radial velocities from the N2K Project: 6 new cold gas giant planets orbiting HD 55696, HD 98736, HD 148164, HD 203473, and HD 211810
The N2K planet search program was designed to exploit the planet-metallicity
correlation by searching for gas giant planets orbiting metal-rich stars. Here,
we present the radial velocity measurements for 378 N2K target stars that were
observed with the HIRES spectrograph at Keck Observatory between 2004 and 2017.
With this data set, we announce the discovery of six new gas giant exoplanets:
a double-planet system orbiting HD 148164 ( of 1.23 and 5.16 M) and single planet detections around HD 55696 ( = 3.87 M), HD 98736 ( = 2.33 M), HD 203473 ( = 7.8
M), and HD 211810 ( = 0.67 M). These gas
giant companions have orbital semi-major axes between 1.0 and 6.2 AU and
eccentricities ranging from 0.13 to 0.71. We also report evidence for three
gravitationally bound companions with between 20 to 30 M, placing them in the mass range of brown dwarfs, around HD 148284, HD
214823, and HD 217850, and four low mass stellar companions orbiting HD 3404,
HD 24505, HD 98630, and HD 103459. In addition, we present updated orbital
parameters for 42 previously announced planets. We also report a nondetection
of the putative companion HD 73256 b. Finally, we highlight the most promising
candidates for direct imaging and astrometric detection, and find that many hot
Jupiters from our sample could be detectable by state-of-the-art telescopes
such as Gaia.Comment: Accepted by the Astronomical Journal. 75 pages, 49 figure
Transiting Exoplanet Survey Satellite
The Transiting Exoplanet Survey Satellite (TESS) will search for planets transiting bright and nearby stars. TESS has been selected by NASA for launch in 2017 as an Astrophysics Explorer mission. The spacecraft will be placed into a highly elliptical 13.7-day orbit around the Earth. During its 2-year mission, TESS will employ four wide-field optical charge-coupled device cameras to monitor at least 200,000 main-sequence dwarf stars with I_C â 4 â 13 for temporary drops in brightness caused by planetary transits. Each star will be observed for an interval ranging from 1 month to 1 year, depending mainly on the starâs ecliptic latitude. The longest observing intervals will be for stars near the ecliptic poles, which are the optimal locations for follow-up observations with the James Webb Space Telescope. Brightness measurements of preselected target stars will be recorded every 2 min, and full frame images will be recorded every 30 min. TESS stars will be 10 to 100 times brighter than those surveyed by the pioneering Kepler mission. This will make TESS planets easier to characterize with follow-up observations. TESS is expected to find more than a thousand planets smaller than Neptune, including dozens that are comparable in size to the Earth. Public data releases will occur every 4 months, inviting immediate community-wide efforts to study the new planets. The TESS legacy will be a catalog of the nearest and brightest stars hosting transiting planets, which will endure as highly favorable targets for detailed investigations
Spin-Orbit Alignment for the Circumbinary Planet Host Kepler-16 A
Kepler-16 is an eccentric low-mass eclipsing binary with a circumbinary transiting planet. Here, we investigate the
angular momentum of the primary star, based on Kepler photometry and Keck spectroscopy. The primary starâs
rotation period is 35.1 ± 1.0 days, and its projected obliquity with respect to the stellar binary orbit is 1°.6 ± 2°.4. Therefore, the three largest sources of angular momentumâthe stellar orbit, the planetary orbit, and the primaryâs rotationâare all closely aligned. This finding supports a formation scenario involving accretion from a single disk. Alternatively, tides may have realigned the stars despite their relatively wide separation (0.2 AU), a hypothesis that is supported by the agreement between the measured rotation period and the âpseudosynchronousâ period of tidal evolution theory. The rotation period, chromospheric activity level, and fractional light variations suggest a main-sequence age of 2â4 Gyr. Evolutionary models of low-mass stars can match the observed masses and radii of the primary and secondary stars to within about 3%
Occurrence and core-envelope structure of 1--4x Earth-size planets around Sun-like stars
Small planets, 1-4x the size of Earth, are extremely common around Sun-like
stars, and surprisingly so, as they are missing in our solar system. Recent
detections have yielded enough information about this class of exoplanets to
begin characterizing their occurrence rates, orbits, masses, densities, and
internal structures. The Kepler mission finds the smallest planets to be most
common, as 26% of Sun-like stars have small, 1-2 R_e planets with orbital
periods under 100 days, and 11% have 1-2 R_e planets that receive 1-4x the
incident stellar flux that warms our Earth. These Earth-size planets are
sprinkled uniformly with orbital distance (logarithmically) out to 0.4 AU, and
probably beyond. Mass measurements for 33 transiting planets of 1-4 R_e show
that the smallest of them, R < 1.5 R_e, have the density expected for rocky
planets. Their densities increase with increasing radius, likely caused by
gravitational compression. Including solar system planets yields a relation:
rho = 2.32 + 3.19 R/R_e [g/cc]. Larger planets, in the radius range 1.5-4.0
R_e, have densities that decline with increasing radius, revealing increasing
amounts of low-density material in an envelope surrounding a rocky core,
befitting the appellation "mini-Neptunes." Planets of ~1.5 R_e have the highest
densities, averaging near 10 g/cc. The gas giant planets occur preferentially
around stars that are rich in heavy elements, while rocky planets occur around
stars having a range of heavy element abundances. One explanation is that the
fast formation of rocky cores in protoplanetary disks enriched in heavy
elements permits the gravitational accumulation of gas before it vanishes,
forming giant planets. But models of the formation of 1-4 R_e planets remain
uncertain. Defining habitable zones remains difficult, without benefit of
either detections of life elsewhere or an understanding of life's biochemical
origins.Comment: 11 pages, 6 figures, accepted for publication Proc. Natl. Acad. Sc
Determining the Mass of Kepler-78b With Nonparametric Gaussian Process Estimation
Kepler-78b is a transiting planet that is 1.2 times the radius of Earth and
orbits a young, active K dwarf every 8 hours. The mass of Kepler-78b has been
independently reported by two teams based on radial velocity measurements using
the HIRES and HARPS-N spectrographs. Due to the active nature of the host star,
a stellar activity model is required to distinguish and isolate the planetary
signal in radial velocity data. Whereas previous studies tested parametric
stellar activity models, we modeled this system using nonparametric Gaussian
process (GP) regression. We produced a GP regression of relevant Kepler
photometry. We then use the posterior parameter distribution for our
photometric fit as a prior for our simultaneous GP + Keplerian orbit models of
the radial velocity datasets. We tested three simple kernel functions for our
GP regressions. Based on a Bayesian likelihood analysis, we selected a
quasi-periodic kernel model with GP hyperparameters coupled between the two RV
datasets, giving a Doppler amplitude of 1.86 0.25 m s and
supporting our belief that the correlated noise we are modeling is
astrophysical. The corresponding mass of 1.87 M
is consistent with that measured in previous studies, and more robust due to
our nonparametric signal estimation. Based on our mass and the radius
measurement from transit photometry, Kepler-78b has a bulk density of
6.0 g cm. We estimate that Kepler-78b is 3226% iron
using a two-component rock-iron model. This is consistent with an Earth-like
composition, with uncertainty spanning Moon-like to Mercury-like compositions.Comment: 10 pages, 5 figures, accepted to ApJ 6/16/201
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