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Ronald L. Gilliland,
William J. Chaplin,
Edward W. Dunham,
Vic S. Argabright,
William J. Borucki, Gibor Basri,
Stephen T. Bryson,
Derek L. Buzasi,
Douglas A. Caldwell,
Yvonne P. Elsworth,
Jon M. Jenkins,
David G. Koch,
Jeffrey Kolodziejczak,
Andrea Miglio,
Jeffrey van Cleve,
Lucianne M. Walkowicz,
and William F. Welsh
[show abstract]
[hide abstract]
ABSTRACT: Kepler mission results are rapidly contributing to fundamentally new discoveries in both the exoplanet and asteroseismology fields. The data returned from Kepler are unique in terms of the number of stars observed, precision of photometry for time series observations, and the temporal extent of high duty cycle observations. As the first mission to provide extensive time series measurements on thousands of stars over months to years at a level hitherto possible only for the Sun, the results from Kepler will vastly increase our knowledge of stellar variability for quiet solar-type stars. Here, we report on the stellar noise inferred on the timescale of a few hours of most interest for detection of exoplanets via transits. By design the data from moderately bright Kepler stars are expected to have roughly comparable levels of noise intrinsic to the stars and arising from a combination of fundamental limitations such as Poisson statistics and any instrument noise. The noise levels attained by Kepler on-orbit exceed by some 50% the target levels for solar-type, quiet stars. We provide a decomposition of observed noise for an ensemble of 12th magnitude stars arising from fundamental terms (Poisson and readout noise), added noise due to the instrument and that intrinsic to the stars. The largest factor in the modestly higher than anticipated noise follows from intrinsic stellar noise. We show that using stellar parameters from galactic stellar synthesis models, and projections to stellar rotation, activity, and hence noise levels reproduce the primary intrinsic stellar noise features.
The Astrophysical Journal Supplement Series 10/2011; 197(1):6. · 13.46 Impact Factor
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Søren Meibom,
Sydney A. Barnes,
David W. Latham,
Natalie Batalha,
William J. Borucki,
David G. Koch, Gibor Basri,
Lucianne M. Walkowicz,
Kenneth A. Janes,
Jon Jenkins,
Jeffrey Van Cleve,
Michael R. Haas,
Stephen T. Bryson,
Andrea K. Dupree,
Gabor Furesz,
Andrew H. Szentgyorgyi,
Lars A. Buchhave,
Bruce D. Clarke,
Joseph D. Twicken,
and Elisa V. Quintana
[show abstract]
[hide abstract]
ABSTRACT: We present rotation periods for 71 single dwarf members of the open cluster NGC 6811 determined using photometry from NASA's Kepler mission. The results are the first from The Kepler Cluster Study, which combines Kepler's photometry with ground-based spectroscopy for cluster membership and binarity. The rotation periods delineate a tight sequence in the NGC 6811 color-period diagram from ~1 day at mid-F to ~11 days at early-K spectral type. This result extends to 1 Gyr similar prior results in the ~600 Myr Hyades and Praesepe clusters, suggesting that rotation periods for cool dwarf stars delineate a well-defined surface in the three-dimensional space of color (mass), rotation, and age. It implies that reliable ages can be derived for field dwarf stars with measured colors and rotation periods, and it promises to enable further understanding of various aspects of stellar rotation and activity for cool stars.
The Astrophysical Journal Letters 05/2011; 733(1):L9. · 5.53 Impact Factor
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Søren Meibom,
Sydney A. Barnes,
David W. Latham,
Natalie Batalha,
William J. Borucki,
David G. Koch, Gibor Basri,
Lucianne M. Walkowicz,
Kenneth A. Janes,
Jon Jenkins,
Jeffrey Van Cleve,
Michael R. Haas,
Stephen T. Bryson,
Andrea K. Dupree,
Gabor Furesz,
Andrew H. Szentgyorgyi,
Lars A. Buchhave,
Bruce D. Clarke,
Joseph D. Twicken,
Elisa V. Quintana
[show abstract]
[hide abstract]
ABSTRACT: We present rotation periods for 71 single dwarf members of the open cluster
NGC6811 determined using photometry from NASA's Kepler Mission. The results are
the first from The Kepler Cluster Study which combine Kepler's photometry with
ground-based spectroscopy for cluster membership and binarity. The rotation
periods delineate a tight sequence in the NGC6811 color-period diagram from ~1
day at mid-F to ~11 days at early-K spectral type. This result extends to ~1
Gyr similar prior results in the ~600 Myr Hyades and Praesepe clusters,
suggesting that rotation periods for cool dwarf stars delineate a well-defined
surface in the 3-dimensional space of color (mass), rotation, and age. It
implies that reliable ages can be derived for field dwarf stars with measured
colors and rotation periods, and it promises to enable further understanding of
various aspects of stellar rotation and activity for cool stars.
04/2011;
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David W. Latham,
Jason F. Rowe,
Samuel N. Quinn,
Natalie M. Batalha,
William J. Borucki,
Timothy M. Brown,
Stephen T. Bryson,
Lars A. Buchhave,
Douglas A. Caldwell,
Joshua A. Carter, [......],
David G. Koch,
Jack J. Lissauer,
Geoffrey W. Marcy,
Elisa V. Quintana,
Darin Ragozzine,
Dimitar D. Sasselov,
Avi Shporer,
Jason H. Steffen,
William F. Welsh,
Bill Wohler
[show abstract]
[hide abstract]
ABSTRACT: In this letter we present an overview of the rich population of systems with
multiple candidate transiting planets found in the first four months of Kepler
data. The census of multiples includes 115 targets that show 2 candidate
planets, 45 with 3, 8 with 4, and 1 each with 5 and 6, for a total of 170
systems with 408 candidates. When compared to the 827 systems with only one
candidate, the multiples account for 17 percent of the total number of systems,
and a third of all the planet candidates. We compare the characteristics of
candidates found in multiples with those found in singles. False positives due
to eclipsing binaries are much less common for the multiples, as expected.
Singles and multiples are both dominated by planets smaller than Neptune; 69
+2/-3 percent for singles and 86 +2/-5 percent for multiples. This result, that
systems with multiple transiting planets are less likely to include a
transiting giant planet, suggests that close-in giant planets tend to disrupt
the orbital inclinations of small planets in flat systems, or maybe even to
prevent the formation of such systems in the first place.
03/2011;
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William J. Borucki,
David G. Koch, Gibor Basri,
Natalie Batalha,
Timothy M. Brown,
Stephen T. Bryson,
Douglas Caldwell,
Jørgen Christensen-Dalsgaard,
William D. Cochran,
Edna DeVore, [......],
Samuel N. Quinn,
Elisa V. Quintana,
Darin Ragozzine,
William Sherry,
Avi Shporer,
Peter Tenenbaum,
Guillermo Torres,
Joseph D. Twicken,
Jeffrey Van Cleve,
Lucianne Walkowicz
[show abstract]
[hide abstract]
ABSTRACT: On 1 February 2011 the Kepler Mission released data for 156,453 stars
observed from the beginning of the science observations on 2 May through 16
September 2009. There are 1235 planetary candidates with transit like
signatures detected in this period. These are associated with 997 host stars.
Distributions of the characteristics of the planetary candidates are separated
into five class-sizes; 68 candidates of approximately Earth-size (radius < 1.25
Earth radii), 288 super-Earth size (1.25 Earth radii < radius < 2 Earth radii),
662 Neptune-size (2 Earth radii < radius < 6 Earth radii), 165 Jupiter-size (6
Earth radii < radius < 15 Earth radii), and 19 up to twice the size of Jupiter
(15 Earth radii < radius < 22 Earth radii). In the temperature range
appropriate for the habitable zone, 54 candidates are found with sizes ranging
from Earth-size to larger than that of Jupiter. Five are less than twice the
size of the Earth. Over 74% of the planetary candidates are smaller than
Neptune. The observed number versus size distribution of planetary candidates
increases to a peak at two to three times Earth-size and then declines
inversely proportional to area of the candidate. Our current best estimates of
the intrinsic frequencies of planetary candidates, after correcting for
geometric and sensitivity biases, are 6% for Earth-size candidates, 7% for
super-Earth size candidates, 17% for Neptune-size candidates, and 4% for
Jupiter-size candidates. Multi-candidate, transiting systems are frequent; 17%
of the host stars have multi-candidate systems, and 33.9% of all the candidates
are part of multi-candidate systems.
02/2011;
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William J. Borucki,
David G. Koch, Gibor Basri,
Natalie Batalha,
Alan Boss,
Timothy M. Brown,
Douglas Caldwell,
Jørgen Christensen-Dalsgaard,
William D. Cochran,
Edna DeVore, [......],
Andrej Prsa,
Elisa V. Quintana,
Jason Rowe,
William Sherry,
Peter Tenenbaum,
Guillermo Torres,
Joseph D. Twicken,
Jeffrey Van Cleve,
Lucianne Walkowicz,
and Hayley Wu
[show abstract]
[hide abstract]
ABSTRACT: In the spring of 2009, the Kepler Mission commenced high-precision photometry on nearly 156,000 stars to determine the frequency and characteristics of small exoplanets, conduct a guest observer program, and obtain asteroseismic data on a wide variety of stars. On 2010 June 15, the Kepler Mission released most of the data from the first quarter of observations. At the time of this data release, 705 stars from this first data set have exoplanet candidates with sizes from as small as that of Earth to larger than that of Jupiter. Here we give the identity and characteristics of 305 released stars with planetary candidates. Data for the remaining 400 stars with planetary candidates will be released in 2011 February. More than half the candidates on the released list have radii less than half that of Jupiter. Five candidates are present in and near the habitable zone; two near super-Earth size, and three bracketing the size of Jupiter. The released stars also include five possible multi-planet systems. One of these has two Neptune-size (2.3 and 2.5 Earth radius) candidates with near-resonant periods.
The Astrophysical Journal 01/2011; 728(2):117. · 6.02 Impact Factor
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Lucianne M. Walkowicz, Gibor Basri,
Natalie Batalha,
Ronald L. Gilliland,
Jon Jenkins,
William J. Borucki,
David Koch,
Doug Caldwell,
Andrea K. Dupree,
David W. Latham,
Soeren Meibom,
Steve Howell,
Timothy M. Brown,
and Steve Bryson
[show abstract]
[hide abstract]
ABSTRACT: We present the results of a search for white-light flares on ~23,000 cool dwarfs in the Kepler Quarter 1 long cadence data. We have identified 373 flaring stars, some of which flare multiple times during the observation period. We calculate relative flare energies, flare rates, and durations and compare these with the quiescent photometric variability of our sample. We find that M dwarfs tend to flare more frequently but for shorter durations than K dwarfs and that they emit more energy relative to their quiescent luminosity in a given flare than K dwarfs. Stars that are more photometrically variable in quiescence tend to emit relatively more energy during flares, but variability is only weakly correlated with flare frequency. We estimate distances for our sample of flare stars and find that the flaring fraction agrees well with other observations of flare statistics for stars within 300 pc above the Galactic plane. These observations provide a more rounded view of stellar flares by sampling stars that have not been pre-selected by their activity, and are informative for understanding the influence of these flares on planetary habitability.
The Astronomical Journal 01/2011; 141(2):50. · 4.03 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: The Kepler space telescope monitors over 156.000 stars with an unprecedented photometric precision. We are interested in stellar rotational periods which we find using Lomb-Scargle periodograms. This work focuses on the 306 exoplanet candidate host stars released on June 15, 2010. We present statistics on how many of them show periodic photometric variability, providing preliminary periods and estimates of stellar activity. In the future, our work will focus on spot evolution and differential rotation. Comment: 6 pages, 6 figures, Cool Stars 16 proceeding
12/2010;
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Gibor Basri,
Lucianne M. Walkowicz,
Natalie Batalha,
Ronald L. Gilliland,
Jon Jenkins,
William J. Borucki,
David Koch,
Doug Caldwell,
Andrea K. Dupree,
David W. Latham,
Geoffrey W. Marcy,
Soeren Meibom,
and Tim Brown
[show abstract]
[hide abstract]
ABSTRACT: We provide an overview of stellar variability in the first quarter data from the Kepler mission. The intent of this paper is to examine the entire sample of over 150,000 target stars for periodic behavior in their light curves and relate this to stellar characteristics. This data set constitutes an unprecedented study of stellar variability given its great precision and complete time coverage (with a half hour cadence). Because the full Kepler pipeline is not currently suitable for a study of stellar variability of this sort, we describe our procedures for treating the "raw" pipeline data. About half of the total sample exhibits convincing periodic variability up to two weeks, with amplitudes ranging from differential intensity changes of less than 10–4 up to more than 10%. K and M dwarfs have a greater fraction of period behavior than G dwarfs. The giants in the sample have distinctive quasi-periodic behavior, but are not periodic in the way we define it. Not all periodicities are due to rotation, and the most significant period is not necessarily the rotation period. We discuss properties of the light curves, and in particular look at a sample of very clearly periodic G dwarfs. It is clear that a large number of them do vary because of rotation and starspots, but it will take further analysis to fully exploit this.
The Astronomical Journal 12/2010; 141(1):20. · 4.03 Impact Factor
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Jon M. Jenkins,
William J. Borucki,
David G. Koch,
Geoffrey W. Marcy,
William D. Cochran,
William F. Welsh, Gibor Basri,
Natalie M. Batalha,
Lars A. Buchhave,
Timothy M. Brown, [......],
Bill Wohler,
Jennifer R. Hall,
Khadeejah Ibrahim,
AKM Kamal Uddin,
Michael S. Wu,
Paresh A. Bhavsar,
Jeffrey Van Cleve,
David L. Pletcher,
Jessie L. Dotson,
and Michael R. Haas
[show abstract]
[hide abstract]
ABSTRACT: We report on the discovery and the Rossiter-McLaughlin (R-M) effect of Kepler-8b, a transiting planet identified by the NASA Kepler Mission. Kepler photometry and Keck-HIRES radial velocities yield the radius and mass of the planet around this F8IV subgiant host star. The planet has a radius R P = 1.419 R J and a mass M P = 0.60 M J, yielding a density of 0.26 g cm–3, one of the lowest planetary densities known. The orbital period is P = 3.523 days and the orbital semimajor axis is 0.0483+0.0006 –0.0012 AU. The star has a large rotational vsin i of 10.5 ± 0.7 km s–1 and is relatively faint (V 13.89 mag); both properties are deleterious to precise Doppler measurements. The velocities are indeed noisy, with scatter of 30 m s–1, but exhibit a period and phase that are consistent with those implied by transit photometry. We securely detect the R-M effect, confirming the planet's existence and establishing its orbit as prograde. We measure an inclination between the projected planetary orbital axis and the projected stellar rotation axis of λ = –264 ± 101, indicating a significant inclination of the planetary orbit. R-M measurements of a large sample of transiting planets from Kepler will provide a statistically robust measure of the true distribution of spin-orbit orientations for hot Jupiters around F and early G stars.
The Astrophysical Journal 11/2010; 724(2):1108. · 6.02 Impact Factor
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Matthew J Holman,
Daniel C Fabrycky,
Darin Ragozzine,
Eric B Ford,
Jason H Steffen,
William F Welsh,
Jack J Lissauer,
David W Latham,
Geoffrey W Marcy,
Lucianne M Walkowicz, [......],
Michael Endl,
Debra Fischer,
Gábor Fürész,
Joel D Hartman,
Howard Isaacson,
John A Johnson,
Phillip J MacQueen,
Althea V Moorhead,
Robert C Morehead,
Jerome A Orosz
[show abstract]
[hide abstract]
ABSTRACT: The Kepler spacecraft is monitoring more than 150,000 stars for evidence of planets transiting those stars. We report the detection of two Saturn-size planets that transit the same Sun-like star, based on 7 months of Kepler observations. Their 19.2- and 38.9-day periods are presently increasing and decreasing at respective average rates of 4 and 39 minutes per orbit; in addition, the transit times of the inner body display an alternating variation of smaller amplitude. These signatures are characteristic of gravitational interaction of two planets near a 2:1 orbital resonance. Six radial-velocity observations show that these two planets are the most massive objects orbiting close to the star and substantially improve the estimates of their masses. After removing the signal of the two confirmed giant planets, we identified an additional transiting super-Earth-size planet candidate with a period of 1.6 days.
Science 10/2010; 330(6000):51-4. · 31.20 Impact Factor
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Jason F. Rowe,
William J. Borucki,
David Koch,
Steve B. Howell, Gibor Basri,
Natalie Batalha,
Timothy M. Brown,
Douglas Caldwell,
William D. Cochran,
Edward Dunham, [......],
Jonathan J. Fortney,
Thomas N. Gautier III,
Ronald L. Gilliland,
Jon Jenkins,
David W. Latham,
Jack J. Lissauer,
Geoff Marcy,
David G. Monet,
Dimitar Sasselov,
and William F. Welsh
[show abstract]
[hide abstract]
ABSTRACT: Kepler photometry has revealed two unusual transiting companions: one orbiting an early A-star and the other orbiting a late B-star. In both cases, the occultation of the companion is deeper than the transit. The occultation and transit with follow-up optical spectroscopy reveal a 9400 K early A-star, KOI-74 (KIC 6889235), with a companion in a 5.2 day orbit with a radius of 0.08 R ☉ and a 10,000 K late B-star KOI-81 (KIC 8823868) that has a companion in a 24 day orbit with a radius of 0.2 R ☉. We infer a temperature of 12,250 K for KOI-74b and 13,500 K for KOI-81b. We present 43 days of high duty cycle, 30 minute cadence photometry, with models demonstrating the intriguing properties of these objects, and speculate on their nature.
The Astrophysical Journal Letters 03/2010; 713(2):L150. · 5.53 Impact Factor
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Gibor Basri,
Lucianne M. Walkowicz,
Natalie Batalha,
Ronald L. Gilliland,
Jon Jenkins,
William J. Borucki,
David Koch,
Doug Caldwell,
Andrea K. Dupree,
David W. Latham,
Søren Meibom,
Steve Howell,
and Tim Brown
[show abstract]
[hide abstract]
ABSTRACT: The Kepler mission provides an exciting opportunity to study the light curves of stars with unprecedented precision and continuity of coverage. This is the first look at a large sample of stars with photometric data of a quality that has heretofore been only available for our Sun. It provides the first opportunity to compare the irradiance variations of our Sun to a large cohort of stars ranging from very similar to rather different stellar properties, at a wide variety of ages. Although Kepler data are in an early phase of maturity, and we only analyze the first month of coverage, it is sufficient to garner the first meaningful measurements of our Sun's variability in the context of a large cohort of main-sequence stars in the solar neighborhood. We find that nearly half of the full sample is more active than the active Sun, although most of them are not more than twice as active. The active fraction is closer to a third for the stars most similar to the Sun, and rises to well more than half for stars cooler than mid-K spectral types.
The Astrophysical Journal Letters 03/2010; 713(2):L155. · 5.53 Impact Factor
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William J. Borucki,
David G. Koch,
Timothy M. Brown, Gibor Basri,
Natalie M. Batalha,
Douglas A. Caldwell,
William D. Cochran,
Edward W. Dunham,
Thomas N. Gautier III,
John C. Geary,
Ronald L. Gilliland,
Steve B. Howell,
Jon M. Jenkins,
David W. Latham,
Jack J. Lissauer,
Geoffrey W. Marcy,
David Monet,
Jason F. Rowe,
and Dimitar Sasselov
[show abstract]
[hide abstract]
ABSTRACT: Early time-series photometry from NASA's Kepler spacecraft has revealed a planet transiting the star we term Kepler-4, at R.A. = 19h02m27.s68, δ = +50°08'087. The planet has an orbital period of 3.213 days and shows transits with a relative depth of 0.87 × 10–3 and a duration of about 3.95 hr. Radial velocity (RV) measurements from the Keck High Resolution Echelle Spectrometer show a reflex Doppler signal of 9.3+1.1 –1.9 m s–1, consistent with a low-eccentricity orbit with the phase expected from the transits. Various tests show no evidence for any companion star near enough to affect the light curve or the RVs for this system. From a transit-based estimate of the host star's mean density, combined with analysis of high-resolution spectra, we infer that the host star is near turnoff from the main sequence, with estimated mass and radius of 1.223+0.053 –0.091 M ☉ and 1.487+0.071 –0.084 R ☉. We estimate the planet mass and radius to be {M P, R P} = {24.5 ± 3.8 M ⊕, 3.99 ± 0.21 R ⊕}. The planet's density is near 1.9 g cm–3; it is thus slightly denser and more massive than Neptune, but about the same size.
The Astrophysical Journal Letters 03/2010; 713(2):L126. · 5.53 Impact Factor
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David W. Latham,
William J. Borucki,
David G. Koch,
Timothy M. Brown,
Lars A. Buchhave, Gibor Basri,
Natalie M. Batalha,
Douglas A. Caldwell,
William D. Cochran,
Edward W. Dunham, [......],
Thomas N. Gautier III,
John C. Geary,
Ronald L. Gilliland,
Steve B. Howell,
Jon M. Jenkins,
Jack J. Lissauer,
Geoffrey W. Marcy,
David G. Monet,
Jason F. Rowe,
and Dimitar D. Sasselov
[show abstract]
[hide abstract]
ABSTRACT: We report on the discovery and confirmation of Kepler-7b, a transiting planet with unusually low density. The mass is less than half that of Jupiter, M P = 0.43 M J, but the radius is 50% larger, R P = 1.48 R J. The resulting density, ρP = 0.17 g cm–3, is the second lowest reported so far for an extrasolar planet. The orbital period is fairly long, P = 4.886 days, and the host star is not much hotter than the Sun, T eff = 6000 K. However, it is more massive and considerably larger than the Sun, M = 1.35 M ☉ and R = 1.84 R ☉, and must be near the end of its life on the main sequence.
The Astrophysical Journal Letters 03/2010; 713(2):L140. · 5.53 Impact Factor
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David G. Koch,
William J. Borucki, Gibor Basri,
Natalie M. Batalha,
Timothy M. Brown,
Douglas Caldwell,
Jørgen Christensen-Dalsgaard,
William D. Cochran,
Edna DeVore,
Edward W. Dunham, [......],
Jason F. Rowe,
Jeffrey E. Van Cleve,
Christopher Allen,
Hema Chandrasekaran,
Bruce D. Clarke,
Jie Li,
Elisa V. Quintana,
Peter Tenenbaum,
Joseph D. Twicken,
and Hayley Wu
[show abstract]
[hide abstract]
ABSTRACT: The Kepler Mission, launched on 2009 March 6, was designed with the explicit capability to detect Earth-size planets in the habitable zone of solar-like stars using the transit photometry method. Results from just 43 days of data along with ground-based follow-up observations have identified five new transiting planets with measurements of their masses, radii, and orbital periods. Many aspects of stellar astrophysics also benefit from the unique, precise, extended, and nearly continuous data set for a large number and variety of stars. Early results for classical variables and eclipsing stars show great promise. To fully understand the methodology, processes, and eventually the results from the mission, we present the underlying rationale that ultimately led to the flight and ground system designs used to achieve the exquisite photometric performance. As an example of the initial photometric results, we present variability measurements that can be used to distinguish dwarf stars from red giants.
The Astrophysical Journal Letters 03/2010; 713(2):L79. · 5.53 Impact Factor
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William J Borucki,
David Koch, Gibor Basri,
Natalie Batalha,
Timothy Brown,
Douglas Caldwell,
John Caldwell,
Jørgen Christensen-Dalsgaard,
William D Cochran,
Edna DeVore, [......],
Elisa V Quintana,
Bruce D Clarke,
Christopher Allen,
Jie Li,
Haley Wu,
Peter Tenenbaum,
Ekaterina Verner,
Frederick Bruhweiler,
Jason Barnes,
Andrej Prsa
[show abstract]
[hide abstract]
ABSTRACT: The Kepler mission was designed to determine the frequency of Earth-sized planets in and near the habitable zone of Sun-like stars. The habitable zone is the region where planetary temperatures are suitable for water to exist on a planet's surface. During the first 6 weeks of observations, Kepler monitored 156,000 stars, and five new exoplanets with sizes between 0.37 and 1.6 Jupiter radii and orbital periods from 3.2 to 4.9 days were discovered. The density of the Neptune-sized Kepler-4b is similar to that of Neptune and GJ 436b, even though the irradiation level is 800,000 times higher. Kepler-7b is one of the lowest-density planets (approximately 0.17 gram per cubic centimeter) yet detected. Kepler-5b, -6b, and -8b confirm the existence of planets with densities lower than those predicted for gas giant planets.
Science 02/2010; 327(5968):977-80. · 31.20 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We have analyzed the rotational broadening and chromospheric activity in a sample of 123 M-dwarfs, using spectra taken at the W.M. Keck Observatory as part of the California Planet Search program. We find that only seven of these stars are rotating more rapidly than our detection threshold of v sin i 2.5 km s–1. Rotation appears to be more common in stars later than M3 than in the M0-M2.5 mass range: we estimate that less than 10% of early-M stars are detectably rotating, whereas roughly a third of those later than M4 show signs of rotation. These findings lend support to the view that rotational braking becomes less effective in fully convective stars. By measuring the equivalent widths of the Ca II H and K lines for the stars in our sample, and converting these to approximate L Ca/L bol measurements, we also provide constraints on the connection between rotation and magnetic activity. Measurable rotation is a sufficient, but not necessary condition for activity in our sample: all the detectable rotators show strong Ca II emission, but so too do a small number of non-rotating stars, which we presume may lie at high inclination angles relative to our line of sight. Our data are consistent with a "saturation-type" rotation-activity relationship, with activity roughly independent of rotation above a threshold velocity of less than 6 km s–1. We also find weak evidence for a "gap" in L Ca/L bol between a highly active population of stars, which typically are detected as rotators, and another much less active group.
The Astronomical Journal 01/2010; 139(2):504. · 4.03 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We present radial velocity measurements of a sample of L0-L8 dwarfs observed with VLT/UVES and Keck/HIRES. We combine these measurements with distance and proper motion from the literature to determine space motions for 43 of our targets. We identify nine candidate members of young moving groups, which have ages of 50-600 Myr according to their space motion. From the total velocity dispersion of the 43 L dwarfs, we calculate a kinematic age of ~5 Gyr for our sample. This age is significantly higher than the ~3 Gyr age known for late M dwarfs in the solar neighbourhood. We find that the distributions of the U and V velocity components of our sample are clearly non-Gaussian, placing the age estimate inferred from the full space motion vector into question. The W-component exhibits a distribution more consistent with a normal distribution, and from W alone we derive an age of ~3 Gyr, which is the same age found for late-M dwarf samples. Our brightness-limited sample is probably contaminated by a number of outliers that predominantly bias the U and V velocity components. The origin of the outliers remain unclear, but we suggest that these brown dwarfs may have gained their high velocities by means of ejection from multiple systems during their formation. Comment: 8 pages, accepted for publication in A&A
01/2010;
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[show abstract]
[hide abstract]
ABSTRACT: In a volume-limited sample of 63 ultracool dwarfs of spectral type M7-M9.5, we have obtained high-resolution spectroscopy with UVES at the Very Large Telescope and HIRES at Keck Observatory. In this second paper, we present projected rotation velocities, average magnetic field strengths, and chromospheric emission from the Halpha line. We confirm earlier results that the mean level of normalized Halpha luminosity decreases with lower temperature, and we find that the scatter among Halpha luminosities is larger at lower temperature. We measure average magnetic fields between 0 and 4kG with no indication for a dependence on temperature between M7 and M9.5. For a given temperature, Halpha luminosity is related to magnetic field strength, consistent with results in earlier stars. A few very slowly rotating stars show very weak magnetic fields and Halpha emission, all stars rotating faster than our detection limit show magnetic fields of at least a few hundred Gauss. In contrast to earlier-type stars, we observe magnetic fields weaker than 1kG in stars rotating faster than ~3km/s, but we find no correlation between rotation and magnetic flux generation among them. We interpret this as a fundamental change in the dynamo mechanism; in ultracool dwarfs, magnetic field generation is predominantly achieved by a turbulent dynamo, while other mechanisms can operate more efficiently at earlier spectral type. Comment: accepted by ApJ
12/2009;
