Timothy M. Brown

Kepler-9: A System of Multiple Planets Transiting a Sun-Like Star, Confirmed by Timing Variations

Extra Exoplanet? A planet is said to transit its star if it can be seen to pass in front of the star; 19% of the known extrasolar planets are transiting planets. A lone planet will transit in an exactly periodic manner; if other planets are present, however, variations in transit duration are expected because of gravitational interactions. Holman et al. (p. 51, published online 26 August; see the cover; see the Perspective by Laughlin) report timing variations in the transits of two exoplanets detected by the Kepler space telescope. The planets have masses similar to that of Saturn and transit the same Sun-like star. A third planet several times the mass of Earth may also transit the star in an interior orbit. Two Saturn-size planets show variations in the times they take to transit their star due to gravitational interaction. 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.

KOI-54: The Kepler Discovery of Tidally Excited Pulsations and Brightenings in a Highly Eccentric Binary

Kepler observations of the star HD 187091 (KIC 8112039, hereafter KOI-54) revealed a remarkable light curve exhibiting sharp periodic brightening events every 41.8 days with a superimposed set of oscillations forming a beating pattern in phase with the brightenings. Spectroscopic observations revealed that this is a binary star with a highly eccentric orbit, e = 0.83. We are able to match the Kepler light curve and radial velocities with a nearly face-on (i = 55) binary star model in which the brightening events are caused by tidal distortion and irradiation of nearly identical A stars during their close periastron passage. The two dominant oscillations in the light curve, responsible for the beating pattern, have frequencies that are the 91st and 90th harmonic of the orbital frequency. The power spectrum of the light curve, after removing the binary star brightening component, reveals a large number of pulsations, 30 of which have a signal-to-noise ratio 7. Nearly all of these pulsations have frequencies that are either integer multiples of the orbital frequency or are tidally split multiples of the orbital frequency. This pattern of frequencies unambiguously establishes the pulsations as resonances between the dynamic tides at periastron and the free oscillation modes of one or both of the stars. KOI-54 is only the fourth star to show such a phenomenon and is by far the richest in terms of excited modes.

TrES-4: A Transiting Hot Jupiter of Very Low Density

We report the discovery of TrES-4, a hot Jupiter that transits the star GSC 02620-00648 every 3.55 days. From high-resolution spectroscopy of the star, we estimate a stellar effective temperature of K, and T p 6100 150 eff from high-precision z and B photometry of the transit we constrain the ratio of the semimajor axis a and the stellar radius to be . We compare these values to model stellar isochrones to constrain the stellar Ra /R p 6.03 0.13 ∗∗ mass to be . Based on this estimate and the photometric time series, we constrain the stellar M p 1.22 0.17 M ∗ , radius to be and the planet radius to be . We model our radial R p 1.738 0.092 RR p 1.674 0.094 R ∗ , p Jup velocity data assuming a circular orbit and find a planetary mass of . Our radial velocity observations 0.84 0.10 MJup rule out line-bisector variations that would indicate a specious detection resulting from a blend of an eclipsing binary system. TrES-4 has the largest radius and lowest density of any of the known transiting planets. It presents a challenge to current models of the physical structure of hot Jupiters and indicates that the diversity of physical properties among the members of this class of exoplanets has yet to be fully explored. Subject headings: planetary systems — techniques: photometric — techniques: radial velocities — techniques: spectroscopic

TrES-3: A Nearby, Massive, Transiting Hot Jupiter in a 31 Hour Orbit*

We describe the discovery of a massive transiting hot Jupiter with a very short orbital period (1.30619 days), which we name TrES-3. From spectroscopy of the host star GSC 03089-00929, we measure T_(eff) = 5720 ± 150 K, log g = 4.6 ± 0.3, and v sin i < 2 km s^(-1) and derive a stellar mass of 0.90 ± 0.15 M_☉. We estimate a planetary mass of 1.92 ± 0.23 M_(Jup), based on the sinusoidal variation of our high-precision radial velocity measurements. This variation has a period and phase consistent with our transit photometry. Our spectra show no evidence of line bisector variations that would indicate a blended eclipsing binary star. From detailed modeling of our B and z photometry of the 2.5% deep transits, we determine a stellar radius 0.802 ± 0.046 R_☉ and a planetary radius 1.295 ± 0.081 R_(Jup). TrES-3 has one of the shortest orbital periods of the known transiting exoplanets, facilitating studies of orbital decay and mass loss due to evaporation, and making it an excellent target for future studies of infrared emission and reflected starlight.

On using the beaming effect to measure spin–orbit alignment in stellar binaries with Sun-like components

Abstract The beaming effect (aka Doppler boosting) induces a variation in the observed flux of a luminous object, following its observed radial velocity variation. We describe a photometric signal induced by the beaming effect during eclipse of binary systems, where the stellar components are late type Sun-like stars. The shape of this signal is sensitive to the angle between the eclipsed star’s spin axis and the orbital angular momentum axis, thereby allowing its measurement. We show that during eclipse there are in fact two effects, superimposed on the known eclipse light curve. One effect is produced by the rotation of the eclipsed star, and is the photometric analog of the spectroscopic Rossiter–McLaughlin effect, thereby it contains information about the sky-projected spin–orbit angle. The other effect is produced by the varying weighted difference, during eclipse, between the beaming signals of the two stars. We give approximated analytic expressions for the amplitudes of the two effects, and present a numerical simulation where we show the light curves for the two effects for various orbital orientations, for a low mass ratio stellar eclipsing binary system. We show that although the overall signal is small, it can be detected in the primary eclipse when using Kepler Long Cadence data of bright systems accumulated over the mission lifetime.

Improved methodology for the automated classification of periodic variable stars

We present a novel automated methodology to detect and classify periodic variable stars in a large database of photometric time series. The methods are based on multivariate Bayesian statistics and use a multi-stage approach. We applied our method to the ground-based data of the TrES Lyr1 field, which is also observed by the Kepler satellite, covering ~26000 stars. We found many eclipsing binaries as well as classical non-radial pulsators, such as slowly pulsating B stars, Gamma Doradus, Beta Cephei and Delta Scuti stars. Also a few classical radial pulsators were found.

Multi-Site Observations Of Pulsation In The Accreting White Dwarf SDSS J161033.64-010223.3 (V386 Ser)

Non-radial pulsations in the primary white dwarfs of cataclysmic variables can now potentially allow us to explore the stellar interior of these accretors using stellar seismology. In this context, we conducted a multi-site campaign on the accreting pulsator SDSSxa0J161033.64–010223.3 (V386 Ser) using seven observatories located around the world in 2007 May over a duration of 11 days. We report the best-fit periodicities here, which were also previously observed in 2004, suggesting their underlying stability. Although we did not uncover a sufficient number of independent pulsation modes for a unique seismological fit, our campaign revealed that the dominant pulsation mode at 609xa0s is an evenly spaced triplet. The even nature of the triplet is suggestive of rotational splitting, implying an enigmatic rotation period of about 4.8 days. There are two viable alternatives assuming the triplet is real: either the period of 4.8 days is representative of the rotation period of the entire star with implications for the angular momentum evolution of these systems, or it is perhaps an indication of differential rotation with a fast rotating exterior and slow rotation deeper in the star. Investigating the possibility that a changing period could mimic a triplet suggests that this scenario is improbable, but not impossible. Using time-series spectra acquired in 2009 May, we determine the orbital period of SDSSxa0J161033.64–010223.3 to be 83.8xa0±xa02.9xa0minutes. Three of the observed photometric frequencies from our 2007 May campaign appear to be linear combinations of the 609xa0s pulsation mode with the first harmonic of the orbital period at 41.5xa0minutes. This is the first discovery of a linear combination between non-radial pulsation and orbital motion for a variable white dwarf.

An observation of a mutual event between two satellites of Uranus

We present observations of the occultation of Umbriel by Oberon on 2007 May 4. We believe this is the first observed mutual event between satellites of Uranus. Fitting a simple geometric model to the light curve, we measure the mid-event time with a precision of 4 s. We assume previously measured values for the albedos of the two satellites, and measure the impact parameter to be 500 ± 80 km. These measurements are more precise than estimates based on current ephemerides for these satellites. Therefore observations of additional mutual events during the 2007–2008 Uranian equinox will provide improved estimates of their orbital and physical parameters.

Astrophysical False Positives Encountered in Wide‐Field Transit Searches

Wide-field photometric transit surveys for Jupiter-sized planets are inundated by astrophysical false positives, namely systems that contain an eclipsing binary and mimic the desired photometric signature. We discuss several examples of such false alarms. These systems were initially identified as candidates by the PSST instrument at Lowell Observatory. For three of the examples, we present follow-up spectroscopy that demonstrates that these systems consist of (1) an M-dwarf in eclipse in front of a larger star, (2) two main-sequence stars presenting grazing-incidence eclipses, and (3) the blend of an eclipsing binary with the light of a third, brighter star. For an additional candidate, we present multi-color follow-up photometry during a subsequent time of eclipse, which reveals that this candidate consists of a blend of an eclipsing binary and a physically unassociated star. We discuss a couple indicators from publicly-available catalogs that can be used to identify which candidates are likely giant stars, a large source of the contaminants in such surveys.

A decade of extrasolar planets around normal stars: The Kepler Mission: Design, expected science results, opportunities to participate

Kepler is a Discovery-class mission designed to determine the frequency of Earth-size and smaller planets in and near the habitable zone (HZ) of spectral type F through M dwarf stars. The instrument consists of a 0.95 m aperture photometer to do high precision photometry of 100,000 solar-like stars to search for patterns of transits. The depth and repetition time of transits provide the size of the planet relative to the star and its orbital period. Multi-band ground-based observation of these stars is currently underway to estimate the stellar parameters and to choose appropriate targets. With these parameters, the true planet radius and orbit scale, hence the relation to the HZ can be determined. These spectra are also used to discover the relationships between the characteristics of planets and the stars they orbit. In particular, the association of planet size and occurrence frequency with stellar mass and metallicity will be investigated. At the end of the four year mission, several hundred terrestrial planets should be discovered with periods between 1 day and 400 days if such planets are common. A null result would imply that terrestrial planets are rare. Based on the results of the recent Doppler-velocity discoveries, over a thousand giant planets will also be found. Information on the albedos and densities of those giants showing transits will be obtained. The mission is now in Phase C/D development and is scheduled for launch in 2008 into a 372-day heliocentric orbit.