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Register a new userA method for the direct determination of the surface gravities of transiting extrasolar planets
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John Southworth
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We show that the surface gravity of a transiting extrasolar planet can be calculated from only the spectroscopic orbit of its parent star and the analysis of its transit light curve. This does not require additional constraints, such as are often inferred from theoretical stellar models or model atmospheres. The planets surface gravity can therefore be measured precisely and from only directly observable quantities. We outline the method and apply it to the case of the first known transiting extrasolar planet, HD 209458b. We find a surface gravity of g_p = 9.28 +/- 0.15 m/s, which is an order of magnitude more precise than the best available measurements of its mass, radius and density. This confirms that the planet has a much lower surface gravity that that predicted by published theoretical models of gas giant planets. We apply our method to all fourteen known transiting extrasolar planets and find a significant correlation between surface gravity and orbital period, which is related to the known correlation between mass and period. This correlation may be the underlying effect as surface gravity is a fundamental parameter in the evaporation of planetary atmospheres.
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We revisit the tidal stability of extrasolar systems harboring a transiting planet and demonstrate that, independently of any tidal model, none but one (HAT-P-2b) of these planets has a tidal equilibrium state, which implies ultimately a collision of these objects with their host star. Consequently, conventional circularization and synchronization timescales cannot be defined because the corresponding states do not represent the endpoint of the tidal evolution. Using numerical simulations of the coupled tidal equations for the spin and orbital parameters of each transiting planetary system, we confirm these predictions and show that the orbital eccentricity and the stellar obliquity do not follow the usually assumed exponential relaxation but instead decrease significantly, reaching eventually a zero value, only during the final runaway merging of the planet with the star. The only characteristic evolution timescale of {it all} rotational and orbital parameters is the lifetime of the system, which crucially depends on the magnitude of tidal dissipation within the star. These results imply that the nearly circular orbits of transiting planets and the alignment between the stellar spin axis and the planetary orbit are unlikely to be due to tidal dissipation. Other dissipative mechanisms, for instance interactions with the protoplanetary disk, must be invoked to explain these properties.
(Abridged) We discuss the design considerations of the EXPLORE (EXtra-solar PLanet Occultation REsearch) project, a series of transiting planet searches using 4-m-class telescopes to continuously monitor a single field of stars in the Galactic Plane in each ~2 week observing campaign. We discuss the general factors which determine the efficiency and the number of planets found by a transit search, including time sampling strategy and field selection. The primary goal is to select the most promising planet candidates for radial velocity follow-up observations. We show that with very high photometric precision light curves that have frequent time sampling and at least two detected transits, it is possible to uniquely solve for the main parameters of the eclipsing system (including planet radius) based on several important assumptions about the central star. Together with a measured spectral type for the star, this unique solution for orbital parameters provides a powerful method for ruling out most contaminants to transiting planet candidates. For the EXPLORE project, radial velocity follow-up observations for companion mass determination of the best candidates are done on 8-m-class telescopes within two or three months of the photometric campaigns. This same-season follow-up is made possible by the use of efficient pipelines to produce high quality light curves within weeks of the observations. We conclude by presenting early results from our first search, EXPLORE I, in which we reached <1% rms photometric precision (measured over a full night) on ~37,000 stars to I <= 18.2.
We carried out an imaging survey for extrasolar planets around stars in the Pleiades (125 Myr, 135 pc) in the $H$ and $K_{S}$ bands using HiCIAO combined with the adaptive optics, AO188, on the Subaru telescope. We found 13 companion candidates fainter than 14.5 mag in the $H$ band around 9 stars. Five of these 13 were confirmed to be background stars by measurement of their proper motion. One was not found in the second epoch observation, and thus was not a background or companion object. One had multi-epoch image, but the precision of its proper motion was not sufficient to conclude whether it was background object. Four other candidates are waiting for second epoch observations to determine their proper motion. Finally, the remaining 2 were confirmed to be 60 $M_{J}$ brown dwarf companions orbiting around HD 23514 (G0) and HII 1348 (K5) respectively, as had been reported in previous studies. In our observations, the average detection limit for a point source was 20.3 mag in the $H$ band beyond 1.5 from the central star. On the basis of this detection limit, we calculated the detection efficiency to be 90% for a planet with 6 to 12 Jovian masses and a semi-major axis of 50--1000 AU. For this we extrapolated the distribution of planet mass and semi-major axis derived from RV observations and adopted the planet evolution model of Baraffe et al. (2003). As there was no detection of a planet, we estimated the frequency of such planets to be less than 17.9% ($2sigma$) around one star of the Pleiades cluster.
We show that a consistent fit to observed secondary eclipse data for several strongly irradiated transiting planets demands a temperature inversion (stratosphere) at altitude. Such a thermal inversion significantly influences the planet/star contrast ratios at the secondary eclipse,their wavelength dependences, and, importantly, the day-night flux contrast during a planetary orbit. The presence of the thermal inversion/stratosphere seems to roughly correlate with the stellar flux at the planet. Such temperature
To determine the physical parameters of a transiting planet and its host star from photometric and spectroscopic analysis, it is essential to independently measure the stellar mass. This is often achieved by the use of evolutionary tracks and isochrones, but the mass result is only as reliable as the models used. The recent paper by Torres et al (2009) showed that accurate values for stellar masses and radii could be obtained from a calibration using T_eff, log g and [Fe/H]. We investigate whether a similarly good calibration can be obtained by substituting log rho - the fundamental parameter measured for the host star of a transiting planet - for log g, and apply this to star-exoplanet systems. We perform a polynomial fit to stellar binary data provided in Torres et al (2009) to obtain the stellar mass and radius as functions of T_eff, log rho and [Fe/H], with uncertainties on the fit produced from a Monte Carlo analysis. We apply the resulting equations to measurements for seventeen SuperWASP host stars, and also demonstrate the application of the calibration in a Markov Chain Monte Carlo analysis to obtain accurate system parameters where spectroscopic estimates of effective stellar temperature and metallicity are available. We show that the calibration using log rho produces accurate values for the stellar masses and radii; we obtain masses and radii of the SuperWASP stars in good agreement with isochrone analysis results. We ascertain that the mass calibration is robust against uncertainties resulting from poor photometry, although a good estimate of stellar radius requires good-quality transit light curve to determine the duration of ingress and egress.
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