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Register a new userDynamic Meteorology at the Photosphere of HD 209458b
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We calculate the meteorology of the close-in transiting extrasolar planet HD 209458b using a global, three-dimensional atmospheric circulation model. Dynamics are driven by perpetual irradiation of one hemisphere of this tidally locked planet. The simulation predicts global temperature contrasts of ~500 K at the photosphere and the development of a steady superrotating jet. The jet extends from the equator to mid-latitudes and from the top model layer at 1 mbar down to 10 bars at the base of the heated region. Wind velocities near the equator exceed 4 km/s at 300 mbar. The hottest regions of the atmosphere are blown downstream from the substellar point by 60 degrees of longitude. We predict from these results a factor of ~2 ratio between the maximum and minimum observed radiation from the planet over a full orbital period, with peak infrared emission preceding the time of the secondary eclipse by ~14 hours.
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We present the first three-dimensional magnetohydrodynamic (MHD) simulations of the atmosphere of HD 209458b which self-consistently include reduction of winds due to the Lorentz force and Ohmic heating. We find overall wind structures similar to that seen in previous models of hot Jupiter atmospheres, with strong equatorial jets and meridional flows poleward near the day side and equatorward near the night side. Inclusion of magnetic fields slows those winds and leads to Ohmic dissipation. We find wind slowing ranging from 10%-40% for reasonable field strengths. We find Ohmic dissipation rates ~10^17 W at 100 bar, orders of magnitude too small to explain the inflated radius of this planet. Faster wind speeds, not achievable in these anelastic calculations, may be able to increase this value somewhat, but likely will not be able to close the gap necessary to explain the inflated radius. We demonstrate that the discrepancy between the simulations presented here and previous models is due to inadequate treatment of magnetic field geometry and evolution. Induced poloidal fields become much larger than those imposed, highlighting the need for a self-consistent MHD treatment of these hot atmospheres.
We derive improved system parameters for the HD 209458 system using a model that simultaneously fits both photometric transit and radial velocity observations. The photometry consists of previous Hubble Space Telescope STIS and FGS observations, twelve I-band transits observed between 2001-2003 with the Mt. Laguna Observatory 1m telescope, and six Stromgren b+y transits observed between 2001-2004 with two of the Automatic Photometric Telescopes at Fairborn Observatory. The radial velocities were derived from Keck/HIRES observations. The model properly treats the orbital dynamics of the system, and thus yields robust and physically self-consistent solutions. Our set of system parameters agrees with previously published results though with improved accuracy. For example, applying robust limits on the stellar mass of 0.93-1.20Msun, we find 1.26 < Rplanet < 1.42 Rjup and 0.59 < Mplanet < 0.70 Mjup. We can reduce the uncertainty on these estimates by including a stellar mass-radius relation constraint, yielding Rplanet = 1.35 +/- 0.07 Rjup and Mplanet = 0.66 +/- 0.04 Mjup. Our results verify that the planetary radius is 10-20% larger than predicted by planet evolution models, confirming the need for an additional mechanism to slow the evolutionary contraction of the planet. A revised ephemeris is derived, T0=2452854.82545 + 3.52474554E (HJD), which now contains an uncertainty in the period of 0.016s and should facilitate future searches for planetary satellites and other bodies in the HD 209458 system.
We report the spectroscopic detection of mid-infrared emission from the transiting exoplanet HD 209458b. Using archive data taken with the Spitzer/IRS instrument, we have determined the spectrum of HD 209458b between 7.46 and 15.25 microns. We have used two independent methods to determine the planet spectrum, one differential in wavelength and one absolute, and find the results are in good agreement. Over much of this spectral range, the planet spectrum is consistent with featureless thermal emission. Between 7.5 and 8.5 microns, we find evidence for an unidentified spectral feature. If this spectral modulation is due to absorption, it implies that the dayside vertical temperature profile of the planetary atmosphere is not entirely isothermal. Using the IRS data, we have determined the broad-band eclipse depth to be 0.00315 +/- 0.000315, implying significant redistribution of heat from the dayside to the nightside. This work required development of improved methods for Spitzer/IRS data calibration that increase the achievable absolute calibration precision and dynamic range for observations of bright point sources.
Atomic hydrogen loss at the top of HD 209458bs atmosphere has been recently detected Vidal-Madjar et al. 2003. We have developed a 1-dimensional model to study the chemistry in the upper atmosphere of this extrasolar hot jupiter. The 3 most abundant elements (other than He), as well as 4 parent molecules are included in this model, viz., H, C, O, H2, CO, H2O, and CH4. The higher temperatures (~ 1000 K) and higher stellar irradiance (~6x10^5 W m^{-2}) strongly enhance and modify the chemical reaction rates in this atmosphere. Our two main results are that (a) the production of atomic hydrogen in the atmosphere is mainly driven by H2O photolysis and reaction of OH with H2, and is not sensitive to the exact abundances of CO, H2O, and CH4, and (b) H2O and CH4 can be produced via the photolysis of CO followed by the reactions with H2.
Here we present the analysis of multi-epoch secondary eclipse observations of HD 189733b and HD 209458b as a probe of temporal variability in the planetary climate using both Spitzer channels 1 and 2 (3.6 and 4.5 um). Constraining temporal variability will inform models and identify physical processes occurring at either length scales too small to directly observe or at pressure levels that are inaccessible to transit observations. We do not detect statistically significant variability and are able to place useful upper limits on the IR variability amplitudes in these atmospheres. There are very few planets with multi-epoch observations at the required precision to probe variability in dayside emission. The observations considered in this study span several years, providing insight into temporal variability at multiple timescales. In the case of HD 189733b, the best fit eclipse depths for the channel 2 observations exhibit a scatter of 102 ppm about a median depth of 1827 ppm and in channel 1 exhibit a scatter of 88 ppm about a median depth of 1481 ppm. For HD 209458b, the best fit eclipse depths for the channel 2 observations exhibit a scatter of 22 ppm about a median depth of 1406 ppm and in channel 1 exhibit a scatter of 131 ppm about a median depth of 1092 ppm. The precision and scatter in these observations allow us to constrain variability to less than (5.6% and 6.0%) and (12% and 1.6%) for channels (1,2) of HD 189733b and HD 209458b respectively. There is a difference in the best fit eclipse timing compared to the predicted time consistent with an offset hotspot as predicted by GCMs and confirmed in previous phase curve observations.
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