Using Stellar Limb‐Darkening to Refine the Properties of HD 209458b

We use multi-band photometry to refine estimates for the planetary radius andorbital inclination of the transiting planet system HD 209458. We gathered 1066spectra over four distinct transits with the STIS spectrometer on the HubbleSpace Telescope using two gratings with a resolution R=1500 and a combinedwavelength range of 290-1030 nm. We divide the spectra into tenspectrophotometric bandpasses, five for each grating, of equal wavelength spanwithin each grating, and fit a transit curve over all bandpassessimultaneously. In our fit we use theoretical values for the stellarlimb-darkening to further constrain the planetary radius. We find that theradius of HD 209458b is 1.320 +/- 0.025 R_Jup, which is a factor of two moreprecise than current estimates. We also obtain improved estimates for theorbital period P and time of center of transit T_C. Although in principle thephoton-limited precision of the STIS data should allow us to measure the timingof individual transits to a precision of 2-7 s, we find that systematicinstrumental offsets in the measured flux from one orbit of the spacecraft tothe next degrade these measurements to a typical precision of +/- 14 s. Withinthis level of error, we find no significant variations in the timing of theeight events examined in this work.Comment: final accepted version of pape