Exoplanetary transits of limb-brightened lines: Tentative Si IV absorption by HD 209458b

Transit light curves for stellar continua have only one minimum and a "U" shape. By contrast, transit curves for optically thin chromospheric emission lines can have a "W" shape because of stellar limb brightening.We calculate light curves for an optically thin shell of emission and fit these models to time-resolved observations of Si IV absorption by the planet HD 209458b. We find that the best-fit Si IV absorption model has R_{p,Si iv}/R_* = 0.34_-0.12^+0.07, similar to the Roche lobe of the planet. While the large radius is only at the limit of statistical significance, we develop formulae applicable to transits of all optically thin chromospheric emission lines.