Exo-Planetary Transits of Limb Brightened Lines; Tentative Si IV Absorption by HD209458b

TL;DR

This paper models and analyzes the transit light curves of HD209458b in Si IV emission lines, revealing potential chromospheric absorption features and developing general formulas for optically thin line transits.

Contribution

It introduces a model for chromospheric line transits, fits it to observations of HD209458b, and provides formulas applicable to all optically thin emission lines.

Findings

Best-fit Si IV absorption radius is comparable to the planet's Roche lobe.

Transit light curves can have a 'W' shape due to limb-brightening.

Developed general formulas for optically thin chromospheric line transits.

Abstract

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 HD209458b. We find that the best fit Si IV absorption model has R_p,SIV/R_*= 0.34+0.07-0.12, 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.