Transmission spectroscopy of HAT-P-19b

Hot Jupiter exoplanets are gas giants orbiting their host star in only a few days. If their orbit lie in the same plane as our line of sight, the planet transits its host star once per orbit. This event yields the opportunity to study the atmosphere of this far-away world. We investigated the inflated hot Jupiter HAT-P-19b for spectroscopic signatures of a cloud-free atmosphere.
Transmission spectroscopy of HAT-P-19b

High-precision transit light curve created from the combined spectroscopic light.

Written by M. Mallonn

Thursday, 13 August 2015

We observed a transit event of HAT-P-19b with the 10m telescope Gran Telescopio Canarias spectroscopically. The target has a mass similar to Saturn, but a size as big as Jupiter, causing the atmosphere to be unusually extended due to the low surface gravity. Therefore, the planet was chosen as a favorable target for investigations of its atmosphere. In a first step, the light of all wavelengths was combined to create a high-precision photometric light curve, which was analyzed to refine the planetary parameters and the ephemeris of this planet. In a second step, the spectroscopic data were analyzed for a wavelength-dependent absorption by the planet atmosphere. The measurements achieved sufficient accuracy to distinguish between theoretical atmospheric compositions and allowed for the conclusion that a cloud-free atmoshere is unlikely if the elemental composition is similar to our Solar gas giants Jupiter and Saturn. A plausible explanation is a layer of clouds or hazes of condensed material in the atmosphere. Since the planet has a temperature of about 1000 Kelvin, such condensates (aerosols) would not be cristals of frozen ammonia as for our Jupiter, instead it might be composed of condensed metal silicates. The measurement fully exploits the light-gathering power of a 10m telescope, however the achieved accuracy of the measurement is limited by systematic uncertainties of the instrument and the look through our own Earth atmosphere. One way to improve the accuracy would be to repeat the observation several times, because the systematic measurement uncertainties changes, while any potential signal from the planet remains the same.

The transit observation was supported by a long-term monitoring of the host star HAT-P-19 with the STELLA telescope. These data were successfully used to clean the planetary measurements from brightness inhomogeneities on the surface of the host star (starspots) and to derive a value of the rotation period and the age of the star.


Link to the published article: Mallonn et al. 2015, A&A, 580, 60