Astronomy and Astrophysics, volume 543, L4-4 (2012/7-1)
Temporal variations in the evaporating atmosphere of the exoplanet HD 189733b.
LECAVELIER DES ETANGS A., BOURRIER V., WHEATLEY P.J., DUPUY H., EHRENREICH D., VIDAL-MADJAR A., HEBRARD G., BALLESTER G.E., DESERT J.-M., FERLET R. and SING D.K.
Abstract (from CDS):
Atmospheric escape has been detected from the exoplanet HD209458b
through transit observations of the hydrogen Lyman-α line. Here we present spectrally resolved Lyman-α transit observations of the exoplanet HD189733b
at two different epochs. These HST/STIS observations show for the first time that there are significant temporal variations in the physical conditions of an evaporating planetary atmosphere. While atmospheric hydrogen is not detected in the first epoch observations, it is observed at the second epoch, producing a transit absorption depth of 14.4±3.6% between velocities of -230 to -140km/s. Contrary to HD209458b, these high velocities cannot arise from radiation pressure alone and require an additional acceleration mechanism, such as interactions with stellar wind protons. The observed absorption can be explained by an atmospheric escape rate of neutral hydrogen atoms of about 109
g/s, a stellar wind with a velocity of 190km/s and a temperature of ∼105
K. An X-ray flare from the active star seen with Swift/XRT 8h before the second-epoch observation supports the idea that the observed changes within the upper atmosphere of the planet can be caused by variations in the stellar wind properties, or by variations in the stellar energy input to the planetary escaping gas (or a mix of the two effects). These observations provide the first indication of interaction between the exoplanet's atmosphere and stellar variations.
planetary systems - stars: individual: HD189733 - techniques: spectroscopic - planets and satellites: atmospheres - stars: flare
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