Journal
ASTRONOMY & ASTROPHYSICS
Volume 623, Issue -, Pages -Publisher
EDP SCIENCES S A
DOI: 10.1051/0004-6361/201732419
Keywords
planets and satellites: atmospheres; techniques: photometric; planets and satellites: physical evolution; planet-star interactions; planets and satellites: individual: WASP-121 b; planets and satellites: individual: WASP-12 b
Categories
Funding
- DFG [SCHM 1032/57-1]
- DLR [50OR1710, 50OR1706]
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Close-in gas planets are subject to continuous photoevaporation that can erode their volatile envelopes. Today, ongoing mass loss has been confirmed in a few individual systems via transit observations in the ultraviolet spectral range. We demonstrate that the Ultraviolet/Optical Telescope (UVOT) onboard the Neil Gehrels Swift Observatory enables photometry to a relative accuracy of about 0.5% and present the first near-UV (200-270 nm, NUV) transit observations of WASP-121 b, a hot Jupiter with one of the highest predicted mass-loss rates. The data cover the orbital phases 0.85-1.15 with three visits. We measure a broadband NUV transit depth of 2.10 +/- 0.29%. While still consistent with the optical value of 1.55%, the NUV data indicate excess absorption of 0.55% at a 1.9 sigma level. Such excess absorption is known from the WASP-12 system, and both of these hot Jupiters are expected to undergo mass loss at extremely high rates. With a Cloudy simulation, we show that absorption lines of Fe II in a dense extended atmosphere can cause broadband near-UV absorption at the 0.5% level. Given the numerous lines of low-ionization metals, the NUV range is a promising tracer of photoevaporation in the hottest gas planets.
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