METALS IN THE EXOSPHERE OF THE HIGHLY IRRADIATED PLANET WASP-12b

Abstract

We present near-UV transmission spectroscopy of the highly irradiated transiting exoplanet WASP-12b, obtained with the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST). The spectra cover three distinct wavelength ranges:NUVA (2539-2580 Å); NUVB (2655-2696 Å); and NUVC (2770-2811 Å). Three independent methods all reveal enhanced transit depths attributable to absorption by resonance lines of metals in the exosphere of WASP-12b. Light curves of total counts in the NUVA and NUVC wavelength ranges show a detection at a 2.5σ level. We detect extra absorption in the Mg II λλ2800 resonance line cores at the 2.8σ level.The NUVA, NUVB and NUVC light curves imply effective radii of 2.69±0.24 R J , 2.18±0.18 R J , and 2.66±0.22 R J respectively, suggesting the planet is surrounded by an absorbing cloud which overfills the Roche lobe. We detect enhanced transit depths at the wavelengths of resonance lines of neutral sodium, tin and manganese, and at singly ionised ytterbium, scandium, manganese, aluminum, vanadium and magnesium. We also find the statistically expected number of anomalous transit depths at wavelengths not associated with any known resonance line. Our data are limited by photon noise, but taken as a whole the results are strong evidence for an extended absorbing exosphere surrounding the planet. The NUVA data exhibits an early ingress, contrary to model expectations; we speculate this could be due to the presence of a disk of previously stripped material. Subject headings: stars: individual (WASP-12) 1 http://archive.stsci.edu/ 2 See the COS Data Handbook for more information on CALCOS:

Ground-based near-UV observations of 15 transiting exoplanets: constraints on their atmospheres and no evidence for asymmetrical transits

“…Previous observations of an early ingress on WASP-12b and HD 189733b observe a flux drop difference of about ∼1 per cent and a timing difference of ∼>30 min between the near-UV and optical light curves (Fossati et al 2010;Haswell et al 2012;Ben-Jaffel & Ballester 2013;Nichols et al 2015). Both these properties are well within reach for ground-based metre-sized telescopes (e.g.…”

“…Additionally, near-UV transits may exhibit asymmetries in their light curves such as ingress/egress timing differences, asymmetric transit shapes, longer durations, or significantly deeper transit depths (>∼1 per cent) than the optical (e.g. Vidal-Madjar et al 2003;Fossati et al 2010;Ehrenreich et al 2012;Kulow et al 2014). The physical interpretations of these abnormalities vary, and include bow shocks, tidal interactions, starplanet magnetic interactions, a plasma torus originating from an active satellite, or escaping planetary atmospheres (e.g.…”

“…These observations can be subdivided into two groups: asymmetric and symmetric light curves. There are five exoplanets (55 Cnc b, GJ 436b, HD 189733b, HD 209458b, WASP12b) where asymmetries in their light curves are observed (VidalMadjar et al 2003(VidalMadjar et al , 2004(VidalMadjar et al , 2008(VidalMadjar et al , 2013Ben-Jaffel 2007Fossati et al 2010;Ehrenreich et al 2012Ehrenreich et al , 2015Haswell et al 2012;BenJaffel & Ballester 2013;Kulow et al 2014;Nichols et al 2015). For the symmetric transits, nine hot Jupiters (HAT-P-1b, HAT-P-12b, WASP43b, are observed to have a constant planetary radii from near-UV to optical wavelengths Turner et al 2013;Bento et al 2014;Nikolov et al 2014;Pearson, Turner & Sagan 2014;Mallonn et al 2015;Ricci et al 2015;Sing et al 2016).…”

Thermal escape from extrasolar giant planets

“…The upper atmospheres of three different extrasolar giant planets (EGPs)-HD209458b, HD189733b and WASP12b-were recently probed by UV transit observations obtained by the Hubble Space Telescope (HST) [1][2][3][4][5][6]. These observations indicate that close-in EGPs such as HD209458b are surrounded by a hot thermosphere composed of atomic hydrogen that extends to several planetary radii and provide the required line of [20,21] and their application to close-in EGPs [8,9].…”

Spectroscopy of planetary atmospheres in our Galaxy

“…Observations from space, with HST/STIS-COS, and from the ground, unveiled a population of ions and radicals wrapping the planet like an envelope and partially occulting the star. These observations are suggestive of escape processes: Jeans escape or hydrodynamical mecha-nisms (Vidal-Madjar et al 2003, 2004Ben-Jaffel 2007Ben-Jaffel and Hosseini 2010;Linsky et al 2010;Lecavelier des Etangs et al 2010Fossati et al 2010;Jensen et al 2012). We show in Fig.…”