Near-Ultraviolet Absorption, Chromospheric Activity, and Star-Planet Interactions in the Wasp-12 System

Abstract

Extended gas clouds have been previously detected surrounding the brightest known close-in hot Jupiter exoplanets, HD 209458 b and HD 189733 b; we observed the distant but more extreme close-in hot Jupiter system, WASP-12, with HST. Near-UV (NUV) transits up to three times deeper than the optical transit of WASP-12 b reveal extensive diffuse gas, extending well beyond the Roche lobe. The distribution of absorbing gas varies between visits. The deepest NUV transits are at wavelength ranges with strong photospheric absorption, implying the absorbing gas may have temperature and composition similar to the stellar photosphere. Our spectra reveal significantly enhanced absorption (greater than 3σ below the median) at ∼ 200 individual wavelengths on each of two HST visits; 65 of these wavelengths are consistent between the two visits, using a strict criterion for velocity matching which excludes matches with velocity shifts exceeding ∼ 20 km s −1 . Excess transit depths are robustly detected throughout the inner wings of the Mg II resonance lines independently on both HST visits. We detected absorption in Fe II 2586Å, the heaviest species yet detected in an exoplanet transit. The Mg II line cores have zero flux, emission cores exhibited by every other observed star of similar age and spectral type are conspicuously absent. WASP-12 probably produces normal Mg II profiles, but the inner portions of these strong resonance lines are likely affected by extrinsic absorption. The required Mg + column is an order of magnitude greater than expected from the ISM, though we cannot completely dismiss that possibility. A more plausible source of absorption is gas lost by WASP-12 b. We show that planetary mass loss can produce the required column. Our Visit 2 NUV light curves show evidence for a stellar flare. We show that some of the possible transit detections in resonance lines of rare elements may be due instead to non-resonant transitions in common species. We present optical observations and update the transit ephemeris. Subject headings: stars: individual (WASP-12, HD 189733) -planets and satellites: individual (WASP-12 b, HD189733 b) -planet-star interactions -planets and satellites: composition -planets and satellites: atmospheres -planets and satellites: physical evolution

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.…”

“…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).…”

An evaporating planet in the wind: stellar wind interactions with the radiatively braked exosphere of GJ 436 b

“…The intense stellar X-ray and extreme ultraviolet energy input at the base of a hydrogen-rich thermosphere has been shown to be responsible for the expansion of the upper atmospheric layers (e.g., Lammer et al 2003;Lecavelier des Etangs et al 2004;Koskinen et al 2013a,b). Heavier species can be carried to high altitudes through collisions with the expanding flow of hydrogen, and several metals and ions were detected around these planets , 2013Linsky et al 2010;Ballester & Ben-Jaffel 2015;Ben-Jaffel & Ballester 2013;Fossati et al 2010;Haswell et al 2012), confirming that their atmospheres are in a state of hydrodynamic blow-off.…”

The Mg i line: a new probe of the atmospheres of evaporating exoplanets

“…Heavier elements were identified at high altitudes in the extended exosphere of HD 209458b, in the lines of O , C , and Si  Linsky et al 2010;Ben-Jaffel & Sona Hosseini 2010), Si  (Schlawin et al 2010), and Mg  (Vidal-Madjar et al 2013). Signatures of escaping particles have also been found in the Lyman-α line of the warm Jupiter 55 Cnc b (Ehrenreich et al 2012) and the hot-Neptune GJ436b (Kulow et al 2014), and in the Mg  line of the hot Jupiter WASP-12b (Fossati et al 2010;Haswell et al 2012). A wide variety of models has been developed to explain these observations, which are well explained by the blow-off of the atmosphere (see Bourrier & Lecavelier des Etangs 2013 and references therein): the strong X/extreme-UV (EUV) heating from the star heats the whole upper atmosphere to such a degree that it is placed in a hydrodynamic state, in some cases overflowing the Roche lobe (Lecavelier des .…”

“…We caution that the evaporation of WASP-33 b and HAT-P-32 b is most likely limited by a low heating efficiency becaue with η = 100% these planets would evaporate in 1.6 Gy and 900 My, while they orbit stars aged 10−400 Myr (Collier Cameron et al 2010;Moya et al 2011) and 3.8 +1.5 −0.5 Gy (Hartman et al 2011). There are thirteen other planets whose evaporating atmosphere would be detectable if they had high heating efficiencies (S /N > 2 with η = 100%): HAT-P-6 b, HAT-P-9 b, HAT-P- One of them is HD 209458b, for which a positive detection of escaping magnesium was obtained, and WASP-12b, for which ionized magnesium was detected at high altitude in the exosphere (Fossati et al 2010;Haswell et al 2012). A higher heating efficiency may not necessarily lead to a proportionally higher S/N because the absorption depth depends not only on the escape rate, but also on self-shielding effects and the optical thickness of the extended atmosphere.…”