LYα TRANSIT SPECTROSCOPY AND THE NEUTRAL HYDROGEN TAIL OF THE HOT NEPTUNE GJ 436b

Abstract

To date, more than 750 planets have been discovered orbiting stars other than the Sun. Two subclasses of these exoplanets, "hot Jupiters" and their less massive counterparts "hot Neptunes," provide a unique opportunity to study the extended atmospheres of planets outside of our solar system. We describe here the first far-ultraviolet transit study of a hot Neptune, specifically GJ436b, for which we use HST /STIS Lyman-α spectra to measure stellar flux as a function of time, observing variations due to absorption from the planetary atmosphere during transit. This analysis permits us to derive information about atmospheric extent, mass-loss rate from the planet, and interactions between the star and planet. We observe an evolution of the Lyman-α lightcurve with a transit depth of GJ436b from 8.8 ± 4.5% near mid-transit, to 22.9 ± 3.9% ∼ 2 hours after the nominal geometric egress of the planet. Using data from the time-tag mode and considering astrophysical noise from stellar variability, we calculate a post-egress occultation of 23.7 ± 4.5%, demonstrating that the signature is statistically significant and of greater amplitude than can be attributed to stellar fluctuations alone. The extended egress absorption indicates the probable existence of a comet-like tail trailing the exoplanet. We calculate a mass-loss rate for GJ436b in the range of 3.7 × 10 6 − 1.1 × 10 9 g s −1 , corresponding to an atmospheric lifetime of 4 × 10 11 − 2 × 10 14 years. Subject headings: planets and satellites: atmospheres -planets and satellites: individual (GJ436b)

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

“…It is not possible to use the planetary mass-loss properties derived by Kulow et al (2014), which are biased by their interpretation of Visit 1 (Sect. 1.2).…”

A global analysis ofSpitzerand new HARPS data confirms the loneliness and metal-richness of GJ 436 b

“…They are featureless between 1.14 and 1.65 µm, ruling out cloudfree hydrogen-dominated atmosphere models. Besides, Kulow et al (2014) showed through Lyman-α transit spectroscopy that GJ 436b is probably trailed by a comet-like tail of neutral hydrogen. Stevenson et al (2010, hereafter S10) published their photometric observations (Spitzer programme 40685) of GJ 436b occultations in the 6 available bandpasses of the Spitzer Space Telescope, i.e.…”

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

“…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).…”