Abstract
We present new observations of four closely-spaced NUV transits of the hot Jupiter-like exoplanet WASP-12b using HST/COS, significantly increasing the phase resolution of the observed NUV light curve relative to previous observations, while minimising the temporal variation of the system. We observe significant excess NUV absorption during the transit, with mean normalised in-transit fluxes of $F_\mathrm{norm}\simeq0.97$, i.e. $\simeq$2-5 $\sigma$ deeper than the optical transit level of $\simeq0.986$ for a uniform stellar disk (the exact confidence level depending on the normalisation method used). We further observe an asymmetric transit shape, such that the post-conjunction fluxes are overall $\simeq$2-3 $\sigma$ higher than pre-conjunction values, and characterised by rapid variations in count rate between the pre-conjunction and out of transit levels. We do not find evidence for an early ingress to the NUV transit as suggested by earlier HST observations. However, we show that the NUV count rate observed prior to the optical transit is highly variable, but overall $\simeq$2.2-3.0 $\sigma$ below the post-transit values and comparable in depth to the optical transit, possibly forming a variable region of NUV absorption from at least phase $\phi\simeq$0.83, limited by the data coverage.
Highlights
WASP-12b is a transiting hot Jupiter-like exoplanet orbiting a late-F/early G main-sequence star with an orbital semi-major axis of 0.0229 AU and an orbital period of 1.09 d (Hebb et al 2009)
We have observed four closely spaced NUV transits of WASP-12b using Hubble Space Telescope (HST)/Cosmic Origins Spectrograph (COS), in order to build up a light curve with higher phase resolution, while minimizing the temporal variation of the system, in an effort to examine more closely the transit NUV light curve
Absorption during the planet’s transit at a level 2–5σ deeper than the optical transit level, and we showed that the transit is apparently asymmetric, with the post-conjunction in-transit fluxes significantly larger overall than the pre-conjunction values, and characterized by rapid variations
Summary
WASP-12b is a transiting hot Jupiter-like exoplanet orbiting a late-F/early G main-sequence star with an orbital semi-major axis of 0.0229 AU and an orbital period of 1.09 d (Hebb et al 2009). The radius of the planet Rp 1.74 RJ is comparable to the planet’s Roche lobe radius of 1.37Rp , which has led to the suggestion that the planet may be losing mass through evaporating material overflowing the Roche lobe, as for HD 209458b (e.g., Vidal-Madjar et al 2003, 2008) With this possible evaporation in mind, Hubble Space Telescope (HST) observations of the near-UV (NUV) transit were obtained by Fossati et al (2010) and Haswell et al (2012) (hereafter F10 and H12, respectively, and F-H together), who observed two transits of the planet in 2009 October and 2010 March, with the aim of detecting elements within the planet’s atmosphere. The red lines show the spectra obtained by F10, with which our data are consistent Note that in both cases the center-of-mass velocity of 19.06 km s−1 observed for WASP-12 by Husnoo et al (2011) is corrected for, and that in this plot the spectra are 1 Å longward of those plotted by F-H, who shifted their spectra to fit to a stellar template. In this paper we concentrate on the light curves obtained by summing the counts obtained over these wavebands
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