The shocking transit of WASP-12b: modelling the observed early ingress in the near-ultraviolet

Abstract

Near-ultraviolet (near-UV) observations of WASP-12b have revealed an early ingress compared to the optical transit light curve. This has been interpreted as due to the presence of a magnetospheric bow shock which forms when the relative velocity of the planetary and stellar material is supersonic. We aim to reproduce this observed early ingress by modelling the stellar wind (or coronal plasma) in order to derive the speed and density of the material at the planetary orbital radius. From this, we determine the orientation of the shock and the density of compressed plasma behind it. With this model for the density structure surrounding the planet we perform Monte Carlo radiation transfer simulations of the near-UV transits of WASP-12b with or without bow shock. We find that we can reproduce the transit light curves with a wide range of plasma temperatures, shock geometries and optical depths. Our results support the hypothesis that a bow shock could explain the observed early ingress.

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

“…This same technique is used in Turner et al (2013) and Pearson et al (2014) to search for asymmetries caused by a possible bow shock in TrES-3b and HAT-P-16b, respectively. This technique is useful for possible bow shock detection because bow shock models of WASP-12b (Llama et al 2011) and HD 189733b (Llama et al 2013) predict a distinct asymmetry between the two halves of the transit (Llama et al 2011, see fig. 2;Llama et al 2013, see fig.…”

“…Most notably, it was predicted that a transiting exoplanet can potentially show an earlier transit ingress in the UV than in the optical, while the transit egress times will be unaffected due to the early absorption of star light due to a bow shock (Vidotto, Jardine & Helling 2010, 2011bLlama et al 2011Llama et al , 2013Vidotto et al 2011a). Additionally, the near-UV transit will have a greater drop in flux than the optical transit and will no longer be symmetric about the mid-transit time (Llama et al 2011(Llama et al , 2013Vidotto et al 2011c). This effect is explained by the presence of a bow shock on the leading edge of the planet formed by interactions between the planet's magnetosphere and the stellar coronal plasma.…”

Exoplanet Upper Atmosphere Environment Characterization

“…One can see that the mass loss is highly overestimated if one applies the energy-limited approach with η = 100%. In reality, the ratio of the net heating rate to the rate of stellar energy absorption is ∼15-20% (e.g., Lammer et al 2009;Murray-Clay et al 2009). At HD 209458b's orbit location of 0.047 AU, the planet lost a negligible fraction of its mass.…”

“…First, the upper atmosphere temperatures between 8000-10000 K are based on the assumption that the observed O, C and Si particles are of planetary origin, so that they are dragged upward by the planetary H wind. One should note that the upper atmosphere temperature could be 8000 K if these particles originate within the stellar wind plasma, as now expected for Mg ions around WASP-12b (Llama et al 2011). Second, Koskinen et al (2010) fitted their model to the transit depth measurements covering the full width of the stellar H Lyman-α profile in the λ range 1212-1220Å with absorption depths of 6.6 ± 2.3% (Ben-Jaffel and Hosseini 2010) and not to the early Vidal-Madjar et al (2003) observations.…”

“…Hydrodynamic models HD 209458b is a hot gas giant with a visual radius R pl = 1.38R Jup (Southworth 2010) and a mass M pl = 0.64 ± 0.09M Jup (Snellen et al 2010) which orbits a ∼4 Gyr old solar like G-type star at 0.047 AU. After the discovery of HD 209458b, several hydrodynamic models were applied to investigate the response of the stellar XUV radiation to the planet's thermosphere (Yelle 2004(Yelle , 2006Tian et al 2005;García Munoz 2007;Penz et al 2008;Murray-Clay et al 2009;Guo 2011). These models solved the set of the 1D hydrodynamic fluid equations for mass, momentum and energy conservation…”