Investigation of the environment around close-in transiting exoplanets using cloudy

Abstract

It has been suggested that hot stellar wind gas in a bow shock around an exoplanet is sufficiently opaque to absorb stellar photons and give rise to an observable transit depth at optical and UV wavelengths. In the first part of this paper, we use the CLOUDY plasma simulation code to model the absorption from X-ray to radio wavelengths by 1-D slabs of gas in coronal equilibrium with varying densities (10 4 − 10 8 cm −3 ) and temperatures (2000 − 10 6 K) illuminated by a solar spectrum. For slabs at coronal temperatures (10 6 K) and densities even orders of magnitude larger than expected for the compressed stellar wind (10 4 − 10 5 cm −3 ), we find optical depths orders of magnitude too small (> 3 × 10 −7 ) to explain the ∼ 3% UV transit depths seen with Hubble. Using this result and our modeling of slabs with lower temperatures (2000 − 10 4 K), the conclusion is that the UV transits of WASP-12b and HD 189733b are likely due to atoms originating in the planet, as the stellar wind is too highly ionized. A corollary of this result is that transport of neutral atoms from the denser planetary atmosphere outward must be a primary consideration when constructing physical models. In the second part of this paper, additional calculations using CLOUDY are carried out to model a slab of planetary gas in radiative and thermal equilibrium with the stellar radiation field. Promising sources of opacity from the X-ray to radio wavelengths are discussed, some of which are not yet observed.

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

“…Additionally, the non-detection of asymmetrical transits confirms and expands upon the theoretical modelling done by Ben-Jaffel & Ballester (2014) and Turner et al (2016). These theoretical studies concentrated on modelling the corona around solar-like stars.…”

“…Our data can be used to confirm the predictions by Ben-Jaffel & Ballester (2014) and Turner et al (2016) that an early ingress should not be present in ground-based near-UV transits. Our sample is chosen to contain a wide variety of different system parameters to determine if any system parameters correlate with the existence of a bow shock (Table 1).…”

“…However, recent studies by Ben-Jaffel & Ballester (2014) and Turner et al (2016) cast doubt on observing asymmetries in all ground-and space-based UV wavelengths using the VJH11a bow shock model. Ben-Jaffel & Ballester (2014) use simple recombination and ionization equilibrium calculations for realistic parameters of the stellar corona (T ∼ 10 6 K; Aschwanden 2005) to determine that only highly ionized stages of heavy elements can cause any detectable optical depth.…”

“…Ben-Jaffel & Ballester (2014) use simple recombination and ionization equilibrium calculations for realistic parameters of the stellar corona (T ∼ 10 6 K; Aschwanden 2005) to determine that only highly ionized stages of heavy elements can cause any detectable optical depth. Furthermore, Turner et al (2016) use the plasma photoionization and microphysics code CLOUDY (Ferland et al 1998(Ferland et al , 2013 to investigate all opacity sources at UV and optical wavelengths that could cause an early ingress due to the presence of a bow shock compressing the coronal plasma. Turner et al (2016) also find that the optical depths in the compressed stellar wind (T ∼ 10 6 K; Aschwanden 2005, n ∼ 10 4 cm −3 ; McKenzie, Axford & Banaszkiewicz 1997) are orders of magnitude too small (>3 × 10 −7 ) to cause an observable absorption in space-and ground-based UV and optical observations (even for stellar wind densities 10 4 times higher than what is expected).…”

“…Furthermore, Turner et al (2016) use the plasma photoionization and microphysics code CLOUDY (Ferland et al 1998(Ferland et al , 2013 to investigate all opacity sources at UV and optical wavelengths that could cause an early ingress due to the presence of a bow shock compressing the coronal plasma. Turner et al (2016) also find that the optical depths in the compressed stellar wind (T ∼ 10 6 K; Aschwanden 2005, n ∼ 10 4 cm −3 ; McKenzie, Axford & Banaszkiewicz 1997) are orders of magnitude too small (>3 × 10 −7 ) to cause an observable absorption in space-and ground-based UV and optical observations (even for stellar wind densities 10 4 times higher than what is expected). Knutson, Howard & Isaacson (2010); (d) Pont et al (2010); (e) Johnson et al (2008); (f) Winn et al (2010); (g) Moutou et al (2011); (h) Winn et al (2008b); (i) Narita et al (2010); (j) Albrecht et al (2011); (k) Albrecht et al (2012); (l) Collier Cameron et al (2010 (Wright et al 2011).…”