Lyα Absorption at Transits of HD 209458b: A Comparative Study of Various Mechanisms Under Different Conditions

Abstract

To shed more light on the nature of the observed Lyα absorption during transits of HD 209458b and to quantify the major mechanisms responsible for the production of fast hydrogen atoms (the so-called energetic neutral atoms, ENAs) around the planet, 2D hydrodynamic multifluid modeling of the expanding planetary upper atmosphere, which is driven by stellar XUV, and its interaction with the stellar wind has been performed. The model selfconsistently describes the escaping planetary wind, taking into account the generation of ENAs due to particle acceleration by the radiation pressure and by the charge exchange between the stellar wind protons and planetary atoms. The calculations in a wide range of stellar wind parameters and XUV flux values showed that under typical Sun-like star conditions, the amount of generated ENAs is too small, and the observed absorption at the level of 6%-8% can be attributed only to the non-resonant natural line broadening. For lower XUV fluxes, e.g., during the activity minima, the number of planetary atoms that survive photoionization and give rise to ENAs increases, resulting in up to 10%-15% absorption at the blue wing of the Lyα line, caused by resonant thermal line broadening. A similar asymmetric absorption can be seen under the conditions realized during coronal mass ejections, when sufficiently high stellar wind pressure confines the escaping planetary material within a kind of bowshock around the planet. It was found that the radiation pressure in all considered cases has a negligible contribution to the production of ENAs and the corresponding absorption.

3D Modeling of absorption by various species for hot jupiter HD209458b

“…The results are given in Table 2 as the simulation run N13. Altogether, the mass loss rate of 8•10 10 g/s, achieved in this case, is about 3.5 times less, as compared to the standard scenario N1, being at the same time very close to the value obtained in 2D modeling under similar conditions (Khodachenko et al 2017, Shaikhislamov et al 2018a. The increase of heating efficiency up to unity (the simulation run N14) doubles the mass loss rate and increases absorption in all considered lines.…”

“…The corresponding SW parameters at the planet orbit are: Vsw=220 km/s, Tsw=0.8 MK, nsw=10 4 cm -3 , psw=10 -5 μbar. The escape regime realized in this case is close to one, we called as 'captured by the star' in our previous 2D simulations (Shaikhislamov et al 2016, 2018a, Khodachenko et al 2017. As the results show, under such moderate SW conditions the absorption in considered lines remains practically the same, as in the case of a very weak SW.…”

“…As it has been pointed out in our works (Shaikhislamov et al 2016, 2018a, Khodachenko et al 2015, 2017, for the typical parameters of plasmaspheres of the close-orbit hot exoplanets, the Coulomb collisions with protons effectively couple all ions. Therefore, there is no need to calculate the dynamics of every charged component of the plasma fluid species, and we assume in the simulations all of them (i.e., H + , He + , O n+ , C n+ , Mg n+ , Si n+ ) to have the same temperature and velocity.…”

“…These are the stellar XUV flux FXUV, helium and other elements' abundances, as well as the base temperature of the upper atmosphere Tbase. The corresponding material escape in this regime appears to be close to one, we called as 'captured by the star' in our previous 2D simulations (Shaikhislamov et al 2016, 2018a, Khodachenko et al 2017. Figure 3 shows the color plots of density distribution of HI, HII, OI and CII in the escaping PW of HD209458b on the whole system scale under the conditions which we consider as the standard ones: FXUV=10 erg cm -2 s -1 at a reference distance of 1 a.u., Solar system abundancies of elements (He/H=0.05, O/H=8.5•10 -4 , C/H=3.6•10 -4 , Si/H=3.7•10 -5 , Mg/H=3.7•10 -5 ), and Tbase=1500 K. This set of the modeling parameters is listed under the number N1 in Table 2, which summarizes all simulations discussed in the paper.…”

“…The same conclusion was also made by Shaikhislamov at al. (2016), Khodachenko et al (2017) and Shaikhislamov et al 2018a, who used a 2D multifluid simulations to treat self-consistently the generation of PW and its interaction with the stellar wind (hereafter SW) plasma.…”