Modeling of Absorption by Heavy Minor Species for the Hot Jupiter HD 209458b

Abstract

The absorption of stellar radiation observed by HD 209458b in the resonant lines of O i and C ii has not yet been satisfactorily explained. We apply a 2D hydrodynamic multi-fluid model that self-consistently describes the expanding planetary wind, driven by stellar XUV radiation and influenced by tidal forces and the surrounding stellar wind. According to this model, HD 209458b has a hydrogen-dominated plasmasphere, expanding beyond the Roche lobe, in the form of two supersonic streams that propagate toward and away from the star. The species heavier than hydrogen and helium are dragged in the escaping material streams and accelerated up to 50 km s−1. Our simulations show that, assuming solar abundances, O i and C ii produce absorption due to the Doppler resonance mechanism at the level of 6%–10%, which is consistent with the observations. Most of this absorption takes place in the streams. The transit depth in the O i and C ii lines is unaffected by the stellar wind, unless it is strong enough to form a compact bowshock around the planet and able to redirect all the escaping material to the tail. In this case, the absorption profile becomes asymmetric due to the prominent blueshifted attenuation. Thus, the spectroscopic measurements enable probing of the planetary wind character, as well as the strength of the stellar wind. The computed absorption at wavelengths of the Si iii, Mg i, and Mg ii lines at solar abundances appears to be much stronger, compared to the observations. This possibly indicates that Si and Mg may be under-abundant in the upper atmosphere of HD 209458b.

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

“…This is significantly less than the values predicted in 2D simulations, but well coincides with the typical widths of the considered spectral lines (see table 1). Therefore, corresponding absorption can be expected to remain at about the same level as in Shaikhislamov et al 2018a. As summarized in Table 2, the absorption for OI triplet is about the same as in 2D simulations, i.e.…”

“…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.…”

“…On the other hand, the temperature and velocity of each neutral component are calculated individually. Nevertheless, the neutral atoms usually are more or less coupled to the main flow as well, due to elastic collisions and charge exchange (Shaikhislamov et al 2016(Shaikhislamov et al , 2018aKhodachenko et al 2017).…”

“…The CII and SiIII lines are sufficiently well resolved in the observed spectra (Linsky et al 2010). For these lines we employ the same analytical fits as those used in Koskinen et al (2013b) and Shaikhislamov et al (2018a). For OI, the intrinsic stellar emission line shapes are taken from the solar spectrum.…”

“…We also considered the case with the same model parameters as those used in our previous 2D modeling Shaikhislamov et al 2018a. In particular, the primary atmosphere of HD209458b was taken consisting of molecular hydrogen with all the hydrogen chemistry involved, such as H2 dissociation, H3 + cooling, as well as fixed heating efficiency of 0.5.…”