In order to predict emission and transmission spectra of HJs we utilized the τ-REx (Tau Retrieval for Exoplanets) software package (Waldmann et al. 2015b,a). This package uses up-to-date molecular cross sections based on line lists provided by EXOMOL6 project (Tennyson & Yurchenko 2012) and HITEMP (Rothman et al. 2010). We additionally used the HCN line list after Harris et al. (2006) provided by the EXOCLIME project7 and generated with the HELIOS-K code (Grimm & Heng 2015). These cross sections are precomputed on a grid of temperatures and pressures and are stored in binary opacity tables that are available for a number of spectral resolutions. The continuum opacity includes Rayleigh scattering on molecules and collisionally-induced absorption due to H2 –H2 and H2–He either after Abel et al. (2011, 2012) or Borysow et al. (2001); Borysow (2002); Borysow & Frommhold (1989), respectively. We extended the public version of τ-REx8 by incorporating additional opacity sources essential for the atmospheres of UHJs. In particular, bound-free and free–free transitions of H− become one of the major continuum opacity contributors for the temperatures hotter than about 2000 K. The cross sections of H− are from John (1988). We also included opacity due to free–free transitions of He− as well as Rayleigh scattering on H I atoms and Thomson scattering on free electrons. The He− cross sections are those originally from John (1968) using the polynomial fit by Carbon et al. (1969). Rayleigh scattering on H I is calculated after Dalgarno (1962). All relevant numerical routines were extracted from the LLMODELS stellar model atmosphere code (Shulyak et al. 2004). At the temperatures of HJs the H− is the dominant continuum opacity source that impacts the observed spectra of these planets (Arcangeli et al. 2018). However, at millibar pressures the He− opacity can become comparable or even stronger than that of H− for wavelengths longer than 1.6 μm, as shown in Fig. 3 (third panel from the bottom), where we display examples of continuum opacity coefficient at different altitudes in the atmosphere of a HJ with Teq = 3000 K. At even smaller pressures, electron scattering and Rayleigh scattering on H I atoms also become important contributors to the continuum opacity at particular wavelengths (top panel in Fig. 3). However, the contribution of H I and e− on the transmission spectra is marginal because their opacity is only strong at low pressures that are hardly probed by transmission spectroscopy. Thus, among all continuum opacity sources, only H− and He− significantly contribute to the predicted amplitude of the transmission spectra, as shown in the bottom plot of Fig. 3. When both continuum and line opacity are included, the effect of He− on predicted spectra is diluted by a much stronger opacity in molecular lines while H− contribution is still significant. Nevertheless, as can be seen from the second plot (from bottom) in Fig. 3, in optically thick layers the He− opacity could still be stronger than, for example, collision-induced absorption due to H2 -H2 and H2-He. We thus conclude that accurate calculation of atmospheric opacity requires He−, H I, and e− opacity included especially at low pressures, following modern stellar atmosphere codes. However, observed transmission and emission spectra of HJs are hardly affected by these three opacity sources. Finally, we updated HELIOS with He− and e− opacity and found out that this has little impact on the atmospheric temperature structure (with a modification in local temperature of at most ΔT ≈ 10 K) and thus can be ignored; the original version of HELIOS already includes H I Rayleigh scattering.
Citation: Shulyak et al. 2004
Citation function: Uses