Na I and Hα absorption features in the atmosphere of MASCARA-2b/KELT-20b

Abstract

We have used the HARPS-North high resolution spectrograph (R = 115 000) at TNG to observe one transit of the highly irradiated planet MASCARA-2b/KELT-20b. Using only one transit observation, we are able to clearly resolve the spectral features of the atomic sodium (Na I) doublet and the Hα line in its atmosphere, which are corroborated with the transmission calculated from their respective transmission light curves. In particular, we resolve two spectral features centered on the atomic sodium (Na I) doublet position with an averaged absorption depth of 0.17±0.03% for a 0.75 Å bandwidth, with line contrasts of 0.44±0.11% (D2) and 0.37±0.08% (D1). The Na I transmission light curves have been also computed, showing a large Rossiter-McLaughlin (RM) effect, with a 0.20 ± 0.05% Na I transit absorption for a 0.75 Å passband, consistent with the absorption depth value measured from the final transmission spectrum. We observe a second feature centered on the Hα line with 0.6 ± 0.1% contrast, and an absorption depth of 0.59 ± 0.08% for a 0.75 Å passband, with consistent absorptions in its transmission light curves, which corresponds to an effective radius of R λ /R P = 1.20±0.04. While S/N of the final transmission spectrum is not sufficient to adjust different temperature profiles to the lines, we find that higher temperatures than the equilibrium (T eq = 2260 ± 50 K) are needed to explain the lines contrast. Particularly, we find that the Na I lines core require a temperature of T = 4210 ± 180 K and that Hα requires a temperature of T = 4330 ± 520 K. MASCARA-2b, like other planets orbiting A-type stars, receives a large amount of UV energy from its host star. This energy excites the atomic hydrogen and produces Hα absorption, leading to the expansion and abrasion of the atmosphere. The study of other Balmer lines in the transmission spectrum would allow the determination of the atmospheric temperature profile and the calculation of the lifetime of the atmosphere with escape rate measurements. In the case of MASCARA-2b, residual features are observed in the H β and H γ lines, but they are not statistically significant. More transit observations are needed to confirm our findings in Na I and Hα, and to build up enough S/N to explore the presence of H β and H γ planetary absorptions.

Detection of neutral atomic species in the ultra-hot Jupiter WASP-121b

“…The effect also averages out for the largest passbands. Minor differences between the observed and modelled CLV and RM effects may be attributed to our LTE assumption (Casasayas-Barris et al 2018); also, we did not account for the effective radius of the planet R λ , which is left as a free parameter in Casasayas- Barris et al (2019). The absorption depths are listed in Table 4.1, and are consistent with the stacked transmission spectrum absorption depths.…”

“…This distorts the average stellar spectrum, and produces a residual in the ratio of intransit and out-of-transit spectra (the distortion is called the Rossiter-McLauglin effect, only considering stellar rotation). The residual can create spurious features in the transmission spectrum and photometric lightcurves of individual lines (Louden & Wheatley 2015;Yan et al 2015Yan et al , 2017Casasayas-Barris et al 2018. In order to evaluate the influence of CLV and RM effects in the observed data, we model them simultaneously in a manner similar to Casasayas- Barris et al (2019).…”

Ionized calcium in the atmospheres of two ultra-hot exoplanets WASP-33b and KELT-9b

“…The Hα line was also detected in two other UHJs: MASCARA-2b (Casasayas-Article number, page 1 of 12 arXiv:1911.00380v1 [astro-ph.EP] 1 Nov 2019 A&A proofs: manuscript no. CaII-paper-acc Barris et al 2018) and WASP-12b (Jensen et al 2018). Some atomic/ionic metal lines are also detected in UHJs, for instance, Fossati et al (2010) detected Mg ii in WASP-12b using UV transmission spectroscopy with the Hubble Space Telescope and various metal elements (including Fe, Ti, Mg, and Na) have been discovered in KELT-9b (Hoeijmakers et al 2018;Cauley et al 2019;Hoeijmakers et al 2019).…”

Detection of Fe I and Fe II in the atmosphere of MASCARA-2b using a cross-correlation method

“…Because of the strong stellar irradiation, the equilibrium temperature of the planet is 2260 K. All stellar and planetary parameters are provided in Table 1. This planet's atmosphere was recently studied using highdispersion transit spectroscopy by Casasayas- Barris et al (2018, where they detected Ca ii, Fe ii, Na i, H α, H β and indications of H γ and Mg i. Here, we analyse the same three transit observations of HARPS-N used in Casasayas- , using the cross-correlation method.…”