Bibcode
Casasayas-Barris, N.; Pallé, E.; Yan, F.; Chen, G.; Albrecht, S.; Nortmann, L.; Van Eylen, V.; Snellen, I.; Talens, G. J. J.; González Hernández, J. I.; Rebolo, R.; Otten, G. P. P. L.
Bibliographical reference
Astronomy and Astrophysics, Volume 616, id.A151, 14 pp.
Advertised on:
9
2018
Journal
Citations
85
Refereed citations
80
Description
We used the HARPS-North high resolution spectrograph (ℛ = 115 000)
at Telescopio Nazionale Galileo (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 (TLC). In particular, we resolve
two spectral features centered on the 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 TLC have also been
computed, showing a large Rossiter-McLaughlin (RM) effect, which has a
0.20 ± 0.05% Na I transit absorption for a 0.75 Å passband
that is 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 that has consistent
absorptions in its TLC, which corresponds to an effective radius of
Rλ/RP = 1.20 ± 0.04. While the
signal-to-noise ratio (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 (Teq =
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 Hβ and
Hγ 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 Hβ and Hγ
planetary absorptions.
The reduced HARPS spectra are only available at the CDS via anonymous
ftp to http://cdsarc.u-strasbg.fr
(ftp://130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/616/A151
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