Issue |
A&A
Volume 658, February 2022
|
|
---|---|---|
Article Number | A133 | |
Number of page(s) | 25 | |
Section | Planets and planetary systems | |
DOI | https://doi.org/10.1051/0004-6361/202142140 | |
Published online | 10 February 2022 |
Near-infrared transmission spectrum of TRAPPIST-1 h using Hubble WFC3 G141 observations★
1
LATMOS, CNRS, Sorbonne Université UVSQ,
11 boulevard d’Alembert,
78280
Guyancourt,
France
e-mail: amelie.gressier@latmos.ipsl.fr
2
Sorbonne Universités, UPMC Université Paris 6 et CNRS, UMR 7095, Institut d’Astrophysique de Paris,
98 bis bd Arago,
75014
Paris,
France
3
LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris,
5 place Jules Janssen,
92195
Meudon,
France
4
Department of Astronomy, University of Tokyo,
Tokyo,
Japan
5
Department of Physics and Astronomy, University College London,
London,
UK
6
AIM, CEA, CNRS, Université Paris-Saclay, Université de Paris,
91191
Gif-sur-Yvette,
France
7
School of Physical Sciences, University of Tasmania,
Private Bag 37 Hobart,
Tasmania
7001
Australia
Received:
1
September
2021
Accepted:
30
November
2021
Context. The TRAPPIST-1 planetary system is favourable for transmission spectroscopy and offers the unique opportunity to study rocky planets with possibly non-primary envelopes. We present here the transmission spectrum of the seventh planet of the TRAPPIST-1 system, TRAPPIST-1 h (RP = 0.752 R⊕, Teq = 173 K) using Hubble Space Telescope (HST), Wide Field Camera 3 Grism 141 (WFC3/G141) data.
Aims. Our purpose is to reduce the HST observations of the seventh planet of the TRAPPIST-1 system and, by testing a simple atmospheric hypothesis, to put a new constraint on the composition and the nature of the planet.
Methods. First we extracted and corrected the raw data to obtain a transmission spectrum in the near-infrared (NIR) band (1.1–1.7 μm). TRAPPIST-1 is a cold M-dwarf and its activity could affect the transmission spectrum. We corrected for stellar modulations using three different stellar contamination models; while some fit the data better, they are statistically not significant and the conclusion remains unchanged concerning the presence or lack thereof of an atmosphere. Finally, using a Bayesian atmospheric retrieval code, we put new constraints on the atmosphere composition of TRAPPIST-1h.
Results. According to the retrieval analysis, there is no evidence of molecular absorption in the NIR spectrum. This suggests the presence of a high cloud deck or a layer of photochemical hazes in either a primary atmosphere or a secondary atmosphere dominated by heavy species such as nitrogen. This result could even be the consequence of the lack of an atmosphere as the spectrum is better fitted using a flat line. Variations in the transit depth around 1.3 μm are likely due to remaining scattering noise and the results do not improve while changing the spectral resolution. TRAPPIST-1 h has probably lost its atmosphere or possesses a layer of clouds and hazes blocking the NIR signal. We cannot yet distinguish between a primary cloudy or a secondary clear envelope using HST/WFC3 data; however, in most cases with more than 3σ confidence, we can reject the hypothesis of a clear atmosphere dominated by hydrogen and helium. By testing the forced secondary atmospheric scenario, we find that a CO-rich atmosphere (i.e. with a volume mixing ratio of 0.2) is one of the best fits to the spectrum with a Bayes factor of 1.01, corresponding to a 2.1σ detection.
Key words: planets and satellites: atmospheres / techniques: photometric / techniques: spectroscopic
The lightcurves are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/658/A133.
© A. Gressier et al. 2022
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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