The Near-Infrared Transmission Spectra of TRAPPIST-1 Planets b, c, d, e, f, and g and Stellar Contamination in Multi-Epoch Transit Spectra

The seven approximately Earth-sized transiting planets in the TRAPPIST-1 system provide a unique opportunity to explore habitable zone and non-habitable zone small planets within the same system.

Its habitable zone exoplanets — due to their favorable transit depths — are also worlds for which atmospheric transmission spectroscopy is within reach with the Hubble Space Telescope (HST) and with the James Webb Space Telescope (JWST). We present here an independent reduction and analysis of two HST Wide Field Camera 3 (WFC3) near-infrared transit spectroscopy datasets for six planets (b through g). Utilizing our physically-motivated detector charge trap correction and a custom cosmic ray correction routine we confirm the general shape of the transmission spectra presented by deWit2016 for planets b and c. Our data reduction approach leads to a 25\% increase in the usable data and reduces the risk of confusing astrophysical brightness variations (e.g., flares) with instrumental systematics.

No prominent absorption features are detected in any individual planet’s transmission spectra; by contrast, the combined spectra of the planets show a clear inverted water absorption feature. We show that this feature — along with the Spitzer transit depth measurements — are fully consistent with stellar contamination, as predicted by Rackham2017b. These spectra demonstrate how stellar contamination can overwhelm planetary absorption features in low-resolution exoplanet transit spectra obtained by HST and JWST and also highlight the challenges in combining multi-epoch observations for planets around rapidly rotating spotted stars.

Zhanbo Zhang, Yifan Zhou, Ben Rackham, Daniel Apai
(Submitted on 6 Feb 2018)

Comments:    30 pages, 16 figures. Under review by AAS journals; submitted on 1/15/2018. Comments and suggestions are welcome
Subjects:    Earth and Planetary Astrophysics (astro-ph.EP)
Cite as:    arXiv:1802.02086 [astro-ph.EP] (or arXiv:1802.02086v1 [astro-ph.EP] for this version)
Submission history
From: Zhanbo Zhang 
[v1] Tue, 6 Feb 2018 17:35:52 GMT (11361kb,D)
https://arxiv.org/abs/1802.02086
Astrobiology