Optical Transmission Spectroscopy of the Terrestrial Exoplanet LHS 3844b from 13 Ground-Based Transit Observations

Transmission spectrum of LHS 3844b compared to model atmospheres. The observed transmission spectrum (1-σ black error bars) is constructed from the inverse-variance weighted mean transit depths across the 13 data sets used in this analysis. They are the same 1-σ black error bars as in Figure 5, and the transit depth values are provided in the grey columns of Table 5. We test two sets of clear atmospheric models: 1) solar composition, and 2) 100% H2O steam. For each model case we test surface pressures from 0.01 - 10 bar. The σ values in the legend are the confidence to which we can rule out each model. We also compare the observed transmission spectrum to a flat line, which is more consistent with the observations.

Atmospheric studies of spectroscopically accessible terrestrial exoplanets lay the groundwork for comparative planetology between these worlds and the Solar System terrestrial planets.

LHS 3844b is a highly-irradiated terrestrial exoplanet (R=1.303+/-0.022R_Earth) orbiting a mid-M dwarf 15 parsecs away. Work based on near-infrared Spitzer phase curves ruled out atmospheres with surface pressures >/=10 bars on this planet. We present 13 transit observations of LHS 3844b taken with the Magellan Clay telescope and the LDSS3C multi-object spectrograph covering 620-1020 nm.

We analyze each of the 13 data sets individually using a Gaussian process regression, and present both white and spectroscopic light curves. In the combined white light curve we achieve an RMS precision of 65 ppm when binning to 10-minutes. The mean white light curve value of (Rp/Rs)^2 is 0.4170+/-0.0046%. To construct the transmission spectrum, we split the white light curves into 20 spectrophotometric bands, each spanning 20 nm, and compute the mean values of (Rp/Rs)^2 in each band.

We compare the transmission spectrum to two sets of atmospheric models. We disfavor a clear, solar composition atmosphere (mu=2.34) with surface pressures >/=0.1 bar to 5.2-sigma confidence. We disfavor a clear, H2O steam atmosphere (mu=18) with surface pressures >/=0.1 bar to low confidence (2.9-sigma).

Our observed transmission spectrum favors a flat line. For solar composition atmospheres with surface pressures >/=1 bar we rule out clouds with cloud-top pressures of 0.1 bar (5.3-sigma), but we cannot address high-altitude clouds at lower pressures. Our results add further evidence that LHS 3844b is devoid of an atmosphere.

Hannah Diamond-Lowe, David Charbonneau, Matej Malik, Eliza M.-R. Kempton, Yuri Beletsky
Comments: 16 pages; 8 figures; accepted to AJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2008.05444 [astro-ph.EP] (or arXiv:2008.05444v1 [astro-ph.EP] for this version)
Submission history
From: Hannah Diamond-Lowe
[v1] Wed, 12 Aug 2020 17:12:07 UTC (1,774 KB)

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