Imaging & Spectroscopy

Ground-based Optical Transmission Spectroscopy of the Nearby Terrestrial Exoplanet LTT 1445Ab

By Keith Cowing
Press Release
October 24, 2022
Filed under , , , , , ,
Ground-based Optical Transmission Spectroscopy of the Nearby Terrestrial Exoplanet LTT 1445Ab
Steps to separate the Hα spectra in time series of LTT 1445B and C from Data Set 1. Panel a: Stacked spectrum of raw images from Data Set 1. Panel b: Zoomed in to the spectral region around the Hα line. Hα emission is visible by eye in stars B and C, but not in A. Panel c: Pixels are summed in the spatial direction, as indicated by the dashed lines in Panel b, and plotted along the dispersion axis. Hα is present in emission for both LTT 1445 B and C. Panel d: We sum the counts in a box of pixels around the Hα lines in both B and C as indicated by the dashed lines. We do this for each exposure across the time span where we see the flare in Data Set 1. Panel e: Counts as a function of time for B and C. It is clear that the flare in Data Set 1 occurring around the 95th minute after the start of the time series originates from LTT 1445C. — astro-ph.EP

Nearby M dwarf systems currently offer the most favorable opportunities for spectroscopic investigations of terrestrial exoplanet atmospheres.

The LTT~1445 system is a hierarchical triple of M dwarfs with two known planets orbiting the primary star, LTT~1445A. We observe four transits of the terrestrial world LTT~1445Ab (R=1.3 R⊕, M=2.9 M⊕) at low resolution with Magellan II/LDSS3C. We use the combined flux of the LTT~1445BC pair as a comparison star, marking the first time that an M dwarf is used to remove telluric variability from time-series observations of another M dwarf.

We find Hα in emission from both LTT~1445B and C, as well as a flare in one of the data sets from LTT~1445C. These contaminated data are removed from the analysis. We construct a broadband transit light curve of LTT~1445Ab from 620–1020 nm. Binned to 3-minute time bins we achieve an rms of 43 ppm for the combined broadband light curve.

We construct a transmission spectrum with 20 spectrophotometric bins each spanning 20 nm and compare it to models of clear, 1× solar composition atmospheres.

We rule out this atmospheric case with a surface pressure of 10 bars to 3.4σ confidence, and with a surface pressure of 1 bar to 2.9σ confidence. Upcoming secondary eclipse observations of LTT~1445Ab with JWST will further probe the cases of a high mean molecular weight atmosphere, a hazy or cloudy atmosphere, or no atmosphere at all on this terrestrial world.

Hannah Diamond-Lowe, Joao M. Mendonca, David Charbonneau, Lars A. Buchhave

Comments: 15 pages, 10 figures, 4 tables, submitted to AJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2210.11809 [astro-ph.EP] (or arXiv:2210.11809v1 [astro-ph.EP] for this version)
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Submission history
From: Hannah Diamond-Lowe
[v1] Fri, 21 Oct 2022 08:34:21 UTC (1,813 KB)

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