The Transit Light Source Effect: False Spectral Features and Incorrect Densities for M-dwarf Transiting Planets

By Benjamin V. Rackham, D. Apai, M. Giampapa. 2017

333 citations recorded by the source corpus.

Transmission spectra are differential measurements that utilize stellar illumination to probe transiting exoplanet atmospheres. Any spectral difference between the illuminating light source and the disk-integrated stellar spectrum due to starspots and faculae will be imprinted in the observed transmission spectrum. However, few constraints exist for the extent of photospheric heterogeneities in M dwarfs. Here we model spot and faculae covering fractions consistent with observed photometric variabilities for M dwarfs and the associated 0.3–5.5 μm stellar contamination spectra. We find that large ranges of spot and faculae covering fractions are consistent with observations and corrections assuming a linear relation between variability amplitude, and covering fractions generally underestimate the stellar contamination. Using realistic estimates for spot and faculae covering fractions, we find that stellar contamination can be more than 10× larger than the transit depth changes expected for atmospheric features in rocky exoplanets. We also find that stellar spectral contamination can lead to systematic errors in radius and therefore the derived density of small planets. In the case of the TRAPPIST-1 system, we show that TRAPPIST-1's rotational variability is consistent with spot covering fractions and faculae covering fractions . The associated stellar contamination signals alter the transit depths of the TRAPPIST-1 planets at wavelengths of interest for planetary atmospheric species by roughly 1–15× the strength of planetary features, significantly complicating JWST follow-up observations of this system. Similarly, we find that stellar contamination can lead to underestimates of the bulk densities of the TRAPPIST-1 planets of , thus leading to overestimates of their volatile contents.

Subfields: Astronomy and Astrophysics, Education, Instrumentation

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Citations in the corpus

High Tide or Riptide on the Cosmic Shoreline? A Water-rich Atmosphere or Stellar Contamination for the Warm Super-Earth GJ 486b from JWST Observations (2023, background): “…spurious features into the transmission spectrum from inhomogenities in the stellar photosphere, a phenomenon called the “transit light source effect” (TLS; Rackham et al. 2018), also known as stellar contamination (Apai et al. 2018; Barclay et al. 2021; Garcia et al. 2022; Barclay et al. 2023).”
The Detectability and Characterization of the TRAPPIST-1 Exoplanet Atmospheres with JWST (2019, background): “Stellar effects such as limb darkening (e.g., Csizmadia et al. 2013) and heterogeneous photospheres (e.g., Rackham et al. 2017, 2018; Zhang et al. 2018) are major concerns for precision exoplanet transmission spectroscopy.”
Exoplanetary Atmospheres: Key Insights, Challenges, and Prospects (2019, background): “This is of particular significance to low-mass exoplanets orbiting M Dwarfs which are known to be active (211).”