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

Benjamin V. Rackham, Daniel Apai, Mark S. Giampapa

Research output: Contribution to journalArticle

49 Citations (Scopus)

Abstract

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.

Original languageEnglish (US)
Article number122
JournalAstrophysical Journal
Volume853
Issue number2
DOIs
StatePublished - Feb 1 2018

Fingerprint

dwarf planets
faculae
transit
light sources
contamination
coverings
planet
planets
extrasolar planets
atmospheric feature
starspots
stellar spectra
James Webb Space Telescope
systematic errors
illuminating
bulk density
effect
illumination
probe
wavelength

Keywords

  • methods: numerical
  • planets and satellites: atmospheres
  • planets and satellites: fundamental parameters
  • stars: activity
  • starspots
  • techniques: spectroscopic

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

The Transit Light Source Effect : False Spectral Features and Incorrect Densities for M-dwarf Transiting Planets. / Rackham, Benjamin V.; Apai, Daniel; Giampapa, Mark S.

In: Astrophysical Journal, Vol. 853, No. 2, 122, 01.02.2018.

Research output: Contribution to journalArticle

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