The Effects of Stellar Activity on Optical High-resolution Exoplanet Transmission Spectra

Abstract

Chromospherically sensitive atomic lines display different spectra in stellar active regions, spots, and the photosphere, raising the possibility that exoplanet transmission spectra are contaminated by the contrast between various portions of the stellar disk. To explore this effect, we performed transit simulations of G and K-type stars for the spectral lines Ca II K at 3933Å, Na I 5890Å, H I 6563 A (Hα), and He I 10830Å. We find that strong facular emission and large coverage fractions can contribute a non-negligible amount to transmission spectra, especially for Hα, Ca II K, and Na I D, while spots and filaments are comparatively unimportant. The amount of contamination depends strongly on the location of the active regions and the intrinsic emission strength. In particular, active regions must be concentrated along the transit chord in order to produce a consistent in-transit signal. Mean absorption signatures in Na I and Hα for example, can reach ≈ 0.2% and 0.3%, respectively, for transits of active latitudes with line emission similar in strength to moderate solar flares. Transmission spectra of planets transiting active stars, such as HD 189733, are likely contaminated by the contrast effect, although the tight constraints on active region geometry and emission strength make it unlikely that consistent in-transit signatures are due entirely to the contrast effect. He I 10830Å is not strongly affected and absorption signatures are likely diluted, rather than enhanced, by stellar activity. He I 10830Å should thus be considered a priority for probing extended atmospheres, even in the case of active stars.

ACCESS: Ground-based Optical Transmission Spectroscopy of the Hot Jupiter WASP-4b

“…Assessing the level of TLSE contamination is critical to correctly interpreting high-precision transmission spectra of transiting exoplanets . This is particularly important for investigating features that could have a planetary or stellar origin, such the He 10,833 Å line (Spake et al 2018) or the Na and K alkali lines (Cauley et al 2018). We have previously demonstrated this effect in the spectrum of GJ 1214b (Rackham et al 2017), which has a featureless near-IR spectrum but a visible spectrum that is apparently offset below the near-IR transit depths.…”

Evidence for He i 10830 Å Absorption during the Transit of a Warm Neptune around the M-dwarf GJ 3470 with the Habitable-zone Planet Finder

“…Stellar Activity: Some M dwarfs can be active, and exhibit time variability in the He 10830 absorption line (we note that not all M dwarfs are active). In comparison to other transmission spectroscopy lines like Ca II K, Na I D, and Hα, active regions typically have He 10830 in absorption (Cauley et al 2018). Thus, when a planet transits an active spot region on the host star, He 10830 is more likely to result in an emission signature, than an absorption signature.…”

Disentangling the Planet from the Star in Late-Type M Dwarfs: A Case Study of TRAPPIST-1g

“…In addition, the time variable nature of stellar activity can have an impact on transmission spectra produced from observations obtained from different epochs (e.g., Zellem et al 2017). Here we adopt the term contrast effect from Cauley et al (2018) to describe the effect that differences in the spectral signatures of various stellar regions -active or otherwise -has on the bulk point source properties of the star.…”