The effects of stellar activity on optical high-resolution exoplanet transmission spectra

TL;DR

This study investigates how stellar activity, like spots and faculae, can contaminate exoplanet transmission spectra, especially in certain spectral lines, affecting the interpretation of planetary atmospheres.

Contribution

It provides detailed simulations showing how stellar active regions influence specific spectral lines in transmission spectra, highlighting the importance of considering stellar activity in exoplanet studies.

Findings

Active regions can cause significant contamination in Hα, Ca II K, and Na I D lines.

Spots and filaments have a minor impact compared to facular emission.

He I 10830 Å is less affected and useful for probing planetary atmospheres.

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 3933 \AA, Na I 5890 \AA, H I 6563 \AA\ (H$\alpha$), and He I 10830 \AA. We find that strong facular emission and large coverage fractions can contribute a non-negligible amount to transmission spectra, especially for H$\alpha$, 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 H$\alpha$ for example, can reach $\approx 0.2\%$ 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 10830 \AA\ is not strongly affected and absorption signatures are likely diluted, rather than enhanced, by stellar activity. He I 10830 \AA\ should thus be considered a priority for probing extended atmospheres, even in the case of active stars.