Impact of stellar spots on the high-resolution transmission spectra of a giant planet around a Sun-like star

TL;DR

Stellar spots significantly affect high-resolution transmission spectra of exoplanets, creating distortions that depend on spot properties and stellar rotation, which can mimic or obscure planetary atmospheric signals.

Contribution

This study models the impact of unocculted stellar spots on transmission spectra using simulations, revealing how spot characteristics influence spectral line distortions.

Findings

Na I and Ca II K lines are most affected by stellar spots.

Spot size and position significantly alter line profile distortions.

Stellar rotation enhances the amplitude and asymmetry of spot-induced signals.

Abstract

Transmission spectroscopy has enabled the analysis of exoplanet atmospheres. However, a major challenge is the noise from host stars, caused by stellar activity such as dark spots and bright plages. This noise can mimic or obscure signals in transmission spectra, complicating the study of exoplanetary atmospheres. We aim to characterize how unocculted stellar spots impact planetary absorption line profiles during transit by analyzing planet-occulted line distortions (POLDs). We used the SOAPv4 tool to simulate transits of a hot Jupiter orbiting a Sun-like star under different spot configurations. We analyze the induced POLDs in the Ca II K, the Na I doublet, and H-alpha lines. Our simulations show that POLDs vary with spot size, position, and stellar rotation. The Na I and Ca II K lines exhibit the strongest distortions, while H-alpha is comparatively less affected. Low-latitude spots and higher values of v sin i enhance both the amplitude and asymmetry of distortions, whereas high-latitude spots have a weaker impact. Larger spots generally lead to more pronounced modifications of line profiles, although their relative effect can decrease due to rotational broadening. Our results show that non-occulted stellar spots imprint structured and line-dependent distortions in high-resolution transmission spectra, with amplitudes and velocity shifts shaped by the combined effects of activity level, stellar rotation, and spot geometry. The projected spot area emerges as the dominant factor controlling the strength of these signatures, while the line response varies, with Ca II K being the most sensitive and H-alpha displaying distinctive asymmetric features. These findings demonstrate that stellar surface heterogeneities can mimic or alter planetary signals, highlighting the importance of detailed modeling for the reliable interpretation of upcoming observations.