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

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.

Figures (15)

• Figure 1: Doppler map of the visible hemisphere of the Sun-like star. Colored open circles indicate the planet positions during transit, moving from red to purple as the transit progresses. For clarity, the data points are undersampled in the figure; the full dataset was used in the analysis. The dotted black line shows the transit path, while the black disk represents a simulated spot with a filling factor of 1% smeared over time.
• Figure 2: Normalized simulated spectra of the quiet Sun (blue) and of a spot (red) for Ca ii, Na i and H$\alpha$ spectral regions.
• Figure 3: Top Panel: Transmission spectrum in the stellar rest frame for a planet-spot configuration with planet orbital phase (position) indicated by the colored circles. The colour coding identical to the one use in Figure \ref{['fig1']}. Bottom Panel: Zoom of the normalized spot and quiet Suns pectra shown in Figure \ref{['fig2']} for the three spectral regions studied.
• Figure 4: Tomography plots of the individual absorption spectra around the Na i doublet for a simulated hot Jupiter. The top panel: shows the case without stellar activity, the middle panel includes the effect of a stellar spot with a coverage fraction of $f_{\mathrm{sp}} = 1\%$, and the bottom panel displays the isolated spot contribution, obtained as the difference between the two upper panels. The color scale represents the absorption $(1 - F_{\mathrm{in}} / F_{\mathrm{out}})$ in percent, while the dashed white line traces the expected planetary trail. The horizontal black lines indicate the four contact points of the transit. For this case we adopt $v \sin i = 2\,\mathrm{km\,s^{-1}}$.
• Figure 5: Top: Mean in-transit absorption spectra in the planetary rest frame around the Na i D1 line. Bottom: Spot contribution as the difference obtained by subtracting the quiet-star profiles from the star with a spot profiles. Each column represents (from left to right) the impact of spot size, spot position (longitude and latitude, respectively), and stellar rotational velocity. According to each case the fixed parameters were $f_{\text{sp}}$ = 1%, $\theta_{lat}$ = $25^\circ$, $\theta_{lon}$ = $20^\circ$ and $v \sin i$ = $2\,\mathrm{km\,s^{-1}}$.