SOAPv4: A new step toward modeling stellar signatures in exoplanet research

TL;DR

SOAPv4 advances exoplanet host-star modeling by integrating time-series spectroscopy with line-by-line and NLTE considerations, enabling realistic simulations of photospheric activity and planet-occulted distortions across wavelengths. By permitting AR-specific input spectra and chromatic contrasts, it produces self-consistent transmission spectra (POLDs) and chromatic RVs, validated against solar observations and the HD 209458 system. Key findings show NLTE spectra better reproduce observed absorption wings, while granulation and center-to-limb variations can masquerade as planetary atmospheric signals, underscoring the need for physically grounded modeling. The work enhances exoplanet atmosphere inferences and RV analyses by providing a flexible, spectrum-wide framework to disentangle stellar activity from planetary signals, with clear paths for future 3D convection integration and expanded solar-analog benchmarking.

Abstract

We present and describe a new version of the spot oscillation and planet code, SOAPv4. Our aim is to demonstrate its capabilities in modeling stellar activity in the context of RV measurements and its effects on transmission spectra. To do this, we employed solar observations alongside synthetic spectra and compared the resulting simulations. We used SOAPv4 to simulate photospheric active regions and planetary transits for a Sun-like star hosting a hot Jupiter. By varying the input spectra, we investigated their impact on the resulting absorption spectra and compared the corresponding simulations. We then assessed how stellar activity deforms these absorption profiles. Finally, we explored the chromatic signatures of stellar activity across different wavelength ranges and discussed how such effects have been employed in the literature to confirm planet detections in radial-velocity measurements. We present the latest updates to SOAP, a tool developed to simulate active regions on the stellar disk while accounting for wavelength-dependent contrast. This functionality enables a detailed study of chromatic effects on radial-velocity measurements. In addition, SOAPv4 models planet-occulted line distortions and quantifies the influence of active regions on absorption spectra. Our simulations indicate that granulation can introduce line distortions that mimic planetary absorption features, potentially leading to misinterpretations of atmospheric dynamics. Furthermore, comparisons with ESPRESSO observations suggest that models incorporating non-local thermodynamic equilibrium effects provide an improved match to the absorption spectra of HD 209458 b, although they do not fully reproduce all observed distortions.

Figures (16)

• Figure 1: Depiction of the SOAP projection of a stellar disk onto a grid of $20 \times 20$ pixels for illustration purposes. The color map represents the local normalized flux in each position, which is determined by the limb-darkening. The white circumference outlines the original disk for reference
• Figure 2: Position of a spot with a radius of $15\%$ of the stellar radius at a latitude and longitude of 45° and projected onto a $300\times300$ pixel grid. The vertical and horizontal green lines indicate the precomputed bounding box that marks the minimum and maximum Cartesian coordinates within the grid. The brown points represent the calculated positions that define the spot boundary in its final projected location.
• Figure 3: Local line profiles centered on the sodium $\text{D}_2$ line at different limb angles for a solar-like star that rotates as a solid body at 20 $\text{km s}^{-1}$. The representation illustrates velocity variations from the disk center to the eastern limb for a star rotating from west to east. As the pixels approach the limb, the flux decreases due to limb darkening, while the projected velocity component increases.
• Figure 4: Variation in the sodium doublet line properties as a function of temperature contrast in regard to the solar case. Positive contrasts correspond to spectra of faculae, and negative contrasts represent spectra of spots. The quiet-star spectrum is indicated by the dashed white line at the zero level. All spectra used to construct the figure were taken from the PHOENIX library.
• Figure 5: IAGNaI D$_2$ (top) and NaI D$_1$ (bottom) NaI lines as a function of limb position. The wavelength interval was selected to represent a 3$\AA$ region surrounding each line. Contaminated regions that are affected by neighboring lines were excluded, as demonstrated by the comparison with the integrated star spectrum shown in black.