Exoplanets and Stellar Physics

Abstract:

As radial velocity (RV) spectrographs reach unprecedented precision and stability below 1 m s, the challenge of granulation in the context of exoplanet detection has intensified. Despite promising advancements in post-processing tools, granulation remains a significant concern for the EPRV (extremely precise radial velocity) community. We present a pilot study to detect and characterize granulation using the High-Accuracy Radial-velocity Planet Searcher for the Northern hemisphere (HARPS-N) spectrograph. We observed HD 166620, a K2 star in the Maunder Minimum phase, intensely for two successive nights, expecting granulation to be the dominant nightly noise source in the absence of strong magnetic activity. After correcting for a newly identified instrumental signature, originating from CCD illumination variations under optimal seeing conditions, we detected the granulation signal using structure-function (SF) analysis and a single-component Gaussian process (GP) model. The granulation signal has a characteristic time-scale of min, within 1, and a standard deviation of cm s, within 3 of the predicted value. By examining spectra and RVs as a function of line formation temperature, we investigated the sensitivity of granulation-induced RV variations across different photospheric layers. We extracted RVs from various photospheric depths using both the line-by-line and cross-correlation function methods to mitigate any extraction method biases. Our findings indicate that granulation variability is detectable in both temperature bins, with the cooler bins, corresponding to the shallower layers of the photosphere, aligning more closely with predicted values.

Abstract:

The search for atmospheres on rocky exoplanets is a crucial step in understanding the processes driving atmosphere formation, retention, and loss. Past studies have revealed the existence of planets interior to the radius valley with densities lower than would be expected for pure-rock compositions, indicative of the presence of large volatile inventories, which could facilitate atmosphere retention. Here, we present an analysis of the JWST/NIRSpec G395H transmission spectrum of the warm ( Teq, AB=0=569 K) super-Earth TOI-270 b (Rp = 1.306 R⊕), captured alongside the transit of TOI-270 d. The JWST white light-curve transit depth updates TOI-270 b’s density to ρp = 3.7 ± 0.5 g cm−3, inconsistent at 4.4σ with an Earth-like composition. Instead, the planet is best explained by a nonzero, percent-level water mass fraction, possibly residing on the surface or stored within the interior. The JWST transmission spectrum shows possible spectroscopic evidence for the presence of this water as part of an atmosphere on TOI-270 b, favoring an H2O-rich steam atmosphere model over a flat spectrum ( lnB=0.3–3.2 , inconclusive to moderate), with the exact significance depending on whether an offset parameter between the NIRSpec detectors is included. We leverage the transit of the twice-larger TOI-270 d crossing the stellar disk almost simultaneously to rule out the alternative hypothesis that the transit light source effect could have caused the water feature in TOI-270 b’s observed transmission spectrum. Planetary evolution modeling furthermore shows that TOI-270 b could sustain a significant atmosphere on gigayear timescales, despite its high stellar irradiation, if it formed with a large initial volatile inventory.