Granulation in Color: The Chromatic Stellar Flicker of LTT-9779 and WASP-121

Abstract

Granulation is a persistent feature of stars with outer convective layers and is the photospheric signature of near-surface convective energy transport. In disk-integrated observations, it produces stochastic brightness variations that contribute substantially to stellar variability on timescales of minutes to hours. High-precision, observations with JWST provide an opportunity to study the chromatic signatures of stellar granulation. This work aims to measure the amplitude and characteristic timescale of granulation-induced variability for solar-type stars and to investigate whether the granulation signal shows significant chromatic variation in the near-infrared. We accessed publicly available JWST/NIRISS SOSS time-series observations of LTT-9779 (G7V) and WASP-121 (F6V) and de-trended the light curves by fitting the transits, eclipses, and planetary phase curves. We computed power spectra from the de-trended light curves and fit them with a Harvey granulation profile. We first analyzed the broadband light curves to measure and fix the granulation timescale and then repeated the power-spectrum analysis in individual wavelength channels to measure the chromatic dependence of the granulation amplitude. For LTT-9779, we measure a broadband granulation amplitude of σ$=53.8^{+4.1}_{-4.0}$ppm at a timescale of τ$=4.8^{+1.5}_{-1.3}$min. For WASP-121, we measure σ$=59.3\pm4.3$ppm and τ$=6.3^{+2.1}_{-1.8}$min. We also measure peak oscillation frequencies of ν$=3382^{+166}_{-191}μ$Hz and $1649^{+55}_{-69}μ$Hz in the bandpass-integrated power spectra of LTT-9779 and WASP-121, respectively. In the spectroscopic fits, we find $σ$ decreases with wavelength. Our results demonstrate a new observational window on short-timescale stellar variability and provides a way to characterize chromatic stellar noise in transit light curves.