Kavli Affiliate: Sara Seager
| Summary:
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$=59.3pm4.3$ppm and τ$=6.3^+2.1_-1.8$min. We also measure peak oscillation frequencies of ν$=3382^+166_-191μ$Hz and $1649^+55_-69μ$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.
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