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+4.1−4.0ppm at a timescale of τ=4.8+1.5−1.3min. For WASP-121, we measure σ=59.3±4.3ppm and τ=6.3+2.1−1.8min. We also measure peak oscillation frequencies of ν=3382+166−191μHz and 1649+55−69μHz 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.
Related
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BibTeX
@misc{waalkes2026granulation,
title = {{Granulation in Color: The Chromatic Stellar Flicker of LTT-9779 and WASP-121}},
author = {William C. Waalkes and Nadiia Kostogryz and Valeriy Vasilyev and Rafael Luque and Alexander Shapiro and Jordi Blanco-Pozo and Ana Glidden and Jeff Valenti and Sami Solanki and Ignasi Ribas and Sara Seager and Laurent Gizon},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.24550},
eprint = {2610.24550},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.24550},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Granulation in Color: The Chromatic Stellar Flicker of LTT-9779 and WASP-121
AU - William C. Waalkes
AU - Nadiia Kostogryz
AU - Valeriy Vasilyev
AU - Rafael Luque
AU - Alexander Shapiro
AU - Jordi Blanco-Pozo
AU - Ana Glidden
AU - Jeff Valenti
AU - Sami Solanki
AU - Ignasi Ribas
AU - Sara Seager
AU - Laurent Gizon
PY - 2026
DA - 2026/10/17
PB - arXiv
DO - 10.48550/arxiv.2610.24550
UR - https://arxiv.org/abs/2610.24550
AB - 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_−4.0ppm at a timescale of τ=4.8^+1.5_−1.3min. For WASP-121, we measure σ=59.3±4.3ppm and τ=6.3^+2.1_−1.8min. We also measure peak oscillation frequencies of ν=3382^+166_−191μHz and 1649^+55_−69μHz 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.
ER -