Scaling relations of convective granulation noise across the HR diagram from 3D stellar atmosphere models

Abstract

High-precision photometric data from space missions have improved our understanding of stellar granulation. These observations have shown with precision the stochastic brightness fluctuations of stars across the HR diagram, allowing us to better understand how stellar surface convection reacts to a change in stellar parameters. These fluctuations need to be understood and quantified in order to improve the detection and characterization of exoplanets. In this work, we provide new scaling relations of two characteristic properties of the brightness fluctuations time series, the standard deviation (σ) and the auto-correlation time ($\tau \rm _{ACF}$). This was done by using long time series of 3D stellar atmosphere models at different metallicities and across the HR diagram, generated with a 3D radiative hydrodynamical code: the STAGGER code. We compared our synthetic granulation properties with the values of a large sample of Kepler stars, and analyzed selected stars with accurate stellar parameters from the Kepler LEGACY sample. Our 3D models showed that σ $\propto \nu \rm _{max}^{-0.567\pm 0.012}$ and $\tau \rm _{ACF}$$\propto \nu \rm _{max}^{-0.997\pm 0.018}$ for stars at solar metallicity. We showed that both σ and $\tau \rm _{ACF}$ decrease with metallicity, although the metallicity dependence is more significant on σ. Unlike previous studies, we found very good agreement between σ from Kepler targets and the 3D models at $\rm {\log }g$ ≤3.5, and a good correlation between the stars and models with $\rm {\log }g$ ≥3.5. For $\tau \rm _{ACF}$, we found that the 3D models reproduced well the Kepler LEGACY star values. Overall, this study shows that 3D stellar atmosphere models reproduce the granulation properties of stars across the HR diagram.

Connecting photometric and spectroscopic granulation signals with CHEOPS and ESPRESSO

“…Through photometric observations, the properties of granulation as a function of stellar parameters have been revealed: CoRoT observations have shown that the granulation timescale and amplitudes decrease with the increasing characteristic frequency of the acoustic modes, the so-called frequency at maximum power ν max , which depends on the stellar surface gravity and temperature (Michel et al 2008). In line with model predictions (see e.g., Svensson & Ludwig 2005;Ludwig 2006;Trampedach et al 2013;Tremblay et al 2013;Beeck et al 2013;Samadi et al 2013;Beeck et al 2015), numerous studies based on Kepler observations (Mathur et al 2011;Bastien et al 2013;Kallinger et al 2014;Bastien et al 2016;Pande et al 2018;Bugnet et al 2018;Tayar et al 2019;Sulis et al 2020a;Rodríguez Díaz et al 2022) have also shown that the Article number, page 1 of 30 arXiv:2211.14398v1 [astro-ph.SR] 25 Nov 2022 granulation properties are dependent on the stellar fundamental parameters (T eff , logg, and [Fe/H]). Granule size increases with lower stellar surface gravities and/or larger effective temperatures, and therefore with a decreasing ν max .…”

“…16 The logg for the Kepler targets was obtained by different techniques, from the highest priority to the lowest ones, depending on availability: asteroseismology, high-resolution spectroscopy, low-resolution spectroscopy, flicker, photometric observations, and finally the KIC (see https://exoplanetarchive.ipac.caltech.edu/docs/KSCI-19097-004.pdf). able agreement with bolometric light variations of KEPLER targets (Rodríguez Díaz et al 2022). Previous studies have also used these kind of models to study the granulation signal (see e.g., Ludwig 2006;Trampedach et al 2013;Tremblay et al 2013;Samadi et al 2013), and compared their results with observations.…”

“…With this scaling, Rodríguez Díaz et al ( 2022) was able to determine the standard deviation of the stellar disk-integrated intensity from the small box models. We refer the reader to Rodríguez Díaz et al (2022) for a more detailed description of the method.…”

“…While computationally expensive (leading to long time series difficult to generate), they have been recently used to generate disk-integrated spectra (Cegla et al 2019) and RV time series (Sulis et al 2020b) of solar granulation. They have also been used to generate synthetic brightness fluctuations whose standard deviation and autocorrelation time match those of Kepler targets from dwarfs to red giants with an overall very good agreement (Rodríguez Díaz et al 2022). Moreover, Cegla et al (2019) predicted correlations between the photometric and spectroscopic signals of stellar granulation, but this has yet to be confirmed with high-precision observations.…”

55 Cancri e’s occultation captured with CHEOPS

“…8 of Kallinger et al 2014). Finally, 3D hydrodynamical models of stellar granulation predict an RMS of 45.2 ppm for 55 Cnc A (see Table 2 of Rodríguez Díaz et al 2022). To compare this value with our dataset, we binned the occultation time series into 3-minute intervals and filtered the periods > 27 min (see details in Sulis et al 2022).…”