Hydrodynamical simulations of convection-related stellar micro-variability. I. Statistical relations for photometric and photocentric variability

TL;DR

This paper develops statistical relations to quantify how convective surface flows in late-type stars cause brightness and photocenter fluctuations, impacting high-precision astrometry and providing insights into stellar micro-variability.

Contribution

It introduces new formulae linking local hydrodynamical model intensities to observable stellar brightness and photocenter fluctuations, enhancing understanding of stellar micro-variability.

Findings

Brightness fluctuations scale with the inverse square root of the number of convective cells.

Photocentric position fluctuations are independent of the number of cells and limb-darkening.

Fluctuations can limit high-precision astrometry in giant stars.

Abstract

Local-box hydrodynamical model atmospheres provide statistical information about the spatial dependence, as well as temporal evolution, of a star's emergent radiation field. Here, we consider late-type stellar atmospheres for which temporal changes of the radiative output are primarily related to convective (granular) surface flows. We derived relations for evaluating the granulation-induced, disk-integrated thus observable fluctuations of the stellar brightness and location of the photocenter from radiation intensities available from a local model. Apart from their application in the context of hydrodynamical stellar atmospheres, these formulae provide some broader insight into the nature of the fluctuations under consideration. Brightness fluctuations scale inversely proportional to the square root of the number of convective cells (the statistically independently radiating surface elements) present on the stellar surface and increase with more pronounced limb-darkening. Fluctuations of the stellar photocentric position do not depend on the number of cells and are largely insensitive to the degree of limb-darkening. They amount to a small fraction of the typical cell size, and can become a limiting factor for high-precision astrometry in the case of extreme giants. The temporal brightness and positional fluctuations are statistically uncorrelated but closely related in magnitude.