Internship report: Exploring the chromospheric emission and photometric variability of low-activity main-sequence stars

The log R HK , a measure of ultraviolet chromospheric emission, is used as a stellar sample selection tool in exoplanet surveys to reduce activity-induced noise. It has indeed been found that above log R HK = −4.7, the Vaughan-Preston gap, photospheric activity increases quite linearly with chromospheric emission [Radick et al., 1998]. In this project, the validity of the log R HK as a photospheric activity indicator below this gap is explored. For this purpose, stellar spectroscopy and photometry are retrieved simultaneously on a stellar sample considered mostly inactive. The log R HK is computed in a parallel study, Vivien [2018], while this project focuses on retrieving the amplitude of variability of the stellar light curves. Autocorrelation Functions, RMS and Gaussian Processes techniques are performed on the data and their relevance is evaluated. It is found that in the low activity range, the relationship between amplitude of variability and log R HK is quite spread, although an increasing trend remains. This casts doubt on the usefulness of the indicator, and calls for a reassesment of chromospheric and photospheric activity correlation.