An observational correlation between stellar brightness variations and surface gravity

TLDR: An analysis of archival data reveals an observational correlation between surface gravity and root mean squared brightness variations on timescales of less than eight hours for inactive Sun-like stars at main-sequence to giant stages of evolution.

Abstract: An analysis of archival data reveals an observational correlation between stellar brightness variations and surface gravity, allowing a determination of surface gravity with a precision of better than 25 per cent for inactive Sun-like stars at main-sequence to giant stages of evolution. Variations in the brightness of solar-type stars are driven by many factors including granulation, a consequence of heat convection below the photosphere. And as granulation is correlated with surface gravity, variations in brightness can be used as a measure of surface gravity. Fabienne Bastien et al. analyse archival data from NASA's Kepler mission and show that brightness fluctuations on timescales of less than 8 hours are correlated with the surface gravity in Sun-like stars in various evolutionary phases. Using straightforward measurements of this type it should be possible to determine the surface gravities of many of the stars observed by Kepler. Surface gravity is a basic stellar property, but it is difficult to measure accurately, with typical uncertainties of 25 to 50 per cent if measured spectroscopically1,2 and 90 to 150 per cent if measured photometrically3. Asteroseismology measures gravity with an uncertainty of about 2 per cent but is restricted to relatively small samples of bright stars, most of which are giants4,5,6. The availability of high-precision measurements of brightness variations for more than 150,000 stars7,8 provides an opportunity to investigate whether the variations can be used to determine surface gravities. The Fourier power of granulation on a star’s surface correlates physically with surface gravity9,10: if brightness variations on timescales of hours arise from granulation11, then such variations should correlate with surface gravity. Here we report an analysis of archival data that reveals an observational correlation between surface gravity and root mean squared brightness variations on timescales of less than eight hours for stars with temperatures of 4,500 to 6,750 kelvin, log surface gravities of 2.5 to 4.5 (cgs units) and overall brightness variations of less than three parts per thousand. A straightforward observation of optical brightness variations therefore allows a determination of the surface gravity with a precision of better than 25 per cent for inactive Sun-like stars at main-sequence to giant stages of evolution.