The Effect of Spots on the Luminosity Spread of the Pleiades

Abstract Cool spots on the surface of magnetically active stars modulate their observed brightnesses and temperatures, thereby affecting the stellar locus on the H-R diagram. Recent high-precision space-based photometric surveys reveal the rotational modulation from spots on stars in young clusters, including K2 monitoring of the 125 Myr old Pleiades cluster. However, light curves reveal only the asymmetries in the visible spot distributions rather than the total sizes of spots on stellar surfaces, which causes a discrepancy between the spot coverage measured by photometric and spectroscopic observations. In this paper, we simulate photometric variability introduced by randomly distributed starspots on a 125 Myr old coeval cluster. Our simulation results show that randomly distributed small spots on the stellar surface would explain the discrepancy that the photometric observations only reveal 10%–40% of the spot coverage measured by spectra. The colors and luminosities of photospheres are modeled for a range of photospheric temperature, spot coverage, and spot temperature. The colors and luminosities of a simulated population are then compared to the luminosity spread of Pleiades members, excluding the 25% of candidates that are identified as non-members with Gaia DR2 astrometry. The observed luminosities of Pleiades members have a standard deviation of 0.05 dex, which could be entirely explained by spots with a star-to-star standard deviation of spot coverage of 10%, but with an average coverage area that is not well constrained.