Are Starspots and Plages Co-Located on Active G and K Stars?

TL;DR

This study investigates the spatial relationship between starspots and plages on active G and K stars, revealing co-location on a young G star and more dispersed plages on older K stars, through simultaneous photometry and spectroscopy.

Contribution

It provides new observational evidence on the co-location of starspots and plages on young G stars and the distribution of plages on older K stars, using multi-epoch data.

Findings

Starspots and plages are co-located on the young G star KIC 9652680.

On older K stars, plages are more extended and not centered on starspots.

Chromospheric activity varies differently with rotation in G and K stars.

Abstract

We explore the connection between starspots and plages of three main-sequence stars by studying the chromospheric and photospheric activity over several rotation periods. We present simultaneous photometry and high-resolution ($R\sim 31,500$) spectroscopy of KIC 9652680, a young, superflare-producing G1 star with a rotation period of 1.4 days. Its Kepler light curve shows rotational modulation consistent with a bright hemisphere followed by a relatively dark hemisphere, generating photometric variability with a semi-amplitude of 4%. We find that KIC 9652680 is darkest when its $S$-index of Ca II H & K emission is at its maximum. We interpret this anti-correlation between flux and $S$ to indicate that dark starspots in the photosphere are co-located with the bright plages in the chromosphere, as they are on the Sun. Moving to lower masses and slower rotators, we present K2 observations with simultaneous spectroscopy of EPIC 211928486 (K5V) and EPIC 211966629 (K4V), two active stars in the 650 Myr-old open cluster Praesepe. The K2 photometry reveals that both stars have rotation periods of 11.7 days; while their flux varies by 1 and 2% respectively, their Ca II H & K $S$-indices seem to hold relatively constant as a function of rotational phase. This suggests that extended chromospheric networks of plages are not concentrated into regions of emission centered on the starspots that drive rotational modulation, unlike KIC 9652680. We also note that the Ca II emission of EPIC 211928486 dipped and recovered suddenly over the duration of one rotation, suggesting that the evolution timescale of plages may be of order the rotation period.