Double Dipping: A New Relation between Stellar Rotation and Starspot Activity

We report the discovery of a new relationship between a simple morphological characteristic of light curves produced by starspots and stellar rotation periods. The characteristic we examine is whether the light curve exhibits one dip or two during a single rotation. We analyze thousands of Kepler light curves of main-sequence stars with temperatures from 3200 to 6200 K. Almost all the stars exhibit segments of their light curve that contain either single-or doubledip segments (very few have more than two significant dips per rotation). We define a variable, the single/double ratio (SDR) that expresses the ratio of the time spent in single mode to the time spent in double mode. Unexpectedly, there is a strong relationship between the SDR and the stellar rotation period, in the sense that longer periods come with a larger fraction of double segments. Even more unexpectedly, the slopes of the SDR-period relations are a clear function of stellar temperature. We also show that the relationships of spot variability amplitude (Rvar) to rotation period have similar levels of scatter, slopes, and dependence on temperature as the SDR-period relations. Finally, the median Rvar of single segments tends to be about twice that of double segments in a given light curve. We offer some tentative interpretations of these new results in terms of starspot coverage and lifetimes. It will be fruitful to look further into this novel rotation-activity relation, and better understand what information these aspects of the morphology of light curves bring to our knowledge of stellar magnetic activity.