Butterfly diagram for starspots on LQ Hya

Abstract

Information on the latitude distribution of starspots and changes in this distribution from year to year is very important for our understanding of the nature of stellar activity and for developing dynamo theory. The concept of butterfly diagrams is introduced for highly spotted stars of late spectral types, by analogy to the Maunder diagrams for the Sun. Our approach is based on the zonal spottedness models constructed by Alekseev and Gershberg. A detailed analysis is given for the single active star LQ Hya, and a comparison is made to similar analyses for several stars with two well-separated spot belts—EK Dra, VY Ari, V775 Her, and V833 Tau. The lower boundary of the butterfly diagram drifts toward the equator during the activity-rise phase, i.e., during years when the relative spotted area increases. This effect is clearly expressed for LQ Hya and other stars whose orientation enables observation of both hemispheres and virtually vanishes for V833 Tau, which is viewed nearly pole-on. The upper boundary of the diagram is virtually unchanged for all the considered spotted stars except V775 Her, for which it moves toward the pole. The drift rate of the lower boundary is −1 to −2 deg/yr, a factor of two to three smaller in magnitude than the corresponding solar value. Our analysis provides an independent confirmation of the occurrence of high-latitude spots on stars that are younger than the Sun and whose activity is high but less regular than the solar activity; it also enables the identification of the starting times of stellar cycles.