Revisiting the cycle-rotation connection for late-type stars

Abstract

Aims. We analyse the relation between the activity cycle length and the Rossby number, which serves as a ‘normalised’ rotation period and appears to be the natural parameter in any cycle relation. Methods. We collected a sample of 44 main sequence stars with well-known activity cycle periods and rotation periods. To compute the Rossby numbers from the observed rotation periods, we used the respective B − V-dependent empirical turnover-times and derived the empirical relation between the cycle length and Rossby number. Results. We found a linear behaviour in the double-logarithmic relation between the Rossby number and cycle period. The bifurcation into a long and a short period branch is clearly real but it depends, empirically, on the colour index B − V, indicating a physical dependence on effective temperature and position on the main sequence. Furthermore, there is also a correlation between cycle length and convective turnover time with the relative depth of the convection zone. Based on this, we derived empirical relations between cycle period and Rossby number individually for narrow B − V ranges, for both cycle branches, as well as a global relation for the short-period branch. For the short period cycle branch relations, we estimated a scatter of the relative deviation between 14% and 28% on the long-period cycle branch. With these relations derived purely from stellar data, we obtained a good match with the 10.3−1.0+1.1 yr period for the well known 11-year solar Schwabe cycle and a long-period branch value of 104−34+50 yr for the Gleissberg cycle of the Sun. Finally, we suggest that the cycles on the short-period branch appear to be generated in the deeper layers of the convective zone, while long-period branch cycles seem to be related to fewer deep layers in that zone. Conclusions. We show that for a broader B − V range, the Rossby number is a more suitable parameter for universal relation with cycle-rotation than just the rotation period alone. As proof, we demonstrate that our empirical stellar relations are consistent with the 11-year solar Schwabe cycle, in contrast to earlier studies using just the rotation period in their relations. Previous studies have tried to explain the cycle position of the Sun in the cycle-rotation presentation via other kinds of dynamo, however, in our study, no evidence is found that would suggest another type of dynamo for the Sun and other stars.