Activity-rotation in the dM4 star Gl 729. A possible chromospheric cycle

TL;DR

This study investigates the magnetic activity and potential activity cycle of the young dM4 star Gl 729, revealing a four-year cycle and suggesting a non-solar dynamo mechanism.

Contribution

It provides the first evidence of a long-term activity cycle in a fully convective star, supporting the possibility of an alpha-squared dynamo.

Findings

Detected a ~4-year activity cycle in Gl 729.

Found no evidence of differential rotation.

Established correlations between activity indexes.

Abstract

Recently, new debates about the role of layers of strong shear have emerged in stellar dynamo theory. Further information on the long-term magnetic activity of fully convective stars could help determine whether their underlying dynamo could sustain activity cycles similar to the solar one. We performed a thorough study of the short- and long-term magnetic activity of the young active dM4 star Gl 729. First, we analyzed long-cadence $K2$ photometry to characterize its transient events (e.g., flares) and global and surface differential rotation. Then, from the Mount Wilson $S$-indexes derived from CASLEO spectra and other public observations, we analyzed its long-term activity between 1998 and 2020 with four different time-domain techniques to detect cyclic patterns. Finally, we explored the chromospheric activity at different heights with simultaneous measurements of the H$\alpha$ and the Na I D indexes, and we analyzed their relations with the $S$-Index. We found that the cumulative flare frequency follows a power-law distribution with slope $\sim- 0.73$ for the range $10^{32}$ to $10^{34}$ erg. We obtained $P_{rot} = (2.848 \pm 0.001)$ days, and we found no evidence of differential rotation. We also found that this young active star presents a long-term activity cycle with a length of $\text{about four}$ years; there is less significant evidence of a shorter cycle of $0.8$ year. The star also shows a broad activity minimum between 1998 and 2004. We found a correlation between the S index, on the one hand, and the H$\alpha$ the Na I D indexes, on the other hand, although the saturation level of these last two indexes is not observed in the Ca lines. Because the maximum-entropy spot model does not reflect migration between active longitudes, this activity cycle cannot be explained by a solar-type dynamo. It is probably caused by an $\alpha^2$-dynamo.