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 Hα 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 ∼−0.73 for the range 1032 to 1034 erg. We obtained
Prot=(2.848±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 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 Hα 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 α2-dynamo