Activity-rotation in the dM4 star Gl 729. A possible chromospheric cycle
Journal
Date Issued
2020
Author(s)
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.
Volume
644
Issue
A2
Start page
1
Issn Identifier
0004-6361
Rights
open.access
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