Activity and rotation in M type stars. The study “HADES RV programme with HARPS-N at TNG. VII. Rotation and activity of M-dwarfs from time-series high-resolution spectroscopy of chromospheric indicators” of A. Suárez Mascareño (Instituto de Astrofísica de Canarias) published on Astronomy & Astrophysics

of Mario Giuseppe Guarcello    ( follow mguarce)

 

We know that the intensity of the magnetic activity in the Sun (e.g.: the family of magnetic phenomena including spots, faculae, flares, etc…) varies with an 11-years cycle. Do all the stars have such a magnetic cycle? If yes, how does it change with stellar properties (mainly mass, rotation and evolutionary phase)?

 

Observations of activity indicators spanning several years of baseline are required in order to answer these questions. One of these indicators is the X-ray emission from the coronal activity. However, X-ray observations of single stars over long temporal baseline are rare. Indicators of stellar activity are instead available in optical medium/high resolution spectra, such as the emission peaks in the Ca II H&K absorption lines due to the chromospheric activity. However, only a few projects exist with observation of indicators of stellar activity over long time baseline, such as the “Olin Wilson’s HK Project” of the Mount Wislon Observatory, with observations of the chromospheric activity of about 100 F to M stars since 1966.

 

Today, high resolution spectroscopic observations of large stellar samples and over long time baselines are available thanks to the projects dedicated to the search of exoplanets with the radial velocity technique (link), such as the  “HADES Radial Velocity Program” based on observations with the spectrograph HARPS-N on the Telescopio Nazionale Galileo. Stellar activity, in fact, can produce periodic radial velocity signals which can mimic the presence of exoplanets. In the study “HADES RV programme with HARPS-N at TNG. VII. Rotation and activity of M-dwarfs from time-series high-resolution spectroscopy of chromospheric indicators” of A. Suárez Mascareño (Instituto de Astrofísica de Canarias), recently published by Astronomy & Astrophysics with the collaboration of the astronomers L. Affer, G. Micela, J. Maldonado, E. González-Alvarez e A. Maggio of the Astronomical Observatory of Palermo, is focused on the chromospheric activity and rotation of a sample of 71 M type stars. The authors of this study have combined HARPS-N observations with archival observations taken with the spectrograph HARPS of the European Southern Observatory and photometric observations in V and I bands from the All Sky Automated Survey (ASAS), covering a long time baseline which goes from 4 to 12 years. This allowed to determine the length of the cycle in 13 stars, the rotation period in 33 stars and the amplitude of the radial velocity signals due to stellar activity in 16 stars, and to find an interesting relation between the rotation period (which in this sample is in the 20-150 days range with a mean value of 40 days) and the indicator of chromospheric activity log10(RHK), which supports the role of stellar rotation in the intensity of stellar activity also in M type stars.

 

The figure (link) shows the relation between the indicator of chromospheric activity log10(RHK) and the rotation period found in this study.