Year-end Sale % OFF 
Abstract
Aims. Other stars are known to have magnetic cycles similar to that of the Sun. The relationship between these activity variations and the observed radial-velocity is still not satisfactorily understood. In this first paper our aim is to assess the long-term activity variations in the low end of the main sequence, having in mind a planetary search perspective. Methods. We used a sample of 30 M0-M5.5 stars from the HARPS M-dwarf planet search program with a median timespan of observations of 5.2 years. We computed chromospheric activity indicators based on the Ca II H and K, H{\alpha}, He I D3, and Na I D1 and D2 lines. All data were binned in to average out undesired effects such as rotationally modulated atmospheric inhomogeneities. We searched for long-term variability of each index and determined the correlations between them. Results. While the S_CaII, H{\alpha}, and Na I indices showed significant variability for a fraction of our stellar sample (39%, 33%, and 37%, respectively), only 10% of our stars presented significant variability in the He i index. We therefore conclude that this index is a poor activity indicator at least for this type of stars. Although the H{\alpha} shows good correlation with S_Ca II for the most active stars, the correlation is lost when the activity level decreases. This result appears to indicate that the Ca II-H{\alpha} correlation is dependent on the activity level of the star. The Na I lines correlate very well with the S_Ca II index for the stars with low activity levels we used, and are thus a good chromospheric activity proxy for early-M dwarfs. We therefore strongly recommend the use of the Na I activity index because the signal-to-noise ratio in the sodium lines spectral region is always higher than for the calcium lines.
Highlights
The increase in precision of the radial-velocity instruments is leading to the detection of smaller reflex semi-amplitude signals induced by extra-solar planets
The sample comes from the HARPS (Mayor et al 2003) Mdwarf planet search program, which corresponds to a volume limited selection of stars brighter than V = 14 mag and with a projected rotational velocity v sin i 6.5 km s−1
A trend can be observed when the correlation coefficient is plotted against the mean S Ca II: for values of S lower than around 0.035 there are no significant correlations and for S higher than ∼0.035, there are only positive correlations and some of them with statistical significance. This trend is not observed when the correlation is plotted against the mean values of Hα. An interpretation of this could be that because the Hα is more sensitive to filaments than the S -index, as the activity gets stronger the contribution of plages becomes more important to the Hα index than the contribution coming from filaments, because their contribution saturates at a certain activity level (Meunier & Delfosse 2009)
Summary
The increase in precision of the radial-velocity instruments is leading to the detection of smaller reflex semi-amplitude signals induced by extra-solar planets (e.g. the 1.9 M⊕ planet discovered by Mayor et al 2009). In a recent paper we studied a sample of seven late-G and early-K dwarfs with well-known magnetic cycles and compared the activity level of these stars with their radial-velocity variations over a timespan of five years (Santos et al 2010). The long-term variations of the S index could be detected in the cross-correlation function parameter’s line bisector span, full-width-at-half-maximum and contrast, implying that these parameters could be used to follow the activity cycles of these stars In this first paper we extend this study to the lower end of the main sequence and use a more extended sample.
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
