Long-term magnetic activity of a sample of M-dwarf stars from the HARPS program

Abstract

Context. The search for extra-solar planets similar to Earth is becoming a reality, but as the level of the measured radial-velocity reaches the sub-m s −1 , stellar intrinsic sources of noise capable of hiding the signal of these planets from scrutiny become more important. 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. Following our previous work, which studied the correlation between activity cycles and long-term velocity variations for K dwarfs, we now expand it to the lower end of the main sequence. 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, Hα, He i D3, and Na i D1 and D2 lines. All data were binned 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 Ca II , Hα, 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 Hα 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−Hα 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.

Modelling the RV jitter of early-M dwarfs using tomographic imaging

“…This index is defined as the ratio between the flux in the Hα absorption line and that in the surrounding continuum, as described in Boisse et al (2011). We use a 0.16 nm window centered at 656.2808 nm for the central line, and two windows of 1.075 and 0.875 nm around 655.087 and 658.031 nm respectively for the continuum as presented in Gomes Da Silva et al (2011).…”

Metallicity of M dwarfs

“…We suspected that this behaviour was due to activity or rotation and did a a posteriori study of the impact of the activity with our technique. To this end, we used the normalised Hα luminosity, log L H α /L bol , from Reiners et al (2012), for the stars in common with our full sample, as well as the median of individual measurements of the H α index defined by Gomes da Silva et al (2011), kindly provided by the author. Table 4 lists both activity indicators, in Cols.…”

“…2 and 3, the right ascension and declination of the star, respectively. Column 4 depicts the S/N, Col. 5 the normalised Hα index taken from Reiners et al (2012), and Col. 6 the Hα index described by Gomes da Silva et al (2011). Column 7 shows the metallicity calculated with the coefficient matrix used in our previous work, Neves et al (2013), Col. 8 the [Fe/H] obtained in our work, along with its associated error in Col. 9.…”

A vigorous activity cycle mimicking a planetary system in HD 200466

“…Gomes da Silva et al (2011) concluded that H α shows good correlation with S Ca II for the most active stars, indicating that the correlation between Ca II and H α depends on the activity level of the star. The correlation between different activity indicators is complex and can be different depending on the star (Cincunegui et al 2007; see also Meunier & Delfosse 2009).…”

“…Various authors explained such observations by means of theoretical modelling (Saar & Donahue 1997;Hatzes 2002;Desort et al 2007). On the other hand, the impact of solar-like activity cycles on high-precision RVs has been studied only recently by analysing the correlations between observed jitter and chromospheric activity for the stars in the California Planet Search (Isaacson & Fischer 2010) or by computing chromospheric activity indicators based on some element lines as in the HARPS program (Gomes da Silva et al 2011). Some authors proposed several methods and models to reduce the associated noise Meunier & Lagrange 2013) and predict the activity-induced RV variations (Lanza et al 2011).…”