X-ray emission regimes and rotation sequences in the M34 open cluster

Abstract

Abstract. I report on a correlation between the saturated and non-saturated regimes of Xray emission and the rotation sequences that have been observed in the M34 open cluster. An interpretation of this correlation in term of magnetic activity evolution in the early stage of evolution on the main sequence is presented. X-ray fluxes were derived from the source count rates using energy conversion factors (ECF) calculated in the 0.5-4.5 keV range (Gondoin 2006). The X-ray fluxes were converted into stellar X-ray luminosities assuming a distance of 470 pc (Jones & Prosser 1996). The X-ray luminosity distribution of the sample stars rolls off at luminosities lower than L X ≈ 10 29 erg s −1 , which provides a sensitivity limit estimate of the XMM − Newton observation. The mass of the sample stars ranges from 0.4 M to 1.3 M and reaches a maximum around 0.8 M . Their rotation periods are between 0.49 days and 11 days. X-ray observations of the M34 open clusterRecent studies have shown that stars tend to group into two main sub-populations that lie on narrow sequences in diagrams where the measured rotation periods of the members of a young stellar cluster are plotted against their B − V colors. Figure 1 (left) shows the rotational periods P of the sample stars as a function of their reddening corrected (B − V) 0 indices. The color-period diagram also displays the I and C rotational sequences of M34 along the form established by Barnes (2007) and Meibom et al. (2011). The proximity of the M34 data points to these curves was used to determine their membership to the I sequence, to the C sequence or to the gap. Figure 1 (right) displays the X-ray to bolometric luminosity ratio L X /L bol of the sample stars as a function of their Rossby number (Ro = P /τ c ) distinguishing members of the I sequence, of the C sequence and of the gap. Figure 1 shows a correlation between the X-ray activity regimes and the rotation sequences. Indeed, members of the C sequence have small Rossby numbers (Ro < 0.1), and an X-ray to bolometric luminosity level close to the 10 −3 saturation level. Members of the I sequence, in contrast, have larger Rossby numbers (Ro 0.17), and an X-ray to bolometric luminosity ratio significantly smaller than the saturation limit. Remarkably, 106at https://www.cambridge.org/core/terms. https://doi