Name/Affiliation: Philippe Gondoin (European Space Agency)
The transition between X-ray emission regimes in the M34 open cluster
The X-ray emission from late-type stars in open clusters exhibits two kinds of dependences on stellar rotation. Fast rotators show a relatively constant X-ray to bolometric luminosity ratio at a so-called saturation level. Slower rotators show a decline of their X-ray emission with decreasing rotation rate. The physical significance of the transition between these two regimes is a matter of debate. Photometric monitoring programs indicate that young stars tend to group into two main populations that lie on narrow sequences in diagrams where the measured rotation periods of the members of a stellar cluster are plotted against their (B - V) colours. I report on a correlation between the saturated and non-saturated regimes of X-ray emission and the rotation sequences that have been observed in the M34 open cluster from extensive rotation periods surveys. An interpretation of this correlation in term of magnetic activity evolution in the early stage of evolution on the main sequence is presented.