Dynamo regime transition among Sun-like stars in M 34 ^⋆,⋆⋆

A time evolution model of X-ray activity on the main sequence

European Space Agency, ESTEC – Postbus 299, 2200 AG, Noordwijk, The Netherlands
e-mail: pgondoin@rssd.esa.int

Received: 15 June 2012
Accepted: 16 August 2012

Abstract

Context. The X-ray emission from late-type stars in open clusters exhibits two kinds of dependences on stellar rotation. While fast rotators have a relatively constant X-ray emission level, slower rotators show a decline of their X-ray emission with decreasing rotation rate. The physical significance of the transition between these two X-ray emission regimes is a matter of debate. During the  ~500 Myr period of stellar evolution that separates the Pleiades from the Hyades, late-type stars such as those present in the M 34 open cluster undergo significant changes in their rotation rates. These could affect the dynamo processes in their interiors, possibly altering their surface magnetic fluxes.

Aims. The purpose of the present study is to look for the X-ray signatures of such possible modifications, to search for correlations with rotation and stellar parameters, and to propose a parameterization of the X-ray activity evolution on the main sequence. The aim is to provide observational constraints on the time evolution of dynamo processes in the interiors of late-type stars.

Methods. The distributions of stellar X-ray luminosities vs. rotation periods and Rossby numbers of a sample of M 34 late-type stars are compared with rotation-activity relationships established in a large sample of cluster members and field stars. A model of X-ray activity evolution is developed by combining an X-ray activity-rotation relationship with a recent model of stellar rotation evolution on the main sequence. The distribution of stellar X-ray luminosities in M 34 is compared with this model.

Results. A correlation is observed between the saturated and non-saturated regime of X-ray emission and the C- and I-rotational sequences that have been observed in M 34 from extensive rotational periods surveys. M 34 sample stars show a steep transition in X-ray to bolometric luminosity ratio between the C-sequence and gap stars that emit close to the 10^-3 saturation level, and the I-sequence stars, whose L_X/L_bol ratio is significantly lower for similar values of the Rossby number. A comparison between X-ray emission vs. mass distribution in M 34 and the X-ray luminosity evolution model suggests that the transition between the saturated and non-saturated regime of X-ray emission occurs in M 34 cluster members depending on their convective turnover time and period of rotation.

Conclusions. I argue that the drop of (L_X/L_bol) by one order of magnitude observed in M 34 around a Rossby number of 0.3 is indicative of a change in dynamo efficiency. I conclude that the transition from the saturated to the non-saturated regime of X-ray emission among main-sequence stars is the result of a dynamo regime transition, possibly between a turbulent dynamo and an interface-type dynamo.