XMM-NEWTONOBSERVATION OF THE α PERSEI CLUSTER

Abstract

We report on the analysis of an archival observation of part of the α Persei cluster obtained with XMM-Newton. We detected 102 X-ray sources in the band 0.3-8.0 keV, of which 39 of them are associated with the cluster as evidenced by appropriate magnitudes and colors from 2MASS photometry. We extend the X-ray Luminosity Distribution (XLD) for M dwarfs, to add to the XLD found for hotter dwarfs from spatially extensive surveys of the whole cluster by ROSAT. Some of the hotter stars are identified as a background, possible slightly older group of stars at a distance of approximately 500 pc.1 1 Corresponding Author composition, at least to a first approximation. Their members display a range of masses, temperatures, luminosities, rotation rates, and multiplicity which can be explored. Comparison of cluster morphology provides an age sequence as a context for the evolution of stars. The well-known spindown of low mass stars with age due to magnetic braking provides a good example of insight from clusters. This slowing of rotation results in the decrease of coronal X-rays due to their connection with stellar dynamos. Good summaries of the decrease in stellar activity as stars age for a range of masses are provided by Favata and Micela (2003) and Güdel (2004).α Per is a young open cluster, found to be 50 Myr old from upper main sequence turnoff morphology (Meynet, Mermilliod, and Maeder, 1993). More recently, Stauffer, et al. (1999) have found an age of 90 Myr from the low mass lithium depletion boundary. Although there is some dispersion in the exact calibration of the age of the cluster, the sequence of age (increasing from the Orion Nebula Cluster through the α Per cluster through the Pleiades) is generally agreed. Thus, studies find a range in age from 50 to 90 Myr. We will use the shorthand "50 Myr" for the age of the cluster.This age makes it an excellent comparison for Cepheids and their companions. Indeed, α Per itself is a yellow supergiant, with similar parameters to Cepheids, except for its location at a temperature outside the instability strip. As an example of usage of the cluster, we have observed a number of Cepheids with the Hubble Space Telescope Wide Field Camera 3 to identify a population of resolved low-mass stars close to Cepheids which are probable physical companions. X-ray observations showing an activity level comparable to that of α Per dwarfs would confirm that they are young companions rather than chance alignments with old field stars (e.g. Evans, et al. 2012). Another interesting aspect of a cluster of this age is that it is the period when young planets have just finished forming, and thus we gain insight into the X-ray environment during the early formation of atmospheres.Because the cluster is nearby, it covers a wide area in the sky. It was observed with a raster of pointings by ROSAT . Essentially all the late F, G, and K members from the membership studies of Prosser (1992) were detected. Three deeper pointed ROSAT observations (22-25 ks) were subsequently made covering part of the ...

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