THE X-RAY ENVIRONMENT DURING THE EPOCH OF TERRESTRIAL PLANET FORMATION:CHANDRAOBSERVATIONS OF h PERSEI

Abstract

We describe Chandra/ACIS-I observations of the massive ~ 13-14 Myr-old cluster, h Persei, part of the famous Double Cluster (h and ? Persei) in Perseus. Combining the list of Chandra-detected sources with new optical/IR photometry and optical spectroscopy reveals ~ 165 X-ray bright stars with V 23. Roughly 142 have optical magnitudes and colors consistent with cluster membership. The observed distribution of Lx peaks at Lx ~ 1030.3 erg s?1 and likely traces the bright edge of a far larger population of 0.4-2 M ? X-ray active stars. From a short list of X-ray active stars with IRAC 8 ?m excess from warm, terrestrial zone dust, we derive a maximum X-ray flux incident on forming terrestrial planets. Although there is no correlation between X-ray activity and IRAC excess, the fractional X-ray luminosity correlates with optical colors and spectral type. By comparing the distribution of Lx /L versus spectral type and V ? I in h Per with results for other 1-100 Myr-old clusters, we show that stars slightly more massive than the Sun ( 1.5 M ?) fall out of X-ray saturation by 10-15 Myr. Changes in stellar structure for 1.5 M ? stars likely play an important role in this decline of X-ray emission.