The X-ray environment during the epoch of terrestrial planet formation: Chandra observations 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 χ 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 μm 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. © 2009. The American Astronomical Society. All rights reserved.