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

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 L x peaks at L x ∼ 10 30.3 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 L x / 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.