Stellar Activity on the Young Suns of Orion: COUP Observations of K5‐7 Pre–Main‐Sequence Stars

In January 2003, the Chandra Orion Ultradeep Project (COUP) detected about1400 young stars during a 13.2 day observation of the Orion Nebula Cluster(ONC). This paper studies a well-defined sample of 28 solar-mass COUP sourcesto characterize the magnetic activity of analogs of the young Sun and therebyto improve understanding of the effects of solar X-rays on the solar nebuladuring the era of planet formation. We find that active young Suns spend 70% of their time in a characteristicstate with relatively constant flux and magnetically confined plasma withtemperatures kT_2 = 2.1 * kT_1. During characteristic periods, the 0.5-8 keVX-ray luminosity is about 0.03% of the bolometric luminosity. One or twopowerful flares per week with peak luminosities logL_x ~ 30-32 erg/s aretypically superposed on this characteristic emission accompanied by heating ofthe hot plasma component from ~2.4 keV to ~7 keV at the flare peak. The energydistribution of flares superposed on the characteristic emission level followsthe relationship dN/dE ~ E^-1.7. The flare rates are consistent with theproduction of sufficiently energetic protons to spawn a spallogenic origin ofsome important short-lived radionuclides found in ancient meteorites. TheX-rays can ionize gas in the circumstellar disk at a rate of 6 10^-9ionizations per second at 1 AU from the central star, orders of magnitude abovecosmic ray ionization rates. The estimated energetic particle fluences aresufficient to a account for many isotopic anomalies observed in meteoriticinclusions.