Abstract
About 1000 X-ray emitting young pre–main-sequence (PMS) stars distributed in mass from ~0.05 M☉ brown dwarfs to a ~50 M☉ O star are detected in an image of the Orion Nebula obtained with the Advanced CCD Imaging Spectrometer on board the Chandra X-Ray Observatory. This is the richest field of sources ever obtained in X-ray astronomy. Individual X-ray luminosities in the Orion Nebula cluster range from the sensitivity limit of 2 × 10^(28) ergs s^(-1) to ~10^(32) ergs s^(-1). ACIS sources include 85%–90% of V < 20 stars, plus a lower but substantial fraction of deeply embedded stars with extinctions as high as A_V ≃ 60. The relationships between X-ray and other PMS stellar properties suggest that X-ray luminosity of lower-mass PMS stars depends more on mass, and possibly stellar rotation, than on bolometric luminosity, as widely reported. In a subsample of 17 unabsorbed stars with mass ≃ 1 M☉, X-ray luminosities are constant at a high level around L_x ≃ 2 × 10^(30) ergs s^(-1) for the first ≃ 2 Myr while descending the convective Hayashi track, but diverge during the 2–10 Myr phase with X-ray emission plummeting in some stars but remaining high in others. This behavior is consistent with the distribution of X-ray luminosities on the zero-age main sequence and with current theories of their rotational history and magnetic dynamos. The sources in the Becklin-Neugebauer/Kleinman-Low region of massive star formation are discussed in detail. They include both unabsorbed and embedded low-mass members of the Orion Nebula cluster, the luminous infrared Source n, and a class of sources without optical or infrared counterparts that may be new magnetically active embedded PMS stars. Several X-ray sources are also variable radio emitters, an association often seen in magnetically active PMS stars. Faint X-ray emission is seen close to, but apparently not coincident with, the Becklin-Neugebauer object. Its nature is not clear.
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