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Description
We observed the Monoceros R2 molecular cloud with the ACIS-I array on board the Chandra X-Ray Observatory. From the central 3.2'x3.2' region, we detect 154 sources above the detection limit of ~5x10^-16^erg/s/cm^2^ with a 100ks exposure. About 85% of the X-ray sources are identified with an infrared counterpart, including four high-mass stars in the zero-age main-sequence (ZAMS) and/or premain-sequence (PMS) phase. The X-ray spectra of the high-mass ZAMS and PMS stars are represented by a thin thermal plasma model of a temperature above ~2keV. The X-rays are time variable and exhibit rapid flares. This high-temperature plasma and flaring activity is similar to that seen in low-mass PMS stars and is contrary to the behavior observed in high-mass main-sequence stars.The X-ray luminosity increases as the intrinsic K-band flux increases. However, the X-ray luminosity saturates at a level of ~10^31^erg/s. We conclude that high-mass ZAMS and PMS stars emit X-rays, possibly because of magnetic activity such as that of low-mass stars.
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