ChandraObservations of High‐Mass Young Stellar Objects in the Monoceros R2 Molecular Cloud

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Abstract
We observed the Monoceros R2 molecular cloud with the ACIS-I array on board the Chandra X-Ray Observatory. From the central 32 × 32 region, we detect 154 sources above the detection limit of ~5 × 10-16 ergs s -1 cm -2 with a 100 ks 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 pre-main-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 ~2 keV. 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 ~1031 ergs s-1. 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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