Chandra Observations of High Mass Young Stellar Objects in the Monoceros R2 molecular cloud

Abstract

We observed the Monoceros R2 molecular cloud with the ACIS-I array onboard the Chandra X-ray Observatory. From the central 3.2' x 3.2' region, we detect 154 sources above the detection limit of ~5x10^-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 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 with a thin thermal plasma model of a temperature above 2 keV. The X-rays are time-variable and exhibit rapid flares. These high temperature plasma and flaring activity are similar to those seen in low mass PMS stars and 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 ergs s^-1. We conclude that high mass ZAMS and PMS emit X-rays, possibly due to the magnetic activity like those of low mass stars.