Possible Detection of X-Ray Emitting Circumstellar Material in the Synchrotron-Dominated Supernova Remnant RX J1713.7-3946

X-ray emission from the circumstellar material in the supernova remnant (SNR) can be a good probe to understand the activity of the progenitor star of the supernova. We report on a discovery of an X-ray emitting circumstellar material knot inside the synchrotron dominant SNR RX J1713.7-3946. This knot was previously thought to be a Wolf-Rayet star (WR 85), but we realized that it is in fact ~40'' away from WR 85, indicating no relation to WR 85. We performed high-resolution X-ray spectroscopy with the Reflection Grating Spectrometer (RGS) onboard XMM-Newton. The RGS spectrum clearly resolves a number of emission lines, such as N VII, O VIII, Fe XVIII, Ne X, Mg XI, and Si XIII. The spectrum can be well represented by an absorbed thermal emission model with a temperature of k_{B}T_{e}=0.65+/-0.02 keV. The elemental abundances are obtained to be N/H=3.5+/-0.8 (N/H)_solar, O/H=0.5+/-0.1 (O/H)_ solar, Ne/H=0.9+/-0.1 (Ne/H)_ solar, Mg/H=1.0+/-0.1 (Mg/H)_ solar, Si/H=1.0+/-0.2 (Si/H)_ solar, and Fe/H=1.3+/-0.1 (Fe/H)_ solar. The enhanced N abundance with others being about the solar values allows us to infer that this knot is circumstellar material ejected when the progenitor star evolved into a red supergiant. The abundance ratio of N to O is obtained to be N/O=6.8_{-2.1}^{+2.5}(N/O)_ solar. By comparing this to those in outer layers of red supergiant stars expected from stellar evolution simulations, we estimate the initial mass of the progenitor star that produced RX J1713.7-3946 to be about 15 - 20M_sun.