We report on a discovery of an X-ray emitting circumstellar material knot inside the synchrotron dominant supernova remnant (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 $\sim$40$^{ \prime\prime}$ away from WR 85, indicating no relation to WR 85. We performed high-resolution X-ray spectroscopy with the Reflection Grating Spectrometer (RGS) on board XMM-Newton. The RGS spectrum clearly resolves a number of emission lines, such as N Ly$\alpha$, O Ly$\alpha$, 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 $k_{ \rm B}T_{ \rm e} = 0.65\pm 0.02$ keV. The elemental abundances are obtained to be ${ \rm N/H} = 3.5\pm 0.8{ \rm \left(N/H\right)_{ \odot}}$, ${ \rm O/H} = 0.5\pm0.1{ \rm \left(O/H\right)_{ \odot}}$, ${ \rm Ne/H} = 0.9\pm0.1{ \rm \left(Ne/H\right)_{ \odot}}$, ${ \rm Mg/H} = 1.0\pm0.1{ \rm \left(Mg/H\right)_{ \odot}}$, ${ \rm Si/H} = 1.0\pm0.2{ \rm \left(Si/H\right)_{ \odot}}$, and ${ \rm Fe/H} = 1.3\pm0.1{ \rm \left(Fe/H\right)_{ \odot}}$. 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 $\rm N/O = 6.8_{ -2.1}^{ +2.5}\left(N/O\right)_{ \odot}$. 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 to be $15\, \rm M_{ \odot} \lesssim \rm M \lesssim 20\, \rm M_{ \odot}$.
Categories: astro-ph.HE