2021ApJ...923..187T

We report on a discovery of an X-ray emitting circumstellar material (CSM) knot inside the synchrotron dominant supernova remnant 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) on board XMM-Newton. The RGS spectrum clearly resolves a number of emission lines, such as N Lyα, O Lyα, 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_BT_e = 0.65 ± 0.02 keV. The elemental abundances are obtained to be N/H=3.5±0.8(N/H)_☉, O/H=0.5±0.1(O/H)_☉, Ne/H=0.9±0.1(Ne/H)_☉, Mg/H=1.0±0.1(Mg/H)_☉, Si/H=1.0±0.2(Si/H)_☉, and Fe/H=1.3±0.1(Fe/H)_☉. The enhanced N abundance with others being about the solar values allows us to infer that this knot is CSM 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)_☉. 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 M_☉ <= M <= 20 M_☉.