We report on robust measurements of elemental abundances of the Type IIn supernova SN 1978K , based on the high-resolution X-ray spectrum obtained with the Reflection Grating Spectrometer ( RGS ) onboard XMM-Newton .
The RGS clearly resolves a number of emission lines , including N Ly \alpha , O Ly \alpha , O Ly \beta , Fe XVII , Fe XVIII , Ne He \alpha and Ne Ly \alpha for the first time from SN 1978K .
The X-ray spectrum can be represented by an absorbed , two-temperature thermal emission model , with temperatures of kT \sim 0.6 keV and 2.7 keV .
The elemental abundances are obtained to be N = 2.36 _ { -0.80 } ^ { +0.88 } , O = 0.20 \pm { 0.05 } , Ne = 0.47 \pm { 0.12 } , Fe = 0.15 _ { -0.02 } ^ { +0.01 } times the solar values .
The low metal abundances except for N show that the X-ray emitting plasma originates from the circumstellar medium blown by the progenitor star .
The abundances of N and O are far from CNO-equilibrium abundances expected for the surface composition of a luminous blue variable , and resemble the H-rich envelope of less-massive stars with masses of 10–25 M _ { \odot } .
Together with other peculiar properties of SN 1978K , i.e. , a low expansion velocity of 500–1000 km s ^ { -1 } and SN IIn-like optical spectra , we propose that SN 1978K is a result of either an electron-capture SN from a super asymptotic giant branch star , or a weak Fe core-collapse explosion of a relatively low-mass ( \sim 10 M _ { \odot } ) or high-mass ( \sim 20–25 M _ { \odot } ) red supergiant star .
However , these scenarios can not naturally explain the high mass-loss rate of the order of \dot { M } \sim 10 ^ { -3 } { M _ { \odot } yr ^ { -1 } } over \gtrsim 1000 yr before the explosion , which is inferred by this work as well as many other earlier studies .
Further theoretical studies are required to explain the high mass-loss rates at the final evolutionary stages of massive stars .