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 $\dot{M} \sim 10^{-3} \rm{M_{\odot}\ yr^{-1}}$ 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.

中文翻译：

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使用 XMM-Newton 的 RGS 首次探测到 IIn 型超新星 1978K 的 X 射线线发射

我们报告了对 IIn 型超新星 SN 1978K 元素丰度的稳健测量，基于 XMM-Newton 上的反射光栅光谱仪 (RGS) 获得的高分辨率 X 射线光谱。RGS 清楚地解析了许多发射线，包括 N Ly$\alpha$、O Ly$\alpha$、O Ly$\beta$、Fe XVII、Fe XVIII、Ne He$\alpha$ 和 Ne Ly$\alpha $ 首次从 SN 1978K 开始。X 射线光谱可以用吸收的双温度热发射模型表示，温度分别为 $kT \sim 0.6$ keV 和 $2.7$ keV。得到的元素丰度为 N $=$ $2.36_{-0.80}^{+0.88}$, O $=$ $0.20 \pm{0.05}$, Ne $=$ $0.47 \pm{0.12}$, Fe $ =$ $0.15_{-0.02}^{+0.01}$ 乘以太阳能值。除 N 外的低金属丰度表明，发射 X 射线的等离子体源自前身星吹出的星周介质。N 和 O 的丰度与一个明亮的蓝色变星的表面组成所预期的 CNO 平衡丰度相去甚远，类似于质量为 10-25 M$_\odot$ 的小质量恒星的富含 H 的包层。连同 SN 1978K 的其他特殊性质，即 500-1000 km s$^{-1}$ 的低膨胀速度和类 SN IIn 光谱，我们认为 SN 1978K 是电子捕获的结果来自超级渐近巨星分支星的超新星，或相对低质量（$\sim$10 M$_\odot$）或质量（$\sim$20-25 M$_\）的弱铁核坍缩爆炸odot$) 红超巨星。然而，这些场景并不能自然地解释 $\dot{M} \sim 10^{-3} \rm{M_{\odot}\ yr^{-1}}$ 超过 $\ gtrsim 爆炸前 1000 年，这是通过这项工作以及许多其他早期研究推断出来的。需要进一步的理论研究来解释大质量恒星最后演化阶段的高质量损失率。