2024ApJ...970....4M

It is generally hard to put robust constraints on progenitor masses of supernovae and supernova remnants (SNRs) observationally, while they offer tantalizing clues to understanding explosion mechanisms and mass distribution. Our recent study suggests that “shell merger,” which is theoretically expected for stellar evolution, can appreciably affect the final yields of intermediate-mass elements (IMEs; such as Ne, Mg, and Si). In light of this, here we report the results of the X-ray spectral analysis of a Galactic SNR G359.0-0.9, whose abundance pattern may possibly be anomalous according to a previous study. Our spectroscopy using all the available data taken with XMM-Newton reveals that this remnant is classified as a Mg-rich SNR because of its high Mg-to-Ne ratio (${Z}_{\mathrm{Mg}}/{Z}_{\mathrm{Ne}}={1.90}_{-0.19}^{+0.27};$ mass ratio ${0.66}_{-0.07}^{+0.09}$) and conclude that the result cannot be explained without the shell merger. By comparing the observation with theoretical calculations, we prefer the so-called Ne-burning shell intrusion and in this case, the progenitor mass M_ZAMS is likely <15M_⊙. We confirm the result also with our new molecular-line observations with the Nobeyama 45 m telescope: G359.0-0.9 is located in the Scutum-Centaurus arm (2.66-2.94 kpc) and in this case, the resultant total ejecta mass ∼6.8M_⊙ is indeed consistent with the above estimate. Our method using mass ratios of IMEs presented in this paper will become useful to distinguish the type of shell merger, the Ne-burning shell intrusion, and the O-burning shell merger, for future SNR studies.