  It is generally hard to put robust constraints on progenitor masses of
supernovae (SNe) and 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 final yields of
inter-mediate mass elements (IMEs; such as Ne, Mg, and Si). In light of this,
here we report results of 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 Mg-rich SNRs because of its high
Mg-to-Ne ratio (Z_Mg/Z_Ne=1.90+0.27-0.19; mass ratio 0.66+0.09-0.07) 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_sun. We confirm the result also by our new molecular line
observations with the NRO-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_sun 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 the shell merger, the Ne-burning shell intrusion and
the O-burning shell merger, for future SNR studies.
