  Since 1996 the blast wave driven by SN 1987A has been interacting with the
dense circumstellar material, which provides us with a unique opportunity to
study the early evolution of a newborn supernova remnant (SNR). Based on the
XMM-Newton RGS and EPIC-pn X-ray observations from 2007 to 2019, we
investigated the post-impact evolution of the X-ray emitting gas in SNR 1987A.
The hot plasma is represented by two non-equilibrium ionization components with
temperature of $\sim0.6$ keV and $\sim2.5$ keV. The low-temperature plasma has
a density $\sim2400$ cm$^{-3}$, which is likely dominated by the lower density
gas inside the equatorial ring (ER). The high-temperature plasma with a density
$\sim550$ cm$^{-3}$ could be dominated by the H II region and the high-latitude
material beyond the ring. In the last few years, the emission measure of the
low-temperature plasma has been decreasing, indicating that the blast wave has
left the main ER. But the blast wave is still propagating into the
high-latitude gas, resulting in the steadily increase of the high-temperature
emission measure. In the meantime, the average abundances of N, O, Ne, and Mg
are found to be declining, which may reflect the different chemical
compositions between two plasma components. We also detected the Fe K lines in
most of the observations, showing increasing flux and centroid energy. We
interpret the Fe K lines as from a third hot component, which may come from the
reflected shock-heated gas or originate from Fe-rich ejecta clumps, shocked by
the reverse shock.
