  Spectral analysis of X-ray emission from ejecta in supernova remnants (SNRs)
is hampered by the low spectral resolution of CCD cameras, which creates a
degeneracy between the best-fit values of abundances and emission measure. The
combined contribution of shocked ambient medium and ejecta to the X-ray
emission complicates the determination of the ejecta mass and chemical
composition, leading to big uncertainties in mass estimates and it can
introduce a bias in the comparison between the observed ejecta composition and
the yields predicted by explosive nucleosynthesis. We explore the capabilities
of present and future spectral instruments with the aim of identifying a
spectral feature which may allow us to discriminate between metal-rich and
pure-metal plasmas in X-ray spectra of SNRs. We studied the behavior of the
most common X-ray emission processes of an optically thin plasma in the
high-abundance regime. We investigated spectral features of bremsstrahlung,
radiative recombination continua (RRC) and line emission, by exploring a wide
range of chemical abundances, temperatures and ionization parameters. We
synthesized X-ray spectra from a 3D hydrodynamic (HD) simulation of Cas A, by
using the response matrix from the Chandra/ACIS-S CCD detector and that of the
XRISM/Resolve X-ray calorimeter. We found that a bright RRC shows up when the
plasma is made of pure-metal ejecta, and a high spectral resolution is needed
to identify this ejecta signature. We verified the applicability of our novel
diagnostic tool and we propose a promising target for the future detection of
such spectral feature: the southeastern Fe-rich clump of Cas A. While there is
no way to unambiguously reveal pure-metal ejecta emission with CCD detectors,
X-ray calorimeters will be able to pinpoint the presence of pure-metal RRC and
to recover correctly absolute mass and the chemical composition of the ejecta.
