Herein we analyse late-time ( post-plateau ; 103 < t < 1229 d ) optical spectra of low-redshift ( z < 0.016 ) , hydrogen-rich Type IIP supernovae ( SNe IIP ) .
Our newly constructed sample contains 91 nebular spectra of 38 SNe IIP , which is the largest dataset of its kind ever analysed in one study , and many of the objects have complementary photometric data .
We determined the peak luminosity , total luminosity , velocity of the peak , half-width at half-maximum intensity , and profile shape for many permitted and forbidden emission lines .
Temporal evolution of these values , along with various flux ratios , are studied and compared to previous work .
We also investigate the correlations between these measurements and photometric observables , such as the peak and plateau absolute magnitudes and the late-time light curve decline rates in various optical bands .
The strongest and most robust result we find is that the luminosities of all spectral features ( except those of helium ) tend to be higher in objects with steeper late-time V -band decline rates .
A steep late-time V -band slope likely arises from less efficient trapping of \gamma -rays and positrons , which could be caused by multidimensional effects such as clumping of the ejecta or asphericity of the explosion itself .
Furthermore , if \gamma -rays and positrons can escape more easily , then so can photons via the observed emission lines , leading to more luminous spectral features .
It is also shown that SNe IIP with larger progenitor stars have ejecta with a more physically extended oxygen layer that is well-mixed with the hydrogen layer .
In addition , we find a subset of objects with evidence for asymmetric ^ { 56 } Ni ejection , likely bipolar in shape .
We also compare our observations to theoretical late-time spectral models of SNe IIP from two separate groups and find moderate-to-good agreement with both sets of models .
Our SNe IIP spectra are consistent with models of 12–15 M _ { \odot } progenitor stars having relatively low metallicity ( Z \leq 0.01 ) .