We examine the late-time ( t \gtrsim 200 days after peak brightness ) spectra of Type Iax supernovae ( SNe Iax ) , a low-luminosity , low-energy class of thermonuclear stellar explosions observationally similar to , but distinct from , Type Ia supernovae .
We present new spectra of SN 2014dt , resulting in the most complete published late-time spectral sequence of a SN Iax .
At late times , SNe Iax have generally similar spectra , all with a similar continuum shape and strong forbidden-line emission .
However , there is also significant diversity where some SN Iax spectra display narrow P-Cygni features from permitted lines and a continuum indicative of a photosphere at late times in addition to strong narrow forbidden lines , while others have no obvious P-Cygni features , strong broad forbidden lines , and weak narrow forbidden lines .
Finally , some SNe Iax have spectra intermediate to these two varieties with weak P-Cygni features and broad/narrow forbidden lines of similar strength .
We find that SNe Iax with strong broad forbidden lines also tend to be more luminous and have higher-velocity ejecta at peak brightness .
We find no evidence for dust formation in the SN ejecta or the presence of circumstellar dust , including for the infrared-bright SN 2014dt .
Late-time SN Iax spectra have strong [ \ion NiII ] emission , which must come from stable Ni , requiring electron captures that can only occur at the high densities of a ( nearly ) Chandrasekhar-mass WD .
Therefore , such a star is the likely progenitor of SNe Iax .
We estimate blackbody and kinematic radii of the late-time photosphere , finding the latter an order of magnitude larger than the former for at least one SN Iax .
We propose a two-component model that solves this discrepancy and explains the diversity of the late-time spectra of SNe Iax .
In this model , the broad forbidden lines originate from the SN ejecta , similar to the spectra of all other types of SNe , while the photosphere , P-Cygni lines , and narrow forbidden lines originate from a wind launched from the remnant of the progenitor white dwarf and is driven by the radioactive decay of newly synthesised material left in the remnant .
The relative strength of the two components accounts for the diversity of late-time SN Iax spectra .
This model also solves the puzzle of a long-lived photosphere and slow late-time decline of SNe Iax .