Molecular remnant of Nova 1670 (CK Vulpeculae): I. Properties and
  enigmatic origin of the gas
  CK Vul erupted in 1670 and is considered a stellar-merger candidate. Its
remnant contains a molecular component of surprisingly rich composition. We
present interferometric line surveys with subarcsec resolution with ALMA and
SMA. The observations provide interferometric maps of molecular line emission
at frequencies between 88 and 243 GHz that allow imaging spectroscopy of more
than 180 transitions of 26 species. We present, classify, and analyze the
different morphologies of the emission regions displayed by the molecules. We
also perform a non-LTE radiative-transfer analysis of emission of most of the
observed species, deriving temperatures and column densities in five parts of
the nebula. Non-LTE effects are clearly seen in complex species including
methanol absorption against the CMB. The temperatures are 17 K in the inner
remnant and 14 K in the extended lobes. We find total (hydrogen plus helium)
densities in the range of $10^4-10^6$ cm$^{-3}$. The column densities provide
relative abundance patterns in the remnant which currently are not understood.
Attempts to derive elemental abundances within the assumption of a chemical
equilibrium give only loose constraints on the CNO elements. That the formation
of many of the observed molecules requires a major involvement of circumstellar
shocks remains the preferred possibility. The molecular gas could have formed
350 yr ago or more recently. The molecules are well shielded from the
interstellar radiation field by the circumstellar dust. Their presence alone
indicates that the unobservable central star cannot be a hot object such as a
white dwarf. This excludes some of the proposed scenarios on the nature of CK
Vul. The general characteristics of the molecular environment of CK Vul derived
in this study resemble quite well those of some pre-planetary nebulae and AGB
stars, most notably that of OH231.8.
