Molecular remnant of Nova 1670 (CK Vulpeculae)

I. Properties and enigmatic origin of the gas

¹ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Rabiańska 8, 87-100 Toruń, Poland
e-mail: tomkam@ncac.torun.pl
² Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
³ Max Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁴ Institut de Radioastronomie Millimétrique, 300 Rue de la Piscine, 38406 Saint-Martin-d’Hères, France
⁵ ESO – European Organisation for Astronomical Research in the Southern Hemisphere, Alonso de Cordoba 3107, Vitacura, Santiago, Chile

Received: 12 June 2020
Accepted: 2 September 2020

Abstract

CK Vul erupted in 1670 and is considered a Galactic stellar-merger candidate. Its remnant, observed 350 yr after the eruption, contains a molecular component of surprisingly rich composition, including polyatomic molecules as complex as methylamine (CH₃NH₂). 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 the kinetic temperatures and column densities in five parts of the molecular nebula. Non-LTE effects are clearly seen in complex species including methanol absorption against the cosmic microwave background. The temperatures are about 17 K in the inner remnant and 14 K in the extended lobes, both higher than excitation temperatures estimated earlier in an LTE approach and based on single-dish spectra. We find total (hydrogen plus helium) densities in the range of 10⁴ − 10⁶ cm⁻³. The column densities provide rough 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 asymptotic giant branch stars, most notably that of OH231.8+4.2.