Molecular remnant of Nova 1670 (CK Vulpeculae): I. Properties and enigmatic origin of the gas

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 (CH 3 NH 2 ). 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 10 4 − 10 6 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.

The circumstellar matter of type II intermediate luminosity optical transients (ILOTs)

“…This study aims at eruptive transients, sometimes referred to as gap transients (e.g., Kasliwal 2011;Blagorodnova et al 2017; because they have peak luminosities between luminosities of classical novae and typical luminosities of supernovae (e.g. Mould et al 1990;Bond et al 2003;Rau et al 2007;Ofek et al 2008;Mason et al 2010;Tylenda et al 2013;Kasliwal 2013;Kamiński et al 2018;Pastorello et al 2018;Boian & Groh 2019;Cai et al 2019;Jencson et al 2019;Kashi et al 2019;Andrews et al 2020;Howitt et al 2020;Jones 2020;Kaminski et al 2020;Klencki et al 2020;Kamiński et al 2020). I refer to transients that are powered by gravitational energy due to stellar merger, that includes the onset of a common envelope evolution or mass transfer (e.g., Tylenda et al 2011;Nandez et al 2014;Kamiński et al 2015a;Soker 2016;MacLeod et al 2017;Gilkis et al 2019;Segev et al 2019;Yalinewich & Matzner 2019;Schrøder et al 2020;, as intermediate luminosity optical transients (ILOTs; Berger et al 2009;Muthukrishna et al 2019) 1 .…”

Transients from the Cataclysmic Deaths of Cataclysmic Variables

“…The expanding ejecta will cool to temperatures 1000 − 2000 K, starting in the outer layers on a timescale ∼ t pk (Eq. 31), enabling the formation of molecules and dust, as observed in LRN (e.g., Kamiński et al 2010), classical novae (e.g., Gehrz et al 1998), and CK Vul (e.g., Eyres et al 2018;Kaminski et al 2020; Section 5.5). After dust forms and blocks optical light from the central source, the spectral energy distribution of the emission will shift into the infrared bands.…”

“…Eyres et al (2018) instead favor a merger involving a WD and brown dwarf, based (in part) on the presence of ionized species HCO + and N 2 H + , which require exposure to an intense UV radiation field, such as that supplied by a hot central WD. However, Kamiński et al (2015); Kaminski et al (2020); Kamiński et al (2020) instead argue that shock excitation by outflows from the remnant can generate the emission lines without appealing to an independent source of ionizing photons.…”

“…The historical transient CK Vulpeculae (Nova Vulpeculae 1670), long thought to be a nova, has in recent years been argued to be a stellar merger (Kamiński et al 2015;Kaminski et al 2020;Kamiński et al 2020) or a merger between a brown dwarf and a WD (Eyres et al 2018). Another motivation for our work is thus to explore whether CK Vul is consistent with being a CV merger.…”

The circumstellar matter of type II intermediate luminosity optical transients (ILOTs)

“…This study aims at eruptive transients, sometimes referred to as gap transients (e.g., Kasliwal 2011;Blagorodnova et al 2017; because they have peak luminosities between luminosities of classical novae and typical luminosities of supernovae (e.g. Mould et al 1990;Bond et al 2003;Rau et al 2007;Ofek et al 2008;Mason et al 2010;Tylenda et al 2013;Kasliwal 2013;Kaminski et al 2018;Pastorello et al 2018;Boian, & Groh 2019;Cai et al 2019;Jencson et al 2019;Kashi et al 2019;Andrews et al 2020;Howitt et al 2020;Jones 2020;Kaminski et al 2020;Klencki et al 2020;Kaminski et al 2021). I refer to transients that are powered by gravitational energy due to stellar merger, that includes the onset of a common envelope evolution or mass transfer (e.g., Tylenda et al 2011;Nandez et al 2014;Kamiński et al 2015b;Soker 2016;MacLeod et al 2017;Gilkis et al 2019;Segev et al 2019;Yalinewich, & Matzner 2019;Schrøder et al 2020; as intermediate luminosity optical transients (ILOTs; Berger et al 2009;Muthukrishna et al 2019) 1 .…”