On the nature of Nova 1670 (CK Vulpeculae): a merger of a red giant with a helium white dwarf

Nova 1670 is a historical transient bearing strong similarities to a recently-recognized type of stellar eruptions known as red novae, which are thought to be powered by stellar mergers. The remnant of the transient, CK Vul, is observable today mainly through cool circumstellar gas and dust, and recombining plasma, but we have no direct view on the stellar object. Within the merger hypothesis, we aim to infer the most likely makeup of the progenitor system that resulted in Nova 1670. We collect and summarize the literature data on the physical properties of the outburst and the remnant, and on the chemical composition of the circumstellar material which resulted from optical and submillimeter observations of the circumstellar gas of CK Vul. Simple simulations yield the form and level of mixing of material associated with the merger. Products of nuclear burning are identified, among them ashes of H burning in the CNO cycles and the MgAl chain, as well as of partial He burning. Based on the luminosity and chemical composition of the remnant, we find that the progenitor primary had to be an evolutionarily advanced red-giant branch star of a mass of 1-2 M$_{\odot}$. The secondary was either a very similar giant, or a He white dwarf. While the eruption event was mainly powered by accretion, we estimate that about 12% of total energy might have come from He burning activated during the merger. The coalescence of a first-ascent giant with a He white dwarf created a star with a rather unique internal structure and composition, which resemble these of early R-type carbon stars. Nova 1670 was the result of a merger between a He white dwarf and a first-ascent red giant and is likely now evolving to become an early R-type carbon star.Comment: accepted to A&A, comments welcom

Lithium in coalesced non-compact stars

Context. Galactic red novae are thought to be produced in stellar mergers between non-compact stars, such as main-sequence stars and cool giants. They are hoped to help in explaining physical processes involved in common envelope evolution and stellar binary collisions. Aims. We investigate the presence of lithium in three best-observed Galactic red nova remnants. Explaining the origin of lithium may point to mixing mechanism present before, during, or after the merger. Methods. The lithium line at 6707.81 A was compared to a feature of [Ca I at 6572.78 A to derive relative abundances in circumstellar gas. Absolute abundances were next calculated assuming the Solar calcium to lithium abundance ratio. Results. Lithium abundances were measured in the merger remnants of V838 Mon with A(Li)=2.3, CK Vul with A(Li)=2.5, and V1309 Sco with A(Li)=1.8. Conclusions. Lithium is overabundant in red novae, suggesting that at least some merger products activate the Cameron-Fowler mechanism whereby convective mixing can reach the deep stellar interior. Whether deep convection and associated diffusion alone or some other processes (e.g. spin down) can be responsible for driving the Cameron-Fowler mechanism in the remnants requires further studies. Early observations of lithium in V838 Mon hint that these mechanisms can activate early, perhaps already in the common envelope phase. These observations should be taken into account in modelling these complex systems.Comment: submitted to A&A; comments welcom

Nova 1670 (CK Vulpeculae) was a merger of a red giant with a helium white dwarf

Context. Nova 1670 is a historical transient bearing strong similarities to a recently recognized type of stellar eruption known as the red nova, which is thought to be powered by stellar mergers. The remnant of the transient, CK Vul, is observable today mainly through cool circumstellar gas and dust, and recombining plasma, but we have no direct view on the stellar object. Aims. Within the merger hypothesis, we aim to infer the most likely configuration of the progenitor system that resulted in Nova 1670. Methods. We collected the literature data on the physical properties of the outburst and the remnant (including their energetics), and on the chemical composition of the circumstellar material (including elemental and isotopic abundances). These data, which result from optical and submillimeter observations of the circumstellar gas of CK Vul, are summarized here. We performed simple simulations to analyze the form and the level of mixing within the material associated with the merger. We identified products of nuclear burning, among which we find ashes of hydrogen burning in the CNO cycles and in the MgAl chain, as well as signs of partial helium burning. Results. Based primarily on the luminosity and chemical composition of the remnant, we find that the progenitor primary had to be an evolutionarily advanced red giant branch star of a mass of 1–2 M⊙. The secondary was either a very similar giant, or –more likely– a helium white dwarf. While the eruption event was mainly powered by accretion, we estimate that about 12% of total energy is likely to have come from helium burning activated during the merger. The coalescence of a first-ascent giant with a helium white dwarf created a star with a rather unique internal structure and composition, which resemble those of early R-type carbon stars. Conclusions. Nova 1670 is the result of a merger between a helium white dwarf and a first-ascent red giant and is likely now evolving to become an early R-type carbon star

A search for cool molecular gas in GK Persei and other classical novae

Detecting molecular line emission from classical nova remnants has the potential to reveal information on the composition of the ejecta, in particular accurate isotopic ratios in the matter processed by a thermonuclear runaway. We conducted searches toward more than 100 classical novae for emission in lines of CO or HCN molecules using single-dish telescopes and interferometric arrays at millimeter and submillimeter wavelengths. The survey demonstrates that classical novae, young or old, are not strong sources of molecular emission at submillimeter or millimeter wavelengths. Additionally, we mapped CO emission around Nova Persei 1901 (GK Per), earlier claimed to be of circumstellar origin. Our measurements indicate that the observed emission is from the interstellar medium. Although no molecular emission at millimeter or submillimeter wavelengths has been found in classical novae, it is still likely that some will be detected with high-sensitivity interferometers such as ALMA

Lithium in red novae and their remnants

Context. Galactic red novae are thought to be produced in stellar mergers between non-compact stars, such as main-sequence stars and cool giants. It is hoped that they will help explain the physical processes involved in common-envelope evolution and stellar binary collisions. Aims. We investigate the presence of lithium in the three best observed Galactic red nova remnants. Explaining the origin of lithium may point to a mixing mechanism present before, during, or after the merger. Methods. We compared the lithium line at 6707.81 Å to a feature of Ca I] at 6572.78 Å to derive relative abundances in circumstellar gas. We next calculated absolute abundances, assuming the solar calcium-to-lithium abundance ratio. Results. Our analysis yields the following lithium abundances: A(Li) = 2.3 for V838 Mon, A(Li) = 2.5 for CK Vul, and A(Li) = 1.8 for V1309 Sco. Conclusions. The super-solar lithium abundances in red novae may suggest that at least some merger products activate mixing mechanisms capable of producing lithium, but other interpretations are possible. In particular, the merger product of V838 Mon may be enhanced in lithium by the matter of one of the progenitor stars. Early observations of lithium in V838 Mon and V1309 Sco hint that Li-producing mechanisms, if present, activate early, perhaps already in the common-envelope phase. Determining the origin of lithium in red novae requires further investigation and a larger sample of measured stars

V838 Monocerotis as seen by ALMA: a remnant of a binary merger in a triple system

V838 Mon erupted in 2002 quickly becoming the prototype of a new type of stellar eruptions known today as (luminous) red novae. The red nova outbursts are thought to be caused by stellar mergers. The merger in V838 Mon took place in a triple or higher system involving two B-type stars. We mapped the merger site with ALMA at a resolution of 25 mas in continuum dust emission and in rotational lines of simple molecules, including CO, SiO, SO, SO$_2$, AlOH, and H$_2$S. We use radiative transfer calculations to reproduce the remnant's architecture at the epoch of the ALMA observations. For the first time, we identify the position of the B-type companion relative to the outbursting component of V838 Mon. The stellar remnant is surrounded by a clumpy wind with characteristics similar to winds of red supergiants. The merger product is also associated with an elongated structure, $17.6 \times 7.6$ mas, seen in continuum emission, and which we interpret as a disk seen at a moderate inclination. Maps of continuum and molecular emission show also a complex region of interaction between the B-type star (its gravity, radiation, and wind) and the flow of matter ejected in 2002. The remnant's molecular mass is about 0.1 M$_{\odot}$ and the dust mass is 8.3$\cdot$10$^{-3}$ M$_{\odot}$. The mass of the atomic component remains unconstrained. The most interesting region for understanding the merger of V838 Mon remains unresolved but appears elongated. To study it further in more detail will require even higher angular resolutions. ALMA maps show us an extreme form of interaction between the merger ejecta with a distant (250 au) companion. This interaction is similar to that known from the Antares AB system but at a much higher mass loss rate. The B-type star not only deflects the merger ejecta but also changes its chemical composition with an involvement of circumstellar shocks.Comment: A&A submitted. Comments welcom