Light echo of V838 Monocerotis: properties of the echoing medium

The light echo phenomenon that accompanied the 2002 eruption of V838 Mon allows one to study the properties of the diffuse dusty matter in the vicinity of the object. We are aiming at obtaining estimates of the optical thickness of the circumstellar matter in front of V838 Mon, as well as optical properties of dust grains in the echoing medium. In particular, we are interested in studying whether the echoing medium can be responsible for the observed faintness of the B-type companion of V838 Mon when compared to three B-type stars that are seen in the vicinty of V838 Mon and are believed to be at the same distance as V838 Mon. We used the V838 Mon light echo images obtained by the Hubble Space Telescope (HST) in different filters and epochs. From the images we derived the total brightness of the echo and its surface brightness. The results of the measurements were compared to model light echoes. The present study allowed us to estimate the optical thickness of the matter in front of the object and the mean cosine value of the scattering angle of dust grains in three HST filters. The optical thickness of the echoing matter is not sufficient to explain the observed difference in brightness between the B-type companion of V838 Mon and the other three B-type stars observed in the vicinity of V838 Mon. Implications of this result are discussed. Our estimate of the mass of the diffuse matter seen in the light echo shows that the matter cannot have resulted form a past mass loss activity of V838 Mon. We probably observe remnants of an interstellar cloud from which V838 Mon and other members of the observed cluster were formed.Comment: Accepted for publication in Astronomy & Astrophysic

Violent Stellar Merger Model for Transient Events

We derive the constraints on the mass ratio for a binary system to merge in a violent process. We find that the secondary to primary stellar mass ratio should be ~0.003 < (M_2/M_1) < ~0.15. A more massive secondary star will keep the primary stellar envelope in synchronized rotation with the orbital motion until merger occurs. This implies a very small relative velocity between the secondary star and the primary stellar envelope at the moment of merger, and therefore very weak shock waves, and low flash luminosity. A too low mass secondary will release small amount of energy, and will expel small amount of mass, which is unable to form an inflated envelope. It can however produce a quite luminous but short flash when colliding with a low mass main sequence star. Violent and luminous mergers, which we term mergebursts, can be observed as V838 Monocerotis type events, where a star undergoes a fast brightening lasting days to months, with a peak luminosity of up to ~10^6 Lo followed by a slow decline at very low effective temperatures.Comment: Accepted by MNRA

Main-sequence stellar eruption model for V838 Mon

We propose that the energy source of the outburst of V838 Mon and similar objects is an accretion event, i.e., gravitational energy rather than thermonuclear runaway. We show that the merger of two main sequence stars, of masses 1.5 Mo and 0.1-0.5 Mo can account for the luminosity, large radius, and low effective temperture of V838 Mon and similar objects. Subsequent cooling and gravitational contraction lead such objects to move along the Hayashi limit, as observed. By varying the masses and types of the merging stars, and by considering slowly expanding, rather than hydrostatic, envelopes, this model can account for a large range in luminosities and radii of such outburst events.Comment: 9 page

Magnetic Activity in Stellar Merger Products

We study the expected X-ray luminosity of stellar merger products several years after merger. The X-ray emission is assumed to result from magnetic activity. The extended envelope of the merger product possesses a large convective region and it is expected to rotate fast. The rotation and convection might give rise to an efficient dynamo operation, therefore we expect strong magnetic activity. Using well known relations connecting magnetic activity and X-ray luminosity in other types of magnetically active stars, we estimate that the strong X-ray luminosity will start several years after merger, will reach a maximum of L_x~3x10^{30} erg/sec, and will slowly decline on a time scale of ~100 years. We predict that X-ray emission from V838 Mon which erupted in 2002 will be detected in 2008 with 20 hours of observation.Comment: MNRAS, in pres

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

Nuclear ashes and outflow in the eruptive star Nova Vul 1670

CK Vulpeculae was observed in outburst in 1670-16721, but no counterpart was seen until 1982, when a bipolar nebula was found at its location. Historically, CK Vul has been considered to be a nova (Nova Vul 1670), but a similarity to 'red transients', which are more luminous than classical nova and thought to be the result of stellar collisions, has re-opened the question of CK Vul's status. Red transients cool to resemble late M-type stars, surrounded by circumstellar material rich in molecules and dust. No stellar source has been seen in CK Vul, though a radio continuum source was identified at the expansion centre of the nebula. Here we report CK Vul is surrounded by chemically rich molecular gas with peculiar isotopic ratios, as well as dust. The chemical composition cannot be reconciled with a nova or indeed any other known explosion. In addition, the mass of the surrounding gas is too high for a nova, though the conversion from observations of CO to a total mass is uncertain. We conclude that CK Vul is best explained as the remnant of a merger of two stars.Comment: an older version of an article that appeared in Nature; published in Nature, online version, 23 March 201