Evolution of V838 Monocerotis during and after the 2002 eruption

By fitting the available photometric data on V838 Mon with standard supergiant spectra we have derived principal stellar parameters, i.e. effective temperature, radius and luminosity, and followed the evolution of the object since its discovery in early January 2002. Our analysis shows that the 2002 outburst of V838 Mon consisted of two major phases: pre-eruption which was observed in January 2002 and a major outburst, called eruption, which started in the beginning of February 2002. During pre-eruption the object seemed to be relaxing after an initial event which had presumably taken place in last days of December 2001. The eruption phase, which lasted till mid-April 2002, resulted from a very strong energy burst, which presumably took place in last days of January at the base of the stellar envelope inflated in pre-eruption. The burst produced an energy wave, which was observed as a strong luminosity flash in the beginning of February, followed by a strong mass outflow in form of two shells, which was observed as an expanding photosphere in later epochs. In mid-April, when the outflow became optically transparent and most of its energy radiated away, the object entered the decline phase during which V838 Mon was evolving along the Hayashi track. This we interpret as an evidence that the main energy source during decline was due to gravitational contraction of the object envelope inflated in eruption. Late in 2002 a dust formation started in the expanding shells which gave rise to a strong infrared excess observed in 2003.Comment: 14 pages, 4 figures, 2 tables, accepted in Astronomy & Astrophysic

Evolution of the progenitor binary of V1309 Scorpii before merger

It was recently demonstrated that the eruption of V1309 Sco was a result of a merger of the components of a cool contact binary. We computed a set of evolutionary models of the detached binaries with different initial parameters to compare it with pre-burst observations of V1309 Sco. The models are based on our recently developed evolutionary model of the formation of cool contact binaries. The best agreement with observations was obtained for binaries with initial masses of 1.8-2.0 solar masses and initial periods of 2.5-3.1 d. The evolution of these binaries consists of three phases: at first the binary is detached and both components lose mass and angular momentum through a magnetized wind. This takes almost two thirds of the total evolutionary lifetime. The remaining third is spent in a semi-detached configuration of the Algol-type, following the Roche-lobe overflow by the initially more massive component. When the other component leaves the main sequence and moves toward the giant branch, a contact configuration is formed for a short time, followed by the coalescence of both components.Comment: 5 pages, 1 figure, Astronomy and Astrophysics, in prin

An analysis of a spectrum of V838 Monocerotis in October 2005

V838 Mon erupted at the beginning of 2002. Among various scenarios proposed to explain the nature of the outburst, the most promising is a stellar merger event. The results of spectroscopic observations of the object obtained in October 2005 with the Keck/HIRES instrument, presented in detail in Paper I, are analysed and discussed. Our analysis of the molecular bands and the P-Cyg profiles of atomic lines shows that the object loses matter with a velocity of up to 215 km/s and a rate of 10^{-6} - 10^{-5} M_sun/yr. In the profiles of some atomic lines, we have also found evidence of matter infall. A narrow absorption component, which is particularly strong in some P-Cyg profiles, may indicate that a jet-like outflow has also been formed. We show that the observed emission in the [Fe II] lines and an eclipse-like event observed in November/December 2006 was probably caused by interactions of the expanding matter, ejected by V838 Mon in 2002, with radiation from the B3V companion. In particular, the observed profiles of the [Fe II] lines can be easily modelled in this scenario and allow us to estimate parameters of the system, such as the position of the B3V companion relative to V838 Mon and the line of sight, density in the outflowing matter, and mass lost in the 2002 eruption. The observed appearance of strong H-alpha emission, just before and during the eclipse-like event, can be interpreted as a result of the accretion of the outflowing matter onto the B3V companion: the accreted matter, shocked above the stellar surface, can be a source of extreme-UV and soft X-ray radiation capable of ionizing and exciting H in the outflow.Comment: 9 pages, 5 figures, accepted in Astronomy & Astrophysic

Observations of V838 Mon in the CO rotational lines

We investigate the structure of a field around the position of V838 Mon as seen in the lowest CO rotational transitions. We also measure and analyse emission in the same lines at the position of V838 Mon.Observations have primarily been done in the CO J = 2-1 and J = 3-2 lines using the KOSMA telescope. A field of 3.4 squared degrees has been mapped in the on-the-fly mode in these transitions. Longer integration spectra in the on-off mode have been obtained to study the emission at the position of V838 Mon. Selected positions in the field have also been observed in the CO J = 1-0 transition using the Delingha telescope.In the observed field we have identified many molecular clouds. They can be divided into two groups from the point of view of their observed radial velocities. One, having V(LSR) in the range 18-32 km/s, can be identified with the Perseus Galactic arm. The other one, having V(LSR) between 44-57 km/s, probably belongs to the Norma-Cygnus arm. The radial velocity of V838 Mon is within the second range but the object does not seem to be related to any of the observed clouds. We did not find any molecular buble of a 1 degree dimension around the position of V838 Mon claimed in van Loon et al. An emission has been detected at the position of the object in the CO J = 2-1 and J = 3-2 transitions. The emission is very narrow (FWHM ~ 1.2 km/s) and at V(LSR) = 53.3 km/s. Our analysis of the data suggests that the emission is probably extended.Comment: paper accepted in A&

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

Strong linear polarization of V4332 Sgr: a dusty disc geometry

The eruption of V4332 Sgr was observed in 1994. During the outburst, the object became extremely red, so it is considered as belonging to luminous red transients of the V838 Mon type. It has recently been suggested that the central object in V4332 Sgr is now hidden in a dusty disc and that the photospheric spectrum of this object observed in the optical results from scattering the central star radiation on dust grains in the disc. One expects significant polarization of the spectrum in this case. We investigate this prediction. We present and analyse polarimetric observations of V4332 Sgr in the V and R photometric bands done with the NOT telescope. The optical light of V4332 Sgr is linearly polarized with a degree of ~26% in the V band and ~11% in R. Discussion of the observed polarization leads us to conclude that the photospheric spectrum observed in V4332 Sgr is probably produced by dust scattering not only in the disc but also in the outflow from the object seen in the emission features.Comment: accepted in Astronomy & Astrophysics as a research not

The peculiar nova V1309 Sco/Nova Sco 2008: A candidate twin of V838 Mon

Nova Scorpii 2008 was the target of our Directory Discretionary Time proposal at VLT+UVES in order to study the evolution, origin and abundances of the heavy-element absorption system recently discovered in 80% of classical novae in outburst. The early decline of Nova Scorpii 2008 was monitored with high resolution echelle spectroscopy at 5 different epochs. The analysis of the absorption and the emission lines show many unusual characteristics. Nova Scorpii 2008 is confirmed to differ from a common Classical Nova as well as a Symbiotic Recurrent Nova, and it shows characteristics which are common to the so called, yet debated, red-novae. The origin of this new nova remains uncertain.Comment: 10 pages, 8 figures. Accepted by A&

A molecular cloud within the light echo of V838 Monocerotis

Context. V838 Mon is an eruptive variable, which exploded in 2002. It displayed the most spectacular light echo ever observed. However, neither the origin of the reflecting matter nor the nature of the 2002 outburst have been firmly constrained. Aims. We investigate the nature of the CO radio emission detected in the field of the light echo. In particular, we explore its connection to the echoing dust around V838 Mon. Methods. We observed the echo region in multiple CO rotational transitions. We present and analyse maps of the region obtained in the 12CO(1-0) and (3-2) lines. In addition, deep spectra at several positions were acquired in 12CO(1-0), (2-1), (3-2), and 13CO(1-0), (2-1). Radiative transfer modelling of line intensities is performed for chosen positions to constrain the kinetic temperatures and densities. We derive global parameters (e.g. mass, distance, total column density) of the emitting cloud. Results. We found that a compact molecular cloud is located within the echo region. The molecular emission is physically connected to the dusty environment seen in the optical echo and they both belong to the same translucent cloud. The interstellar nature of the cloud is confirmed by its high mass of 90-150 Msun. We propose that the cloud consists of material remaining after the formation of the cluster to which V838 Mon belongs. This indicates that the eruptive star has young age (3-10 Myr).Comment: accepted for publication in A&