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

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

Optical spectropolarimetry of V4332 Sagittarii

The eruption of V4332 Sgr was observed in 1994. During the outburst, the object became extremely red, so it has been considered as to belong to red transients of the V838 Mon type. Optical spectroscopy obtained a few years after the eruption showed a faint M-type stellar spectrum underlying numerous molecular and atomic emission features. 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 of the radiation of the central star on dust grains in the circumstellar matter. Recent polarimetric photometry has shown that the optical radiation of the object is indeed strongly polarized. We present and analyse our spectropolarimetric observations of V4332 Sgr obtained with the VLT in the optical region. The optical continuum of V4332 Sgr is linearly polarized with a typical degree of 16.6%. A clear depolarization is observed in the spectral regions where emission features contribute significantly to the observed flux. The only prominent exception is the CaI 6573 emission line, which is polarized in 21%. The results of our spectropolarimetric observations are in accord with the proposed geometry of the system and the advocated nature of the observed optical spectrum of V4332 Sgr. The continuum is not seen directly but results from scattering on dust within the disc and polar outflow, while most of the gas emission comes from the polar outflow excited by the radiation field of the central source. Additionally, the observed polarization patterns suggest a stratification of the outflow.Comment: accepted for publication in Astronomy & Astrophysic

Evolution of the stellar-merger red nova V1309 Scorpii: SED analysis

One very important object for understanding the nature of red novae is V1309 Sco. Its pre-outburst observations showed that, before its red-nova eruption in 2008, it was a contact binary quickly evolving to the merger of the components. It thus provided us with a direct evidence that the red novae result from stellar mergers. We analyse the spectral energy distribution (SED) of the object and its evolution with time. From various optical and infrared surveys and observing programmes carried out with OGLE, HST, VVV, Gemini South, WISE, Spitzer, and Herschel we constructed observed SED in 2010 and 2012. Some limited data are also available for the red-nova progenitor in 2007. We analyse the data with our model of a dusty envelope surrounding a central star. Dust was present in the pre-outburst state of V1309 Sco. Its high temperature (900-1000 K) suggests that this was a freshly formed dust in a presumable mass-loss from the spiralling-in binary. Shortly after its 2008 eruption, V1309~Sco became almost completely embedded in dust. The parameters (temperature, dimensions) of the dusty envelope in 2010 and 2012 evidence that we then observed matter lost by the object during the 2008 outburst. Its mass is at least $10^{-3}\,M_\odot$. The object remains quite luminous, although since its maximum brightness in September 2008, it has faded in luminosity by a factor of ~50 (in 2012). Far infrared data from Herschel reveal presence of a cold (~30 K) dust at a distance of a few thousand AU from the object. This conclusion could be verified by submillimeter interferometric observations.Comment: 12 pages, 7 figures, accepted in Astronomy & Astrophysisc

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

High-resolution optical spectroscopy of V838 Monocerotis in 2009

V838 Mon erupted at the beginning of 2002. In the course of the outburst the object evolved to low effective temperatures and declined as a very late M-type supergiant. Among various scenarios proposed to explain the nature of the outburst, the most promising is a stellar merger event. We aim at studying the structure and evolution of the object in the decline from the 2002 eruption. We obtained spectroscopic observations of V838 Mon in January--March 2009 with UVES/VLT. The results are analysed and compared with similar observations obtained in October 2005 with HIRES/Keck. The most striking difference between 2009 and 2005 is a complete absence of the B3V component and of the [FeII] emission lines in 2009. The present spectrum displays only the spectrum of the 2002 eruption remnant. It resembles that of an M6 giant, although the molecular bands in V838 Mon are deeper than those in standard stellar spectra of a similar spectral class. Several atomic lines, which displayed P-Cyg profiles in 2005, are now dominated by pure absorptions. Some of these lines, however, show a narrow emission component, which, as we argue, measures the radial velocity of V838 Mon. The resulting heliocentric velocity is 71 km/s, which very well agrees with the velocity obtained from SiO maser observations. The atomic lines and the molecular bands show very complex kinematics. In some atomic lines and high-excitation molecular bands we observe matter infalling in the object atmosphere. The infall components were already observed in 2005, but were less pronounced and present in fewer lines than in 2009. We argue that the most negative radial velocity components seen in the resonance atomic lines and in the low-excitation molecular bands were formed in the ejecta of the 2002 eruption. The B3V companion most probably became engulfed in an opaque dusty matter of the 2002 V838 Mon ejecta.Comment: A&A, in pres

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&

Post-outburst spectra of a stellar-merger remnant of V1309 Scorpii: from a twin of V838 Monocerotis to a clone of V4332 Sagittarii

We present optical and infrared spectroscopy of V1309 Sco, an object that erupted in 2008 in a stellar-merger event. During the outburst, V1309 Sco displayed characteristics typical of red transients, a class of objects similar to V838 Mon. Our observations were obtained in 2009 and 2012, i.e. months and years after the eruption of V1309 Sco, and illustrate severe changes in the remnant, mainly in its circumstellar surroundings. In addition to atomic gas observed in earlier epochs, we identified molecular bands of TiO, VO, H$_2$O, ScO, AlO, and CrO. The infrared bands of CrO we analyse are the first astronomical identification of the features. Over the whole period covered by our data, the remnant was associated with a cool ($\lesssim$1000 K) outflow with a terminal velocity of about 200 km/s. Signatures of warmer atomic gas, likely to be still dissipating the energy of the 2008 outburst, dramatically decreased their brightness between 2009 and 2012. In addition, the source of optical continuum disappeared sometime before 2012, likely owing to the formation of new dust. The final stage of V1309 Sco's evolution captured by our spectra is an object remarkably similar to an older red transient, V4332 Sgr. In addition to providing a detailed view on the settling of the eruptive object, the observations presented here reinforce the conclusion that all the Galactic red transients are a manifestation of the same phenomenon, i.e. a stellar merger. The late spectra of V1309 Sco also suggest peculiarities in the chemical composition of the remnant, which still need to be explored.Comment: to appear in A&

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&