* Astronomy

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RE: V838 Monocerotis

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Title: Light echo of V838 Monocerotis: properties of the echoing medium
Authors: Romuald Tylenda, Tomasz Kaminski

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 vicinity 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.

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Rare star smash may explain mystery outburst
A star that brightened dramatically in 2002 may have been sent into a spin by another star, X-ray observations suggest

In 2002, a star called V838 Monocerotis brightened dramatically. Some researchers have suggested that a collision with another star produced the fireworks, while others have argued that the star simply "burped" after swallowing some planet-sized objects.
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Title: An analysis of a spectrum of V838 Monocerotis in October 2005
Authors: R. Tylenda, T. Kaminski, M. Schmidt

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 ionising and exciting H in the outflow.

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Title: Peculiar Red Nova V838 Mon: Accretion and Interaction in a Wide Binary System after Explosion of Its Companion
Authors: Vitaly Goranskij, Alla Zharova, Elena Barsukova, Sergei Fabrika, Azamat Valeev

We report the results of recent multicolour photometry and medium resolution spectroscopy of V838 Mon taken in 2007-2008. In the eclipse-like event in December 2006, the hot B3V type companion disappeared. The event accompanied by strengthening emission [FeII]/FeII lines in the spectra. We explain this event as the formation of temporal short-lived accretion disc around the hot companion. Later, in February 2007 the hot star reappeared in its full brightness, but disappeared again for a long time since September 2007. This is the engulf of B3V companion by expanding remnant of 2002 outburst. We assume that the thick accretion disc has formed around B type companion which is moving now inside the envelope of the cool star. There is some evidence of heating this disc and/or cool star envelope. We estimated the radius of expanding cool remnant in December, 2006 of about 150 A.U. or 30000 solar radii.

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Title: V838 Monocerotis: A Geometric Distance from Hubble Space Telescope Polarimetric Imaging of its Light Echo
Authors: William B. Sparks (1), Howard E. Bond (1), Misty Cracraft (1), Zolt Levay (1), Lisa A. Crause (2), Michael A. Dopita (3), Arne A. Henden (4), Ulisse Munari (5), Nino Panagia (1,6), Sumner G. Starrfield (7), Ben E. Sugerman (8), R. Mark Wagner (9), Richard L. White (1), (1-Space Telescope Science Institute, 2-South African Astronomical Observatory, 3-RSAA, Australian National University, 4-AAVSO, 5-INAF Osservatorio Astronomico di Padova, 6-INAF Osservatorio Astrofisico di Catania and Supernova Ltd., 7-Arizona State University, 8-Goucher College, 9-Large Binocular Telescope Observatory)

Following the outburst of the unusual variable star V838 Monocerotis in 2002, a spectacular light echo appeared. A light echo provides the possibility of direct geometric distance determination, because it should contain a ring of highly linearly polarised light at a linear radius of ct, where t is the time since the outburst. We present imaging polarimetry of the V838 Mon light echo, obtained in 2002 and 2005 with the Advanced Camera for Surveys onboard the Hubble Space Telescope, which confirms the presence of the highly polarised ring. Based on detailed modelling that takes into account the outburst light curve, the paraboloidal echo geometry, and the physics of dust scattering and polarisation, we find a distance of 6.1±0.6 kpc. The error is dominated by the systematic uncertainty in the scattering angle of maximum linear polarization, taken to be theta_{max}=90^o ± 5^o. The polarimetric distance agrees remarkably well with a distance of 6.2±1.5 kpc obtained from the entirely independent method of main-sequence fitting to a sparse star cluster associated with V838 Mon. At this distance, V838 Mon at maximum light had M_V ~= -9.8, making it temporarily one of the most luminous stars in the Local Group. Our validation of the polarimetric method offers promise for measurement of extragalactic distances using supernova light echoes.

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Title: Observations of V838 Mon in the CO rotational lines
Authors: T. Kaminski, M. Miller, R. Tylenda

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 bubble 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.

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