The Dust Environment of V838 Mon based on the Light Echo, and Afsar-Bond Star Cluster Vitaly Goranskij Sternberg Astronomical Institute of the Moscow University Fragments of the Nick Risinger Sky Survey. Location of V838 Mon is shown.SAO 1 m Zeiss reflector

Observations of V838 Mon light echo were performed with the 60 cm telescope of the SAI Crimean Station of and CCD PI VersArray and with the SAO 100 cm Zeiss with CCD EEV Both CCDs are liquid-nitrogen cooled to the temperature of about -130 C. Johnson or Cousins R band filters were used. Field of CCD EEV in the R band was partly subjected to fringes, the exposures were accumulated up to 1.5 hours in 300 s each, with the position shifts and then added with star matching. 31 CCD frames of different quality were taken in the time range between 2002 October 4 and 2009 October 21. Additionally used were Hubble Space Telescope, USNO Flagstaff Station, South African Astronomical Observatory and other images with the fixed date (published in the Proceedings of La Palma 2006 Conference “The Nature of V838 Mon and its Light Echo” ASP Conf. Series Vol. 363).

HST SUM SUM SUM

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Cirrus

Cirrus

The last frame. The illuminated region is still visible 1’ north of V838 Mon. Time dependence of echo edges’ angular distance from central object in different directions. The brakes of dependence are seen due to the transit of the light echo into side walls of the dust cloud. Superluminal part Method of distance estimate.

Models of the light echo Ellipsoid fitting used. Profiles of the dust nebula of V838 Mon were taken in two sections with position angles 30 and 120 degrees. With the generally adopted distance value of 6 kpc, -the frontal surface is about plane and inclined by 15 deg to the line of site; - side surface is approximately cylindrical; - cloud has diameter of 3-4 pc, V838 Mon is located in the depth of about 2 pc under the frontal surface and 0.8 pc from the center; - the opposite surface is badly visible. Distance larger than 10 pc excluded. To observer Superluminal part is badly fitted

The nature of a central hole in the first HST frames of the light echo V838 Mon is located in the void near the edge of a dust inhomogeneity. Structural details of the nebula are unrelated to the star's position. Sign + is a star position HST

B3V B4V B6V V838 Mon progenitor Photographic V band image taken 25 years before the explosion with the 50 cm Maksutov АZТ-5 telescope of SAI Crimean Station on 1977 September 21. A pair of B3V type stars of V838 Mon progenitor looks fainter than single B3V type cluster member AB 9. V838 Mon and Afsar-Bond cluster With the magnitudes measured using archive plates, both the exploded star and its companion are plotted in the Color-Magnitude Diagram of the Afsar-Bond cluster. Both components are essentially lower than cluster members of the same Sp type.

Location of both V838 Mon components in the Color-Magnitude Diagram of the cluster Our photometry of stars inside the region of the echo-illuminated cloud is presented. Spectra of marked stars were taken with the Russian 6 m telescope BTA. Location of V838 Mon B3V companion in the Two-Color (U-B) - (B-V) Diagram Lines are fragments of main sequences (unreddened and reddened by E(B-V)=0.77) We did not found new cluster members

Cluster members Star Sp. V* B-V U-B M V B.C. log L/L sun AB 9 B3V AB 8 B4V AB 7 B6V Exploded** B3V Companion B3V *) Magnitudes and colors are measured relative to Munary et al. (2002, A&A 389, L51) standard sequence. **) Measured lost light extracted from archive BVRI photographic photometry.

Location of V838 Mon components in the Spectrum-Luminosity Diagram Basic data sources: Distance 6.2 kpc (Bond et al., 2008; echo); Reddening E(B-V) = 0.87 mag, A V = 2.68 mag (Munari et al., 2005) [our value is 0.77 mag]; Spectra of cluster members and V838 Mon companion from Afsar & Bond (2007); Photometry in the V band is ours; Bolometric corrections of B type stars from Nieva & Przybilla (IAUS 272, Paris, 2010); Evolutionary tracks from Schaller et al. (1992), Y=0.300, Z= Fragment from Fig. 1 by Schaller et al. (1992)

Conclusions Components of V838 Mon system were underluminous B3V stars. Nevertheless, they are cluster members, and associated with a dust cloud. The exploded star was located 0.97 mag below, and its companion B3V engulfed in 2008 by the explosion remnant was 1.32 mag below in the V band. With the generally accepted distance and reddening, the exploded star is located precisely in the zero age main sequence (ZAMS)*, and its companion is below (ZAMS). Both components of V838 Mon are young, whereas other cluster members are evolved stars. *) This is a ZAMS star also based on its chemical composition (Kipper et al., 2004).

What was the cause of explosion of the brighter companion? Hypotheses. I. Started hydrogen burning in the center of a star at the end of gravitational contraction stage. Then the companion stands in line to explode. II. Merging of a binary core inside the common envelope. III. Hydrogen flash due to core merging. Why V838 Mon components have a lag behind in evolution? Hypotheses. I. Rapid rotation. II. Gravitational contraction stage. III. Probable I + II.

Why red novae are cool in the outburst? In classical novae, hydrogen explosion happens in a layer on a white dwarf surface. A small amount of matter located above being ejected into space has a mass of 1/1000 or 1/10000 solar masses. When the ejected gas expands, its density decreases rapidly, it passes into the optically thin state and gets ionized by the hard radiation of the hot dwarf surface. On the contrary, if hydrogen explosion or energy release of another nature happens in the center of a star, the power push makes large volume of mass located above to expand in a nearly adiabatic regime. A lack of hot star’s energy comes out, and the star becomes dark. When the radiation of the explosion gets the expanding surface, its area is already so large that this radiation can not heat the surface to high temperature. So, red novae begin expansion long before the optical outburst. There is a weak confirmation of this hypothesis in the case of V838 Mon: Kimeswenger & Eyres (2006, IBVS 5708) have found fading of the star in the R and I bands in 1998 – 1999 by 0 m.36 and 0 m.40, correspondingly, a few years before the outburst.

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