V838 Mon: Brown supergiant has eclipsed its B3V companion

Russian (KOI-8)
Published in Astronomer's Telegram No. 964.

V838 Mon is a member of a new class of eruptive variables which appear as a peculiar novae having late K-M type spectra in their outburst. Before its outburst, it was a wide binary system of B3V type stars. The bright companion of this system has exploded in 2002. On the base of plate archives it was revealed that the progenitor was 36+/-3 per cent brighter in the B and V bands than its companion, and their B-V colors were indistinguishable within the definition errors. The B3V companion was seen in the outburst in the UV spectrum taken with HST (Rauch et al., Exotic Stars as Challenges of Evolution. ASP Conf. Ser. V.279, 345, 2002), and was discovered by Munari et al. (IAUC 8005) in the post-outburst spectrum. The remnant of the explosion is also seen in the spectrum, it is an unique extremely cool oxygen rich L-type supergiant (sgL) (Evans et al., MNRAS V.343, 1054, 2003). In the latest spectra, the light of B3V companion was predominating in the short-wave photometric bands.

In December 2006, the sudden light decay of V838 Mon has been detected in the short-wavelength photometric bands by Goranskij (ATel 964). The amplitudes of the decay were 0.10, 0.65, 1.16, and 1.6 mag in Rc,V,B, and U, consequently. Figure 1 shows the CCD images taken with 1-m SAO telescope in B band, where this decay is well seen by eye. These observations along with the data published by Bond (ATel 966) indicate that the decay has happened between October 26 and November 22, 2006 suddenly, and probably no observations were covered this time range to fill the descending branch.

Fig. 1. CCD blue frames of V838 Mon before the eclipse (left) and in the eclipse (right).

The fragment of the historical light curve of V838 Mon in the B band given in the upper box of the Figure 2 shows the disappearance of the both blue components of the binary. Red circles are photographic pre-outburst observations indicating the total light of two stars. The main outburst light curve is not shown here, only its decay is drawn by vertical line. The blue and green circles are CCD observations. After the main outburst decay, we see only one B3V star (near JD~2452600). The remnant of the explosion was then so cool, that its light did not reach photometric B band. Later it became hotter, a small amount of flux reached the B band, and weak brightening occured. And finaly, the second B3V star in this system disappeared due to eclipse. The light curve of the eclipse is shown in the bottom box. One point given in parentheses shows an observation of poor quality published by Bond.

Fig. 2. Fragments of the light curves of V838 Mon in the B filter.

Spectral energy distributions (SEDs) of V838 Mon in UBVRcIc bands are shown in the Figure 3. These distributions are corrected for the interstellar reddening with the value of E(B-V)=0.77 mag. White line is the pre-outburst SED of the B-star binary reconstructed from the photographic plate archives. It is well fitted by SED of HD 29763, B3V type star. The violet line is the SED based on the September 2002 post-outburst observations. This SED contains B3V type star in UBV bands with the essential contribution of sgL star in R and I bands. Green line is the distribution on 2004 March when the sgL had maximum brightness. Dark-red and light-red SEDs were taken in the late 2006 before the eclipse, and in eclipse. The difference of these distributions is the light lost in the eclipse, it is drawn by blue line. Note that lost light fits well the B3V star distribution in the UBV bands observed just after the outburst and not distorted by sgL radiation in these bands.

Fig. 3. Spectral energy distributions of V838 Mon.

It is interesting to establish the nature of residual light in the eclipse (light-red line). Mostly this is the radiation of cool sgL star at the long wavelength, but there is a strong blue and UV excess superimposed on the distribution of the cool star. Barsukova et al. (ATel 803) reported the rapid strengthening of [FeII] forbidden- line spectrum in the blue region what they explain with the rarefied gas of ejecta arriving to the vicinities of B3V companion, and this companion was the source of ionizing radiation. This emission-line spectrum may explain the excess. Bond (ATel 966) reported that the blue emission lines of [FeII] continue to be strong, and very strong emission of H-alpha is detected in his October 12, November 29, and December 13, 2006 spectra. H-alpha emission was not seen in May. The evolution of this emission-line spectrum was accompanied by small photometric brightening of the star by 0.13, 0.15, 0.18, and 0.20 mag in Rc,V,B, and U filters, consequently. Bond suggested that the ejecta have reached the vicinity of the B3V companion and are beginning to engulf the star.

Our December 19 photometry (BVRcIc observations were taken by A.N.Burenkov in SAO) shows that the radiation of B3V star has totally disappeared from the star energy distribution. If such a hot star is really engulfed by the L-type supergiant and is now located under its photosphere, the source of ionization should disappear, and [FeII] line spectrum vanish. Otherwise, the eclipse hypothesis does not predict disappearance of [FeII] line spectrum, but partial eclipse of its forming region is possible.

View of current UBVRcRjIcIj light curves of V838 Mon. Java compatible brouser is needed. Use mouse mouseDrag function to examine the details.