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V838 Mon: Radical spectral changes at short wavelengths

ATEL # 1821; V.P. Goranskij (SAI, Moscow Univ.) and E.A. Barsukova (Special Astrophysical Observatory, Russia)
on 30 Oct 2008; 16:29 UT
Password Certification: Vitaly Goranskij (

Subjects: Optical, Ultra-Violet, Binaries, Nova, Variables, Stars

We continue multicolor photometric CCD UBVRI monitoring of the peculiar red nova V838 Mon using CCD photometer and SAO RAS 1-m reflector. New observations were obtained in two nights in October 2008. We noted strong changes in the U and B bands happened between 2008 April and October. The star became invisible in U band frames. Each night we have accumulated 6000 s exposures to get a measurable image of the star. The accuracy we have got was 0.2 mag. The accuracy of magnitudes in other filters was 0.02 mag. The current magnitudes (U,B,V,Rc,Ic) comparing with the April 2008 data are the following:

2008 Apr 10.75 UT (18.50,17.72,15.76,12.94,10.22);
2008 Oct 27.05 UT (20.75,18.48,15.79,12.90,10.17);
2008 Oct 28.03 UT (21.06,18.49,15.78,12.92,10.17).

In such a way the nova declined by 2.4 mag in the U band and by 0.76 mag in the B band. Changes in other filters are small.

To understand these changes, one should take into account that before the explosion in 2002, V838 Mon was a system containing two B3V type stars, and the brighter companion exploded. The remnant of the explosion was an expanding very cool star, a possible L type supergiant. The secondary B3V star remained in the spectrum of the system. Since 2004 till 2006 the forbidden FeII emissions strengthened in the short wavelength spectrum. This was possibly due to an approach of the ejecta to B3V companion (ATEL #803). In Dec 2006, B3V type star disappeared from the spectral energy distribution (ATEL #964) and reappeared in 70 days (Munari et al., 2007). At the same time, the emission line spectrum remained very strong. In the season of 2007-2008, B3V companion was captured by expanding remnant, but its variable radiation leaked through the cloudy structure of the remnant's envelope. This radiation of B star excited weak [FeII] lines. The radius of cool star was estimated as 30.000 solar radii at the moment of the capture (Goranskij et al., 2008).

Evidently, the hot B3V companion has entered now in dense parts of the cool star's envelope, and its radiation does not come out. The engulf of the hot star assumed by Bond (ATEL #966) has happened at last. Additionally, the radiation of excited gas being strong in this spectral range disappeared. So the contribution of the cool remnant stays dominant in all the spectrum. Our current photometry agrees with the Planck energy distribution of a single body with the temperature of 2200+/-200K.

The details.

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