[vsnet-campaign-v838mon 362] the nebula is an old story

[Ulisse Munari <munari@astras.pd.astro.it>]

The nebula around V838 Mon is an old story, already discussed
also within VSNET.

The nebula became visible when a light echo sweeped through it.
The light echo (and the nebula) have been discovered
by Henden, Munari and Schwartz in February and announced in
IAU Circ. 7859 (25 March 2002).

The light echo is made of scattered light, and scattering is enormously
more efficient at shorter wavelengths (the reason why the sky is blue.
This is called Rayleigh scattering, and its dependence with the
wavelength is lambda^[-4]), and so the  light-echo and  the nebula became
visible only in U band in the beginning.
Also the very red colors  of the central star eased the contrast at shorter
wavelengths. Now that the central star had dimmed so much, the nebula is
getting visible even at longer wavelengths (visual), even if it is more
evident at shorter wavelength bands.

However, the feeding of light into the circumstellar nebula is now ceasing
that V838 Mon is rapidly returning toward quiescence brightness, and
a central "hole" (better to call it "brightness drop") is now developping
and expanding outward as much as the light-echo expanded outward when
first became visible. The central hole has been discovered by Henden,
Munari, Marrese, Boschi and Corradi and announced in IAU Circ 7889
(1 May 2002). Its radius was 6 arcsec on Apr 30.

   Ulisse Munari, Padova and Asiago Astronomical Observatories

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