[IAUC] CBET 780: 20061221 : V838 MONOCEROTIS

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                                                  Electronic Telegram No. 780
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
M.S. 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
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V838 MONOCEROTIS
     U. Munari, Istituto Nazionale di Astrofisica, Padova Astronomical
Observatory; R. L. M. Corradi, Isaac Newton Group (ING), La Palma; A. Henden,
AAVSO; and I. G. de la Rosa and A. Vazdekis, Instituto de Astrofisica de
Canarias, report that two high-S/N spectra of V838 Mon were obtained on Dec.
18.09 UT with the 4.2-m William Herschel Telescope of the ING at the
Observatorio del Roque de los Muchachos, La Palma, using the ISIS
spectrograph.  A 600-lines/mm grating was used to cover the spectral range
from 386 to 554 nm at a resolution of 0.16 nm.  The spectrophotometric
standard G191-B2B was also observed to flux-calibrate the spectra of V838 Mon.
For the first time since the eruption in January 2002, the spectra now display
a forest of very strong Fe II and [Fe II] emission lines (far stronger than
Balmer lines) on top of the cool continuum of the L-type supergiant dominated
by molecular absorptions.  Fully developed Fe II multiplets 27, 28, 37, 38,
42, 48, and 49 are observed at heliocentric radial velocity +30.8 (+/- 1.1)
km/s, and [Fe II] multiplets 4, 6, 7, 18, 19, 20, and 21 are observed at +30.3
(+/- 0.7) km/s.  The emission lines are intrinsically sharp enough to be not
resolved.  The integrated flux of the H_beta emission line is 1.6 x 10^(-15)
erg/cm2/s.  The integrated flux of [Fe II] 428.740-nm (multiplet 7) is 5.3
times that of H_beta, and for Fe II 531.661-nm (multiplet 49) is 5.5 times
that of H_beta.  Compared to the situation in December 2005, as described by
Barsukova et al. (http://www.astronomerstelegram.org/?read=803), now [Fe II]
multiplet 21 emission lines are 9.5 times stronger.  The spectrum of the B3 V
companion has weakened sensibly, being now just slightly perceptible.
Magnitudes obtained with the Sonoita Research Observatory 0.35-m robotic
telescope (errors not exceeding 0.02 mag):  Oct. 26.409, V = 15.207, B-V =
+0.918, V-R_c = +2.111, R_c-I_c = +2.767; Nov. 23.337, 15.560, +1.363, +2.408,
+2.869; Dec. 19.276, 15.742, +1.608, +2.568, +2.882; Dec. 20.297, 15.786,
+1.852, +2.595, +2.886.  The sudden fading and reddening at blue wavelengths
announced by Goranskij (http://www.astronomerstelegram.org/?read=964) and Bond
(http://www.astronomerstelegram.org/?read=966) is confirmed.  The average
fading in B since Oct. 26 is 0.025 mag/day, and reddening in B-V is 0.015
mag/day.  The decline in the B band is clearly associated with the
disappearance of the B3 V spectrum at shorter wavelengths, while the L-type
supergiant has remained stable in brightness at R_c within 0.03 magnitude of
around 13.15.


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                         (C) Copyright 2006 CBAT
2006 December 21                 (CBET 780)               Daniel W. E. Green