V838 Mon engulfs its companion

ATel #966; Howard E. Bond (STScI)
on 18 Dec 2006; 17:54 UT
Credential Certification: Howard E. Bond (bond@stsci.edu)

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In ATel #964 Goranskij reported a sudden fading at blue wavelengths of the peculiar variable star V838 Mon. We report that the fading is confirmed by CCD frames obtained with the Andicam camera on the SMARTS Consortium 1.3m telescope at Cerro Tololo (observers: J. Espinoza & D. Gonzalez). Observations during 2006 are as follows (UT dates are for the B observations; the V magnitudes were obtained within 1-2 days of those at B). We also list the observations reported by Goranskij in ATel #964.

Date	B	V
------	-----	-----
Jan 11	16.63	15.40
Jan 21	16.62	15.40
Feb 7	16.62	15.39
Feb 18	16.61	15.38
Mar 26	16.61	15.36
Apr 19	16.60	15.35
May 18	16.62	15.36
Sep 21	16.54	15.33
Oct 21	16.47	15.27
Oct 26	16.42	15.36	Goranskij ATel 964
Nov 22	17.14:	15.70
Dec 2	17.33	15.75	Goranskij ATel 964
Dec 10	17.52	15.76	Goranskij ATel 964

V838 Mon was nearly constant during most of the first half of 2006, with perhaps a slight brightening in V. After emerging from behind the Sun, it appeared to brighten by ~0.1 mag in V and ~0.2 mag in B through the end of October. The sudden decline had its onset sometime in November.

Spectroscopic observations with the SMARTS 1.5m telescope (observers: C. Aguilera & A. Pasten) also show dramatic changes. The blue emission lines of [Fe II] reported in 2006 May by Barsukova et al. (ATel #803) continue to be strong, while the blue continuum due to the B3 companion is fading. A very strong H-alpha emission line was detected in a spectrogram on 2006 Oct 12 and again on Nov 29 and Dec 13, whereas no H-alpha emission was seen on 2006 May 14.

These findings suggest that ejecta from the V838 Mon outburst have now reached the vicinity of the B3 V companion and are beginning to engulf the star. Continued monitoring is encouraged.