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Variable SiO Masers in the Circumstellar
Environment of V838 Monocerotis
M. J Claussen (NRAO), K. H. Healy, S. Starrfield (ASU), and H. E. Bond (STScI)
Introduction
In January 2002, the star V838 Monocerotis erupted, generating intense interest, at least partly because of the exquisite pictures taken by the Hubble Space Telescope (see the
background of this poster and the color picture below) ACS showing what is interpreted as a “light echo” from the eruption (1). A summary of the of the major results of observations since
the eruption are:
• The detailed light curve is unlike that of a supernova, nova, or any other type of variable star (1,2,3). During the outburst, V838 Mon was found to have a maximum effective temperature
of an A – F star at the optical maximum in February 2002. The effective temperature then cooled to a very low ~800 K by 230 days after the outburst, appearing to be that of an M or L
type supergiant (2.4.5.6).
• The distance to the source is estimated to be from 5 to 12 kpc. Analysis of the light echo and the extinction are the keys to these distance estimates (1,7,8,9). The best estimate of the
distance is 9 +/- 2 kpc.
• Evidence for a hot, blue companion (B3 V) was discovered when V838 Mon cooled (10,11). The progenitor of the outbursting component has been estimated to be 1) a very massive (65
Msolar), hot (Teff ~ 50,000 K), evolved star such as a Wolf-Rayet star (7); or 2) a main sequence or pre-main sequence star of 5 – 10 Msolar (11). There appears to be agreement that the
progenitor could not have been an evolved, low-mass, red giant star.
• Near-infrared spectroscopy taken as the star cooled showed very deep bands of H2O and strong absorption features of many metal oxides (12). The near-IR spectra were explained by
considering that the star ejected a shell of material that cooled and became opaque in many molecular bands. A model of the circumstellar shell gives a radius of the shell as 43 AU, a
photospheric radius of 8.8 AU and a distance of 9.2 kpc (12). The derivation of these parameters depends crucially on the cloud expansion velocity which is not well determined.
• SiO masers in the J = 1 -> 0, v = 1 and v = 2 transitions at 43 GHz were initially detected in February and April 2005 with the Nobeyama 45-m radio telescope (13), after a non-detection
using the NRAO Very Large Array in November 2003.
Figure 3
Figure 1
VLA and GBT Observations
Based on the rise of the strength of the SiO
masers in V838 Mon, we began a project of
monitoring the masers with the Very Large
Array. Since September, 2005, we have
observed the masers for 5 epochs with the
VLA, separated by approximately 1-month
intervals. The most recent VLA spectra are
shown in Figure 1. Figure 2 shows the time
history of the peak maser fluxes. In addition,
we observed the SiO masers with the 100-m
Green Bank Telescope, searching for very high
velocity spectral features; no such features
within +/- 350 km/s of the stellar velocity were
found.
Figure 2
Figure 4
VLBA Observations
VLBA observations were made of the SiO masers on October 17, 2005. The angular
resolution of the VLBA observations is 610 x 180 microarcseconds. The v=2 maser was
detected; a spectrum made from the image cube is shown in Figure 3. Only about 60% of
the VLA or GBT flux is recovered in this spectrum. An image of the peak velocity channel
(velocity width 0.11 km/s) is shown in Figure 4. The emission may be barely resolved
spatially. The missing flux density must be either in a “halo” of emission resolved out by the
interferometer or in many unresolved clumps, each undetectable (the rms noise in the
Figure 4 image is about 60 mJy/beam.
Discussion
Although the progenitor star is not thought to be a late-type giant or supergiant, the spectrum of the SiO
masers resembles that of Mira variables or supergiant stars harboring SiO masers just above the
photosphere, and below the dust condensation radius. The flux density of the masers in these late-type
stars typically have the same period as the optical or infrared pulsation period. In V838 Mon we have
seen a steady rise in the flux density of the masers after their first detection to the present epoch. In
nearby Mira variables the maser components are typically found in ring-type structures of several AU in
diameter (1.5 – 4.0 R*). At a distance of 9 kpc, 5 AU corresponds to about 0.5 milliarcseconds. It appears
that most of the maser emission (from the VLBA observations) lies well within the 43 AU dust shell, but the
estimate by Lynch et al. (2004) of Rphot~ 8.8 AU seems inconsistent with the small size of the maser
emission, unless the masers are actually within the photosphere.
SiO masers in the circumstellar shells of Mira variables are thought to be pumped by collisions, and the
infrared dust radiation field seen by the masers is expected to be weak since the masers are usually
separated from the dust condensation zone by at least a few stellar radii. It is not clear how this model
applies to the masers in V838 Mon, but similar physical conditions should apply.
Further VLA and VLBA observations are continuing.
References
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The NRAO is operated by Associated Universities, Inc. under cooperative agreement with the National Science Foundation. SS acknowledges partial support to ASU from the NSF and NASA.