Variable SiO Masers in the Circumstellar Environment of V838 Monocerotis

1
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 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 -gt 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 4
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 mases 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.

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 v2 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.
Figure 2

• 
  
  Referencs
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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.