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 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 ( ).
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 -> 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, 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.
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.