Issue |
A&A
Volume 432, Number 2, March III 2005
|
|
---|---|---|
Page(s) | 531 - 545 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20041658 | |
Published online | 02 March 2005 |
SiO masers in TX Cam
Simultaneous VLBA observations of two 43 GHz masers at four epochs
1
Onsala Space Observatory, Chalmers University of Technology, 43992 Onsala, Sweden e-mail: jiyune@oso.chalmers.se
2
University of Manchester, Jodrell Bank Observatory, Macclesfield, Cheshire SK11 9DL, UK
Received:
13
July
2004
Accepted:
20
October
2004
We present the results of simultaneous high resolution
observations of and
,
SiO masers toward
TX Cam at four epochs covering a stellar cycle. We used a
new observing technique to determine the relative
positions of the two maser maps. Near maser maximum
(Epochs III and IV), the individual components of both
masers are distributed in ring-like structures but the
ring is severely disrupted near stellar maser minimum
(Epochs I and II). In Epochs III and IV there is a large
overlap between the radii at which the two maser
transitions occur. However in both epochs the average
radius of the
maser ring is smaller than for
the
maser ring, the difference being larger for Epoch IV.
The observed relative ring radii in the two transitions,
and the trends on the ring thickness, are close to those
predicted by the model of Humphreys et al.
([CITE], A&A, 386, 256). In many individual features there
is an almost exact overlap in space and velocity of
emission from the two transitions, arguing against pure
radiative pumping. At both Epochs III and IV in many
spectral features only 50% of the flux density is
recovered in our images, implying significant smooth
maser structure. For both transitions we find that
red- and blue-shifted masers occur in all parts of
the rings, with relatively few masers at the systemic
velocity. Thus there is no evidence for rotation, although
the blue-shifted masers are somewhat more prominent to
the west. At all four epochs red-shifted components are
generally brighter than blue-shifted ones. Blue-shifted
masers become very weak at some stellar phases but never
completely disappear. At Epochs III and IV, we see many
filamentary or spoke-like features in both
and
masers, especially in the red-shifted gas. These spokes
show systematic velocity gradients consistent with a
decelerating outward flow with increasing radius. We
outline a possible model to explain why, given the
presence of these spokes, there is a deficit of maser
features at the systemic velocity. The breaking of
spherical symmetry by spoke-like features may explain
the high-velocity wings seen in SiO maser single dish
spectra.
Key words: masers / stars: AGB and post-AGB / stars: circumstellar matter / stars: individual: TX Camelopardalis / instrumentation: interferometers
© ESO, 2005
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