Maser Observations in VY CMa with VERA
Yoon Kyung CHOI (The University of Tokyo) and VERA project team (NAOJ)
ABSTRACT
We present the results of multi-epoch VERA(VLBI Exploration of Radio Astrometry) observations of 22GHz H2O and 43GHz
SiO masers in the supergiant VY CMa(VY Canis Majoris). We estimated the inner motion of H2O masers over 6 months andthat of SiO masers over 1 month. We also compared the distribution of SiO J=1-0 v=1 masers with J=1-0 v=2 masers in VY
CMa using the simultaneous VERA observation.
OBSERVATIONSINTRODUCTION
 VERA 4 stations in Japan
Red supergiants have a high mass-loss rate and are often
surrounded by a shell of gas and dust. H2O and SiO masers arefound in circumstellar envelopes(CSE) of the supergiants. Thesemasers are important tool to study the structure and the dynamics ofCSE, the mass-loss process, and the evolution of these stars.

DateTransition

2003. 05. 20
2003. 10. 23

H2O(616-523)

2003. 12. 18
2006. 04. 23
2006. 05. 26

RESULTS
22GHz H2O Maser Distribution

28SiO J=1-0

v=1 and v=2Duration

Beam size (mas)8 hours2.16 ×0.77

6 hours

2.24 ×0.80

8 hours

2.06 ×0.80

7 hours

1.02 ×0.40

7 hours

1.06 ×0.40

Correlation: Mitaka FX correlator

43GHz SiO Maser Distribution

Figure 1.(a) The spectrum of H2O masers
To align three maps , we assume theposition of the strongest maser
component at each epoch is thesame over three epochs. In order toinvestigate the inner motions, maser
components which are detected at all
three epochs have been chosen.
Figure 1.(b) The distribution of H2O
masers over three epochs.

For chosen components, we haveinvestigated the inner motions over
six months between epoch 1 andepoch 3. Figure 2 shows themovement of these components. The
arrows on the Figure 2 indicate thedirection of the inner motions, but thelength of the arrows do not relate theamount of movement.

Figure 3. (top) SiO maser spectrum for v=1 J=1-0 (left) and for v=2 J=1-0 (right)(bottom) SiO maser distributions for v=1 J=1-0 (left) and for v=2 J=1-0 (right)

The movement of the maser
components are 0.55 mas in R.A. and0.7 mas in DEC on the average.

Figure 2. The inner motion of H2O
masers for six months

Using these inner motions, weestimated the statistical parallax. The
statistical parallax method assume
the standard deviation of the radialvelocities and that of the innermotions are the same. The distanceto VY CMa we estimated is 1.88 kpc.
Figure 4. The aligned map of v=1 and v=2

We aligned both v=1 and v=2 maps withthe maser component which has theVLSR=22.0 km s-1. For the same velocity
components, the difference of thepositions is 0.45 mas in R.A. and 0.19
mas in DEC on the average.
For the transition of J=1-0 v=1 28SiO
masers, we estimated the preliminary
inner motion for one month. The distanceto VY VMa is 1.18 kpc assuming thestatistical parallax. This result isunderestimated to the previousstudies(1.5kpc, Lada & Reid, 1978).

DISCUSSION
We estimated the statistical parallax for both H2O
and SiO masers. Compared with the result from H2O
masers, the distance is underestimated from theinner motion of SiO masers. The possible reason isthat the inner motions are overestimated because ofthe short time scale.

The adopted physical value of the model for VY CMa’s
circumstellar geometry is R*=13 AU (Smith et al. 2001),
and it is 6.91 mas at 1.88 kpc and 11.01 mas at 1.18 kpc.
When we assume the distribution of SiO masers is thering as shown Figure. 5, the diameter is approximately 40mas. Therefore we obtain RSiO ~ 1.81-2.89 R*, and thisresult is consistent with that the SiO maser emit at typicaldistances of 2–4 stellar radii toward Mira variables andsupergiants (Boboltz et al. 1997).

REFERENCE
Boboltz, D. A., Diamond, P. J., & Kemball, A. J. 1997, ApJ, 487, L147
Lada, C. J., & Reid, M. J. 1978, ApJ, 219, 95
Smith et al. 2001, AJ, 121, 1111

Figure 5. The preliminary inner motion of
v=1 J=1-0 SiO masers