1. Multi-transition VLBA observations of circumstellar SiO masers
Rebeca Soria-Ruiz
(Observatorio Astronómico Nacional, Spain)
Francisco Colomer (OAN)
Javier Alcolea (OAN)
Valentín Bujarrabal (OAN)
Jean-Francois Desmurs (OAN)
K. B. Marvel (AAS)
D. Boboltz (USNO)
P. J. Diamond (JBO)
A. J. Kemball (NRAO)

2. SiO observational data
(1) Mass loss values taken from Loup et al. , 1993, A&AS 99, 291
(2) Phase with respect to the NIR/SiO maximum extrapolated from Pardo J.R., Alcolea J. & Bujarrabal V. (in prep.)
(3) Phases with respect to the optical maximum derived from the light curves available at

3. IRC : 28SiO v =1 J = (43 GHz)

4. IRC : 28SiO v =2 J =1 0

5. IRC : v =1 vs. v =2 (J =1 0)
 Similar spatial distribution
 Ring-like structure sizes:
v = 1: d  20 m.a.s.
v = 2: d  18 m.a.s.
 Consistent with Desmurs et al. (2000)

6. IRC : 28SiO v =1 J = (86 GHz)

7.  Spatial distribution quite different in both transitions
IRC : 7 mm vs. 3 mm (v = 1)
 Spatial distribution quite
different in both transitions
and non-coincident.
 Ring-like structure sizes:
7 mm: d  20 m.a.s.
3 mm: d  m.a.s.
Contradiction with the current SiO masers pumping mechamisms 
c f. similar study of R Cas by Phillips et al. (2003)

8. chi Cyg: 28SiO v =1 J = ( 43 GHz)

9. chi Cyg: 28SiO v =1 J = (86 GHz)

10.  Ring-like structure sizes:
chi Cyg: 7 mm vs. 3 mm (v = 1)
 Spatial distribution different in
both transitions: only one
possible “coincident” spot ?
Different flux-velocity distribution
The emission arises in nearby (but
not the same) projected regions.
 Ring-like structure sizes:
7 mm: d  m.a.s.
3 mm: d  45 m.a.s.

11. TX Cam: 28SiO v =1 J = (43 GHz)

12. TX Cam: 28SiO v =2 J =

13. TX Cam: v =1 vs. v = 2 (J =1 0)  Similar spatial distribution
in both transitions.
 Ring-like structure sizes:
v = 1: d  28 m.a.s.
v = 2: d  26 m.a.s.

14. * Results consistent with Desmurs J.F., et al. A&A, 360 , 189 (2000)
Conclusions
Source Comparison
* Results consistent with Desmurs J.F., et al. A&A, 360 , 189 (2000) 
Observational results clearly contradict the theoretical models, where the spatial distributions and “ring” structure sizes of the J =1-0 and J = 2-1 (v = 1) transitions should be similar.

15. Work in progress 
We are investigating the prevalence of these results, on a second epoch already observed (both at 7mm and 3mm) on these sources and R Leo, as well as the detailed study of the possible effects on the pumping mechanism as a function of:
Phase of the stellar pulsation
Mass loss rate
Chemical properties (in particular, line overlaps between ro-vibrational SiO and H2O transitions )