1. Circumstellar SiO masers in long period variable stars
Rebeca Soria Ruiz
F. Colomer, J. Alcolea, V. Bujarrabal & J.-F. Desmurs
OAN

2. Why SiO masers ? TX Cam R Leo Schematic view of an AGB star
Pardo et al 1998
Schematic view of an AGB star
Why SiO masers ?
Produced in the innermost regions of the circumstellar
envelopes around AGB stars
Ring-like structures at 24 R* (Diamond et al 1994) composed
of compact spots, with typical sizes of a few mas, and
high brightness temperatures  perfect targets for VLBI
Maser emission is shown in 28SiO, 29SiO and 30SiO in many
rotational transitions, which have been extensively studied
in single-dish
Multi-line simultaneous observations help in constraining
the current pumping mechanisms

3. Radiative or collisional?
IR stellar radiation: RADIATIVE MODELS
Collisions with gas particles (H2): COLLISIONAL MODELS
Bujarrabal (1994)
Humphreys et al (2000; 2002)
Nph (1043 s-1)
v =1 J =21
v =1 J =10
v =2 J =10
n (H2) (cm-3)
v =3 J =10
v =1 J = 10
v =1 J = 21
v =2 J = 10

4. Observations 28SiO 29SiO SiO transition frequency (MHz) band (MHz)
channels
resolution
(km s-1)
28SiO v=1 J=10 8
256
0.22
28SiO v=2 J=10
29SiO v=0 J=10
28SiO v=2 J=21 16
512
0.11
29SiO v=0 J=21
28SiO v=1 J=21
v =0
28SiO
v =1
v =2
29SiO
 1780 K

5. AGB sample IRC+10011 R Leo TX Cam chi Cyg 7.1 10–6 7.1 10–8 2.5 10–6
source
IRC+10011
R Leo
TX Cam
chi Cyg
variability
OH/IR
Mira
spectral type M9 M8
M8.5 S7
(Msol /año)
7.1 10–6
7.1 10–8
2.5 10–6
3.9 10–7
period (days)
660
310
557
408
distance (pc)
470
101
330
106
VLSR (km/s)
9.2
-0.5
11.8
10.2
phase
1.00
0.25

6. R Leo 28SiO v =1 J =10 28SiO v =2 J =10 28SiO v =1 J =21

7. TX Cam ( I ) 28SiO v =1 J =10 28SiO v =2 J =10 5 UA 28SiO v=2 J=10

8. TX Cam ( II ) 28SiO v =1 J =10 28SiO v =2 J =10 28SiO v =1 J =21
5 UA
28SiO v =2 J =10
28SiO v =1 J =21
28SiO v=2 J=21
28SiO v =2 J =21
28SiO v=1 J=21
28SiO v=2 J=10
28SiO v=1 J=10

9. Summary of results 28SiO 29SiO
Similar spatial distribution of the v =1 and v =2 J =10 28SiO maser emissions in the O-rich
stars. The v =2 emission is located in an inner region, with a separation of  13 mas
In IRC+10011, R Leo y TX Cam, the spatial distribution of the v =1 J=10 and J=21 is completely different, the latter being produced in an outer region BUT in chi Cyg, an S-type star, the sizes are comparable

10. H2O 2 = 0 127,5  2 = 1 116,6 + SiO v = 1 J = 0  v = 2 J = 1
Pumping models: line overlaps
Proposed by Olofsson (1981; 1985) to explain the quenching of the
28SiO v =2 J = 21 maser in O-rich stars
If Tex (H2O) > Tb and the H2O line is optically thick, the v =2 J =1
rotational level is overpopulated
H2O 2 = ,5  2 = , SiO v = 1 J = 0  v = 2 J = 1
The model (based on Bujarrabal et al 1996)
Excitation of both molecules under the Large Velocity Gradient approximation
H2O not including the overlap
SiO including the overlap

11. Line overlaps: results
greys v = 1 J =1 v = 2 J =10
v = 1 J =2 v = 2 J =21
Soria-Ruiz et al 2004
overlap included
overlap not included

12. Summary
28SiO masers
In O-rich envelopes: Similar spatial distribution of the 7mm emissions with the
v=2 located in an inner region. The 3mm is produced further away
(not in S-type envelopes where the 7 & 3mm regions have comparable sizes).
Current models don’t explain the observed results in O-rich stars. The line overlaps
strongly affect the excitation of SiO and should be introduced in the pumping models.
29SiO masers
First maps of the 29SiO v = 0 J = 1– 0 maser transition in a circumstellar envelope.
When detected, the line profile is composed of narrow peaks near or at the stellar
velocity, suggesting tangential amplification as in the 28SiO maser lines.

13. Work in progress in the OH/IR variable IRC+10011
28SiO v=1 and v=2 J=10 43 GHz) with the High Sensitivity Array
in the OH/IR variable IRC+10011
Global VLBI observations of the 28SiO v=1 and v=2 J=21 in O-rich
envelopes
Monitoring of the HCN (0, 20, 0) J=10 maser in C-rich envelopes using
the Pico de Veleta (IRAM, Spain) 30m telescope
VLBA observations of HCN maser transitions in the variable CIT6