Astronomy & Astrophysics manuscript no. Soria-final c© ESO 2018
October 25, 2018
Mapping the circumstellar SiO maser emission in R Leo
R. Soria-Ruiz1, J. Alcolea2, F. Colomer3, V. Bujarrabal3, and J.-F. Desmurs2
1 Joint Institute for VLBI in Europe, Postbus 2, 7990 AA Dwingeloo, The Netherlands
2 Observatorio Astronómico Nacional, Alfonso XII 3, E-28014 Madrid, Spain
3 Observatorio Astronómico Nacional, Apartado 112, E-28803 Alcalá de Henares, Spain
Received ***, 2006; accepted ***, 2006
ABSTRACT
Context. The study of the innermost circumstellar layers around AGB stars is crucial to understand how these envelopes are formed and
evolve. The SiO maser emission occurs at a few stellar radii from the central star, providing direct information on the stellar pulsation and on
the chemical and physical properties of these regions. Our data also shed light on several aspects of the SiO maser pumping theory that are not
well understood yet.
Aims. We aim to determine the relative spatial distribution of the 43 GHz and 86 GHz SiO maser lines in the oxygen-rich evolved star R Leo.
Methods. We have imaged with milliarcsecond resolution, by means of Very Long Baseline Interferometry, the 43 GHz (28SiO v=1, 2 J=1–0
and 29SiO v=0 J=1–0) and 86 GHz (28SiO v=1 J=2–1 and 29SiO v=0 J=2–1) masing regions.
Results. We confirm previous results obtained in other oxygen-rich envelopes. In particular, when comparing the 43 GHz emitting regions, the
28SiO v=2 J=1–0 transition is produced in an inner layer, slightly closer to the central star than the v=1 J=1–0. On the other hand, the 86 GHz
28SiO v=1 J=2–1 line arises in a clearly farther shell. We have also mapped for the first time the 29SiO v=0 J=1–0 emission in R Leo. The al-
ready reported discrepancy between the observed distributions of the different maser lines and the theoretical predictions is also found in R Leo.
Key words. radio lines: stars – masers – technique: interferometric – stars: circumstellar matter – stars: AGB
1. Introduction
Along the Asymptotic Giant Branch phase, many stars exhibit
maser amplification in different molecular lines. In oxygen rich
stars, [O]/[C]>1, O-bearing compounds are mainly formed and
maser emission is presented in SiO, H2O and OH. The study
of these different molecules provides information of the overall
envelope, from the inner layers dominated by the stellar pulsa-
tion (SiO masers) to the outermost regions where the circum-
stellar material is expanding at constant velocity (OH masers).
The very long baseline interferometry is a unique technique
to study the compact and very bright SiO emission, and there-
fore, it is particularly helpful in understanding the different and
complex processes occuring in these inner regions of the enve-
lope. On the other hand, the current models of pumping, either
collisional or radiative, do not reproduce some characteristics
of the SiO masers that have been observed, as for example,
their relative location in the envelope. To test these models and
constrain the physical parameters of these inner shells better, it
is very useful to perform simultaneous observations of several
maser transitions. For this reason, we have carried out multi-
line and multi-epoch observations in a sample of AGB stars.
Send offprint requests to: R. Soria-Ruiz
e-mail: soria@jive.nl
We present in this paper the latest results for the Mira-type vari-
able R Leo.
2. VLBA observations
The observations were made with the NRAO1 Very Long
Baseline Array (VLBA) on 2002 december 7. Nearly simul-
taneous observations of different 43 GHz and 86 GHz 28SiO
and 29SiO maser transitions were performed in the variable star
R Leo. The 86 GHz lines were observed in between the two
43 GHz scans, and, therefore, we can assume for our purpouses
that the observations were simultaneous.
The data correlation was done at the VLBA correlator lo-
cated in Socorro (New Mexico). Left and right circular polar-
izations (LCP & RCP) were measured for the 28SiO v=1 and
v=2 J=1–0 lines, whereas only the LCP was observed in the
other transitions. Since no significant difference was found be-
tween the maps, less than 5%, the final image is the average of
both polarizations.
Standard procedures for spectral line VLBI data reduction
were followed in the calibration and production of the maps.
1 The National Radio Astronomy Observatory is a facility of the
National Science Foundation operated under cooperative agreement
by Associated Universities, Inc.
http://arxiv.org/abs/0704.0682v1
2 R. Soria-Ruiz et al.: Mapping the circumstellar SiO maser emission in R Leo
Table 1. Observed maser transitions and results of the fits.
specie transition restoring R̄ △R center
beam (mas2) (mas) (mas) (Xc,Yc)
28SiO v=1 J=1–0 0.78×0.50 29.24 6.42 (26.9,–21.3)
v=1 J=2–1 0.50×0.50 33.84 4.20 (–35.2,–4.7)
v=2 J=1–0 0.50×0.50 25.92 6.88 (25.5,–16.3)
29SiO v=0 J=1–0 0.78×0.22 one spot — —
v=0 J=2–1 non–det. — — —
The amplitude calibration was done using the system temper-
atures and antenna gain corrections for the 86 GHz and 29SiO
data, and the template method for the other 43 GHz data. The
phase errors were removed in a two-step process: first, the
single-band delay corrections were derived from the continuum
calibrators, OJ287 and 3C273. Second, the fringe-rates were
estimated by selecting a bright and simple-structured channel;
the corrections found were subsequently applied to the maser
source. The maps were produced using the CLEAN deconvo-
lution algorithm.
3. Results and Data Fits
The results are presented as follows (Figs. 1–2): for each
observed line, we show the integrated emission map in
Jy beam−1 km s−1 units (center panel), the spectrum of the
cross-correlated emission for the different maser components
(numbered panels), the total power spectrum (AC) of one of the
VLBA antennas and of the emission in the map (XC) (upper-
left panel), and the ratio of these two magnitudes (upper-right
panel). We have also estimated the size of the total masing re-
gions by fitting our data to rings. Only those components with
SNR≥6 have been included in the fits. The results derived from
the calculations are summarized in Table 1: characteristic ring
radius (R̄), ring width (△R) and center of the ring (Xc,Yc) (see
details on the fitting process in Soria-Ruiz et al., 2005). In par-
ticular, the angular sizes derived for the 28SiO v=1 and v=2
J=1–0 regions (Table 1) are compatible with previous obser-
vations performed in R Leo by Cotton et al. (2004). Among the
six transitions observed only the 29SiO v=0 J=2–1 has not been
detected. A more detailed description of the maps is given in
the subsequent section.
4. Relative spatial distribution and pumping
mechanisms
Our maps show that the spatial distribution of the v=1 and v=2
maser spots is similar although not all the components appear
in both transitions (Fig. 1). Concerning the relative location of
the 43 GHz 28SiO maser layers, the v=2 emission is produced
in a closer region of the envelope, assuming that the centroids
of all the emissions are coincident. This is also consistent with
previously reported results in other oxygen rich envelopes (see
e.g. Desmurs et al., 2000; Cotton et al., 2004; Soria-Ruiz et al.,
2004, 2005). In contrast to the 43 GHz regions, this first map of
the 28SiO v=1 J=2–1 emission in R Leo reveals that the com-
ponents of this maser line are situated in a significantly outer
region of the envelope, with a very different spot distribution.
Since the J=2–1 emission has been imaged only in a very few
sources, this result in R Leo is particularly important to test
the proposed SiO maser mechanisms. Finally, the 29SiO v=0
J=1–0 map consisted of one maser spot, thus making difficult
to derive any spatial information. The total power and recov-
ered emission are shown in Figure 2.
Current pumping models, either radiative (Bujarrabal,
1994a,b) or collisional (Humphreys et al., 2002), predict that
the different rotational maser lines within the same vibra-
tional state are produced under similar conditions and there-
fore are expected to be located in the same region of the en-
velope. As previously mentioned, we find a contradiction be-
tween these theoretical predictions and our observational re-
sults. This discrepancy has also been observed in other oxygen-
rich stars; IRC+10011 (Soria-Ruiz et al., 2005) and TX Cam
(Soria-Ruiz et al., 2006). Further calculations of the excita-
tion of the SiO molecule in AGB stars have shown that the
conditions under which the different maser transitions occur
change drastically when the line overlap between infrared
lines of H2O and
28SiO is introduced in the pumping models
(Bujarrabal et al., 1996; Soria-Ruiz et al., 2004); such a mech-
anism could explain the lack of coincidence between the spots
of different J–transitions within a vibrational state.
Nevertheless, although these new maps support the rele-
vance of line overlaps in the SiO maser pumping in O–rich
shells, we think that similar studies should be performed in a
larger number of evolved stars. In particular, it would be nec-
essary to have data on all types of long-period variable stars,
namely, Mira-type, semirregular and irregular variables, as well
as supergiant stars.
Acknowledgements. This work has been financially supported by the
Spanish DGI (MCYT) under projects AYA2000-0927 and AYA2003-
7584. All plots have been made using the GILDAS software package
(http://www.iram.fr/IRAMFR/GILDAS).
References
Bujarrabal, V. 1994a, A&A, 285, 953
Bujarrabal, V. 1994b, A&A, 285, 971
Bujarrabal, V., Alcolea, J., Sánchez Contreras, C., & Colomer,
F. 1996, A&A, 314, 883
Bujarrabal, V., Gómez-González, J., & Planesas, P. 1989,
A&A, 219, 256
Cotton, W. D., Mennesson, B., Diamond, P. J., et al. 2004,
A&A, 414, 275
Desmurs, J.-F., Bujarrabal, V., Colomer, F., & Alcolea, J. 2000,
A&A, 360, 189
Humphreys, E. M. L., Gray, M. D., Yates, J. A., et al. 2002,
A&A, 386, 256
Soria-Ruiz, R., Alcolea, J., Colomer, F., et al. 2004, A&A, 426,
Soria-Ruiz, R., Colomer, F., Alcolea, J., et al. 2005, A&A, 432,
Soria-Ruiz, R., Colomer, F., Alcolea, J., et al. 2006,
Proceedings of the 8th EVN Symposium
http://www.iram.fr/IRAMFR/GILDAS
R. Soria-Ruiz et al.: Mapping the circumstellar SiO maser emission in R Leo 3
Fig. 1. The 28SiO v=1 (upper panel) and v=2 (lower panel) J=1–0 maser emission in R Leo. Each figure shows the integrated
intensity map in Jy beam−1 km s−1 units, the spectra of the individual maser components, the total power spectrum and the
emission in the map, and their ratio. For some maser components, the intensity has been divided by a factor of 2 or 3 to ease
the comparison with the other spectra. The vertical dashed lines indicate the systemic velocity of the source, VLSR= –0.5 km s
(Bujarrabal et al., 1989). Circles represent the fits for the masing regions (dashed: mean radius R̄, continuous: Rout and Rin defined
as R̄± 12△R) (see Section 3). The peak intensity is 28.45 Jy beam
−1 km s−1 (v=1) and 53.5 Jy beam−1 km s−1 (v=2), and the shown
contour is equivalent to the 5σ level, with σ= 0.4 Jy beam−1 km s−1 (v=1) and σ= 0.7 Jy beam−1 km s−1 (v=2).
4 R. Soria-Ruiz et al.: Mapping the circumstellar SiO maser emission in R Leo
Fig. 2. Same as Figure 1 for the 28SiO v=1 J=2–1 (upper panel) and 29SiO v=0 J=1–0 (lower panel) maser emission in R Leo.
The peak intensity is 6.03 Jy beam−1 km s−1 (28SiO) and 0.26 Jy beam−1 km s−1 (29SiO), and the shown contour is equivalent to
the 5σ level, with σ= 0.08 Jy beam−1 km s−1 (28SiO) and σ= 0.01 Jy beam−1 km s−1 (29SiO).
	Introduction
	VLBA observations
	Results and Data Fits
	Relative spatial distribution and pumping mechanisms

  The study of the innermost circumstellar layers around AGB stars is crucial
to understand how these envelopes are formed and evolve. The SiO maser emission
occurs at a few stellar radii from the central star, providing direct
information on the stellar pulsation and on the chemical and physical
properties of these regions. Our data also shed light on several aspects of the
SiO maser pumping theory that are not well understood yet. We aim to determine}
the relative spatial distribution of the 43 GHz and 86 GHz SiO maser lines in
the oxygen-rich evolved star R Leo. We have imaged with milliarcsecond
resolution, by means of Very Long Baseline Interferometry, the 43 GHz (28SiO
v=1, 2 J=1-0 and 29SiO v=0 J=1-0) and 86 GHz (28SiO v=1 J=2-1 and 29SiO v=0
J=2-1) masing regions. We confirm previous results obtained in other
oxygen-rich envelopes. In particular, when comparing the 43 GHz emitting
regions, the 28SiO v=2 transition is produced in an inner layer, closer to the
central star. On the other hand, the 86 GHz line arises in a clearly farther
shell. We have also mapped for the first time the 29SiO v=0 J=1-0 emission in R
Leo. The already reported discrepancy between the observed distributions of the
different maser lines and the theoretical predictions is also found in R Leo.

Introduction
Along the Asymptotic Giant Branch phase, many stars exhibit
maser amplification in different molecular lines. In oxygen rich
stars, [O]/[C]>1, O-bearing compounds are mainly formed and
maser emission is presented in SiO, H2O and OH. The study
of these different molecules provides information of the overall
envelope, from the inner layers dominated by the stellar pulsa-
tion (SiO masers) to the outermost regions where the circum-
stellar material is expanding at constant velocity (OH masers).
The very long baseline interferometry is a unique technique
to study the compact and very bright SiO emission, and there-
fore, it is particularly helpful in understanding the different and
complex processes occuring in these inner regions of the enve-
lope. On the other hand, the current models of pumping, either
collisional or radiative, do not reproduce some characteristics
of the SiO masers that have been observed, as for example,
their relative location in the envelope. To test these models and
constrain the physical parameters of these inner shells better, it
is very useful to perform simultaneous observations of several
maser transitions. For this reason, we have carried out multi-
line and multi-epoch observations in a sample of AGB stars.
Send offprint requests to: R. Soria-Ruiz
e-mail: soria@jive.nl
We present in this paper the latest results for the Mira-type vari-
able R Leo.
2. VLBA observations
The observations were made with the NRAO1 Very Long
Baseline Array (VLBA) on 2002 december 7. Nearly simul-
taneous observations of different 43 GHz and 86 GHz 28SiO
and 29SiO maser transitions were performed in the variable star
R Leo. The 86 GHz lines were observed in between the two
43 GHz scans, and, therefore, we can assume for our purpouses
that the observations were simultaneous.
The data correlation was done at the VLBA correlator lo-
cated in Socorro (New Mexico). Left and right circular polar-
izations (LCP & RCP) were measured for the 28SiO v=1 and
v=2 J=1–0 lines, whereas only the LCP was observed in the
other transitions. Since no significant difference was found be-
tween the maps, less than 5%, the final image is the average of
both polarizations.
Standard procedures for spectral line VLBI data reduction
were followed in the calibration and production of the maps.
1 The National Radio Astronomy Observatory is a facility of the
National Science Foundation operated under cooperative agreement
by Associated Universities, Inc.
http://arxiv.org/abs/0704.0682v1
2 R. Soria-Ruiz et al.: Mapping the circumstellar SiO maser emission in R Leo
Table 1. Observed maser transitions and results of the fits.
specie transition restoring R̄ △R center
beam (mas2) (mas) (mas) (Xc,Yc)
28SiO v=1 J=1–0 0.78×0.50 29.24 6.42 (26.9,–21.3)
v=1 J=2–1 0.50×0.50 33.84 4.20 (–35.2,–4.7)
v=2 J=1–0 0.50×0.50 25.92 6.88 (25.5,–16.3)
29SiO v=0 J=1–0 0.78×0.22 one spot — —
v=0 J=2–1 non–det. — — —
The amplitude calibration was done using the system temper-
atures and antenna gain corrections for the 86 GHz and 29SiO
data, and the template method for the other 43 GHz data. The
phase errors were removed in a two-step process: first, the
single-band delay corrections were derived from the continuum
calibrators, OJ287 and 3C273. Second, the fringe-rates were
estimated by selecting a bright and simple-structured channel;
the corrections found were subsequently applied to the maser
source. The maps were produced using the CLEAN deconvo-
lution algorithm.
3. Results and Data Fits
The results are presented as follows (Figs. 1–2): for each
observed line, we show the integrated emission map in
Jy beam−1 km s−1 units (center panel), the spectrum of the
cross-correlated emission for the different maser components
(numbered panels), the total power spectrum (AC) of one of the
VLBA antennas and of the emission in the map (XC) (upper-
left panel), and the ratio of these two magnitudes (upper-right
panel). We have also estimated the size of the total masing re-
gions by fitting our data to rings. Only those components with
SNR≥6 have been included in the fits. The results derived from
the calculations are summarized in Table 1: characteristic ring
radius (R̄), ring width (△R) and center of the ring (Xc,Yc) (see
details on the fitting process in Soria-Ruiz et al., 2005). In par-
ticular, the angular sizes derived for the 28SiO v=1 and v=2
J=1–0 regions (Table 1) are compatible with previous obser-
vations performed in R Leo by Cotton et al. (2004). Among the
six transitions observed only the 29SiO v=0 J=2–1 has not been
detected. A more detailed description of the maps is given in
the subsequent section.
4. Relative spatial distribution and pumping
mechanisms
Our maps show that the spatial distribution of the v=1 and v=2
maser spots is similar although not all the components appear
in both transitions (Fig. 1). Concerning the relative location of
the 43 GHz 28SiO maser layers, the v=2 emission is produced
in a closer region of the envelope, assuming that the centroids
of all the emissions are coincident. This is also consistent with
previously reported results in other oxygen rich envelopes (see
e.g. Desmurs et al., 2000; Cotton et al., 2004; Soria-Ruiz et al.,
2004, 2005). In contrast to the 43 GHz regions, this first map of
the 28SiO v=1 J=2–1 emission in R Leo reveals that the com-
ponents of this maser line are situated in a significantly outer
region of the envelope, with a very different spot distribution.
Since the J=2–1 emission has been imaged only in a very few
sources, this result in R Leo is particularly important to test
the proposed SiO maser mechanisms. Finally, the 29SiO v=0
J=1–0 map consisted of one maser spot, thus making difficult
to derive any spatial information. The total power and recov-
ered emission are shown in Figure 2.
Current pumping models, either radiative (Bujarrabal,
1994a,b) or collisional (Humphreys et al., 2002), predict that
the different rotational maser lines within the same vibra-
tional state are produced under similar conditions and there-
fore are expected to be located in the same region of the en-
velope. As previously mentioned, we find a contradiction be-
tween these theoretical predictions and our observational re-
sults. This discrepancy has also been observed in other oxygen-
rich stars; IRC+10011 (Soria-Ruiz et al., 2005) and TX Cam
(Soria-Ruiz et al., 2006). Further calculations of the excita-
tion of the SiO molecule in AGB stars have shown that the
conditions under which the different maser transitions occur
change drastically when the line overlap between infrared
lines of H2O and
28SiO is introduced in the pumping models
(Bujarrabal et al., 1996; Soria-Ruiz et al., 2004); such a mech-
anism could explain the lack of coincidence between the spots
of different J–transitions within a vibrational state.
Nevertheless, although these new maps support the rele-
vance of line overlaps in the SiO maser pumping in O–rich
shells, we think that similar studies should be performed in a
larger number of evolved stars. In particular, it would be nec-
essary to have data on all types of long-period variable stars,
namely, Mira-type, semirregular and irregular variables, as well
as supergiant stars.
Acknowledgements. This work has been financially supported by the
Spanish DGI (MCYT) under projects AYA2000-0927 and AYA2003-
7584. All plots have been made using the GILDAS software package
(http://www.iram.fr/IRAMFR/GILDAS).
References
Bujarrabal, V. 1994a, A&A, 285, 953
Bujarrabal, V. 1994b, A&A, 285, 971
Bujarrabal, V., Alcolea, J., Sánchez Contreras, C., & Colomer,
F. 1996, A&A, 314, 883
Bujarrabal, V., Gómez-González, J., & Planesas, P. 1989,
A&A, 219, 256
Cotton, W. D., Mennesson, B., Diamond, P. J., et al. 2004,
A&A, 414, 275
Desmurs, J.-F., Bujarrabal, V., Colomer, F., & Alcolea, J. 2000,
A&A, 360, 189
Humphreys, E. M. L., Gray, M. D., Yates, J. A., et al. 2002,
A&A, 386, 256
Soria-Ruiz, R., Alcolea, J., Colomer, F., et al. 2004, A&A, 426,
Soria-Ruiz, R., Colomer, F., Alcolea, J., et al. 2005, A&A, 432,
Soria-Ruiz, R., Colomer, F., Alcolea, J., et al. 2006,
Proceedings of the 8th EVN Symposium
http://www.iram.fr/IRAMFR/GILDAS
R. Soria-Ruiz et al.: Mapping the circumstellar SiO maser emission in R Leo 3
Fig. 1. The 28SiO v=1 (upper panel) and v=2 (lower panel) J=1–0 maser emission in R Leo. Each figure shows the integrated
intensity map in Jy beam−1 km s−1 units, the spectra of the individual maser components, the total power spectrum and the
emission in the map, and their ratio. For some maser components, the intensity has been divided by a factor of 2 or 3 to ease
the comparison with the other spectra. The vertical dashed lines indicate the systemic velocity of the source, VLSR= –0.5 km s
(Bujarrabal et al., 1989). Circles represent the fits for the masing regions (dashed: mean radius R̄, continuous: Rout and Rin defined
as R̄± 12△R) (see Section 3). The peak intensity is 28.45 Jy beam
−1 km s−1 (v=1) and 53.5 Jy beam−1 km s−1 (v=2), and the shown
contour is equivalent to the 5σ level, with σ= 0.4 Jy beam−1 km s−1 (v=1) and σ= 0.7 Jy beam−1 km s−1 (v=2).
4 R. Soria-Ruiz et al.: Mapping the circumstellar SiO maser emission in R Leo
Fig. 2. Same as Figure 1 for the 28SiO v=1 J=2–1 (upper panel) and 29SiO v=0 J=1–0 (lower panel) maser emission in R Leo.
The peak intensity is 6.03 Jy beam−1 km s−1 (28SiO) and 0.26 Jy beam−1 km s−1 (29SiO), and the shown contour is equivalent to
the 5σ level, with σ= 0.08 Jy beam−1 km s−1 (28SiO) and σ= 0.01 Jy beam−1 km s−1 (29SiO).
	Introduction
	VLBA observations
	Results and Data Fits
	Relative spatial distribution and pumping mechanisms
