94 research outputs found
Mapping the circumstellar SiO maser emission in R Leo
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.Comment: accepted for publication in A&
SiO masers from AGB stars in the vibrationally excited v=1,v=2, and v=3 states
The v=1 and v=2 J=1-0 (43 GHz), and v=1 J=2-1 (86 GHz) SiO masers are intense
in AGB stars and have been mapped using VLBI showing ring-like distributions.
Those of the v=1, v=2 J=1-0 masers are similar, but the spots are rarely
coincident, while the v=1 J=2-1 maser arises from a well separated region
farther out. These relative locations can be explained by models tools that
include the overlap of two IR lines of SiO and H2O. The v=3 J=1-0 line is not
directly affected by any line overlap and its spot structure and position,
relative to the other lines, is a good test to the standard pumping models. We
present single-dish and simultaneous VLBI observations of the v=1, v=2, and v=3
J=1-0 maser transitions of 28SiO in several AGB stars. The spatial distribution
of the SiO maser emission in the v=3 J=1-0 transition from AGB stars is
systematically composed of a series of spots that occupy a ring-like structure.
The overall ring structure is extremely similar to that found in the other 43
GHz transitions and is very different from the structure of the v=1 J=2-1
maser. The positions of the individual spots of the different 43 GHz lines are,
however, very rarely coincident, which in general is separated by about 0.3 AU
(between 1 and 5 mas). These results are very difficult to reconcile with
standard pumping models, which predict that the masers of rotational
transitions within a given vibrational state require very similar excitation
conditions, while the transitions of different vibrational states should appear
in different positions. However, models including line overlap tend to predict
v=1, v=2, v=3 J=1-0 population inversion to occur under very similar
conditions, while the requirements for v=1 J=2-1 appear clearly different, and
are compatible with the observational results.Comment: 9 pages, 4 figures accepted by A&
Dichroic Masers due to Radiation Anisotropy and the Influence of the Hanle Effect on the Circumstellar SiO Polarization
The theory of the generation and transfer of polarized radiation, mainly
developed for interpreting solar spectropolarimetric observations, allows to
reconsider, in a more rigorous and elegant way, a physical mechanism that has
been suggested some years ago to interpret the high degree of polarization
often observed in astronomical masers. This mechanism, for which the name of
'dichroic maser' is proposed, can operate when a low density molecular cloud is
illuminated by an anisotropic source of radiation (like for instance a nearby
star). Here we investigate completely unsaturated masers and show that
selective stimulated emission processes are capable of producing highly
polarized maser radiation in a non-magnetic environment. The polarization of
the maser radiation is linear and is directed tangentially to a ring
equidistant to the central star. We show that the Hanle effect due to the
presence of a magnetic field can produce a rotation (from the tangential
direction) of the polarization by more that 45 degrees for some selected
combinations of the strength, inclination and azimuth of the magnetic field
vector. However, these very same conditions produce a drastic inhibition of the
maser effect. The rotations of about 90 degrees observed in SiO masers in the
evolved stars TX Cam by Kemball & Diamond (1997) and IRC+10011 by Desmurs et al
(2000) may then be explainedby a local modification of the anisotropy of the
radiation field, being transformed from mainly radial to mainly tangential.Comment: Accepted for publication on Ap
High resolution observations of SiO masers: comparing the spatial distribution at 43 and 86 GHz
We present sub-milliarcsecond observations of SiO masers in the late-type
stars IRC +10011 and Chi Cyg. We have used the NRAO Very Long Baseline Array
(VLBA) to map the 43 GHz (v=1, 2 J=1-0) and the 86 GHz (v=1, 2 J=2-1) SiO
masers. All the transitions have been imaged except the v=2 J=2-1 in IRC
+10011. We report the first VLBI map of the v=1 J=2-1 28SiO maser in IRC +10011
as well as the first VLBA images of SiO masers in an S-type Mira variable, Chi
Cyg. In this paper we have focused on the study of the relative spatial
distribution of the different observed lines. We have found that in some cases
the observational results are not reproduced by the current theoretical pumping
models, either radiative or collisional. In particular, for IRC +10011, the v=1
J=1-0 and J=2-1 28SiO lines have different spatial distributions and emitting
region sizes, the J=2-1 emission being located in an outer region of the
envelope. For Chi Cyg, the distributions also differ, but the sizes of the
masing regions are comparable. We suggest that the line overlaps between
ro-vibrational transitions of two abundant molecular species, H2O and 28SiO, is
a possible explanation for the discrepancies found between the observations and
the theoretical predictions. We have introduced this overlapping process in the
calculations of the excitation of the SiO molecule. We conclude that the line
overlaps can strongly affect the excitation of SiO and may reproduce the
unexpected observational results for the two sources studied.Comment: 16 pages, 12 figure
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