132 research outputs found
Water vapor and silicon monoxide maser observations in the protoplanetary nebula OH 231.8+4
OH 231.8+4.2 is a well studied preplanetary nebula (pPN) around a binary
stellar system that shows a remarkable bipolar outflow. To study the structure
and kinematics of the inner 10-80 AU nebular regions we performed
high-resolution observations of the HO 6--5 and SiO
=2, =1--0 maser emissions with the Very Long Baseline Array. The absolute
position of both emission distributions were recovered using the phase
referencing technique, and accurately registered in HST optical images. HO
maser clumps are found to be distributed in two areas of 20 mas in size
spatially displaced by 60 milli-arcseconds along an axis oriented nearly
north-south. SiO masers are tentatively found to be placed between the two
HO maser emitting regions, probably indicating the position of the Mira
component of the system. The SiO maser emission traces an inner equatorial
component with a diameter of 12 AU, probably a disk rotating around the M-type
star. Outwards, we detect in the HO data a pair of polar caps, separated by
80 AU. We believe that the inner regions of the nebula probably have been
altered by the presence of the companion, leading to an equator-to-pole density
contrast that may explain the lack of HO masers and strong SiO maser
emission in the denser, equatorial regions.Comment: 5 pages, 1 figure, A&A accepte
A high-sensitivity OH 5-cm line survey in late-type stars
We have undertaken a comprehensive search for 5-cm excited OH maser emission
from evolved stars representative of various stages of late stellar evolution.
Observed sources were selected from known 18-cm OH sources. This survey was
conducted with the 100-m Effelsberg telescope to achieve high signal to noise
ratio observations and a sensitivity limit of about 0.05 to 0.1 Jy. A total of
64 stellar sources were searched for both main line and satellite line
emission. We confirm the previous detection of 5 cm OH in Vy 2-2, do not
confirm emission from NML-Cyg and do not report any other new detection within
the above sensitivity limit.
Implications of these results on the pumping mechanism of the OH radical in
circumstellar envelopes are briefly discussed.Comment: 8 pages, 5 figures, A&A in pres
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&
EVLA Observations of OH Masers in ON 1
This Letter reports on initial Expanded Very Large Array (EVLA) observations
of the 6035 MHz masers in ON 1. The EVLA data are of good quality, lending
confidence in the new receiver system. Nineteen maser features, including six
Zeeman pairs, are detected. The overall distribution of 6035 MHz OH masers is
similar to that of the 1665 MHz OH masers. The spatial resolution is sufficient
to unambiguously determine that the magnetic field is strong (~ -10 mG) at the
location of the blueshifted masers in the north, consistent with Zeeman
splitting detected in 13441 MHz OH masers in the same velocity range. Left and
right circularly polarized ground-state features dominate in different regions
in the north of the source, which may be due to a combination of magnetic field
and velocity gradients. The combined distribution of all OH masers toward the
south is suggestive of a shock structure of the sort previously seen in W3(OH).Comment: 4 pages using emulateapj.cls including 2 tables and 2 color figure
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&
Preliminary results on SiO v=3 J=1-0 maser emission from AGB stars
We present the results of SiO maser observations at 43GHz toward two AGB
stars using the VLBA. Our preliminary results on the relative positions of the
different J=1-0 SiO masers (v=1,2 and 3) indicate that the current ideas on SiO
maser pumping could be wrong at some fundamental level. A deep revision of the
SiO pumping models could be necessary.Comment: poster, 2 pages, 2 figures, Proc. IAU Symp. 287 "Cosmic Masers: from
OH to H0", R.S. Booth, E.M.L. Humphreys and W.H.T. Vlemmings, ed
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