368 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&
Warm gas in the rotating disk of the Red Rectangle: accurate models of molecular line emission
We aim to study the excitation conditions of the molecular gas in the
rotating disk of the Red Rectangle, the only post-Asymptotic-Giant-Branch
object in which the existence of an equatorial rotating disk has been
demonstrated. For this purpose, we developed a complex numerical code that
accurately treats radiative transfer in 2-D, adapted to the study of molecular
lines from rotating disks.
We present far-infrared Herschel/HIFI observations of the 12CO and 13CO
J=6-5, J=10-9, and J=16-15 transitions in the Red Rectangle. We also present
our code in detail and discuss the accuracy of its predictions, from comparison
with well-tested codes. Theoretical line profiles are compared with the
empirical data to deduce the physical conditions in the disk by means of model
fitting.
We conclude that our code is very efficient and produces reliable results.
The comparison of the theoretical predictions with our observations reveals
that the temperature of the Red Rectangle disk is typically ~ 100-150 K, about
twice as high as previously deduced from mm-wave observations of lower-J lines.
We discuss the relevance of these new temperature estimates for understanding
the thermodynamics and dynamics of this prototype object, as well as for
interpreting observations of other rarely studied post-AGB disks. Despite our
sophisticated treatment of the line formation, our model cannot explain the
relatively strong line-wing emission for intermediate-J transitions. We argue
that a model including a rotating disk only cannot reproduce these data and
suggest that there is an additional extended (probably bipolar) structure
expanding at about 7--15 km/s.Comment: 18 pages, 21 figure
Relation of SiO maser emission to IR radiation in evolved stars based on the MSX observation
Based on the space MSX observation in bands A(8m), C(12m),
D(15m) and E(21m), and the ground SiO maser observation of evolved
stars by the Nobeyama 45-m telescope in the v=1 and v=2 J=1-0 transitions, the
relation between SiO maser emission and mid-IR continuum radiation is analyzed.
The relation between SiO maser emission and the IR radiation in the MSX bands
A, C, D and E is all clearly correlated. The SiO maser emission can be
explained by a radiative pumping mechanism according to its correlation with
infrared radiation in the MSX band A.Comment: 11 pages, 4 figures, to appear in ApJ
Minkowski's Footprint revisited. Planetary Nebula formation from a single sudden event?
M1-92 can be considered an archetype of bipolar pre-planetary nebulae. It
shows a clear axial symmetry, along with the kinematics and momentum excess
characteristic of this class of envelopes around post-AGB stars. By taking
advantage of the new extended configuration of the IRAM Plateau de Bure
interferometer, we wanted to study the morphology and velocity field of the
molecular gas better in this nebula, particularly in its central part. We
performed sub-arcsecond resolution interferometric observations of the J=2-1
rotational line 13CO M1-92. We found that the equatorial component is a thin
flat disk, which expands radially with a velocity proportional to the distance
to the center. The kinetic age of this equatorial flow is very similar to that
of the two lobes. The small widths and velocity dispersion in the gas forming
the lobe walls confirm that the acceleration responsible for the nebular shape
could not last more than 100-120 yr. The present kinematics of the molecular
gas can be explained as the result of a single brief acceleration event, after
which the nebula reached an expansion velocity field with axial symmetry. In
view of the similarity to other objects, we speculate on the possibility that
the whole nebula was formed as a result of a magneto-rotational explosion in a
common-envelope system.Comment: 4 pages (2 figures
The molecular envelope of CRL 618: A new model based on Herschel/HIFI observations
We study the physical properties and molecular excitation of the different
warm gas components found in the protoplanetary nebula CRL 618. We revise our
previous Herschel/HIFI observations, which consist of several 12CO and 13CO
lines in the far-infrared/sub-mm band. These data have been re-analyzed in
detail by improving calibration, the signal-to-noise-ratio, and baseline
substraction. We identify the contributions of the different nebular components
to the line profiles. We have used a spatio-kinematical model to better
constrain the temperature, density, and kinematics of the molecular components
probed by the improved CO observations. The 12CO and 13CO J=16-15, J=10-9, and
J=6-5 transitions are detected in this source. The line profiles present a
composite structure showing spectacular wings in some cases, which become
dominant as the energy level increases. Our analysis of the high-energy CO
emission with the already known low-energy J=2-1 and J=1-0 lines confirms that
the high-velocity component, or fast bipolar outflow, is hotter than previously
estimated with a typical temperature of ~300 K. This component may then be an
example of a very recent acceleration of the gas by shocks that has not yet
cooled down. We also find that the dense central core is characterized by a
very low expansion velocity, ~5 km/s, and a strong velocity gradient. We
conclude that this component is very likely to be the unaltered circumstellar
layers that are lost in the last AGB phase, where the ejection velocity is
particularly low. The physical properties of the other two nebular components,
the diffuse halo and the double empty shell, more or less agrees with the
estimations derived in previous models.Comment: Accepted for publication in Astronomy & Astrophysics. 8 pages, 3
figure
Modeling the physical and excitation conditions of the molecular envelope of NGC 7027
The link between the shaping of bipolar planetary nebulae and the mass
ejection activity of their central stars is still poorly understood.
Appropriately characterizing the evolution of the shells ejected during the
late stages of evolution is vital to gain insight into the mechanism of nebular
shaping.
Herschel/HIFI provides an invaluable tool by opening a new window from which
to probe warm molecular gas (~50-1000 K). We present a radiative-transfer,
spatio-kinematic modeling of the molecular envelope of the young planetary
nebula NGC 7027 in high- and low-J 12CO and 13CO transitions observed by
Herschel/HIFI and IRAM 30-m, and discuss the structure and dynamics of the
molecular envelope.
We have developed a code which, used along with the existing SHAPE software,
implements spatio-kinematic modeling with accurate non-LTE calculations of line
excitation and radiative transfer in molecular species. We have used this code
to build a relatively simple "russian doll" model of the molecular envelope of
NGC 7027.
The model nebula consists of four nested, mildly bipolar shells plus a pair
of high-velocity blobs. The innermost shell is the thinnest and shows a
significant jump in physical conditions (temperature, density, abundance and
velocity) with respect to the adjacent shell. This is a clear indication of a
shock front in the system. Each of the high-velocity blobs is divided into two
sections with different physical conditions. The presence of H2O in NGC 7027, a
C-rich nebula, is likely due to photo-induced chemistry from the hot central
star. The computed molecular mass of the nebula is 1.3 Msun, compatible with
estimates from previous works.Comment: 11 pages, 3 figures. Accepted by Astronomy & Astrophysics on July
3rd, 201
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
The chemical composition of the circumstellar envelopes around yellow hypergiant stars
The yellow hypergiant stars (YHGs) are extremely luminous and massive objects
whose general properties are poorly known. Only two of this kind of star show
massive circumstellar envelopes, IRC+10420 and AFGL2343. We aim to study the
chemistry of the circumstellar envelopes around these two sources, by
comparison with well known AGB stars and protoplanetary nebulae. We also
estimate the abundances of the observed molecular species. We have performed
single-dish observations of different transitions for twelve molecular species.
We have compared the ratio of the intensities of the molecular transitions and
of the estimated abundances in AFGL2343 and IRC+10420 with those in O-rich and
C-rich AGB stars and protoplanetary nebulae. Both YHGs, AFGL2343, and
IRC+10420, have been found to have an O-rich chemistry similar to that in
O-rich AGB stars, though for AFGL2343 the emission of most molecules compared
with 13CO lines is relatively weak. Clear differences with the other evolved
sources appear when we compare the line intensity corrected for distance and
the profile widths which are, respectively, very intense and very wide in YHGs.
The abundances obtained for IRC+10420 agree with those found in AGB stars, but
in general those found in AFGL2343, except for 13CO, are too low. This
apparently low molecular abundance in AFGL2343 could be due to the fact that
these molecules are present only in an inner region of the shell where the mass
is relatively low.Comment: 14 pages, 12 figure
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