613 research outputs found
Detection of Keplerian dynamics in a disk around the post-AGB star AC Her
So far, only one rotating disk has been clearly identified and studied in AGB
or post-AGB objects (in the Red Rectangle), by means of observations with high
spectral and spatial resolution. However, disks are thought to play a key role
in the late stellar evolution and are suspected to surround many evolved stars.
We aim to extend our knowledge on these structures.
We present interferometric observations of CO J=2-1 emission from the nebula
surrounding the post-AGB star AC Her, a source belonging to a class of objects
that share properties with the Red Rectangle and show hints of Keplerian disks.
We clearly detect the Keplerian dynamics of a second disk orbiting an evolved
star. Its main properties (size, temperature, central mass) are derived from
direct interpretation of the data and model fitting. With this we confirm that
there are disks orbiting the stars of this relatively wide class of post-AGB
objectsComment: 4 pages, 3 figure
Circular polarization from a corner reflector antenna with tilted dipole of arbitrary length
The problem consists of the development of the electric far field equations for a forty-five degree corner reflector antenna with a tilted dipole, of arbitrary length, centered at the corner bisector plane to produce circularly-polarized radiation. The far field expression was developed as a function of the spherical coordinate system variables and placed in Fortran language in order to determine the dipole locations which would produce circularly-polarized radiation. At such dipole locations the field patterns were plotted in the [theta] = [pi]/2 and [phi] = 0.0 planes. Computations were made by the IBM 1620 digital computer --Abstract, page ii
Extended rotating disks around post-AGB stars
There is a group of binary post-AGB stars that show a conspicuous NIR excess,
usually assumed to arise from hot dust in very compact possibly rotating disks.
These stars are surrounded by significantly fainter nebulae than the
"standard", well studied protoplanetary and planetary nebulae (PPNe, PNe).
We present high-sensitivity mm-wave observations of CO lines in 24 objects of
this type. CO emission is detected in most observed sources and the line
profiles show that the emissions very probably come from disks in rotation. We
derive typical values of the disk mass between 1e-3 and 1e-2 Mo, about two
orders of magnitude smaller than the (total) masses of "standard" PPNe. The
high-detection rate (upper limits being in fact not very significant) clearly
confirm that the NIR excess of these stars arises from compact disks in
rotation, very probably the inner parts of those found here. Low-velocity
outflows are also found in about eight objects, with moderate expansion
velocities of ~ 10 km/s, to be compared with the velocities of about 100 km/s
often found in "standard" PPNe. Except for two sources with complex profiles,
the outflowing gas in our objects represents a minor nebular component. Our
simple estimates of the disk typical sizes yields values ~ 0.5 - 1 arcsec, i.e.
between 5e15 and 3e16 cm. Estimates of the linear momenta carried by the
outflows, which can only be performed in a few well studied objects, also yield
moderate values, compared with the linear momenta that can be released by the
stellar radiation pressure (contrary, again, to the case of the very massive
and fast bipolar outflows in "standard" PPNe, that are strongly overluminous).
The mass and dynamics of nebulae around various classes of post-AGB stars
differ very significantly, and we can expect the formation of PNe with very
different properties.Comment: 19 pages, 26 figure
Interferometric observations of SiO thermal emission in the inner wind of M-type AGB stars IK Tauri and IRC+10011
Context. AGB stars go through a process of strong mass-loss that involves
pulsations of the atmosphere, which extends to a region where the conditions
are adequate for dust grains to form. Radiation pressure acts on these grains
which, coupled to the gas, drive a massive outflow. The details of this process
are not clear, including which molecules are involved in the condensation of
dust grains.
Aims. To study the role of the SiO molecule in the process of dust formation
and mass-loss in M-type AGB stars.
Methods. Using the IRAM NOEMA interferometer we observed the SiO and
SiO , emission from the inner circumstellar envelope of the
evolved stars IK Tau and IRC+10011. We computed azimuthally averaged emission
profiles to compare the observations to models using a molecular excitation and
ray-tracing code for SiO thermal emission.
Results. We observed circular symmetry in the emission distribution. We also
found that the source diameter varies only marginally with radial velocity,
which is not the expected behavior for envelopes expanding at an almost
constant velocity. The adopted density, velocity, and abundance laws, together
with the mass-loss rate, which best fit the observations, give us information
on the chemical behavior of the SiO molecule and its role in the dust formation
process.
Conclusions. The results indicate that there is a strong coupling between the
depletion of gas phase SiO and gas acceleration in the inner envelope. This
could be explained by the condensation of SiO into dust grains
Two short mass-loss events that unveil the binary heart of Minkowski's Butterfly Nebula
Studying the appearance and properties of bipolar winds is critical to
understand the stellar evolution from the AGB to the planetary nebula (PN)
phase. Many uncertainties exist regarding the presence and role of binary
stellar systems, mainly due to the deficit of conclusive observational
evidences. We investigate the extended equatorial distribution around the early
bipolar planetary nebula M 2-9 ("Minkowski's Butterfly Nebula") to gather new
information on the mechanism of the axial ejections. Interferometric millimeter
observations of molecular emission provide the most comprehensive view of the
equatorial mass distribution and kinematics in early PNe. Here we present
subarcsecond angular-resolution observations of the 12CO J=2-1 line and
continuum emission with the Plateau de Bure interferometer. The data reveal two
ring-shaped and eccentric structures at the equatorial basis of the two coaxial
optical lobes. The two rings were formed during short mass-loss episodes (~ 40
yr), separated by ~ 500 yr. Their positional and dynamical imprints provide
evidence of the presence of a binary stellar system at the center, which yields
critical information on its orbital characteristics, including a mass estimate
for the secondary of ~< 0.2 \ms. The presence of a stellar system with a
modest-mass companion at the center of such an elongated bipolar PN strongly
supports the binary-based models, because these are more easily able to explain
the frequent axisymmetric ejections in PNe.Comment: 8 page
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
A disk inside the bipolar planetary nebula M2-9
Bipolarity in proto-planetary and planetary nebulae is associated with events
occurring in or around their cores. Past infrared observations have revealed
the presence of dusty structures around the cores, many in the form of disks.
Characterising those dusty disks provides invaluable constraints on the
physical processes that govern the final mass expulsion of intermediate-mass
stars. We focus this study on the famous M2-9 bipolar nebula, where the moving
lighthouse beam pattern indicates the presence of a wide binary. The compact
and dense dusty core in the center of the nebula can be studied by means of
optical interferometry. M2-9 was observed with VLTI/MIDI at 39-47 m baselines
with the UT2-UT3 and UT3-UT4 baseline configurations. These observations are
interpreted using a dust radiative transfer Monte Carlo code. A disk-like
structure is detected perpendicular to the lobes and a good fit is found with a
stratified disk model composed of amorphous silicates. The disk is compact,
2535 mas at 8, and 3746 mas at 13. For
the adopted distance of 1.2 kpc, the inner rim of the disk is 15 AU. The
mass represents a few percent of the mass found in the lobes. The compactness
of the disk puts strong constraints on the binary content of the system, given
an estimated orbital period 90-120yr. We derive masses of the binary components
between 0.6--1.0M_{\sun} for a white dwarf and 0.6--1.4M_{\sun} for an
evolved star. We present different scenarios on the geometric structure of the
disk accounting for the interactions of the binary system, which includes an
accretion disk as well.Comment: 9 figures, A&A accepte
Further ALMA observations and detailed modeling of the Red Rectangle
We present new high-quality ALMA observations of the Red Rectangle (a well
known post-AGB object) in C17O J=6-5 and H13CN J=4-3 line emission and results
from a new reduction of already published 13CO J=3-2 data. A detailed model
fitting of all the molecular line data, including previous maps and single-dish
spectra, was performed using a sophisticated code. These observations and the
corresponding modeling allowed us to deepen the analysis of the nebular
properties. We also stress the uncertainties in the model fitting.
We confirm the presence of a rotating equatorial disk and an outflow, which
is mainly formed of gas leaving the disk. The mass of the disk is ~ 0.01 Mo,
and that of the CO-rich outflow is ~ 10 times smaller. High temperatures of ~
100 K are derived for most components. From comparison of the mass values, we
roughly estimate the lifetime of the rotating disk, which is found to be of
about 10000 yr. Taking data of a few other post-AGB composite nebulae into
account, we find that the lifetimes of disks around post-AGB stars typically
range between 5000 and more than 20000 yr. The angular momentum of the disk is
found to be high, ~ 9 Mo AU km/s, which is comparable to that of the stellar
system at present. Our observations of H13CN show a particularly wide velocity
dispersion and indicate that this molecule is only abundant in the inner
Keplerian disk, at ~ 60 AU from the stellar system. We suggest that HCN is
formed in a dense photodissociation region (PDR) due to the UV excess known to
be produced by the stellar system, following chemical mechanisms that are well
established for interstellar medium PDRs and disks orbiting young stars. We
further suggest that this UV excess could lead to the efficient formation and
excitation of PAHs and other C-bearing macromolecules, whose emission is very
intense in the optical counterpart.Comment: Astronomy & Astrohysics, in press; 17 pages, 18 figures, 1 tabl
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