489 research outputs found
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
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
SHAPEMOL: a 3-D code for calculating CO line emission in planetary and protoplanetary nebulae. Detailed model fitting of the complex nebula NGC 6302
Modern instrumentation in radioastronomy constitutes a valuable tool for
studying the Universe: ALMA has reached unprecedented sensitivities and spatial
resolution, while Herschel/HIFI has opened a new window for probing molecular
warm gas (~50-1000 K). On the other hand, the software SHAPE has emerged in the
past few years as a standard tool for determining the morphology and velocity
field of different kinds of gaseous emission nebulae via spatio-kinematical
modelling. SHAPE implements radiative transfer solving, but it is only
available for atomic species and not for molecules. Being aware of the growing
importance of the development of tools for simplifying the analyses of
molecular data, we introduce shapemol, a complement to SHAPE, with which we
intend to fill the so-far under-developed molecular niche. shapemol enables
user-friendly, spatio-kinematic modelling with accurate non-LTE calculations of
excitation and radiative transfer in CO lines. It allows radiative transfer
solving in the 12CO and 13CO J=1-0 to J=17-16 lines, but its implementation
permits easily extending the code to different molecular species. shapemol
allows easily generating synthetic maps and line profiles to match against
interferometric or single-dish observations. We fully describe shapemol and
discuss its limitations and the sources of uncertainty to be expected in the
final synthetic profiles or maps. As an example of the power and versatility of
shapemol, we build a model of the molecular envelope of the planetary nebula
NGC 6302 and compare it with 12CO and 13CO J=2-1 interferometric maps from SMA
and high-J transitions from Herschel/HIFI. We find the molecular envelope to
have a complex, broken ring-like structure with an inner, hotter region and
several 'fingers' and high-velocity blobs, emerging outwards from the plane of
the ring. We derive a mass of 0.11 Msun for the molecular envelope.Comment: 19 pages, 15 figures. Accepted for publication in Astronomy &
Astrophysic
ALMA observations of the Red Rectangle, a preliminary analysis
We aim to study equatorial disks in rotation and axial outflows in post-AGB
objects, as to disclose the formation and shaping mechanisms in planetary
nebulae. So far, both disks and outflows had not been observed simultaneously.
We have obtained high-quality ALMA observations of 12CO and 13CO J=3-2 and
12CO J=6-5 line emission in the Red Rectangle, the only post-AGB/protoplanetary
object in which a disk in rotation has been mapped up to date.
These observations provide an unprecedented description of the complex
structure of this source. Together with an equatorial disk in rotation, we find
a low-velocity outflow that occupies more or less the region placed between the
disk and the optical X-shaped nebula. From our observations and preliminary
modeling of the data, we confirm the previously known properties of the disk
and obtain a first description of the structure, dynamics, and physical
conditions of the outflow.Comment: 5 pages, 5 figure
New insights into the outflows from R Aquarii
R Aquarii is a symbiotic binary surrounded by a large and complex nebula with
a prominent curved jet. It is one of the closest known symbiotic systems, and
therefore offers a unique opportunity to study the central regions of these
systems and the formation and evolution of astrophysical jets. We studied the
evolution of the central jet and outer nebula of R Aqr taking advantage of a
long term monitoring campaign of optical imaging, as well as of high-resolution
integral field spectroscopy. Narrow-band images acquired over a period of more
than 21 years are compared in order to study the expansion and evolution of all
components of the R Aqr nebula. The magnification method is used to derive the
kinematic ages of the features that appear to expand radially. Integral field
spectroscopy of the OIII 5007A emission is used to study the velocity structure
of the central regions of the jet. New extended features, further out than the
previously known hourglass nebula, are detected. The kinematic distance to R
Aqr is calculated to be 178 pc using the expansion of the large hourglass
nebula. This nebula of R Aqr is found to be roughly 650 years old, while the
inner regions have ages ranging from 125 to 290 years. The outer nebula is
found to be well described by a ballistic expansion, while for most components
of the jet strong deviations from such behaviour are found. We find that the
Northern jet is mostly red-shifted while its Southern part is blue-shifted,
apparently at odds with findings from previous studies but almost certainly a
consequence of the complex nature of the jet and variations in ionisation and
illumination between observations.Comment: 13 pages, 8 figures, accepted for publication in A&
High-resolution observations of IRAS 08544-4431. Detection of a disk orbiting a post-AGB star and of a slow disk wind
We are studying a class of binary post-AGB stars that seem to be
systematically surrounded by equatorial disks and slow outflows. Although the
rotating dynamics had only been well identified in three cases, the study of
such structures is thought to be fundamental to the understanding of the
formation of nebulae around evolved stars. We present ALMA maps of 12CO and
13CO J=3-2 lines in one of these sources, IRAS08544-4431. We analyzed the data
by means of nebula models, which account for the expectedly composite source
and can reproduce the data. From our modeling, we estimated the main nebula
parameters, including the structure and dynamics and the density and
temperature distributions. We discuss the uncertainties of the derived values
and, in particular, their dependence on the distance.
Our observations reveal the presence of an equatorial disk in rotation; a
low-velocity outflow is also found, probably formed of gas expelled from the
disk. The main characteristics of our observations and modeling of
IRAS08544-4431 are similar to those of better studied objects, confirming our
interpretation. The disk rotation indicates a total central mass of about 1.8
Mo, for a distance of 1100 pc. The disk is found to be relatively extended and
has a typical diameter of ~ 4 10^16 cm. The total nebular mass is ~ 2 10^-2 Mo,
of which ~ 90% corresponds to the disk. Assuming that the outflow is due to
mass loss from the disk, we derive a disk lifetime of ~ 10000 yr. The disk
angular momentum is found to be comparable to that of the binary system at
present. Assuming that the disk angular momentum was transferred from the
binary system, as expected, the high values of the disk angular momentum in
this and other similar disks suggest that the size of the stellar orbits has
significantly decreased as a consequence of disk formation.Comment: 10 pages, 7 figures, accepted by A&
Gas infall and possible circumstellar rotation in R Leonis
We present new interferometer molecular observations of R Leo taken at 1.2 mm with the Atacama Large Millimeter Array with an angular resolution up to similar or equal to 0.\u27\u27 026. These observations permitted us to resolve the innermost envelope of this star, which revealed a complex structure that involves extended continuum emission and molecular emission showing a non-radial gas velocity distribution. This molecular emission displays prominent red-shifted absorptions located immediately in front of the star, which are typical footprints of material infall. This emission also shows lateral gas motions compatible with a torus-like structure
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