390 research outputs found
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
Arcsecond-resolution 12CO mapping of the yellow hypergiants IRC +10420 and AFGL 2343
IRC +10420 and AFGL 2343 are the unique, known yellow hypergiants (YHGs)
presenting a heavy circumstellar envelope (CSE). We aim to study the
morphology, exceptional kinematics, and excitation conditions of their CSEs,
and the implications for mass-loss processes. We have mapped the 12CO J=2-1 and
1-0 emission in these YHGs with the IRAM Plateau de Bure interferometer and the
30m telescope. We developed LVG models in order to analyze their circumstellar
characteristics. The maps show that the overall shape of both CSEs is
approximately spherical, although they also reveal several aspherical features.
The CSE around IRC +10420 shows a rounded extended halo surrounding a bright
inner region, with both components presenting aspherical characteristics. It
presents a brightness minimum at the center. The envelope around AFGL 2343 is a
detached shell, showing spherical symmetry and clumpiness at a level of about
15% of the maximum brightness. The envelopes expand isotropically at about 35
km/s, about two or three times faster than typical CSEs around AGB stars. High
temperatures (~ 200 K) are derived for the innermost regions in IRC +10420,
while denser and cooler (~ 30 K) gas is found in AFGL 2343. The mass-loss
processes in these YHGs have been found to be similar. The deduced mass-loss
rates (~ 10E-4 - 10E-3 Msun/yr) are much higher than those obtained in AGB
stars, and they present significant variations on time scales of ~ 1000 yr
Clues to NaCN formation
ALMA is providing us essential information on where certain molecules form.
Observing where these molecules emission arises from, the physical conditions
of the gas, and how this relates with the presence of other species allows us
to understand the formation of many species, and to significantly improve our
knowledge of the chemistry that occurs in the space. We studied the molecular
distribution of NaCN around IRC +10216, a molecule detected previously, but
whose origin is not clear. High angular resolution maps allow us to model the
abundance distribution of this molecule and check suggested formation paths. We
modeled the emission of NaCN assuming local thermal equilibrium (LTE)
conditions. These profiles were fitted to azimuthal averaged intensity profiles
to obtain an abundance distribution of NaCN. We found that the presence of NaCN
seems compatible with the presence of CN, probably as a result of the
photodissociation of HCN, in the inner layers of the ejecta of IRC +10216.
However, similar as for CH 3 CN, current photochemical models fail to reproduce
this CN reservoir. We also found that the abundance peak of NaCN appears at a
radius of 3 x 10 15 cm, approximately where the abundance of NaCl, suggested to
be the parent species, starts to decay. However, the abundance ratio shows that
the NaCl abundance is lower than that obtained for NaCN. We expect that the LTE
assumption might result in NaCN abundances higher than the real ones. Updated
photochemical models, collisional rates, and reaction rates are essential to
determine the possible paths of the NaCN formation.Comment: 7 pages, 10 figures. Accepted for publication in A&A letter
Through the magnifying glass: ALMA acute viewing of the intricate nebular architecture of OH231.8+4.2
We present continuum and molecular line emission ALMA observations of OH
231.8+4.2, a well studied bipolar nebula around an asymptotic giant branch
(AGB) star. The high angular resolution (~0.2-0.3 arcsec) and sensitivity of
our ALMA maps provide the most detailed and accurate description of the overall
nebular structure and kinematics of this object to date. We have identified a
number of outflow components previously unknown. Species studied in this work
include 12CO, 13CO, CS, SO, SO2, OCS, SiO, SiS, H3O+, Na37Cl, and CH3OH. The
molecules Na37Cl and CH3OH are first detections in OH 231.8+4.2, with CH3OH
being also a first detection in an AGB star. Our ALMA maps bring to light the
totally unexpected position of the mass-losing AGB star (QX Pup) relative to
the large-scale outflow. QX Pup is enshrouded within a compact (<60 AU) parcel
of dust and gas (clump S) in expansion (V~5-7 km/s) that is displaced by
0.6arcsec to the south of the dense equatorial region (or waist) where the
bipolar lobes join. Our SiO maps disclose a compact bipolar outflow that
emerges from QX Pup's vicinity. This outflow is oriented similarly to the
large-scale nebula but the expansion velocities are about ten times lower (~35
km/s). We deduce short kinematical ages for the SiO outflow, ranging from
~50-80 yr, in regions within ~150 AU, to ~400-500 yr at the lobe tips (~3500
AU). Adjacent to the SiO outflow, we identify a small-scale hourglass-shaped
structure (mini-hourglass) that is probably made of compressed ambient material
formed as the SiO outflow penetrates the dense, central regions of the nebula.
The lobes and the equatorial waist of the mini-hourglass are both radially
expanding with a constant velocity gradient. The mini-waist is characterized by
extremely low velocities, down to ~1 km/s at ~150 AU, which tentatively suggest
the presence of a stable structure. (abridged
Herschel/HIFI observations of molecular emission in protoplanetary nebulae and young planetary nebulae
We performed Herschel/HIFI observations of intermediate-excitation molecular
lines in the far-infrared/submillimeter range in a sample of ten protoplanetary
nebulae and young planetary nebulae. The high spectral resolution provided by
HIFI yields accurate measurements of the line profiles. The observation of
these high-energy transitions allows an accurate study of the excitation
conditions, particularly in the warm gas, which cannot be properly studied from
the low-energy lines.
We have detected FIR/sub-mm lines of several molecules, in particular of
12CO, 13CO, and H2O. Emission from other species, like NH3, OH, H2^{18}O, HCN,
SiO, etc, has been also detected. Wide profiles showing sometimes spectacular
line wings have been found. We have mainly studied the excitation properties of
the high-velocity emission, which is known to come from fast bipolar outflows.
From comparison with general theoretical predictions, we find that CRL 618
shows a particularly warm fast wind, with characteristic kinetic temperature Tk
>~ 200 K. In contrast, the fast winds in OH 231.8+4.2 and NGC 6302 are cold, Tk
~ 30 K. Other nebulae, like CRL 2688, show intermediate temperatures, with
characteristic values around 100 K. We also discuss how the complex structure
of the nebulae can affect our estimates, considering two-component models. We
argue that the differences in temperature in the different nebulae can be due
to cooling after the gas acceleration (that is probably due to shocks); for
instance, CRL 618 is a case of very recent acceleration, less than ~ 100 yr
ago, while the fast gas in OH 231.8+4.2 was accelerated ~ 1000 yr ago. We also
find indications that the densest gas tends to be cooler, which may be
explained by the expected increase of the radiative cooling efficiency with the
density.Comment: 24 pages, 31 figure
A contribution to the quantification of crustal shortening and kinematics of deformation across the Western Andes ( ∼ 20–22° S)
The Andes are an emblematic active Cordilleran orogen.
Mountain building in the Central Andes (∼20∘ S)
started by the Late Cretaceous to early Cenozoic along the subduction margin
and propagated eastward. In general, the structures sustaining the uplift of
the western flank of the Andes are dismissed, and their contribution to
mountain building remains poorly constrained. Here, we focus on two sites
along the Western Andes at ∼20–22∘ S in the
Atacama desert, where structures are well exposed. We combine mapping from
high-resolution satellite images with field observations and numerical
trishear forward modeling to provide quantitative constraints on the
kinematic evolution of the investigated field sites. When upscaling our
local field interpretations to the regional scale, we identify two main
structures: (1)Â the Andean Basement Thrust, a west-vergent thrust system
placing Andean Paleozoic basement over Mesozoic strata, and (2)Â a series of
west-vergent thrusts pertaining to the West Andean Thrust System, deforming
primarily Mesozoic units. From our interpreted sections, we estimate that
both structures together accommodate at least ∼6–9 km of
shortening across the sole investigated ∼7–17 km wide field
sites. This multi-kilometric shortening represents only a fraction of the
total shortening accommodated across the whole Western Andes. The timing of
the main deformation recorded in the folded Mesozoic series can be bracketed
between ∼68 and ∼29 Ma – and possibly between
∼68 and ∼44 Ma – from dated deformed
geological layers, with a subsequent significant slowing-down of shortening
rates. Even though the structures forming the Western Andes only absorbed a
small fraction of the total shortening across the whole orogen, their
contribution was relatively significant at the earliest stages of
Andean mountain building before deformation proceeded eastward.</p
The structure and chemistry of the massive shell around AFGL 2343: 29SiO and HCN as tracers of high-excitation regions
The yellow hypergiant stars (YHGs) are very massive objects that are expected
to pass through periods of intense mass loss during their evolution. Despite of
this, massive circumstellar envelopes have been found only in two of them,
IRC+10420 and AFGL 2343. The envelopes around these objects and the processes
that form them are poorly known. We aim to study the structure, dynamics and
chemistry of the envelope around AFGL 2343. We have obtained interferometric
maps of the rotational lines 29SiO J= 2-1, HCN J= 1-0 and SO J(K)= 2(2)-1(1)
towards AFGL 2343. We have used an LVG excitation model to analyze the new
observations and some previously published line profiles of AFGL 2343. The
analysis of the observational data and the fitting results show the presence of
a thin, hot and dense component within the previously identified CO shell. This
component can be associated with recently shocked gas, but it could also be due
to a phase of extremely copious mass loss. We suggest that this shell is the
responsible for the whole 29SiO emission and significantly contributes to the
HCN emission. The presence of such a dense shell rich in SiO can be related
with that previously found for IRC+10420, which was also suggested to result
from a shock. This may be a common feature in the evolution of these stars, as
a consequence of the episodic mass loss periods that they pass during their
evolution. We present new results for the mass loss pattern, the total mass of
the circumstellar envelope and the molecular abundances of some species in AFGL
2343.Comment: 9 pages, 9 figure
Experimental Study for the Stripping of PTFE Coatings on Al-Mg Substrates Using Dry Abrasive Materials
Polytetrafluoroethylene (PTFE) coatings are used in many applications and processing industries. With their use, they wear out and lose properties and must be replaced by new ones if the cost of the element so advises. There are different stripping techniques, but almost all of them are very difficult and require strict environmental controls. It is a challenge to approach the process through efficient and more sustainable techniques. In the present work, we have studied the stripping of PTFE coatings by projection with abrasives (1 step) as an alternative to carbonization + sandblasting procedures (2 steps). For this purpose, different types of abrasives have been selected: brown corundum, white corundum, glass microspheres, plastic particles, and a walnut shell. The tests were performed at pressures from 0.4 to 0.6 MPa on PTFE-coated aluminium substrates of EN AW-5182 H111 alloy. Stripping rates, surface roughness, and substrate hardness have been studied. Scanning electron microscopy (SEM) images of sandblasted specimens have also been obtained. All abrasives improved mechanical and surface properties in one-step vs. two-step processes. The abrasives of plastic and glass microspheres are the most appropriate for the one-step process, which increases the hardness and roughness level Ra in the substrate. Corundum abrasives enable the highest stripping rates
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