122 research outputs found
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
The Abundance of SiC2 in Carbon Star Envelopes: Evidence that SiC2 is a gas-phase precursor of SiC dust
Silicon carbide dust is ubiquitous in circumstellar envelopes around C-rich
AGB stars. However, the main gas-phase precursors leading to the formation of
SiC dust have not yet been identified. The most obvious candidates among the
molecules containing an Si--C bond detected in C-rich AGB stars are SiC2, SiC,
and Si2C. We aim to study how widespread and abundant SiC2, SiC, and Si2C are
in envelopes around C-rich AGB stars and whether or not these species play an
active role as gas-phase precursors of silicon carbide dust in the ejecta of
carbon stars. We carried out sensitive observations with the IRAM 30m telescope
of a sample of 25 C-rich AGB stars to search for emission lines of SiC2, SiC,
and Si2C in the 2 mm band. We performed non-LTE excitation and radiative
transfer calculations based on the LVG method to model the observed lines of
SiC2 and to derive SiC2 fractional abundances in the observed envelopes. We
detect SiC2 in most of the sources, SiC in about half of them, and do not
detect Si2C in any source, at the exception of IRC +10216. Most of these
detections are reported for the first time in this work. We find a positive
correlation between the SiC and SiC2 line emission, which suggests that both
species are chemically linked, the SiC radical probably being the
photodissociation product of SiC2 in the external layer of the envelope. We
find a clear trend in which the denser the envelope, the less abundant SiC2 is.
The observed trend is interpreted as an evidence of efficient incorporation of
SiC2 onto dust grains, a process which is favored at high densities owing to
the higher rate at which collisions between particles take place. The observed
behavior of a decline in the SiC2 abundance with increasing density strongly
suggests that SiC2 is an important gas-phase precursor of SiC dust in envelopes
around carbon stars.Comment: Published in A&A. 16 pages and 10 figure
ALMA Observations of the Water Fountain Pre-Planetary Nebula IRAS 16342-3814: High-velocity bipolar jets and an Expanding Torus
R. Sahai, W.H.T. Vlemmings, T. Gledhill, C. Sanchez Contreras, E. Lagadec, L-A Nyman, and G. Quintana-Lacaci, âALMA Observations of the Water Fountain Pre-planetary Nebula IRAS 16342-3814: High-velocity Bipolar Jets and an Expanding Torusâ, The Astrophysical Journal Letters, Vol 835: L13 (6 pp), published 20 January 2017. The version of record is available online via doi: 10.3847/2041-8213/835/1/L13 © 2017. The American Astronomical Society. All rights reserved.We have mapped 12CO J=3â2 and other molecular lines from the âwater fountainâ bipolar pre-planetary nebula (PPN) IRAS 16342-3814 with âŒ0.35âł resolution using Atacama Large Millimeter/submillimeter Array. We find (i) two very high-speed knotty, jet-like molecular outflows; (ii) a central high-density (>few x106 cmâ3), expanding torus of diameter 1300 au; and (iii) the circumstellar envelope of the progenitor AGB, generated by a sudden, very large increase in the mass-loss rate to >3.5 x10-4 Ms yrâ1 in the past âŒ455 years. Strong continuum emission at 0.89 mm from a central source (690 mJy), if due to thermally emitting dust, implies a substantial mass (0.017 Ms) of very large (âŒmillimeter-sized) grains. The measured expansion ages of the above structural components imply that the torus (ageâŒ160 years) and the younger high-velocity outflow (ageâŒ110 years) were formed soon after the sharp increase in the AGB mass-loss rate. Assuming a binary model for the jets in IRAS 16342, the high momentum rate for the dominant jet-outflow in IRAS 16342 implies a high minimum accretion rate, ruling out standard BondiâHoyleâLyttleton wind accretion and wind Roche-lobe overflow (RLOF) models with white-dwarf or main-sequence companions. Most likely, enhanced RLOF from the primary or accretion modes operating within common-envelope evolution are needed.Peer reviewedFinal Published versio
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
Zooming in on VY CMa ejecta
The Atacama Large Millimeter/Submillimeter Array (ALMA), Cagliari, Italy 14â18 October 2019ALMA has allowed us to study the ejecta around evolved stars with unprecedented resolution. This extremely high resolution at the millimeter domain provides a unique tool to study the processes taking place in the innermost regions of these evolved stars. In particular, the processes leading to the mass ejections of the Red Supergiant stars are unknown. The pulsation process responsible for mass ejection in the intermediate mass AGB phase does not work in the high mass evolved stars. Therefore, studying the characteristics of the mass ejections near the photosphere of the massive stars is essential to constrain the processes leading to the observed gas ejection. In this sense, we have obtained interferometric maps in the range 231.7 Âż 235.3 GHz of the ejecta around the Red Supergiant star VY CMa with an spatial resolution of 0.02>. These maps revealed a level of complexity higher than previously anticipated from previous observations. The complexity seems to be due both to structural and chemical processes. The molecular lines covered within these maps range from upper energies 19 up to 3400 K, tracing different excitation conditions. We will present a global view of the different structures observedThe research leading to these results has received funding from the European
Research Council under the European Unionâs Seventh Framework Programme
(FP/2007-2013) / ERC Grant Agreement n. 610256 (NANOCOSMOS). We would also
like to thank the Spanish MINECO for funding support from grants CSD2009-00038,
AYA2012-32032, AYA2016-75066-C2-1-P & AYA2016-78994-P. M.A. also thanks for
funding support from the RamĂłn y Cajal programme of Spanish MINECO (RyC-2014-
16277)
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
The circumstellar envelope around the S-type AGB star W Aql Effects of an eccentric binary orbit
The CO(J=3-2) emission from the CSE of the binary S-type AGB star W Aql has
been observed at subarcsecond resolution using ALMA. The aim of this paper is
to investigate the wind properties of the AGB star and to analyse how the known
companion has shaped the CSE. The average mass-loss rate during the creation of
the detected CSE is estimated through modelling, using the ALMA brightness
distribution and previously published single-dish measurements as observational
constraints. The ALMA observations are presented and compared to the results
from a 3D smoothed particle hydrodynamics (SPH) binary interaction model with
the same properties as the W Aql system and with two different orbital
eccentricities. Three-dimensional radiative transfer modelling is performed and
the response of the interferometer is modelled and discussed. The estimated
average mass-loss rate of W~Aql agrees with previous results. The size of the
emitting region is consistent with photodissociation models. The CO(J=3-2)
emission is dominated by a smooth component overlayed with two weak arc
patterns with different separations. The larger pattern is predicted by the
binary interaction model with separations of 10" and therefore likely due to
the known companion. It is consistent with a binary orbit with low
eccentricity. The smaller separation pattern is asymmetric and coincides with
the dust distribution, but the separation timescale (200 yrs) is not consistent
with any known process of the system. The separation of the known companions of
the system is large enough to not have a very strong effect on the
circumstellar morphology. The density contrast across the envelope of a binary
with an even larger separation will not be easily detectable, even with ALMA,
unless the orbit is strongly asymmetric or the AGB star has a much larger
mass-loss rate.Comment: 10 pages, 8 figure
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