99 research outputs found
An ALMA view of CS and SiS around oxygen-rich AGB stars
We aim to determine the distributions of molecular SiS and CS in the
circumstellar envelopes of oxygen-rich asymptotic giant branch stars and how
these distributions differ between stars that lose mass at different rates. In
this study we analyse ALMA observations of SiS and CS emission lines for three
oxygen-rich galactic AGB stars: IK Tau, with a moderately high mass-loss rate
of M yr, and W Hya and R Dor with low mass loss
rates of M yr. These molecules are usually
more abundant in carbon stars but the high sensitivity of ALMA allows us to
detect their faint emission in the low mass-loss rate AGB stars. The high
spatial resolution of ALMA also allows us to precisely determine the spatial
distribution of these molecules in the circumstellar envelopes. We run
radiative transfer models to calculate the molecular abundances and abundance
distributions for each star. We find a spread of peak SiS abundances with
for R Dor, for W Hya, and for
IK Tau relative to H. We find lower peak CS abundances of
for R Dor, for W Hya and
for IK Tau, with some stratifications in the abundance
distributions. For IK Tau we also calculate abundances for the detected
isotopologues: CS, SiS, SiS, SiS, SiS,
SiS, and SiS. Overall the isotopic ratios we derive
for IK Tau suggest a lower metallicity than solar.Comment: 16 page
The unusual 3D distribution of NaCl around the AGB star IK Tau
NaCl is a diatomic molecule with a large dipole moment, which allows for its
detection even at relatively small abundances. It has been detected towards
several evolved stars, among which is the AGB star IK Tau, around which it is
distributed in several clumps that lie off-center from the star. We aim to
study the three-dimensional distribution of NaCl around the AGB star IK Tau,
and to obtain the abundance of NaCl relative to H for each of the clumps.
First, a new value for the maximum expansion velocity is determined. The
observed ALMA channel maps are then deprojected to create a three-dimensional
model of the distribution of NaCl. This model is then used as input for the
radiative transfer modelling code magritte, which is used to obtain the NaCl
abundances of each of the clumps by comparing the observations with the results
of the magritte simulations. Additionally, the rotational temperature of the
clumps is determined using population diagrams. We derive an updated value for
the maximum expansion velocity of IK Tau = 28.4 km/s. A
spiral-like shape can be discerned in our three-dimensional distribution model
of the NaCl. This spiral lies more or less in the plane of the sky. The
distribution is also flatter in the line-of-sight direction than in the plane
of the sky. We find clump abundances between and relative to H, where the relative abundance is typically lower for
clumps closer to the star. For the first time, we used deprojection to
understand the three-dimensional environment of an AGB star and calculated the
fractional abundance of NaCl in clumps surrounding the star.Comment: Accepted for publication in A&
Determining the effects of clumping and porosity on the chemistry in a non-uniform AGB outflow
Determining the effects of clumping and porosity on the chemistry in a non-uniform AGB outflow
(abridged) In the inner regions of AGB outflows, several molecules have been
detected with abundances much higher than those predicted from thermodynamic
equilibrium (TE) chemical models. The presence of the majority of these species
can be explained by shock-induced non-TE chemical models, where shocks caused
by the pulsating star take the chemistry out of TE in the inner region.
Moreover, a non-uniform density structure has been detected in several AGB
outflows. A detailed parameter study on the quantitative effects of a
non-homogeneous outflow has so far not been performed. We implement a porosity
formalism for treating the increased leakage of light associated with radiation
transport through a clumpy, porous medium. The effects from the altered UV
radiation field penetration on the chemistry, accounting also for the increased
reaction rates of two-body processes in the overdense clumps, are examined. We
present a parameter study of the effect of clumping and porosity on the
chemistry throughout the outflow. Both the higher density within the clumps and
the increased UV radiation field penetration have an important impact on the
chemistry, as they both alter the chemical pathways. The increased amount of UV
radiation in the inner region leads to photodissociation of parent species,
releasing the otherwise deficient elements. We find an increased abundance in
the inner region of all species not expected to be present assuming TE
chemistry, such as HCN in O-rich outflows, HO in C-rich outflows, and
NH in both. Outflows whose clumps have a large overdensity and that are
very porous to the interstellar UV radiation field yield abundances comparable
to those observed in O- and C-rich outflows for most of the unexpected species
investigated. The inner wind abundances of HO in C-rich outflows and of
NH in O- and C-rich outflows are however underpredicted.Comment: 33 pages, 20 figures, 15 tables, accepted for publication in
Astronomy & Astrophysic
Triplet Exciton Generation in Bulk-Heterojunction Solar Cells based on Endohedral Fullerenes
Organic bulk-heterojunctions (BHJ) and solar cells containing the trimetallic
nitride endohedral fullerene 1-[3-(2-ethyl)hexoxy
carbonyl]propyl-1-phenyl-Lu3N@C80 (Lu3N@C80-PCBEH) show an open circuit voltage
(VOC) 0.3 V higher than similar devices with [6,6]-phenyl-C[61]-butyric acid
methyl ester (PC61BM). To fully exploit the potential of this acceptor molecule
with respect to the power conversion efficiency (PCE) of solar cells, the short
circuit current (JSC) should be improved to become competitive with the state
of the art solar cells. Here, we address factors influencing the JSC in blends
containing the high voltage absorber Lu3N@C80-PCBEH in view of both
photogeneration but also transport and extraction of charge carriers. We apply
optical, charge carrier extraction, morphology, and spin-sensitive techniques.
In blends containing Lu3N@C80-PCBEH, we found 2 times weaker photoluminescence
quenching, remainders of interchain excitons, and, most remarkably, triplet
excitons formed on the polymer chain, which were absent in the reference
P3HT:PC61BM blends. We show that electron back transfer to the triplet state
along with the lower exciton dissociation yield due to intramolecular charge
transfer in Lu3N@C80-PCBEH are responsible for the reduced photocurrent
(Sub)stellar companions shape the winds of evolved stars
Binary interactions dominate the evolution of massive stars, but their role is less clear for low- and intermediate-mass stars. The evolution of a spherical wind from an asymptotic giant branch (AGB) star into a nonspherical planetary nebula (PN) could be due to binary interactions. We observed a sample of AGB stars with the Atacama Large Millimeter/submillimeter Array (ALMA) and found that their winds exhibit distinct nonspherical geometries with morphological similarities to planetary nebulae (PNe). We infer that the same physics shapes both AGB winds and PNe; additionally, the morphology and AGB mass-loss rate are correlated. These characteristics can be explained by binary interaction. We propose an evolutionary scenario for AGB morphologies that is consistent with observed phenomena in AGB stars and PNe
(Sub)stellar companions shape the winds of evolved stars
Binary interactions dominate the evolution of massive stars, but their role is less clear for low- and intermediate-mass stars. The evolution of a spherical wind from an asymptotic giant branch (AGB) star into a nonspherical planetary nebula (PN) could be due to binary interactions. We observed a sample of AGB stars with the Atacama Large Millimeter/submillimeter Array (ALMA) and found that their winds exhibit distinct nonspherical geometries with morphological similarities to planetary nebulae (PNe). We infer that the same physics shapes both AGB winds and PNe; additionally, the morphology and AGB mass-loss rate are correlated. These characteristics can be explained by binary interaction. We propose an evolutionary scenario for AGB morphologies that is consistent with observed phenomena in AGB stars and PNe
ATOMIUM: ALMA tracing the origins of molecules in dust forming oxygen rich M-type stars: Motivation, sample, calibration, and initial results
This overview paper presents atomium, a Large Programme in Cycle 6 with the Atacama Large Millimeter/submillimeter Array (ALMA). The goal of atomium is to understand the dynamics and the gas phase and dust formation chemistry in the winds of evolved asymptotic giant branch (AGB) and red supergiant (RSG) stars. A more general aim is to identify chemical processes applicable to other astrophysical environments. Seventeen oxygen-rich AGB and RSG stars spanning a range in (circum)stellar parameters and evolutionary phases were observed in a homogeneous observing strategy allowing for an unambiguous comparison. Data were obtained between 213.83 and 269.71 GHz at high (0.025-0.050), medium (0.13-0.24), and low (~1) angular resolution. The sensitivity per ~1.3 km s-1 channel was 1.5-5 mJy beam-1, and the line-free channels were used to image the millimetre wave continuum. Our primary molecules for studying the gas dynamics and dust formation are CO, SiO, AlO, AlOH, TiO, TiO2, and HCN; secondary molecules include SO, SO2, SiS, CS, H2O, and NaCl. The scientific motivation, survey design, sample properties, data reduction, and an overview of the data products are described. In addition, we highlight one scientific result - the wind kinematics of the atomium sources. Our analysis suggests that the atomium sources often have a slow wind acceleration, and a fraction of the gas reaches a velocity which can be up to a factor of two times larger than previously reported terminal velocities assuming isotropic expansion. Moreover, the wind kinematic profiles establish that the radial velocity described by the momentum equation for a spherical wind structure cannot capture the complexity of the velocity field. In fifteen sources, some molecular transitions other than 12CO v = 0 J = 2 - 1 reach a higher outflow velocity, with a spatial emission zone that is often greater than 30 stellar radii, but much less than the extent of CO. We propose that a binary interaction with a (sub)stellar companion may (partly) explain the non-monotonic behaviour of the projected velocity field. The atomium data hence provide a crucial benchmark for the wind dynamics of evolved stars in single and binary star models
ATOMIUM: halide molecules around the S-type AGB star W Aquilae
Context. S-type asymptotic giant branch (AGB) stars are thought to be intermediates in the evolution of oxygen- to carbon-rich AGB stars. The chemical compositions of their circumstellar envelopes are also intermediate but have not been studied in as much detail as their carbon- and oxygen-rich counterparts. W Aql is a nearby S-type star, with well-known circumstellar parameters, making it an ideal object for in-depth study of less common molecules.
Aims. We aim to determine the abundances of AlCl and AlF from rotational lines, which have been observed for the first time towards an S-type AGB star. In combination with models based on PACS observations, we aim to update our chemical kinetics network based on these results.
Methods. We analyse ALMA observations towards W Aql of AlCl in the ground and first two vibrationally excited states and AlF in the ground vibrational state. Using radiative transfer models, we determine the abundances and spatial abundance distributions of Al35Cl, Al37Cl, and AlF. We also model HCl and HF emission and compare these models to PACS spectra to constrain the abundances of these species.
Results. AlCl is found in clumps very close to the star, with emission confined within 0′′.1 of the star. AlF emission is more extended, with faint emission extending 0′′.2 to 0′′.6 from the continuum peak. We find peak abundances, relative to H2, of 1.7 × 10−7 for Al35Cl, 7 × 10−8 for Al37Cl, and 1 × 10−7 for AlF. From the PACS spectra, we find abundances of 9.7 × 10−8 and ≤10−8, relative to H2, for HCl and HF, respectively.
Conclusions. The AlF abundance exceeds the solar F abundance, indicating that fluorine synthesised in the AGB star has already been dredged up to the surface of the star and ejected into the circumstellar envelope. From our analysis of chemical reactions in the wind, we conclude that AlF may participate in the dust formation process, but we cannot fully explain the rapid depletion of AlCl seen inthe wind
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