483 research outputs found
CO and HCN isotopologue ratios in the outflows of AGB stars
Isotopologue line intensity ratios of circumstellar molecules have been
widely used to trace the photospheric elemental isotopic ratios of evolved
stars. However, depending on the molecular species and the physical conditions
of the environment, the circumstellar isotopologue ratio may deviate
considerably from the stellar atmospheric value. In this paper, we aim to
examine how the CO and HCN abundance ratios vary radially due to chemical
reactions in the outflows of AGB stars and the effect of excitation and optical
depth on the resulting line intensity ratios. We find that the circumstellar
12CO/13CO can deviate from its atmospheric value by up to 25-94% and 6-60% for
C- and O-type CSEs, respectively. We show that variations of the intensity of
the ISRF and the gas kinetic temperature can significantly influence the CO
isotopologue ratio in the outer CSEs. On the contrary, the H12CN/H13CN ratio is
stable for all tested mass-loss rates. The RT modeling shows that the
integrated line intensity ratio of CO of different rotational transitions
varies significantly for stars with intermediate mass-loss rates due to
combined chemical and excitation effects. In contrast, the excitation
conditions for the both HCN isotopologues are the same. We demonstrate the
importance of using the isotopologue abundance profiles from chemical models as
inputs to RT models in the interpretation of isotopologue observations.
Previous studies of CO isotopologue ratios are based on multi-transition data
for individual sources and it is difficult to estimate the errors in the
reported values due to assumptions that are not entirely correct according to
this study. If anything, previous studies may have overestimated the
circumstellar 12CO/13CO abundance ratio. The use of the HCN as a tracer of C
isotope ratios is affected by fewer complicating problems, provided one
accounts corrections for high optical depths.Comment: 14 pages, 11 figure
ALMA observations of the vibrationally-excited rotational CO transition towards five AGB stars
We report the serendipitous detection with ALMA of the vibrationally-excited
pure-rotational CO transition towards five asymptotic giant branch
(AGB) stars, Cet, R Aqr, R Scl, W Aql, and Gru. The observed lines
are formed in the poorly-understood region located between the stellar surface
and the region where the wind starts, the so-called warm molecular layer. We
successfully reproduce the observed lines profiles using a simple model. We
constrain the extents, densities, and kinematics of the region where the lines
are produced. R Aqr and R Scl show inverse P-Cygni line profiles which indicate
infall of material onto the stars. The line profiles of Cet and R Scl show
variability. The serendipitous detection towards these five sources shows that
vibrationally-excited rotational lines can be observed towards a large number
of nearby AGB stars using ALMA. This opens a new possibility for the study of
the innermost regions of AGB circumstellar envelopes.Comment: 6 pages, 2 figures, 2 tables, 2016MNRAS.463L..74
HD101584: Circumstellar characteristics and evolutionary status
We have performed a study of the characteristics of the circumstellar
environment of the binary object HD101584, that provides information on a
likely evolutionary scenario. We have obtained and analysed ALMA observations,
complemented with observations using APEX, of a large number of molecular
lines. An analysis of the spectral energy distribution has also been performed.
Emissions from 12 molecular species (not counting isotopologues) have been
observed, and most of them mapped with angular resolutions in the range 0.1" to
0.6". Four circumstellar components are identified: i) a central compact source
of size 0.15", ii) an expanding equatorial density enhancement (a flattened
density distribution in the plane of the orbit) of size 3", iii) a bipolar
high-velocity outflow (150 km/s), and iv) an hourglass structure. The outflow
is directed almost along the line of sight. There is evidence of a second
bipolar outflow. The mass of the circumstellar gas is 0.5[D/1 kpc]^2 Msun,
about half of it lies in the equatorial density enhancement. The dust mass is
0.01[D/1 kpc]^2 Msun, and a substantial fraction of this is in the form of
large-sized, up to 1 mm, grains. The estimated kinetic age of the outflow is
770[D/1 kpc] yr. The kinetic energy and the scalar momentum of the accelerated
gas are estimated to be 7x10^(45)[D/1 kpc]^2 erg and 10^(39)[D/1 kpc]^2 g cm/s,
respectively. We provide good evidence that the binary system HD101584 is in a
post-common-envelope-evolution phase, that ended before a stellar merger.
Isotope ratios combined with stellar mass estimates suggest that the primary
star's evolution was terminated already on the first red giant branch (RGB).
Most of the energy required to drive the outflowing gas was probably released
when material fell towards the companion.Comment: Accepted for publication in A&
Study of the inner dust envelope and stellar photosphere of the AGB star R Doradus using SPHERE/ZIMPOL
We use high-angular-resolution images obtained with SPHERE/ZIMPOL to study
the photosphere, the warm molecular layer, and the inner wind of the close-by
oxygen-rich AGB star R Doradus. We present observations in filters V,
cntH, and cnt820 and investigate the surface brightness distribution of
the star and of the polarised light produced in the inner envelope. Thanks to
second-epoch observations in cntH, we are able to see variability on
the stellar photosphere. We find that in the first epoch the surface brightness
of R Dor is asymmetric in V and cntH, the filters where molecular
opacity is stronger, while in cnt820 the surface brightness is closer to being
axisymmetric. The second-epoch observations in cntH show that the
morphology of R Dor changes completely in a timespan of 48 days to a more
axisymmetric and compact configuration. The polarised intensity is asymmetric
in all epochs and varies by between a factor of 2.3 and 3.7 with azimuth for
the different images. We fit the radial profile of the polarised intensity
using a spherically symmetric model and a parametric description of the dust
density profile, . On average, we find exponents of
that correspond to a much steeper density profile than that of
a wind expanding at constant velocity. The dust densities we derive imply an
upper limit for the dust-to-gas ratio of at 5.0
. Given the uncertainties in observations and models, this value is
consistent with the minimum values required by wind-driving models for the
onset of a wind, of . However, if the steep density
profile we find extends to larger distances from the star, the dust-to-gas
ratio will quickly become too small for the wind of R Dor to be driven by the
grains that produce the scattered light.Comment: 10 pages, 8 figures, 4 table
Anisotropic and strong negative magneto-resistance in the three-dimensional topological insulator Bi2Se3
We report on high-field angle-dependent magneto-transport measurements on
epitaxial thin films of Bi2Se3, a three-dimensional topological insulator. At
low temperature, we observe quantum oscillations that demonstrate the
simultaneous presence of bulk and surface carriers. The magneto- resistance of
Bi2Se3 is found to be highly anisotropic. In the presence of a parallel
electric and magnetic field, we observe a strong negative longitudinal
magneto-resistance that has been consid- ered as a smoking-gun for the presence
of chiral fermions in a certain class of semi-metals due to the so-called axial
anomaly. Its observation in a three-dimensional topological insulator implies
that the axial anomaly may be in fact a far more generic phenomenon than
originally thought.Comment: 6 pages, 4 figure
Processing of the papain precursor. Purification of the zymogen and characterization of its mechanism of processing.
The precursor of the cysteine protease papain has been expressed and secreted as propapain from insect cells infected with a recombinant baculovirus expressing a synthetic gene coding for prepropapain. This 39-kDa secreted propapain zymogen molecule is glycosylated and can be processed in vitro into an enzymatically active authentic papain molecule of 24.5 kDa (Vernet, T., Tessier, D.C., Richardson, C., Laliberte, F., Khouri, H. E., Bell, A. W., Storer, A. C., and Thomas, D. Y. (1990) J. Biol. Chem. 265, 16661-16666). Recombinant propapain was stabilized with Hg2+ and purified to homogeneity using affinity chromatography, gel filtration, and ion-exchange chromatographic procedures. The maximum rate of processing in vitro was achieved at approximately pH 4.0, at a temperature of 65 degrees C and under reducing conditions. Precursor processing is inhibited by a variety of reversible and irreversible cysteine protease inhibitors but not by specific inhibitors of serine, metallo or acid proteases. Replacement by site-directed mutagenesis of the active site cysteine with a serine at position 25 also prevents processing. The inhibitor 125I-N-(2S,3S)-3-trans-hydroxycarbonyloxiran-2-carbonyl-L-tyrosine benzyl ester covalently labeled the wild type papain precursor, but not the C25S mutant, indicating that the active site is accessible to the inhibitor and is in a native conformation within the precursor. Based on biochemical and kinetic analyses of the activation and processing of propapain we have shown that the papain precursor is capable of autoproteolytic cleavage (intramolecular). Once free papain is released processing can then occur in trans (intermolecular)
Secretion of functional papain precursor from insect cells. Requirement for N-glycosylation of the pro-region.
The synthetic gene coding for the precursor of the cysteine protease papain (EC 3.4.22.2) has been expressed using the baculovirus/insect cell system. The prepropapain gene was cloned into the transfer vector IpDC125 behind the polyhedrin promoter. The recombinant construct was then incorporated by homologous recombination into the Autographa californiaca nuclear polyhedrosis virus genome. The host Spodoptera frugiperda Sf9 cells infected with the recombinant baculovirus secrete an enzymatically inactive N-glycosylated papain precursor. This zymogen could be activated in vitro to yield about 400 nmol of active papain per liter of culture. The recombinant active mature papain was enzymatically indistinguishable from natural papain but the precursor was not processed to the same amino acid residue. The insect cells also accumulated prepropapain and glycosylated propapain intracellularly. This accumulation was an indication that there are rate-limiting steps in the secretion of proteins from insect cells in this expression system. Characterization of mutants of the precursor has shown that entry into the secretory pathway and addition of carbohydrate are prerequisite conditions for the production and secretion of functional propapain
VLTI/PIONIER reveals the close environment of the evolved system HD101584
Context: The observed orbital characteristics of post-asymptotic giant branch
(post-AGB) and post-red giant branch (post-RGB) binaries are not understood. We
suspect that the missing ingredients to explain them probably lie in the
continuous interaction of the central binary with its circumstellar
environment. Aims: We aim at studying the circumbinary material in these
complex systems by investigating the connection between the innermost and
large-scale structures. Methods: We perform high-angular resolution
observations in the near-infrared continuum of HD101584, which has a complex
structure as seen at millimeter wavelengths with a disk-like morphology and a
bipolar outflow due to an episode of strong binary interaction. To account for
the complexity of the target we first perform an image reconstruction and use
this result to fit a geometrical model to extract the morphological and thermal
features of the environment. Results: The image reveals an unexpected
double-ring structure. We interpret the inner ring to be produced by emission
from dust located in the plane of the disk and the outer ring to be produced by
emission from dust that is located 1.6[D/1kpc] au above the disk plane. The
inner ring diameter (3.94[D/1kpc] au), and temperature (T=154010K) are
compatible with the dust sublimation front of the disk. The origin of the
out-of-plane ring (with a diameter of 7.39[D/1kpc] au and a temperature of
1014K) could be due to episodic ejection or a dust condensation front in
the outflow. Conclusion: The observed outer ring is possibly linked with the
blue-shifted side of the large scale outflow seen by ALMA and is tracing its
launching location to the central star. Such observations give morphological
constraints on the ejection mechanism. Additional observations are needed to
constrain the origin of the out-of-plane structure.Comment: Accepted to A&A. 14 pages, 13 figure
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