810 research outputs found
Resolving the extended stellar atmospheres of Asymptotic Giant Branch stars at (sub-)millimetre wavelengths
The initial conditions for the mass loss during the asymptotic giant branch
(AGB) phase are set in their extended atmospheres, where, among others,
convection and pulsation driven shocks determine the physical conditions. High
resolution observations of AGB stars at (sub)millimetre wavelengths can now
directly determine the morphology, activity, density, and temperature close to
the stellar photosphere. We used the Atacama Large Millimeter/submillimeter
Array (ALMA) high angular resolution observations to resolve the extended
atmospheres of four of the nearest AGB stars: W Hya, Mira A, R Dor and R Leo.
We interpreted the observations using a parameterised atmosphere model. We
resolve all four AGB stars and determine the brightness temperature structure
between and stellar radii. For W Hya and R Dor we confirm the existence
of hotspots with brightness temperatures to ~K. All four stars
show deviations from spherical symmetry. We find variations on a timescale of
days to weeks, and for R Leo we directly measure an outward motion of the
millimetre wavelength surface with a velocity of at least
~km~s. For all objects but W Hya we find that the
temperature-radius and size-frequency relations require the existence of a
(likely inhomogeneous) layer of enhanced opacity. The ALMA observations provide
a unique probe of the structure of the extended AGB atmosphere. We find highly
variable structures of hotspots and likely convective cells. In the future,
these observations can be directly compared to multi-dimensional chromosphere
and atmosphere models that determine the temperature, density, velocity, and
ionisation structure between the stellar photosphere and the dust formation
region. However, our results show that for the best interpretation, both very
accurate flux calibration and near-simultaneous observations are essential.Comment: 18 pages, 13 figures, Accepted to A&A, final version after language
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Pattern matching techniques applied to human computer interfaces for virtual environments applications
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
Influence of additives on decrease of temperature of slag flow from energy coal in wet bottom boiler
This paper describes the features of the energy coal combusted in a power plant, its impact on energy production, while the possibility of using natural and secondary raw materials to modify the properties of energy coal. All selected types of additives (fluorspar, de-metallized steelmaking slag (DSS), dolomite, and limestone) in admixture of coal have clearly proved their ability to reduce the pour point of the ash in the laboratory experiments. The highest decrease of the temperature at 5 % of the additive was achieved by fluorspar and dolomite from the temperature of 1 593 °C to the temperature of 1 307 °C. In terms of the economy and the availability of the additives the most suitable seems to be DSS
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&
Modelling of copper and zinc adsorption onto zeolite
Adsorption of Cu(II) and Zn(II) ions from metallurgical solutions has been studied and the adsorption capacity of zeolite (Nižný Hrabovec, SK) has been determined. Zeolites are characterized by relatively high sorption capacity, i.e. Cu(II) and Zn(II) can be removed even at relatively low concentrations. The experiments were realised in a batch system and evaluated using isotherms. According to the results of the experiments the adsorption equilibrium of Cu(II) and Zn(II) on zeolite was best described by Freundlich isotherm. The maximum sorption capacity was 1,48 and 1,49 mg/g for Cu(II) and Zn(II), respectively. The experimental results of this study demonstrate that zeolite is suitable for adsorption of copper and zinc from aqueous solutions at low concentrations
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