579 research outputs found
The abundance of HCN in circumstellar envelopes of AGB stars of different chemical types
A multi-transition survey of HCN (sub-) millimeter line emission from a large
sample of AGB stars of different chemical type is presented. The data are
analysed and circumstellar HCN abundances are estimated. The sample stars span
a large range of properties such as mass-loss rate and photospheric C/O-ratio.
The analysis of the new data allows for more accurate estimates of the
circumstellar HCN abundances and puts new constraints on chemical models. In
order to constrain the circumstellar HCN abundance distribution a detailed
non-LTE excitation analysis, based on the Monte Carlo method, is performed.
Effects of line overlaps and radiative excitation from dust grains are
included. The median values for the derived abundances of HCN (with respect to
H2) are 3x10-5, 7x10-7 and 10-7 for carbon stars (25 stars), S-type AGB stars
(19 stars) and M-type AGB stars (25 stars), respectively. The estimated sizes
of the HCN envelopes are similar to those obtained in the case of SiO for the
same sample of sources and agree well with previous results from
interferometric observations, when these are available. We find that there is a
clear dependence of the derived circumstellar HCN abundance on the C/O-ratio of
the star, in that carbon stars have about two orders of magnitude higher
abundances than M-type AGB stars, on average. The derived HCN abundances of the
S-type AGB stars have a larger spread and typically fall in between those of
the two other types, however, slightly closer to the values for the M-type AGB
stars. For the M-type stars, the estimated abundances are much higher than what
would be expected if HCN is formed in thermal equilibrium. However, the results
are also in contrast to predictions from recent non-LTE chemical models, where
very little difference is expected in the HCN abundances between the various
types of AGB stars.Comment: Accepted for publication in A&
Interactions of Heavy Hadrons using Regge Phenomenology and the Quark Gluon String Model
The search for stable heavy exotic hadrons is a promising way to observe new
physics processes at collider experiments. The discovery potential for such
particles can be enhanced or suppressed by their interactions with detector
material. This paper describes a model for the interactions in matter of stable
hadrons containing an exotic quark of charges or
using Regge phenomenology and the Quark Gluon String Model. The influence of
such interactions on searches at the LHC is also discussed
The detached dust and gas shells around the carbon star U Ant
Context: Geometrically thin, detached shells of gas have been found around a
handful of carbon stars. --Aims: Previous observations of scattered stellar
light in the circumstellar medium around the carbon star U Ant were taken
through filters centred on the resonance lines of K and Na. These observations
could not separate the scattering by dust and atoms. The aim of this paper is
to remedy this situation. --Methods: We have obtained polarization data on
stellar light scattered in the circumstellar medium around U Ant through
filters which contain no strong lines, making it possible to differentiate
between the two scattering agents. Kinematic, as well as spatial, information
on the gas shells were obtained through high-resolution echelle spectrograph
observations of the KI and NaD lines. --Results: We confirm the existence of
two detached shells around U Ant. The inner shell (at a radius of approx 43"
and a width of approx 2") consists mainly of gas, while the outer shell (at a
radius of approx 50" and a width of approx 7") appears to consist exclusively
of dust. Both shells appear to have an over-all spherical geometry. The gas
shell mass is estimated to be 2x10^-3 M(Sun), while the mass of the dust shell
is estimated to be 5x10^-5 M(Sun). The derived expansion velocity, from the KI
and NaD lines, of the gas shell, 19.5 km/s, agrees with that obtained from CO
radio line data. The inferred shell age is 2700 years. There is structure, e.g.
in the form of arcs, inside the gas shell, but it is not clear whether these
are due to additional shells. --Conclusions: Our results support the hypothesis
that the observed geometrically thin, detached shells around carbon stars are
the results of brief periods of intense mass loss, probably associated with
thermal pulses, and subsequent wind-wind interactions
The ATLAS tile calorimeter digitizer
The ATLAS Tile Calorimeter digitizer system samples photomultiplier signals from the scintillating tiles of the hadronic calorimeter. For each channel a pair of 10-bit ADCs digitize high and low gain signals at 40.08 MHz to provide the necessary 16-bit dynamic range. The sampled data is temporarily stored in digital pipelines for up to 6.375 \muÂs, awaiting a level-1 accept. For each accept received, the corresponding sampled pulse is transferred to a derandomizer buffer for subsequent readout to the data acquisition system (DAQ). The main functionality of the digitizer is implemented in radiation tolerant ASICs, using a fault tolerant architecture to minimize the consequences of radiation induced faults
Circumstellar water vapour in M-type AGB stars: Constraints from H2O(1_10 - 1_01) lines obtained with Odin
Aims: Spectrally resolved circumstellar H2O(1_10 - 1_01) lines have been
obtained towards three M-type AGB stars using the Odin satellite. This provides
additional strong constrains on the properties of circumstellar H2O and the
circumstellar envelope. Methods: ISO and Odin satellite H2O line data are used
as constraints for radiative transfer models. Special consideration is taken to
the spectrally resolved Odin line profiles, and the effect of excitation to the
first excited vibrational states of the stretching modes (nu1=1 and nu3=1) on
the derived abundances is estimated. A non-local, radiative transfer code based
on the ALI formalism is used. Results: The H2O abundance estimates are in
agreement with previous estimates. The inclusion of the Odin data sets stronger
constraints on the size of the H2O envelope. The H2O(1_10 - 1_01) line profiles
require a significant reduction in expansion velocity compared to the terminal
gas expansion velocity determined in models of CO radio line emission,
indicating that the H2O emission lines probe a region where the wind is still
being accelerated. Including the nu3=1 state significantly lowers the estimated
abundances for the low-mass-loss-rate objects. This shows the importance of
detailed modelling, in particular the details of the infrared spectrum in the
range 3 to 6 micron, to estimate accurate circumstellar H2O abundances.
Conclusions: Spectrally resolved circumstellar H2O emission lines are important
probes of the physics and chemistry in the inner regions of circumstellar
envelopes around asymptotic giant branch stars. Predictions for H2O emission
lines in the spectral range of the upcoming Herschel/HIFI mission indicate that
these observations will be very important in this context.Comment: accepted in A&A, 10 pages, 8 figure
The Herschel Planetary Nebula Survey (HerPlaNS) I. Data Overview and Analysis Demonstration with NGC 6781
This is the first of a series of investigations into far-IR characteristics
of 11 planetary nebulae (PNs) under the Herschel Space Observatory Open Time 1
program, Herschel Planetary Nebula Survey (HerPlaNS). Using the HerPlaNS data
set, we look into the PN energetics and variations of the physical conditions
within the target nebulae. In the present work, we provide an overview of the
survey, data acquisition and processing, and resulting data products. We
perform (1) PACS/SPIRE broadband imaging to determine the spatial distribution
of the cold dust component in the target PNs and (2) PACS/SPIRE
spectral-energy-distribution (SED) and line spectroscopy to determine the
spatial distribution of the gas component in the target PNs. For the case of
NGC 6781, the broadband maps confirm the nearly pole-on barrel structure of the
amorphous carbon-richdust shell and the surrounding halo having temperatures of
26-40 K. The PACS/SPIRE multi-position spectra show spatial variations of
far-IR lines that reflect the physical stratification of the nebula. We
demonstrate that spatially-resolved far-IR line diagnostics yield the (T_e,
n_e) profiles, from which distributions of ionized, atomic, and molecular gases
can be determined. Direct comparison of the dust and gas column mass maps
constrained by the HerPlaNS data allows to construct an empirical gas-to-dust
mass ratio map, which shows a range of ratios with the median of 195+-110. The
present analysis yields estimates of the total mass of the shell to be 0.86
M_sun, consisting of 0.54 M_sun of ionized gas, 0.12 M_sun of atomic gas, 0.2
M_sun of molecular gas, and 4 x 10^-3 M_sun of dust grains. These estimates
also suggest that the central star of about 1.5 M_sun initial mass is
terminating its PN evolution onto the white dwarf cooling track.Comment: 27 pages, 16 figures, accepted for publication in A&
Synthetic photometry for carbon-rich giants II. The effects of pulsation and circumstellar dust
By using self-consistent dynamic model atmospheres which simulate
pulsation-enhanced dust-driven winds of AGB stars we studied in detail the
influence of (i) pulsations of the stellar interiors, and (ii) the development
of dusty stellar winds on the spectral appearance of long period variables with
carbon-rich atmospheric chemistry. While the pulsations lead to large-amplitude
photometric variability, the dusty envelopes cause pronounced circumstellar
reddening. Based on one selected dynamical model which is representative of
C-type Mira variables with intermediate mass loss rates, we calculated
synthetic spectra and photometry for standard broad-band filters from the
visual to the near-infrared. Our modelling allows to investigate in detail the
substantial effect of circumstellar dust on the resultant photometry. The
pronounced absorption of amorphous carbon dust grains leads to colour indices
which are significantly redder than the corresponding ones based on hydrostatic
dust-free models. Only if we account for this circumstellar reddening we get
synthetic colours that are comparable to observations of evolved AGB stars. The
photometric variations of the dynamical model were compared to observed
lightcurves of the C-type Mira RU_Vir which appears to be quite similar to the
model. We found good agreement concerning the principal behaviour of the
BVRIJHKL lightcurves and also quantitatively fitting details. The analysed
model is able to reproduce the variations of RU_Vir and other Miras in (J-H)
vs. (H-K) diagrams throughout the light cycle. Contrasting the model photometry
with observational data for a variety of galactic C-rich giants in such
colour-colour diagrams proved that the chosen atmospheric model fits well into
a sequence of objects with increasing mass loss rates, i.e., redder colour
indices.Comment: Accepted for publication in A&
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