1,408 research outputs found
Stellar populations in the Galactic bulge
AIMS:The aim of this paper is to study the characteristics of the stellar
populations and the metallicity distribution in the Galactic bulge. We study
the entire stellar population, but also retrieve information using only the red
clump stars. METHODS: To study the characteristics of the stellar populations
and the metallicity distribution in the Galactic bulge, we compared the output
of the galaxy model TRILEGAL, which implements the Binney et al. (1997) bulge
model, with observations from 2MASS and OGLE-II. A minimisation procedure has
been set up to retrieve the best fitting model with different stellar
populations and metallicity distributions. RESULTS: Using the TRILEGAL code we
find that the best model resembling the characteristics of the Galactic bulge
is a model with the distance to the Galactic centre kpc, the major axis ratios of the bar , and the angle between the
Sun-centre line and the bar . Using these
parameters the best model is found for a burst of 8 Gyr, although it is almost
indistinguishable from models with ages of 9 and 10 Gyr. The metallicity
distribution found is consistent with metallicity distributions in the
literature based on spectroscopic results.Comment: A&A accepte
Dynamic System Adaptation by Constraint Orchestration
For Paradigm models, evolution is just-in-time specified coordination
conducted by a special reusable component McPal. Evolution can be treated
consistently and on-the-fly through Paradigm's constraint orchestration, also
for originally unforeseen evolution. UML-like diagrams visually supplement such
migration, as is illustrated for the case of a critical section solution
evolving into a pipeline architecture.Comment: 19 page
PACS and SPIRE range spectroscopy of cool, evolved stars
Context: At the end of their lives AGB stars are prolific producers of dust
and gas. The details of this mass-loss process are still not understood very
well. Herschel PACS and SPIRE spectra offer a unique way of investigating
properties of AGB stars in general and the mass-loss process in particular.
Methods: The HIPE software with the latest calibration is used to process the
available PACS and SPIRE spectra of 40 evolved stars. The spectra are convolved
with the response curves of the PACS and SPIRE bolometers and compared to the
fluxes measured in imaging data of these sources. Custom software is used to
identify lines in the spectra, and to determine the central wavelengths and
line intensities. Standard molecular line databases are used to associate the
observed lines. Because of the limited spectral resolution of the spectrometers
several known lines are typically potential counterparts to any observed line.
To help identifications the relative contributions in line intensity of the
potential counterpart lines are listed for three characteristic temperatures
based on LTE calculations and assuming optically thin emission. Result: The
following data products are released: the reduced spectra, the lines that are
measured in the spectra with wavelength, intensity, potential identifications,
and the continuum spectra, i.e. the full spectra with all identified lines
removed. As simple examples of how this data can be used in future studies we
have fitted the continuum spectra with three power laws and find that the few
OH/IR stars seem to have significantly steeper slopes than the other oxygen-
and carbon-rich objects in the sample. As another example we constructed
rotational diagrams for CO and fitted a two-component model to derive
rotational temperatures.Comment: A&A accepte
A purely geometric distance to the binary star Atlas, a member of the Pleiades
We present radial velocity and new interferometric measurements of the double
star Atlas, which permit, with the addition of published interferometric data,
to precisely derive the orbital parameters of the binary system and the masses
of the components. The derived semi-major axis, compared with its measured
angular size, allows to determine a distance to Atlas of 132+-4 pc in a purely
geometrical way. Under the assumption that the location of Atlas is
representative of the average distance of the cluster, we confirm the distance
value generally obtained through main sequence fitting, in contradiction with
the early Hipparcos result (118.3+-3.5 pc).Comment: 5 pages, 3 figures, accepted for publication in A&A Letter
Exploring the causes of adverse events in hospitals and potential prevention strategies
Objectives
To examine the causes of adverse events
(AEs) and potential prevention strategies to minimise the
occurrence of AEs in hospitalised patients.
Methods
For the 744 AEs identified in the patient record
review study in 21 Dutch hospitals, trained reviewers
were asked to select all causal factors that contributed
to the AE. The results were analysed together with data
on preventability and consequences of AEs. In addition,
the reviewers selected one or more prevention strategies
for each preventable AE. The recommended prevention
strategies were analysed together with four general
causal categories: technical, human, organisational and
patient-related factors.
Results
Human causes were predominantly involved in
the causation of AEs (in 61% of the AEs), 61% of those
being preventable and 13% leading to permanent
disability. In 39% of the AEs, patient-related factors were
involved, in 14% organisational factors and in 4%
technical factors. Organisational causes contributed
relatively often to preventable AEs (93%) and AEs
resulting in permanent disability (20%). Recommended
strategies to prevent AEs were quality assurance/peer
review, evaluation of safety behaviour, training and
procedures. For the AEs with human and patient-related
causes, reviewers predominantly recommended quality
assurance/peer review. AEs caused by organisational
factors were considered preventable by improving
procedures.
Discussion
Healthcare interventions directed at human
causes are recommended because these play a large
role in AE causation. In addition, it seems worthwhile to
direct interventions on organisational causes because the
AEs they cause are nearly always believed to be
preventable. Organisational factors are thus relatively
easy to tackle. Future research designs should allow
researchers to interview healthcare providers that were
involved in the event, as an additional source of
information on contributing factors.
Circumstellar molecular line emission from S-type AGB stars: Mass-loss rates and SiO abundances
The main aim is to derive reliable mass-loss rates and circumstellar SiO
abundances for a sample of 40 S-type AGB stars based on new multi-transitional
CO and SiO radio line observations. In addition, the results are compared to
previous results for M-type AGB stars and carbon stars to look for trends with
chemical type. The circumstellar envelopes are assumed to be spherically
symmetric and formed by a constant mass-loss rate. The mass-loss rates are
estimated from fitting the CO observations using a non-local, non-LTE radiative
transfer code. Once the physical properties of the circumstellar envelopes are
determined, the same radiative transfer code is used to model the observed SiO
lines in order to derive circumstellar abundances and the sizes of the SiO
line-emitting regions. We have estimated mass-loss rates of 40 S-type AGB stars
and find that the derived mass-loss rates have a distribution that resembles
those previously derived for similar samples of M-type AGB stars and carbon
stars. The estimated mass-loss rates also correlate well with the corresponding
expansion velocity. In all, this indicates that the mass loss is driven by the
same mechanism in all three chemical types of AGB stars. In addition, we have
estimated the circumstellar fractional abundance of SiO relative to H2 in 26 of
the sample S-type AGB stars. The derived SiO abundances are, on average, about
an order of magnitude higher than predicted by stellar atmosphere thermal
equilibrium chemistry, indicating that non-equilibrium chemical processes
determines the abundance of SiO in the circumstellar envelope. Moreover, a
comparison with the results for M-type AGB stars and carbon stars show that for
a certain mass-loss rate, the circumstellar SiO abundance seems independent
(although with a large scatter) of the C/O-ratio.Comment: 24 pages, 11 figure
De ontwikkeling op potplantenbedrijven in de periode 1970 - 1974 in de centra Aalsmeer, Lent en het Zuidhollands glasdistrict
The distribution of H13CN in the circumstellar envelope around IRC+10216
H13CN J=8-7 sub-millimetre line emission produced in the circumstellar
envelope around the extreme carbon star IRC+10216 has been imaged at
sub-arcsecond angular resolution using the SMA. Supplemented by a detailed
excitation analysis the average fractional abundance of H13CN in the inner wind
(< 5E15 cm) is estimated to be about 4E-7, translating into a total HCN
fractional abundance of 2E-5 using the isotopic ratio 12C/13C=50.
Multi-transitional single-dish observations further requires the H13CN
fractional abundance to remain more or less constant in the envelope out to a
radius of about 4E16 cm, where the HCN molecules are effectively destroyed,
most probably, by photodissociation. The large amount of HCN present in the
inner wind provides effective line cooling that can dominate over that
generated from CO line emission. It is also shown that great care needs to be
taken in the radiative transfer modelling where non-local, and non-LTE, effects
are important and where the radiation field from thermal dust grains plays a
major role in exciting the HCN molecules. The amount of HCN present in the
circumstellar envelope around IRC+10216 is consistent with predicted
photospheric values based on equilibrium chemical models and indicates that any
non-equilibrium chemistry occurring in the extended pulsating atmosphere has no
drastic net effect on the fractional abundance of HCN molecules that enters the
outer envelope. It further suggests that few HCN molecules are incorporated
into dust grains.Comment: Accepted for publication in ApJ. 20 pages, 7 figure
The ALMA detection of CO rotational line emission in AGB stars in the Large Magellanic Cloud
Context. Low- and intermediate-mass stars lose most of their stellar mass at the end of their lives on the asymptotic giant branch (AGB). Determining gas and dust mass-loss rates (MLRs) is important in quantifying the contribution of evolved stars to the enrichment of the interstellar medium.
Aims: This study attempts to spectrally resolve CO thermal line emission in a small sample of AGB stars in the Large Magellanic Cloud (LMC).
Methods: The Atacama Large Millimeter Array was used to observe two OH/IR stars and four carbon stars in the LMC in the CO J = 2-1 line.
Results: We present the first measurement of expansion velocities in extragalactic carbon stars. All four C stars are detected and wind expansion velocities and stellar velocities are directly measured. Mass-loss rates are derived from modelling the spectral energy distribution and Spitzer/IRS spectrum with the DUSTY code. The derived gas-to-dust ratios allow the predicted velocities to agree with the observed gas-to-dust ratios. The expansion velocities and MLRs are compared to a Galactic sample of well-studied relatively low MLRs stars supplemented with extreme C stars with properties that are more similar to the LMC targets. Gas MLRs derived from a simple formula are significantly smaller than those derived from dust modelling, indicating an order of magnitude underestimate of the estimated CO abundance, time-variable mass loss, or that the CO intensities in LMC stars are lower than predicted by the formula derived for Galactic objects. This could be related to a stronger interstellar radiation field in the LMC.
Conclusions: Although the LMC sample is small and the comparison to Galactic stars is non-trivial because of uncertainties in their distances (hence luminosities), it appears that for C stars the wind expansion velocities in the LMC are lower than in the solar neighbourhood, while the MLRs appear to be similar. This is in agreement with dynamical dust-driven wind models
Pulsating stars in the VMC survey
The VISTA survey of the Magellanic Clouds system (VMC) began observations in
2009 and since then, it has collected multi-epoch data at Ks and in addition
multi-band data in Y and J for a wide range of stellar populations across the
Magellanic system. Among them are pulsating variable stars: Cepheids, RR Lyrae,
and asymptotic giant branch stars that represent useful tracers of the host
system geometry.Comment: 8 pages, 7 figures, proceeding contribution of invited presentation
at "Wide-field variability surveys: a 21st-century perspective", San Pedro de
Atacama (Chile
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