1,147 research outputs found
The Hottest Horizontal-Branch Stars in omega Centauri - Late Hot Flasher vs. Helium Enrichment
UV observations of some massive globular clusters uncovered a significant
population of very hot stars below the hot end of the horizontal branch (HB),
the so-called blue hook stars. This feature might be explained either as
results of the late hot flasher scenario where stars experience the helium
flash while on the white dwarf cooling curve or by the progeny of the
helium-enriched sub-population recently postulated to exist in some clusters.
Moderately high resolution spectra of stars at the hot end of the blue HB in
omega Cen were analysed for atmospheric parameters and abundances using LTE and
Non-LTE model atmospheres. In the temperature range 30,000K to 50,000K we find
that 35% of our stars are helium-poor (log(n_He/n_H) < -2), 51% have solar
helium abundance within a factor of 3 (-1.5 <= log(n_He/n_H) <= -0.5) and 14%
are helium-rich (log(n_He/n_H)> -0.4). We also find carbon enrichment in step
with helium enrichment, with a maximum carbon enrichment of 3% by mass. At
least 14% of the hottest HB stars in omega Cen show helium abundances well
above the highest predictions from the helium enrichment scenario (Y = 0.42
corresponding to log(n_He/n_H) ~ -0.74). In addition, the most helium-rich
stars show strong carbon enrichment as predicted by the late hot flasher
scenario. We conclude that the helium-rich HB stars in omega Cen cannot be
explained solely by the helium-enrichment scenario invoked to explain the blue
main sequence. (Abridged)Comment: 4 pages, 3 figures, uses aa.cls (enclosed), accepted as A&A Lette
On The Importance Of The Interclump Medium For Superionization: O VI Formation In The Wind Of Zeta Puppis
We have studied superionization and X-ray line formation in the spectra of zeta Pup using our new stellar atmosphere code (XCMFGEN) that can be used to simultaneously analyze optical, UV, and X-ray observations. Here, we present results on the formation of the O VI lambda lambda 1032, 1038 doublet. Our simulations, supported by simple theoretical calculations, show that clumped wind models that assume void in the interclump space cannot reproduce the observed O vi profiles. However, enough O vi can be produced if the voids are filled by a low-density gas. The recombination of O vi is very efficient in the dense material, but in the tenuous interclump region an observable amount of O vi can be maintained. We also find that different UV resonance lines are sensitive to different density regimes in z Pup: C IV is almost exclusively formed within the densest regions, while the majority of O vi resides between clumps. N v is an intermediate case, with contributions from both the tenuous gas and clumps
Studying the evolution of galaxies in compact groups over the past 3 Gyr - II. The importance of environment in the suppression of star formation
We present an in depth study on the evolution of galaxy properties in compact
groups over the past 3 Gyr. We are using the largest multi-wavelength sample
to-date, comprised 1770 groups (containing 7417 galaxies), in the redshift
range of 0.01<z<0.23. To derive the physical properties of the galaxies we rely
on ultraviolet (UV)-to-infrared spectral energy distribution modeling, using
CIGALE. Our results suggest that during the 3 Gyr period covered by our sample,
the star formation activity of galaxies in our groups has been substantially
reduced (3-10 times). Moreover, their star formation histories as well as their
UV-optical and mid-infrared colors are significantly different from those of
field and cluster galaxies, indicating that compact group galaxies spend more
time transitioning through the green valley. The morphological transformation
from late-type spirals into early-type galaxies occurs in the mid-infrared
transition zone rather than in the UV-optical green valley. We find evidence of
shocks in the emission line ratios and gas velocity dispersions of the
late-type galaxies located below the star forming main sequence. Our results
suggest that in addition to gas stripping, turbulence and shocks might play an
important role in suppressing the star formation in compact group galaxies.Comment: (Accepted for publication in MNRAS, date of submission November 18,
2015
Properties of Galactic early-type O-supergiants: A combined FUV-UV and optical analysis
We aim to constrain the properties and evolutionary status of early and
mid-spectral type supergiants (from O4 to O7.5). These posses the highest
mass-loss rates among the O stars, and exhibit conspicuous wind profiles. Using
the non-LTE wind code CMFGEN, we simultaneously analyzed the FUV-UV and optical
spectral range to determine the photospheric properties and wind parameters. We
derived effective temperatures, luminosities, surface gravities, surface
abundances, mass-loss rates, wind terminal velocities, and clumping filling
factors. The supergiants define a very clear evolutionary sequence, in terms of
ages and masses, from younger and more massive stars to older stars with lower
initial masses. O4 supergiants cluster around the 3 Myr isochrone and are more
massive than 60 Msun, while the O5 to O7.5 stars have masses in the range 50 -
40 Msun and are 4 +/- 0.3 Myr old. The surface chemical composition is typical
of evolved O supergiants (nitrogen-rich, carbon- and oxygen-poor). While the
observed ranges of carbon and nitrogen mass-fractions are compatible with those
expected from evolutionary models for the measured stellar masses, the N/C
ratios as a function of age are inconsistent with the theoretical predictions
for the four earliest (O4 spectral type) stars of the sample. We question the
efficiency of rotational mixing as a function of age for these stars and
suggest that another mechanism may be needed to explain the observed abundance
patterns. Mass-loss rates derived with clumped-models range within a factor of
three of the theoretical mass-loss rates. The corresponding volume-filling
factors associated with small-scale clumping are 0.05 +/- 0.02. Clumping is
found to start close to the photosphere for all but three stars, two of which
are fast rotators.Comment: 30 pages, 11 figures, accepted for publication in A&A 201
Radial abundance gradients in the outer Galactic disk as traced by main-sequence OB stars
Using a sample of 31 main-sequence OB stars located between galactocentric
distances 8.4 - 15.6 kpc, we aim to probe the present-day radial abundance
gradients of the Galactic disk. The analysis is based on high-resolution
spectra obtained with the MIKE spectrograph on the Magellan Clay 6.5-m
telescope on Las Campanas. We used a non-NLTE analysis in a self-consistent
semi-automatic routine based on TLUSTY and SYNSPEC to determine atmospheric
parameters and chemical abundances. Stellar parameters (effective temperature,
surface gravity, projected rotational velocity, microturbulence, and
macroturbulence) and silicon and oxygen abundances are presented for 28 stars
located beyond 9 kpc from the Galactic centre plus three stars in the solar
neighborhood. The stars of our sample are mostly on the main-sequence, with
effective temperatures between 20800 - 31300 K, and surface gravities between
3.23 - 4.45 dex. The radial oxygen and silicon abundance gradients are negative
and have slopes of -0.07 dex/kpc and -0.09 dex/kpc, respectively, in the region
\,kpc. The obtained gradients are compatible with the
present-day oxygen and silicon abundances measured in the solar neighborhood
and are consistent with radial metallicity gradients predicted by
chemodynamical models of Galaxy Evolution for a subsample of young stars
located close to the Galactic plane.Comment: Accepted for publication in the A&
Clinical Processes - The Killer Application for Constraint-Based Process Interactions?
For more than a decade, the interest in aligning information
systems in a process-oriented way has been increasing. To enable operational
support for business processes, the latter are usually specified in
an imperative way. The resulting process models, however, tend to be too
rigid to meet the flexibility demands of the actors involved. Declarative
process modeling languages, in turn, provide a promising alternative in
scenarios in which a high level of flexibility is demanded. In the scientific
literature, declarative languages have been used for modeling rather simple
processes or synthetic examples. However, to the best of our knowledge,
they have not been used to model complex, real-world scenarios
that comprise constraints going beyond control-flow. In this paper, we
propose the use of a declarative language for modeling a sophisticated
healthcare process scenario from the real world. The scenario is subject to
complex temporal constraints and entails the need for coordinating the
constraint-based interactions among the processes related to a patient
treatment process. As demonstrated in this work, the selected real process
scenario can be suitably modeled through a declarative approach.Ministerio de Economía y Competitividad TIN2016-76956-C3-2-RMinisterio de Economía y Competitividad TIN2015-71938-RED
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