12,306 research outputs found
The IACOB project: A grid-based automatic tool for the quantitative spectroscopic analysis of O-stars
We present the IACOB grid-based automatic tool for the quantitative
spectroscopic analysis of O-stars. The tool consists of an extensive grid of
FASTWIND models, and a variety of programs implemented in IDL to handle the
observations, perform the automatic analysis, and visualize the results. The
tool provides a fast and objective way to determine the stellar parameters and
the associated uncertainties of large samples of O-type stars within a
reasonable computational time.Comment: 8 pages, 2 figures, 1 table. Proceedings of the "GREAT-ESF Stellar
Atmospheres in the Gaia Era Workshop
Spectroscopic and physical parameters of Galactic O-type stars. II. Observational constraints on projected rotational and extra broadening velocities as a function of fundamental parameters and stellar evolution
Rotation is of key importance for the evolution of hot massive stars,
however, the rotational velocities of these stars are difficult to determine.
Based on our own data for 31 Galactic O stars and incorporating similar data
for 86 OB supergiants from the literature, we aim at investigating the
properties of rotational and extra line-broadening as a function of stellar
parameters and at testing model predictions about the evolution of stellar
rotation. Fundamental stellar parameters were determined by means of the code
FASTWIND. Projected rotational and extra broadening velocities originate from a
combined Ft + GOF method. Model calculations published previously were used to
estimate the initial evolutionary masses. The sample O stars with Minit > 50
Msun rotate with less that 26% of their break-up velocity, and they also lack
objects with v sin i 35 Msun on the
hotter side of the bi-stability jump, the observed and predicted rotational
rates agree quite well; for those on the cooler side of the jump, the measured
velocities are systematically higher than the predicted ones. In general, the
derived extra broadening velocities decrease toward cooler Teff, whilst for
later evolutionary phases they appear, at the same v sin i, higher for
high-mass stars than for low-mass ones. None of the sample stars shows extra
broadening velocities higher than 110 km/s. For the majority of the more
massive stars, extra broadening either dominates or is in strong competition
with rotation. Conclusions: For OB stars of solar metallicity, extra broadening
is important and has to be accounted for in the analysis. When appearing at or
close to the zero-age main sequence, most of the single and more massive stars
rotate slower than previously thought. Model predictions for the evolution of
rotation in hot massive stars may need to be updated.Comment: 15 pages, 10 figures, accepted for publication in A &
The chemical composition of the Orion star forming region: stars, gas and dust
We present a summary of main results from the studies performed in the series
of papers "The chemical composition of the Orion star forming region". We
reinvestigate the chemical composition of B-type stars in the Orion OB1
association by means of state-of-the-art stellar atmosphere codes, atomic
models and techniques, and compare the resulting abundances with those obtained
from the emission line spectra of the Orion nebula (M42), and recent
determinations of the Solar chemical composition.Comment: 5 pages, 4 figures, 2 tables. Poster contribution to the proceedings
of the LIAC2010 conference "The multi-wavelength view of hot, massive stars
The spectroscopic Hertzsprung-Russell diagram of Galactic massive stars
The distribution of stars in the Hertzsprung-Russell diagram narrates their
evolutionary history and directly assesses their properties. Placing stars in
this diagram however requires the knowledge of their distances and interstellar
extinctions, which are often poorly known for Galactic stars. The spectroscopic
Hertzsprung-Russell diagram (sHRD) tells similar evolutionary tales, but is
independent of distance and extinction measurements. Based on spectroscopically
derived effective temperatures and gravities of almost 600 stars, we derive for
the first time the observational distribution of Galactic massive stars in the
sHRD. While biases and statistical limitations in the data prevent detailed
quantitative conclusions at this time, we see several clear qualitative trends.
By comparing the observational sHRD with different state-of-the-art stellar
evolutionary predictions, we conclude that convective core overshooting may be
mass-dependent and, at high mass (), stronger than previously
thought. Furthermore, we find evidence for an empirical upper limit in the sHRD
for stars with between 10000 and 32000 K and, a strikingly large
number of objects below this line. This over-density may be due to inflation
expanding envelopes in massive main-sequence stars near the Eddington limit.Comment: 5 pages, 2 figures, 1 table; accepted for publication in A&A Letter
Structures of bacterial kynurenine formamidase reveal a crowded binuclear zinc catalytic site primed to generate a potent nucleophile
Tryptophan is an important precursor for chemical entities that ultimately support the biosynthesis of key metabolites. The second stage of tryptophan catabolism is catalysed by kynurenine formamidase, an enzyme that is different between eukaryotes and prokaryotes. In the present study, we characterize the catalytic properties and present the crystal structures of three bacterial kynurenine formamidases. The structures reveal a new amidase protein fold, a highly organized and distinctive binuclear Zn2+ catalytic centre in a confined, hydrophobic and relatively rigid active site. The structure of a complex with 2-aminoacetophenone delineates aspects of molecular recognition extending to the observation that the substrate itself may be conformationally restricted to assist binding in the confined space of the active site and for subsequent processing. The cations occupy a crowded environment, and, unlike most Zn2+ -dependent enzymes, there is little scope to increase co-ordination number during catalysis.We propose that the presence of a bridging water/hydroxide ligand in conjunction with the placement of an active site histidine supports a distinctive amidation mechanism.</p
Saturation properties of helium drops from a Leading Order description
Saturation properties are directly linked to the short-range scale of the
two-body interaction of the particles. The case of helium is particular, from
one hand the two-body potential has a strong repulsion at short distances. On
the other hand, the extremely weak binding of the helium dimer locates this
system very close to the unitary limit allowing for a description based on an
effective theory. At leading order of this theory a two- and a three-body term
appear, each one characterized by a low energy constant. In a potential model
this description corresponds to a soft potential model with a two-body term
purely attractive plus a three-body term purely repulsive constructed to
describe the dimer and trimer binding energies. Here we analyse the capability
of this model to describe the saturation properties making a direct link
between the low energy scale and the short-range correlations. We will show
that the energy per particle, , can be obtained with reasonable accuracy
at leading order extending the validity of this approximation, characterizing
universal behavior in few-boson systems close to the unitary limit, to the
many-body system.Comment: 5 pages, 3 figure
Haro15: Is it actually a low metallicity galaxy?
We present a detailed study of the physical properties of the nebular
material in multiple knots of the blue compact dwarf galaxy Haro 15. Using long
slit and echelle spectroscopy, obtained at Las Campanas Observatory, we study
the physical conditions (electron density and temperature), ionic and total
chemical abundances of several atoms, reddening and ionization structure. The
latter was derived by comparing the oxygen and sulphur ionic ratios to their
corresponding observed emission line ratios (the eta and eta' plots) in
different regions of the galaxy. Applying direct and empirical methods for
abundance determination, we perform a comparative analysis between these
regions.Comment: (Poster paper) 2 pages, 2 figure
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