809 research outputs found
The shape of the initial cluster mass function: what it tells us about the local star formation efficiency
We explore how the expulsion of gas from star-cluster forming cloud-cores due
to supernova explosions affects the shape of the initial cluster mass function,
that is, the mass function of star clusters when effects of gas expulsion are
over. We demonstrate that if the radii of cluster-forming gas cores are roughly
constant over the core mass range, as supported by observations, then more
massive cores undergo slower gas expulsion. Therefore, for a given star
formation efficiency, more massive cores retain a larger fraction of stars
after gas expulsion. The initial cluster mass function may thus differ from the
core mass function substantially, with the final shape depending on the star
formation efficiency. A mass-independent star formation efficiency of about 20
per cent turns a power-law core mass function into a bell-shaped initial
cluster mass function, while mass-independent efficiencies of order 40 per cent
preserve the shape of the core mass function.Comment: accepted in Ap
Variations in Stellar Clustering with Environment: Dispersed Star Formation and the Origin of Faint Fuzzies
The observed increase in star formation efficiency with average cloud
density, from several percent in whole giant molecular clouds to ~30 or more in
cluster-forming cores, can be understood as the result of hierarchical cloud
structure if there is a characteristic density as which individual stars become
well defined. Also in this case, the efficiency of star formation increases
with the dispersion of the density probability distribution function (pdf).
Models with log-normal pdf's illustrate these effects. The difference between
star formation in bound clusters and star formation in loose groupings is
attributed to a difference in cloud pressure, with higher pressures forming
more tightly bound clusters. This correlation accounts for the observed
increase in clustering fraction with star formation rate and with the
observation of Scaled OB Associations in low pressure environments. ``Faint
fuzzie'' star clusters, which are bound but have low densities, can form in
regions with high Mach numbers and low background tidal forces. The proposal by
Burkert, Brodie & Larsen (2005) that faint fuzzies form at large radii in
galactic collisional rings, satisfies these constraints.Comment: 14 pages, 2 figures, ApJ, 672, January 10th 200
Repositioning the Catalytic Triad Aspartic Acid of Haloalkane Dehalogenase: Effects on Stability, Kinetics, and Structure
Haloalkane dehalogenase (DhlA) catalyzes the hydrolysis of haloalkanes via an alkyl-enzyme intermediate. The covalent intermediate, which is formed by nucleophilic substitution with Asp124, is hydrolyzed by a water molecule that is activated by His289. The role of Asp260, which is the third member of the catalytic triad, was studied by site-directed mutagenesis. Mutation of Asp260 to asparagine resulted in a catalytically inactive D260N mutant, which demonstrates that the triad acid Asp260 is essential for dehalogenase activity. Furthermore, Asp260 has an important structural role, since the D260N enzyme accumulated mainly in inclusion bodies during expression, and neither substrate nor product could bind in the active-site cavity. Activity for brominated substrates was restored to D260N by replacing Asn148 with an aspartic or glutamic acid. Both double mutants D260N+N148D and D260N+N148E had a 10-fold reduced kcat and 40-fold higher Km values for 1,2-dibromoethane compared to the wild-type enzyme. Pre-steady-state kinetic analysis of the D260N+N148E double mutant showed that the decrease in kcat was mainly caused by a 220-fold reduction of the rate of carbon-bromine bond cleavage and a 10-fold decrease in the rate of hydrolysis of the alkyl-enzyme intermediate. On the other hand, bromide was released 12-fold faster and via a different pathway than in the wild-type enzyme. Molecular modeling of the mutant showed that Glu148 indeed could take over the interaction with His289 and that there was a change in charge distribution in the tunnel region that connects the active site with the solvent. On the basis of primary structure similarity between DhlA and other α/β-hydrolase fold dehalogenases, we propose that a conserved acidic residue at the equivalent position of Asn148 in DhlA is the third catalytic triad residue in the latter enzymes.
Radial velocities of early-type stars in the Perseus OB2 association
We present radial velocities for 29 B- and A-type stars in the field of the
nearby association Perseus OB2. The velocities are derived from spectra
obtained with AURELIE, via cross correlation with radial velocity standards
matched as closely as possible in spectral type. The resulting accuracy is ~2 -
3 km s. We use these measurements, together with published values for a
few other early-type stars, to study membership of the association. The mean
radial velocity (and measured velocity dispersion) of Per OB2 is 23.5 \pm 3.9
km s, and lies ~15 km s away from the mean velocity of the local
disk field stars. We identify a number of interlopers in the list of possible
late-B- and A-type members which was based on Hipparcos parallaxes and proper
motions, and discuss the colour-magnitude diagram of the association.Comment: 20 pages, 9 figures, accepted for publication in A&A, minor revision
The effect of the dynamical state of clusters on gas expulsion and infant mortality
The star formation efficiency (SFE) of a star cluster is thought to be the
critical factor in determining if the cluster can survive for a significant
(>50 Myr) time. There is an often quoted critical SFE of ~30 per cent for a
cluster to survive gas expulsion. I reiterate that the SFE is not the critical
factor, rather it is the dynamical state of the stars (as measured by their
virial ratio) immediately before gas expulsion that is the critical factor. If
the stars in a star cluster are born in an even slightly cold dynamical state
then the survivability of a cluster can be greatly increased.Comment: 6 pages, 2 figures. Review talk given at the meeting on "Young
massive star clusters - Initial conditions and environments", E. Perez, R. de
Grijs, R. M. Gonzalez Delgado, eds., Granada (Spain), September 2007,
Springer: Dordrecht. Replacement to correct mistake in a referenc
Patterns and determinants of response to novel therapies in juvenile and adult-onset polyarthritis
Biological and targeted synthetic disease-modifying anti-rheumatic drugs (b/tsDMARDs) have revolutionized the management of multiple rheumatic inflammatory conditions. Amongst these, polyarticular Juvenile-Idiopathic Arthritis (pJIA) and Rheumatoid Arthritis (RA) display similarities in terms of disease pathophysiology and response pattern to b/tsDMARDs. Indeed, therapeutic efficacy of novel targeted drugs is variable among individual patients, in both RA and pJIA. Mechanisms and determinants of this heterogeneous response are diverse and complex, such that development of true “precision”-medicine strategies has proven highly challenging. In this review, we will discuss pathophysiological, patient-specific, drug-specific and environmental factors contributing to individual therapeutic response in polyarticular JIA in comparison to what is known in RA. Although some biomarkers have been identified that stratify for the likelihood of either therapeutic response or non-response, few have proved useful in clinical practice so far, likely due to the complexity of treatment-response mechanisms. Consequently, we propose a pragmatic, patient-centered and clinically-based approach, i.e. personalized instead of biomarker-based precision medicine in JIA
Spectra disentangling applied to the Hyades binary Theta^2 Tau AB: new orbit, orbital parallax and component properties
Theta^2 Tauri is a detached and single-lined interferometric-spectroscopic
binary as well as the most massive binary system of the Hyades cluster. The
system revolves in an eccentric orbit with a periodicity of 140.7 days. The
secondary has a similar temperature but is less evolved and fainter than the
primary. It is also rotating more rapidly. Since the composite spectra are
heavily blended, the direct extraction of radial velocities over the orbit of
component B was hitherto unsuccessful. Using high-resolution spectroscopic data
recently obtained with the Elodie (OHP, France) and Hermes (ORM, La Palma,
Spain) spectrographs, and applying a spectra disentangling algorithm to three
independent data sets including spectra from the Oak Ridge Observatory (USA),
we derived an improved spectroscopic orbit and refined the solution by
performing a combined astrometric-spectroscopic analysis based on the new
spectroscopy and the long-baseline data from the Mark III optical
interferometer. As a result, the velocity amplitude of the fainter component is
obtained in a direct and objective way. Major progress based on this new
determination includes an improved computation of the orbital parallax. Our
mass ratio is in good agreement with the older estimates of Peterson et al.
(1991, 1993), but the mass of the primary is 15-25% higher than the more recent
estimates by Torres et al. (1997) and Armstrong et al. (2006). Due to the
strategic position of the components in the turnoff region of the cluster,
these new determinations imply stricter constraints for the age and the
metallicity of the Hyades cluster. The location of component B can be explained
by current evolutionary models, but the location of the more evolved component
A is not trivially explained and requires a detailed abundance analysis of its
disentangled spectrum.Comment: in press, 13 pages, 10 Postscript figures, 5 tables. Table~4 is
available as online material. Keywords: astrometry - techniques: high angular
resolution - stars: binaries: visual - stars: binaries: spectroscopic -
stars: fundamental parameter
Surviving infant mortality in the hierarchical merging scenario
We examine the effects of gas expulsion on initially sub-structured and
out-of-equilibrium star clusters. We perform -body simulations of the
evolution of star clusters in a static background potential before removing
that potential to model gas expulsion. We find that the initial star formation
efficiency is not a good measure of the survivability of star clusters. This is
because the stellar distribution can change significantly, causing a large
change in the relative importance of the stellar and gas potentials. We find
that the initial stellar distribution and velocity dispersion are far more
important parameters than the initial star formation efficiency, and that
clusters with very low star formation efficiencies can survive gas expulsion.
We suggest that it is variations in cluster initial conditions rather than in
their star formation efficiencies that cause some clusters to be destroyed
while a few survive.Comment: 9 pages, 10 figures, 1 tabl
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