1,913 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
Modeling Network Populations via Graph Distances
This article introduces a new class of models for multiple networks. The core
idea is to parametrize a distribution on labelled graphs in terms of a
Fr\'{e}chet mean graph (which depends on a user-specified choice of metric or
graph distance) and a parameter that controls the concentration of this
distribution about its mean. Entropy is the natural parameter for such control,
varying from a point mass concentrated on the Fr\'{e}chet mean itself to a
uniform distribution over all graphs on a given vertex set. We provide a
hierarchical Bayesian approach for exploiting this construction, along with
straightforward strategies for sampling from the resultant posterior
distribution. We conclude by demonstrating the efficacy of our approach via
simulation studies and two multiple-network data analysis examples: one drawn
from systems biology and the other from neuroscience.Comment: 33 pages, 8 figure
The Cocoon Nebula and its ionizing star: do stellar and nebular abundances agree?
(Abridged) Main sequence massive stars embedded in an HII region should have
the same chemical abundances as the surrounding nebular gas+dust. The Cocoon
nebula, a close-by Galactic HII region ionized by a narrow line B0.5 V single
star (BD+46 3474), is an ideal target to perform a detailed comparison of
nebular and stellar abundances in the same Galactic HII region. We investigate
the chemical content of O, N and S in the Cocoon nebula from two different
points of view: an empirical analysis of the nebular spectrum and a detailed
spectroscopic analysis of its ionizing B-type star using state-of-the-art
stellar atmosphere modeling. By comparing the stellar and nebular abundances,
we aim to indirectly address the long-standing problem of the discrepancy found
between abundances obtained from collisionally excited lines (CELs) and optical
recombination lines in photoionized nebulae. We collect spatially resolved
spectroscopy of the Cocoon nebula and a high resolution optical spectrum of its
ionizing star. Standard nebular techniques are used to compute the physical
conditions and gaseous abundances of O, N and S. We perform a self-consistent
spectroscopic abundance analysis of BD+46 3474 based on the atmosphere code
FASTWIND to determine the stellar parameters and Si, O, and N abundances. The
Cocoon nebula and its ionizing star, located at a distance of 800+-80 pc, have
a very similar chemical composition as the Orion nebula and other B-type stars
in the solar vicinity. This result agrees with the high degree of homogeneity
of the present-day composition of the solar neighbourhood as derived from the
study of the local cold-gas ISM. The comparison of stellar and nebular CELs
abundances in the Cocoon nebula indicates that O and N gas+dust nebular values
are in better agreement with stellar ones assuming small temperature
fluctuations, of the order of those found in the Orion nebula.Comment: Accepted for publication in A&A. 13 pages, 7 tables and 6 figure
Observational evidence for a correlation between macroturbulent broadening and line-profile variations in OB Supergiants
The spectra of O and B supergiants are known to be affected by a significant
form of extra line broadening (usually referred to as macroturbulence) in
addition to that produced by stellar rotation. Recent analyses of high
resolution spectra have shown that the interpretation of this line broadening
as a consequence of large scale turbulent motions would imply highly supersonic
velocity fields in photospheric regions, making this scenario quite improbable.
Stellar oscillations have been proposed as a likely alternative explanation. As
part of a long term observational project, we are investigating the
macroturbulent broadening in O and B supergiants and its possible connection
with spectroscopic variability phenomena and stellar oscillations. In this
letter, we present the first encouraging results of our project, namely firm
observational evidence for a strong correlation between the extra broadening
and photospheric line-profile variations in a sample of 13 supergiants with
spectral types ranging from O9.5 to B8.Comment: 8 pages, 3 figures, accepted for publication in ApJ
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Mesenchymal stem cells and their use as cell replacement therapy and disease modelling tool.
Mesenchymal stem cells (MSCs) from adult somatic tissues may differentiate in vitro and in vivo into multiple mesodermal tissues including bone, cartilage, adipose tissue, tendon, ligament or even muscle. MSCs preferentially home to damaged tissues where they exert their therapeutic potential. A striking feature of the MSCs is their low inherent immunogenicity as they induce little, if any, proliferation of allogeneic lymphocytes and antigen-presenting cells. Instead, MSCs appear to be immunosuppressive in vitro. Their multilineage differentiation potential coupled to their immuno-privileged properties is being exploited worldwide for both autologous and allogeneic cell replacement strategies. Here, we introduce the readers to the biology of MSCs and the mechanisms underlying immune tolerance. We then outline potential cell replacement strategies and clinical applications based on the MSCs immunological properties. Ongoing clinical trials for graft-versus-host-disease, haematopoietic recovery after co-transplantation of MSCs along with haematopoietic stem cells and tissue repair are discussed. Finally, we review the emerging area based on the use of MSCs as a target cell subset for either spontaneous or induced neoplastic transformation and, for modelling non-haematological mesenchymal cancers such as sarcomas
The Library of Babel
We show that heavy pure states of gravity can appear to be mixed states to
almost all probes. Our arguments are made for Schwarzschild black
holes using the field theory dual to string theory in such spacetimes. Our
results follow from applying information theoretic notions to field theory
operators capable of describing very heavy states in gravity. For certain
supersymmetric states of the theory, our account is exact: the microstates are
described in gravity by a spacetime ``foam'', the precise details of which are
invisible to almost all probes.Comment: 7 pages, 1 figure, Essay receiving honorable mention in the 2005
Gravity Research Foundation essay competitio
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