238 research outputs found
Constraints on the distance to SGR 1806-20 from HI absorption
The giant flare detected from the magnetar SGR 1806-20 on 2004 December 27
had a fluence more than 100 times higher than the only two other SGR flares
ever recorded. Whereas the fluence is independent of distance, an estimate for
the luminosity of the burst depends on the source's distance, which has
previously been argued to be ~15 kpc. The burst produced a bright radio
afterglow, against which Cameron et al. (2005) have measured an HI absorption
spectrum. This has been used to propose a revised distance to SGR 1806-20 of
between 6.4 and 9.8 kpc. Here we analyze this absorption spectrum, and compare
it both to HI emission data from the Southern Galactic Plane Survey and to
archival 12-CO survey data. We confirm ~6 kpc, as a likely lower limit on the
distance to SGR 1806-20, but argue that it is difficult to place an upper limit
on the distance to SGR 1806-20 from the HI data currently available. The
previous value of ~15 kpc thus remains the best estimate of the distance to the
source.Comment: 3 pages, 1 embedded EPS figure. Added sentences to end of Abstract
and Conclusion, clarifying that most likely distance is 15 kpc. ApJ Letters,
in pres
The Distance Scale of Planetary Nebulae
By collecting distances from the literature, a set of 73 planetary nebulae
with mean distances of high accuracy is derived. This sample is used for
recalibration of the mass-radius relationship, used by many statistical
distance methods. An attempt to correct for a statistical peculiarity, where
errors in the distances influences the mass--radius relationship by increasing
its slope, has been made for the first time. Distances to PNe in the Galactic
Bulge, derived by this new method as well as other statistical methods from the
last decade, are then used for the evaluation of these methods as distance
indicators. In order of achieving a Bulge sample that is free from outliers we
derive new criteria for Bulge membership. These criteria are much more
stringent than those used hitherto, in the sense that they also discriminate
against background objects. By splitting our Bulge sample in two, one with
optically thick (small) PNe and one with optically thin (large) PNe, we find
that our calibration is of higher accuracy than most other calibrations.
Differences between the two subsamples, we believe, are due to the
incompleteness of the Bulge sample, as well as the dominance of optical
diameters in the ``thin'' sample and radio diameters in the ``thick'' sample.
Our final conclusion is that statistical methods give distances that are at
least as accurate as the ones obtained from many individual methods. Also, the
`long' distance scale of Galactic PNe is confirmed.Comment: 15 pages, 9 figures, accepted for publication in A&
Multiscale strength homogenization : application to shale nanoindentation
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2008.Includes bibliographical references (p. 236-246).Shales are one of the most encountered materials in sedimentary basins. Because of their highly heterogeneous nature, their strength prediction for oil and gas exploitation engineering has long time been an enigma. In this thesis, we propose a two-scale non-linear procedure for the homogenization of their yield design strength properties, based on the Linear Comparison Composite Theory. At Level 0, the intrinsic friction of shales is captured via a cohesive-frictional strength criterion for the clay particles (Drucker-Prager). Level I is composed of a porous clay phase and Level II incorporates silt and quartz grains. Homogenization yields either an elliptical or an hyperbolc strength criterion, depending on the packing density of the porous clay phase. These criteria are employed in an original reverse algorithm of indentation hardness to develop hardness-packing density scaling relations that allow a separation of constituent properties and volume fraction and morphology parameters, including interface conditions between the porous clay matrix and the (rigid) silt inclusions. The application of this algorithm to 11 shale samples from the GeoGenome project data base allows us to identify: (i) an invariant value of the solid hardness of clay particles, which is independent of clay mineralogy, porosity, etc.; and (ii) shale independent scaling relations of the cohesion and of the friction coefficient with the mean clay packing density, which provides some evidence that the elementary building block of shale is a clay polycrystal. The use of these scaling relations in the Level II-homogenization provides a first-order model for the prediction of the macroscopic strength properties of shale, based on only two parameters that delineate shale's macroscopic diversity: clay packing density and silt inclusion volume fraction.by Benjamin Gathier.S.M
Spitzer detection of PAH and silicate features in post-AGB stars and young Planetary Nebulae
We have observed a small sample of hot post-AGB stars with the InfraRed Array
Camera (IRAC) and the InfraRed Spectrograph (IRS) on-board the Spitzer Space
Telescope. The stars were selected from the literature on the basis of their
far-Infrared excess (i.e., post-AGB candidates) and B spectral type (i.e.,
close to the ionization of the envelope). The combination of our IRAC
observations with 2MASS and IRAS catalog data, along with previous radio
observations in the cm range (where available) allowed us to model the SEDs of
our targets and find that in almost all of them at least two shells of dust at
different temperatures must be present, the hot dust component ranging up to
1000 K. In several targets grains larger than 1 micron are needed to match the
far-IR data points. In particular, in IRAS 17423-1755 grains up to 100 micron
must be introduced to match the emission in the mm range.
We obtained IRS spectra to identify the chemistry of the envelopes and found
that more than 1/3 of the sources in our sample have mixed chemistry, showing
both mid-IR bands attributed to Polycyclic Aromatic Hydrocarbons (PAH) and
silicate features. The analysis of the PAH features indicates that these
molecules are located in the outflows, far away from the central stars. We
consider the larger than expected percentage of mixed-chemistry targets as a
selection bias towards stars with a disk or torus around them. Our results
strengthen the current picture of mixed chemistry being due to the spatial
segregation of different dust populations in the envelopes.Comment: 20 pages, 18 figures, accepted for publication on Ap
Planetary nebulae abundances and stellar evolution II
Context: In recent years mid- and far infrared spectra of planetary nebulae
have been analysed and lead to more accurate abundances. It may be expected
that these better abundances lead to a better understanding of the evolution of
these objects. Aims: The observed abundances in planetary nebulae are compared
to those predicted by the models of Karakas (2003) in order to predict the
progenitor masses of the various PNe used. The morphology of the PNe is
included in the comparison. Since the central stars play an important role in
the evolution, it is expected that this comparison will yield additional
information about them. Methods: First the nitrogen/oxygen ratio is discussed
with relation to the helium/hydrogen ratio. The progenitor mass for each PNe
can be found by a comparison with the models of Karakas. Then the present
luminosity of the central stars is determined in two ways: first by computing
the central star effective temperature and radius, and second by computing the
nebular luminosity from the hydrogen and helium lines. This luminosity is also
a function of the initial mass so that these two values of initial mass can be
compared. Results: Six of the seven bipolar nebulae can be identified as
descendants of high mass stars (4Msun - 6Msun) while the seventh is ambiguous.
Most of the elliptical PNe have central stars which descend from low initial
mass stars, although there are a few caveats which are discussed. There is no
observational evidence for a higher mass for central stars which have a high
carbon/oxygen ratio. The evidence provided by the abundance comparison with the
models of Karakas is consistent with the HR diagram to which it is compared. In
the course of this discussion it is shown how `optically thin' nebulae can be
separated from those which are 'optically thick'.Comment: 12 pages, 4 tables, 4 figures. Accepted for publication in A&
IC 4406: a radio-infrared view
IC 4406 is a large (about 100'' x 30'') southern bipolar planetary nebula,
composed of two elongated lobes extending from a bright central region, where
there is evidence for the presence of a large torus of gas and dust. We show
new observations of this source performed with IRAC (Spitzer Space Telescope)
and the Australia Telescope Compact Array. The radio maps show that the flux
from the ionized gas is concentrated in the bright central region and
originates in a clumpy structure previously observed in H_alpha, while in the
infrared images filaments and clumps can be seen in the extended nebular
envelope, the central region showing toroidal emission. Modeling of the
infrared emission leads to the conclusion that several dust components are
present in the nebula.Comment: 22 pages, 7 figures, accepted for publication in The Astrophysical
Journal; v.2 has changes in both figures and content; preprint forma
Violences conjugales à Antananarivo (Madagascar): un enjeu de santé publique
Introduction: La violence conjugale a été étudiée dans beaucoup de pays développés mais peu en Afrique subsaharienne. Madagascar est un pays où ce phénomène est peu documenté. Méthodes: En 2007, une enquête sur la violence conjugale à Antananarivo (ELVICA) a été menée sur la violence conjugale envers les femmes dans la capitale malgache. ELVICA a interrogé 400 femmes en union, de 15 à 59 ans. Des informations sur les caractéristiques démographiques, socioéconomiques des couples ont été collectées ainsi que sur les actes de violences physiques des hommes sur leurs épouses. L’objectif de cet article est d’identifier les facteurs de risques de la violence conjugale grave, celle qui a des conséquences sur la santé physique des femmes. Résultats: Trente-cinq pour cent des femmes qui ont déclaré avoir subi au moins une forme de violence physique au cours des 12 mois précédent l’enquête. Presque la moitié (46%) des femmes violentées ont déclaré avoir déjà eu des hématomes, et environ un quart (23%) des plaies avec saignement. Vingt-deux pour cent ont déjà dû consulter un médecin. Parmi les nombreuses variables socioéconomiques et démographiques testées, quelques-unes sont associées positivement au risque de violence conjugale grave: le fait pour une femme d’être en union consensuelle et d’avoir une activité professionnelle. Il y aussi un lien entre la violence subie et l’autonomie des femmes (liberté accordée par le mari de travailler, de circuler, de voir sa famille). Conclusion: A Madagascar, comme ailleurs, la lutte contre les violences conjugales est un élément majeur de l’amélioration du statut et de la santé des femmes
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