373 research outputs found
2D gel probes for Fe2+, ∑PO4 and S(-II). Distribution studies in aquatic sediments
Date du colloque : 01/2009International audienc
The short-duration GRB 050724 host galaxy in the context of the long-duration GRB hosts
We report optical and near-infrared broad band observations of the
short-duration GRB 050724 host galaxy, used to construct its spectral energy
distribution (SED). Unlike the hosts of long-duration gamma-ray bursts (GRBs),
which show younger stellar populations, the SED of the GRB 050724 host galaxy
is optimally fitted with a synthetic elliptical galaxy template based on an
evolved stellar population (age ~2.6 Gyr). The SED of the host is difficult to
reproduce with non-evolving metallicity templates. In contrast, if the short
GRB host galaxy metallicity enrichment is considered, the synthetic templates
fit the observed SED satisfactorily. The internal host extinction is low (A_v
\~< 0.4 mag) so it cannot explain the faintness of the afterglow. This short
GRB host galaxy is more massive (~5x10^10 Mo) and luminous (~1.1 L*) than most
of the long-duration GRB hosts. A statistical comparison based on the ages of
short- and long-duration GRB host galaxies strongly suggests that
short-duration GRB hosts contain, on average, older progenitors. These findings
support a different origin for short- and long-duration GRBs.Comment: A&A in pres
EROS Variable Stars : Discovery of Beat Cepheids in the Small Magellanic Cloud and the effect of metallicity on pulsation
We report the discovery of eleven beat Cepheids in the Small Magellanic
Cloud, using data obtained by the EROS microlensing survey. Four stars are
beating in the fundamental and first overtone mode (F/1OT), seven are beating
in the first and second overtone (1OT/2OT). The SMC F/1OT ratio is
systematically higher than the LMC F/1OT, while the 1OT/2OT period ratio in the
SMC Cepheids is the same as the LMC one.Comment: 4 pages, Latex file with 4 .ps figures. accepted for publication in A
A Letter
The effect of metallicity on the Cepheid distance scale and its implications for the Hubble constant () determination
Recent HST determinations of the expansion's rate of the Universe (the Hubble
constant, H_0) assumed that the Cepheid Period-Luminosity relation at V and I
are independent of metallicity (Freedman, et al., 1996, Saha et al., 1996,
Tanvir et al., 1995). The three groups obtain different vales for H_0. We note
that most of this discrepancy stems from the asumption (by both groups) that
the Period-Luminosity relation is independent of metallicity. We come to this
conclusion as a result of our study of the Period-Luminosity relation of 481
Cepheids with 3 millions two colour measurements in the Large Magellanic Cloud
and the Small Magellanic Cloud obtained as a by-product of the EROS
microlensing survey. We find that the derived interstellar absorption
corrections are particularly sensitive to the metallicity and when our result
is applied to recent estimates based on HST Cepheids observations it makes the
low-H_0 values higher and the high-H_0 value lower, bringing those discrepant
estimates into agrement around .Comment: 4 pages, Latex, with 2 .ps accepted for publication astronomy and
astrophysics Letter
A Detailed Look at Chemical Abundances in Magellanic Cloud Planetary Nebulae. I. The Small Magellanic Cloud
We present an analysis of elemental abundances of He, N, O, Ne, S, and Ar in
Magellanic Cloud planetary nebulae (PNe), and focus initially on 14 PNe in the
Small Magellanic Cloud (SMC). We derived the abundances from a combination of
deep, high dispersion optical spectra, as well as mid-infrared (IR) spectra
from the Spitzer Space Telescope. A detailed comparison with prior SMC PN
studies shows that significant variations among authors of relative emission
line flux determinations lead to systematic discrepancies in derived elemental
abundances between studies that are >~0.15 dex, in spite of similar analysis
methods. We used ionic abundances derived from IR emission lines, including
those from ionization stages not observable in the optical, to examine the
accuracy of some commonly used recipes for ionization correction factors
(ICFs). These ICFs, which were developed for ions observed in the optical and
ultraviolet, relate ionic abundances to total elemental abundances. We find
that most of these ICFs work very well even in the limit of substantially
sub-Solar metallicities, except for PNe with very high ionization. Our
abundance analysis shows enhancements of He and N that are predicted from prior
dredge-up processes of the progenitors on the AGB, as well as the well known
correlations among O, Ne, S, and Ar that are little affected by nucleosynthesis
in this mass range. We identified MG_8 as an interesting limiting case of a PN
central star with a ~3.5 M_sun progenitor in which hot-bottom burning did not
occur in its prior AGB evolution. We find no evidence for O depletion in the
progenitor AGB stars via the O-N cycle, which is consistent with predictions
for lower-mass stars. We also find low S/O ratios relative to SMC H_II regions,
with a deficit comparable to what has been found for Galactic PNe.Comment: 9 figures, 6 tables; to be published in Ap
Reddening law and interstellar dust properties along Magellanic sight-lines
This study establishes that SMC, LMC and Milky Way extinction curves obey the
same extinction law which depends on the 2200A bump size and one parameter, and
generalizes the Cardelli, Clayton and Mathis (1989) relationship. This suggests
that extinction in all three galaxies is of the same nature. The role of linear
reddening laws over all the visible/UV wavelength range, particularly important
in the SMC but also present in the LMC and in the Milky Way, is also
highlighted and discussed.Comment: accepted for publication in Astrophysics and Space Science. 16 pages,
12 figures. Some figures are colour plot
ISM Properties in Low-Metallicity Environments II. The Dust Spectral Energy Distribution of NGC 1569
We present new 450 and 850 microns SCUBA data of the dwarf galaxy NGC 1569.
We construct the mid-infrared to millimeter SED of NGC 1569, using ISOCAM,
ISOPHOT, IRAS, KAO, SCUBA and MAMBO data, and model the SED in order to explore
the nature of the dust in low metallicity environments. The detailed modeling
is performed in a self-consistent way, synthesizing the global ISRF of the
galaxy using an evolutionary synthesis model with further constraints provided
by the observed MIR ionic lines and a photoionisation model. Our results show
that the dust properties are different in this low metallicity galaxy compared
to other more metal rich galaxies. The results indicate a paucity of PAHs
probably due to the destructive effects of the ISRF penetrating a clumpy
environment and a size-segregation of grains where the emission is dominated by
small grains of size ~3 nm, consistent with the idea of shocks having a
dramatic effect on the dust properties in NGC 1569. A significant millimetre
excess is present in the dust SED which can be explained by the presence of
ubiquitous very cold dust (T = 5-7 K). This dust component accounts for 40 to
70 % of the total dust mass in the galaxy (1.6 - 3.4 10^5 Msol) and could be
distributed in small clumps (size a few pc) throughout the galaxy. We find a
gas-to-dust mass ratio of 740 - 1600, larger than that of the Galaxy and a
dust-to-metals ratio of 1/4 to 1/7. We generate an extinction curve for NGC
1569, consistent with the modeled dust size distribution. This extinction curve
has relatively steep FUV rise and smaller 2175 Angstroms bump, resembling the
observed extinction curve of some regions in the Large Magellanic Cloud.Comment: 20 pages, 20 figures, accepted by A&
Observation of periodic variable stars towards the galactic spiral arms by EROS II
We present the results of a massive variability search based on a photometric
survey of a six square degree region along the Galactic plane at (, ) and (, ). This
survey was performed in the framework of the EROS II (Exp\'erience de Recherche
d'Objets Sombres) microlensing program. The variable stars were found among
1,913,576 stars that were monitored between April and June 1998 in two
passbands, with an average of 60 measurements. A new period-search technique is
proposed which makes use of a statistical variable that characterizes the
overall regularity of the flux versus phase diagram. This method is well suited
when the photometric data are unevenly distributed in time, as is our case.
1,362 objects whose luminosity varies were selected. Among them we identified 9
Cepheids, 19 RR Lyrae, 34 Miras, 176 eclipsing binaries and 266 Semi-Regular
stars. Most of them are newly identified objects. The cross-identification with
known catalogues has been performed. The mean distance of the RR Lyrae is
estimated to be kpc undergoing an average absorption of
magnitudes. This distance is in good agreement with the one
of disc stars which contribute to the microlensing source star population.Our
catalogue and light curves are available electronically from the CDS,
Strasbourg and from our Web site http://eros.in2p3.fr.Comment: 15 pages, 11 figures, accepted in A&A (april 2002
Towards an online-coupled chemistry-climate model: evaluation of trace gases and aerosols in COSMO-ART
Peer reviewe
An interesting candidate for isolated massive star formation in the Small Magellanic Cloud
The region of the Small Magellanic Cloud (SMC) with which this paper is
concerned contains the highest concentration of IRAS/Spitzer sources, H I
emission, and molecular clouds in this neighboring galaxy. However very few
studies have been devoted to it, despite these signs of star formation. We
present the first detailed study of the compact H II region N33 in the SMC by
placing it in a wider context of massive star formation. Moreover, we show that
N33 is a particularly interesting candidate for isolated massive star
formation. This analysis is based mainly on optical ESO NTT observations, both
imaging and spectroscopy, coupled with other archive data, notably Spitzer
images (IRAC 3.6, 4.5, 5.8, and 8.0 mic) and 2MASS observations. We derive a
number of physical characteristics of the compact H II region N33 for the first
time. This gas and dust formation of 7".4 (2.2 pc) in diameter is powered by a
massive star of spectral type O6.5-O7 V. The compact H II region belongs to a
rare class of H II regions in the Magellanic Clouds, called high-excitation
blobs (HEBs). We show that this H II region is not related to any star cluster.
Specifically, we do not find any traces of clustering around N33 on scales
larger than 10" (~ 3 pc). On smaller scales, there is a marginal stellar
concentration, the low density of which, below the 3 sigma level, does not
classify it as a real cluster. We also verify that N33 is not a member of any
large stellar association. Under these circumstances, N33 is also therefore
attractive because it represents a remarkable case of isolated massive-star
formation in the SMC. Various aspects of the relevance of N33 to the topic of
massive-star formation in isolation are discussed.Comment: 17 pages, 9 figures, 5 tables; Accepted for publication in A&
- …