6,539 research outputs found
Nitrogen isotopic ratios in Barnard 1: a consistent study of the N2H+, NH3, CN, HCN and HNC isotopologues
The 15N isotopologue abundance ratio measured today in different bodies of
the solar system is thought to be connected to 15N-fractionation effects that
would have occured in the protosolar nebula. The present study aims at putting
constraints on the degree of 15N-fractionation that occurs during the
prestellar phase, through observations of D, 13C and 15N-substituted
isotopologues towards B1b. Both molecules from the nitrogen hydride family,
i.e. N2H+ and NH3, and from the nitrile family, i.e. HCN, HNC and CN, are
considered in the analysis. As a first step, we model the continuum emission in
order to derive the physical structure of the cloud, i.e. gas temperature and
H2 density. These parameters are subsequently used as an input in a non-local
radiative transfer model to infer the radial abundances profiles of the various
molecules. Our modeling shows that all the molecules are affected by depletion
onto dust grains, in the region that encompasses the B1-bS and B1-bN cores.
While high levels of deuterium fractionation are derived, we conclude that no
fractionation occurs in the case of the nitrogen chemistry. Independently of
the chemical family, the molecular abundances are consistent with 14N/15N~300,
a value representative of the elemental atomic abundances of the parental gas.
The inefficiency of the 15N-fractionation effects in the B1b region can be
linked to the relatively high gas temperature ~17K which is representative of
the innermost part of the cloud. Since this region shows signs of depletion
onto dust grains, we can not exclude the possibility that the molecules were
previously enriched in 15N, earlier in the B1b history, and that such an
enrichment could have been incorporated into the ice mantles. It is thus
necessary to repeat this kind of study in colder sources to test such a
possibility.Comment: accepted in A&
Optical evidence for heavy charge carriers in FeGe
The optical spectrum of the cubic helimagnetic metal FeGe has been
investigated in the frequency range from 0.01 - 3.1 eV for different
temperatures from 30 K to 296 K. The optical conductivity shows the evolution
of a low energy (0.22 eV) interband transition and the development of a narrow
free carrier response with a strong energy and temperature dependence. The
frequency dependent effective mass and scattering rate derived from the optical
data indicate the formation of dressed quasi-particles with a mass
renormalization factor of 12. Similar to FeSi the spectral weight in FeGe is
not recovered over a broad frequency range, an effect usually attributed to the
influence of the on-site Coulomb interaction.Comment: 5 pages, 5 figure
Antimatter cosmic rays from dark matter annihilation: First results from an N-body experiment
[Abridged]. We take advantage of the galaxy-like 3D dark matter map extracted
from the HORIZON Project results to calculate the positron and antiproton
fluxes from dark matter annihilation, in a model-independent approach as well
as for dark matter particle benchmarks relevant at the LHC scale (from
supersymmetric and extra-dimensional theories). Such a study is dedicated to a
better estimate of the theoretical uncertainties affecting predictions, while
the PAMELA and GLAST satellites are currently taking data which will soon
provide better observational constraints. We discuss the predictions of the
antiproton and positron fluxes, and of the positron fraction as well, as
compared to the current data. We finally discuss the limits of the Nbody
framework in describing the dark matter halo of our Galaxy.Comment: 19 pages, 9 figures. Backgrounds included and additional comments and
figures on the positron fraction. Accepted for publication in PR
Mott transition and collective charge pinning in electron doped Sr2IrO4
We studied the in-plane dynamic and static charge conductivity of electron
doped Sr2IrO4 using optical spectroscopy and DC transport measurements. The
optical conductivity indicates that the pristine material is an indirect
semiconductor with a direct Mott-gap of 0.55 eV. Upon substitution of 2% La per
formula unit the Mott-gap is suppressed except in a small fraction of the
material (15%) where the gap survives, and overall the material remains
insulating. Instead of a zero energy mode (or Drude peak) we observe a soft
collective mode (SCM) with a broad maximum at 40 meV. Doping to 10% increases
the strength of the SCM, and a zero-energy mode occurs together with metallic
DC conductivity. Further increase of the La substitution doesn't change the
spectral weight integral up to 3 eV. It does however result in a transfer of
the SCM spectral weight to the zero-energy mode, with a corresponding reduction
of the DC resistivity for all temperatures from 4 to 300 K. The presence of a
zero-energy mode signals that at least part of the Fermi surface remains
ungapped at low temperatures, whereas the SCM appears to be caused by pinning a
collective frozen state involving part of the doped electrons
Nitrogen isotopic ratios in Barnard 1: a consistent study of the N_2H^+, NH_3, CN, HCN, and HNC isotopologues
Context. The ^(15)N isotopologue abundance ratio measured today in different bodies of the solar system is thought to be connected to ^(15)N-fractionation effects that would have occurred in the protosolar nebula.
Aims. The present study aims at putting constraints on the degree of 15N-fractionation that occurs during the prestellar phase, through observations of D, ^(13)C, and ^(15)N-substituted isotopologues towards B1b. Molecules both from the nitrogen hydride family, i.e. N2H+, and NH3, and from the nitrile family, i.e. HCN, HNC, and CN, are considered in the analysis.
Methods. As a first step, we modelled the continuum emission in order to derive the physical structure of the cloud, i.e. gas temperature and H_2 density. These parameters were subsequently used as input in a non-local radiative transfer model to infer the radial abundance profiles of the various molecules.
Results. Our modelling shows that all the molecules are affected by depletion onto dust grains in the region that encompasses the B1-bS and B1-bN cores. While high levels of deuterium fractionation are derived, we conclude that no fractionation occurs in the case of the nitrogen chemistry. Independently of the chemical family, the molecular abundances are consistent with ^(14)N/^(15)N ~ 300, a value representative of the elemental atomic abundances of the parental gas.
Conclusions. The inefficiency of the ^(15)N-fractionation effects in the B1b region can be linked to the relatively high gas temperature ~17 K, which is representative of the innermost part of the cloud. Since this region shows signs of depletion onto dust grains, we cannot exclude the possibility that the molecules were previously enriched in ^(15)N, earlier in the B1b history and that such an enrichment could have been incorporated into the ice mantles. It is thus necessary to repeat this kind of study in colder sources to test such a possibility
Birth, life and survival of Tidal Dwarf Galaxies
Advances on the formation and survival of the so-called Tidal Dwarf Galaxies
(TDGs) are reviewed. The understanding on how objects of the mass of dwarf
galaxies may form in debris of galactic collisions has recently benefited from
the coupling of multi-wavelength observations with numerical simulations of
galaxy mergers. Nonetheless, no consensual scenario has yet emerged and as a
matter of fact the very definition of TDGs remains elusive. Their real
cosmological importance is also a matter of debate, their presence in our Local
Group of galaxies as well. Identifying old, evolved, TDGs among the population
of regular dwarf galaxies and satellites may not be straightforward. However a
number of specific properties (location, dark matter and metal content) that
objects of tidal origin should have are reminded here. Examples of newly
discovered genuine old TDGs around a nearby elliptical galaxy are finally
presented.Comment: 9 pages, 5 figures, invited talk at JENAM 2010 symposium on "Dwarf
Galaxies", v2:reference and acknowledgements update
Spitzer's mid-infrared view on an outer Galaxy Infrared Dark Cloud candidate toward NGC 7538
Infrared Dark Clouds (IRDCs) represent the earliest observed stages of
clustered star formation, characterized by large column densities of cold and
dense molecular material observed in silhouette against a bright background of
mid-IR emission. Up to now, IRDCs were predominantly known toward the inner
Galaxy where background infrared emission levels are high. We present Spitzer
observations with the Infrared Camera Array toward object G111.80+0.58 (G111)
in the outer Galactic Plane, located at a distance of ~3 kpc from us and ~10
kpc from the Galactic center. Earlier results show that G111 is a massive, cold
molecular clump very similar to IRDCs. The mid-IR Spitzer observations
unambiguously detect object G111 in absorption. We have identified for the
first time an IRDC in the outer Galaxy, which confirms the suggestion that
cluster-forming clumps are present throughout the Galactic Plane. However,
against a low mid-IR back ground such as the outer Galaxy it takes some effort
to find them.Comment: Accepted for publication in ApJL -- 11 pages, 2 figures (1 colour
The wind of W Hya as seen by Herschel. II. The molecular envelope of W Hya
The evolution of low- and intermediate-mass stars on the asymptotic giant
branch (AGB) is mainly controlled by the rate at which these stars lose mass in
a stellar wind. Understanding the driving mechanism and strength of the stellar
winds of AGB stars and the processes enriching their surfaces with products of
nucleosynthesis are paramount to constraining AGB evolution and predicting the
chemical evolution of galaxies. In a previous paper we have constrained the
structure of the outflowing envelope of W Hya using spectral lines of the
CO molecule. Here we broaden this study by modelling an extensive set of
HO and SiO lines observed by the three instruments on board
Herschel using a state-of-the-art molecular excitation and radiative transfer
code. The oxygen isotopic ratios and the SiO abundance profile can be
connected to the initial stellar mass and to crucial aspects of dust formation
at the base of the stellar wind, respectively. The modelling of HO and
SiO confirms the properties of the envelope model of W Hya derived from
CO lines. We find an HO ortho-to-para ratio of
2.5\,, consistent with what is expected for an AGB wind. The
O/O ratio indicates that W Hya has an initial mass of about 1.5
M. Although the ortho- and para-HO lines observed by HIFI appear
to trace gas of slightly different physical properties, a turbulence velocity
of km s fits the HIFI lines of both spin isomers and those
of SiO well. The ortho- and para-HO and SiO abundances
relative to H are , , and , respectively. Assuming a solar
silicon-to-carbon ratio, the SiO line emission model is consistent with
about one-third of the silicon atoms being locked up in dust particles
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