4,052 research outputs found
Carbon and oxygen in HII regions of the Magellanic Clouds: abundance discrepancy and chemical evolution
We present C and O abundances in the Magellanic Clouds derived from deep
spectra of HII regions. The data have been taken with the Ultraviolet-Visual
Echelle Spectrograph at the 8.2-m VLT. The sample comprises 5 HII regions in
the Large Magellanic Cloud (LMC) and 4 in the Small Magellanic Cloud (SMC). We
measure pure recombination lines (RLs) of CII and OII in all the objects,
permitting to derive the abundance discrepancy factors (ADFs) for O^2+, as well
as their O/H, C/H and C/O ratios. We compare the ADFs with those of other HII
regions in different galaxies. The results suggest a possible metallicity
dependence of the ADF for the low-metallicity objects, but more uncertain for
high-metallicity objects. We compare nebular and B-type stellar abundances and
we find that the stellar abundances agree better with the nebular ones derived
from collisionally excited lines (CELs). Comparing these results with other
galaxies we observe that stellar abundances seem to agree better with the
nebular ones derived from CELs in low-metallicity environments and from RLs in
high-metallicity environments. The C/H, O/H and C/O ratios show almost flat
radial gradients, in contrast with the spiral galaxies where such gradients are
negative. We explore the chemical evolution analysing C/O vs. O/H and comparing
with the results of HII regions in other galaxies. The LMC seems to show a
similar chemical evolution to the external zones of small spiral galaxies and
the SMC behaves as a typical star-forming dwarf galaxy.Comment: Accepted for publication in MNRAS, 17 pages, 11 figures, 8 table
Testing the chemical tagging technique with open clusters
Context. Stars are born together from giant molecular clouds and, if we
assume that the priors were chemically homogeneous and well-mixed, we expect
them to share the same chemical composition. Most of the stellar aggregates are
disrupted while orbiting the Galaxy and most of the dynamic information is
lost, thus the only possibility of reconstructing the stellar formation history
is to analyze the chemical abundances that we observe today.
Aims. The chemical tagging technique aims to recover disrupted stellar
clusters based merely on their chemical composition. We evaluate the viability
of this technique to recover co-natal stars that are no longer gravitationally
bound.
Methods. Open clusters are co-natal aggregates that have managed to survive
together. We compiled stellar spectra from 31 old and intermediate-age open
clusters, homogeneously derived atmospheric parameters, and 17 abundance
species, and applied machine learning algorithms to group the stars based on
their chemical composition. This approach allows us to evaluate the viability
and efficiency of the chemical tagging technique.
Results. We found that stars at different evolutionary stages have distinct
chemical patterns that may be due to NLTE effects, atomic diffusion, mixing,
and biases. When separating stars into dwarfs and giants, we observed that a
few open clusters show distinct chemical signatures while the majority show a
high degree of overlap. This limits the recovery of co-natal aggregates by
applying the chemical tagging technique. Nevertheless, there is room for
improvement if more elements are included and models are improved.Comment: accepted for publication in Astronomy and Astrophysics. Corrected
typo
A microscopic model for thin film spreading
A microscopic, driven lattice gas model is proposed for the dynamics and
spatio-temporal fluctuations of the precursor film observed in spreading
experiments. Matter is transported both by holes and particles, and the
distribution of each can be described by driven diffusion with a moving
boundary. This picture leads to a stochastic partial differential equation for
the shape of the boundary, which agrees with the simulations of the lattice
gas. Preliminary results for flow in a thermal gradient are discussed.Comment: 4 pages, 3 figures. Submitte
Photon capture cones and embedding diagrams of the Ernst spacetime
The differences between the character of the Schwarzschild and Ernst
spacetimes are illustrated by comparing the photon capture cones, and the
embedding diagrams of the sections of the equatorial planes
of both the ordinary and optical reference geometry of these spacetimes. The
non-flat asymptotic character of the Ernst spacetime reflects itself in two
manifest facts: the escape photon cones correspond to purely outward radial
direction, and the embedding diagrams of both the ordinary and optical geometry
shrink to zero radius asymptotically. Using the properties of the embedding
diagrams, regions of these spacetimes which could have similar character are
estimated, and it is argued that they can exist for the Ernst spacetimes with a
sufficiently low strength of the magnetic field.Comment: 12 pages, 7 figure
Interplay between collective effects and nonstandard interactions of supernova neutrinos
We consider the effect of non-standard neutrino interactions (NSI, for short) on the propagation of neutrinos through the supernova (SN) envelope within a three-neutrino framework and taking into account the presence of a neutrino background. We find that for given NSI parameters, with strength generically denoted by Δij, neutrino evolution exhibits a significant time dependence. For |ΔÏÏ|âł 10â3 the neutrino survival probability may become sensitive to the Ξ23 octant and the sign of ΔÏÏ. In particular, if ΔÏÏâł10â2 an internal I-resonance may arise independently of the matter density. For typical values found in SN simulations this takes place in the same dense-neutrino region above the neutrinosphere where collective effects occur, in particular during the synchronization regime. This resonance may lead to an exchange of the neutrino fluxes entering the bipolar regime. The main consequences are (i) bipolar conversion taking place for normal neutrino mass hierarchy and (ii) a transformation of the flux of low-energy Îœe, instead of the usual spectral swap
Dark Matter and the Chemical Evolution of Irregular Galaxies
We present three types of chemical evolution models for irregular galaxies:
closed-box with continuous star formation rates (SFRs), closed-box with
bursting SFRs, and O-rich outflow with continuous SFRs. We discuss the chemical
evolution of the irregular galaxies NGC 1560 and II Zw 33, and a ``typical''
irregular galaxy. The fraction of low-mass stars needed by our models is larger
than that derived for the solar vicinity, but similar to that found in globular
clusters. For our typical irregular galaxy we need a mass fraction of about 40%
in the form of substellar objects plus non baryonic dark matter inside the
Holmberg radius, in good agreement with the results derived for NGC 1560 and II
Zw 33 where we do have an independent estimate of the mass fraction in non
baryonic dark matter. Closed-box models are better than O-rich outflow models
in explaining the C/O and Z/O observed values for our typical irregular galaxy.Comment: 14 pages, 2 figure, uses emulateapj.sty package. ApJ in press. New
models were added. The order of Tables has been correcte
S_3-flavour symmetry as realized in lepton flavour violating processes
A variety of lepton flavour violating effects related to the recent discovery
of neutrino oscillations and mixings is here systematically discussed in terms
of an S_3-flavour permutational symmetry. After a brief review of some relevant
results on lepton masses and mixings, that had been derived in the framework of
a Minimal S_3-Invariant Extension of the Standard Model, we derive explicit
analytical expressions for the matrices of the Yukawa couplings and compute the
branching ratios of some selected flavour changing neutral current (FCNC)
processes, as well as, the contribution of the exchange of neutral flavour
changing scalars to the anomaly of the muon's magnetic moment as functions of
the masses of the charged leptons and the neutral Higgs bosons. We find that
the S_3 x Z_2 flavour symmetry and the strong mass hierarchy of the charged
leptons strongly suppress the FCNC processes in the leptonic sector well below
the present experimental upper bounds by many orders of magnitude. The
contribution of FCNC to the anomaly of the muon's magnetic moment is small but
non-negligible.Comment: 23 pages, one figure. To appear in J. Phys A: Mathematical and
Theoretical (SPE QTS5
The Gaia-ESO Survey: Tracing interstellar extinction
Large spectroscopic surveys have enabled in the recent years the computation
of three-dimensional interstellar extinction maps thanks to accurate stellar
atmospheric parameters and line-of-sight distances. Such maps are complementary
to 3D maps extracted from photometry, allowing a more thorough study of the
dust properties. Our goal is to use the high-resolution spectroscopic survey
Gaia-ESO in order to obtain with a good distance resolution the interstellar
extinction and its dependency as a function of the environment and the
Galactocentric position. We use the stellar atmospheric parameters of more than
5000 stars, obtained from the Gaia-ESO survey second internal data release, and
combine them with optical (SDSS) and near-infrared (VISTA) photometry as well
as different sets of theoretical stellar isochrones, in order to calculate
line-of-sight extinction and distances. The extinction coefficients are then
compared with the literature to discuss their dependancy on the stellar
parameters and position in the Galaxy. Within the errors of our method, our
work does not show that there is any dependence of the interstellar extinction
coefficient on the atmospheric parameters of the stars. We do not find any
evidence of the variation of E(J-H)/E(J-K) with the angle from the Galactic
centre nor with Galactocentric distance. This suggests that we are dealing with
a uniform extinction law in the SDSS ugriz bands and the near-IR JHKs bands.
Therefore, extinction maps using mean colour-excesses and assuming a constant
extinction coefficient can be used without introducing any systematic errors.Comment: 13 pages, 14 figures, 1 Appendix accepted for publication in
Astronomy&Astrophysic
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