243 research outputs found
Recovering 3D structural properties of galaxies from SDSS-like photometry
Because of the 3D nature of galaxies, an algorithm for constructing spatial
density distribution models of galaxies on the basis of galaxy images has many
advantages over surface density distribution approximations. We present a
method for deriving spatial structure and overall parameters of galaxies from
images and estimate its accuracy and derived parameter degeneracies on a sample
of idealised model galaxies. The test galaxies consist of a disc-like component
and a spheroidal component with varying proportions and properties. Both
components are assumed to be axially symmetric and coplanar. We simulate these
test galaxies as if observed in the SDSS project through ugriz filters, thus
gaining a set of realistically imperfect images of galaxies with known
intrinsic properties. These artificial SDSS galaxies were thereafter remodelled
by approximating the surface brightness distribution with a 2D projection of a
bulge+disc spatial distribution model and the restored parameters were compared
to the initial ones. Down to the r-band limiting magnitude 18, errors of the
restored integral luminosities and colour indices remain within 0.05 mag and
errors of the luminosities of individual components within 0.2 mag. Accuracy of
the restored bulge-to-disc ratios (B/D) is within 40% in most cases, and
becomes worse for galaxies with low B/D, but the general balance between bulges
and discs is not shifted systematically. Assuming that the intrinsic disc axial
ratio is < 0.3, the inclination angles can be estimated with errors < 5deg for
most of the galaxies with B/D < 2 and with errors < 15deg up to B/D = 6. Errors
of the recovered sizes of the galactic components are below 10% in most cases.
In general, models of disc components are more accurate than models of
spheroidal components for geometrical reasons.Comment: 15 pages, 13 figures, accepted for publication in RA
Near-Infrared Spectroscopy of Carbon-Enhanced Metal-Poor Stars. I. A SOAR/OSIRIS Pilot Study
We report on an abundance analysis for a pilot study of seven Carbon-Enhanced
Metal-Poor (CEMP) stars, based on medium-resolution optical and near-infrared
spectroscopy. The optical spectra are used to estimate [Fe/H], [C/Fe], [N/Fe],
and [Ba/Fe] for our program stars. The near-infrared spectra, obtained during a
limited early science run with the new SOAR 4.1m telescope and the Ohio State
Infrared Imager and Spectrograph (OSIRIS), are used to obtain estimates of
[O/Fe] and 12C/13C. The chemical abundances of CEMP stars are of importance for
understanding the origin of CNO in the early Galaxy, as well as for placing
constraints on the operation of the astrophysical s-process in very
low-metallicity Asymptotic Giant Branch (AGB) stars.
This pilot study includes a few stars with previously measured [Fe/H],
[C/Fe], [N/Fe],[O/Fe], 12C/13C, and [Ba/Fe], based on high-resolution optical
spectra obtained with large-aperture telescopes. Our analysis demonstrates that
we are able to achieve reasonably accurate determinations of these quantities
for CEMP stars from moderate-resolution optical and near-infrared spectra. This
opens the pathway for the study of significantly larger samples of CEMP stars
in the near future. Furthermore, the ability to measure [Ba/Fe] for (at least
the cooler) CEMP stars should enable one to separate stars that are likely to
be associated with s-process enhancements (the CEMP-s stars) from those that do
not exhibit neutron-capture enhancements (the CEMP-no stars).Comment: 27 pages, including 5 tables, 6 figures, accepted for publication in
The Astronomical Journa
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Antiferromagnetic Ising spin glass competing with BCS pairing interaction in a transverse field
The competition among spin glass (SG), antiferromagnetism (AF) and local
pairing superconductivity (PAIR) is studied in a two-sublattice fermionic Ising
spin glass model with a local BCS pairing interaction in the presence of an
applied magnetic transverse field . In the present approach, spins in
different sublattices interact with a Gaussian random coupling with an
antiferromagnetic mean and standard deviation . The problem is
formulated in the path integral formalism in which spin operators are
represented by bilinear combinations of Grassmann variables. The saddle-point
Grand Canonical potential is obtained within the static approximation and the
replica symmetric ansatz. The results are analysed in phase diagrams in which
the AF and the SG phases can occur for small ( is the strength of the
local superconductor coupling written in units of ), while the PAIR phase
appears as unique solution for large . However, there is a complex line
transition separating the PAIR phase from the others. It is second order at
high temperature that ends in a tricritical point. The quantum fluctuations
affect deeply the transition lines and the tricritical point due to the
presence of .Comment: 16 pages, 6 figures, accepted Eur. Phys. J.
Levantamento florÃstico de um componente arbóreo de mata ciliar do Rio Paraná, Marechal Cândido Rondon, PR
The objective this study was to conduct a floristic survey in the tree component of riparian forest area in the left bank of the Paraná river, Marechal Cândido Rondon, district of Iguiporã, PR. The study was conducted in an area of 600 m2, in six parcels of 20m x 5m, and were used to sample trees with DAP ≥ 10 cm. Frequency, density parameters and Shannon-Weaver index were calculated for each species. The survey resulted in 54 individuals belonging to ten species, ten genera and eight families. The most frequent and abundant species was Parapiptadenia rigida. The family with greater frequency of species was the Leguminosae-Mimosoideae. The Shannon-Weaver index found was 2.05.Objetivou-se realizar um levantamento florístico no componente arbóreo de uma área de mata ciliar na margem esquerda do rio Paraná, município de Marechal Cândido Rondon, distrito de Iguiporã, PR. O estudo foi realizado em uma área de 600 m2, em seis parcelas amostrais de 5 x 20 m, tendo-se como critério de inclusão um diâmetro à altura do peito (DAP) mínimo de 10 cm. Para cada espécie amostrada foram estimados parâmetros relativos à frequência, densidade e índice de diversidade de Shannon-Weaver. O levantamento resultou em 54 indivíduos pertencentes a dez espécies, dez gêneros e oito famílias. A espécie mais abundante e frequente foi Parapiptadenia rigida. A família com maior frequência de espécies foi a Leguminosae-Mimosoideae. O índice de diversidade de Shannon-Weaver obtido foi de 2,05
The Kr85 s-process Branching and the Mass of Carbon Stars
We present new spectroscopic observations for a sample of C(N)-type red
giants. These objects belong to the class of Asymptotic Giant Branch stars,
experiencing thermal instabilities in the He-burning shell (thermal pulses).
Mixing episodes called third dredge-up enrich the photosphere with newly
synthesized C12 in the He-rich zone, and this is the source of the high
observed ratio between carbon and oxygen (C/O > 1 by number). Our spectroscopic
abundance estimates confirm that, in agreement with the general understanding
of the late evolutionary stages of low and intermediate mass stars, carbon
enrichment is accompanied by the appearance of s-process elements in the
photosphere. We discuss the details of the observations and of the derived
abundances, focusing in particular on rubidium, a neutron-density sensitive
element, and on the s-elements Sr, Y and Zr belonging to the first s-peak. The
critical reaction branching at Kr85, which determines the relative enrichment
of the studied species, is discussed. Subsequently, we compare our data with
recent models for s-processing in Thermally Pulsing Asymptotic Giant Branch
stars, at metallicities relevant for our sample. A remarkable agreement between
model predictions and observations is found. Thanks to the different neutron
density prevailing in low and intermediate mass stars, comparison with the
models allows us to conclude that most C(N) stars are of low mass (M < 3Mo). We
also analyze the C12/C13 ratios measured, showing that most of them cannot be
explained by canonical stellar models. We discuss how this fact would require
the operation of an ad hoc additional mixing, currently called Cool Bottom
Process, operating only in low mass stars during the first ascent of the red
giant branch and, perhaps, also during the asymptotic giant branch.Comment: 54 pages + 6 figures + 6 tables. ApJ accepte
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