4,236 research outputs found
An estimate of the time variation of the O/H radial gradient from planetary nebulae
Radial abundance gradients are a common feature of spiral galaxies, and in
the case of the Galaxy both the magnitude of the gradients and their variations
are among the most important constraints of chemical evolution models.
Planetary nebulae (PN) are particularly interesting objects to study the
gradients and their variations. Owing to their bright emission spectra, they
can be observed even at large galactocentric distances, and the derived
abundances are relatively accurate, with uncertainties of about 0.1 to 0.2 dex,
particularly for the elements that are not synthesized in their progenitor
stars. On the other hand, as the offspring of intermediate mass stars, with
main sequence masses in the interval of 1 to 8 solar masses, they are
representative of objects with a reasonable age span. In this paper, we present
an estimate of the time variation of the O/H radial gradient in a sample
containing over 200 nebulae with accurate abundances. Our results are
consistent with a flattening of the O/H gradient roughly from -0.11 dex/kpc to
-0.06 dex/kpc during the last 9 Gyr, or from -0.08 dex/kpc to -0.06 dex/kpc
during the last 5 Gyr.Comment: 9 pages, 7 encapsulated postscript figures, LaTeX, uses Astronomy and
Astrophysics macro aa.cls, graphicx package, to be published in Astronomy and
Astrophysics (2002), Also available at: http://www.astro.iag.usp.br/~macie
The Electron Temperature Gradient in the Galactic Disk
We derive the electron temperature gradient in the Galactic disk using a
sample of HII regions that spans Galactocentric distances 0--17 kpc. The
electron temperature was calculated using high precision radio recombination
line and continuum observations for more than 100 HII regions. Nebular
Galactocentric distances were calculated in a consistent manner using the
radial velocities measured by our radio recombination line survey. The large
number of nebulae widely distributed over the Galactic disk together with the
uniformity of our data provide a secure estimate of the present electron
temperature gradient in the Milky Way. Because metals are the main coolants in
the photoionized gas, the electron temperature along the Galactic disk should
be directly related to the distribution of heavy elements in the Milky Way. Our
best estimate of the electron temperature gradient is derived from a sample of
76 sources for which we have the highest quality data. The present gradient in
electron temperature has a minimum at the Galactic Center and rises at a rate
of 287 +/- 46 K/kpc. There are no significant variations in the value of the
gradient as a function of Galactocentric radius or azimuth. The scatter we find
in the HII region electron temperatures at a given Galactocentric radius is not
due to observational error, but rather to intrinsic fluctuations in these
temperatures which are almost certainly due to fluctuations in the nebular
heavy element abundances. Comparing the HII region gradient with the much
steeper gradient found for planetary nebulae suggests that the electron
temperature gradient evolves with time, becoming flatter as a consequence of
the chemical evolution of the Milky Way's disk.Comment: 43 pages, 9 figures (accepted for publication in the ApJ
Analise semiotica de um ava (ambiente virtual de ensino aprendizagem) : características cts através da escada semotica de Stamper
Este artigo demonstra um estudo e aplicação da análise semiótica, para a avaliação de interfaces de ambientes virtuais de ensino-aprendizagem (AVA), de acordo com a escada semiótica de Stamper. Com base nas características CTS (Ciência,Tecnologia e Sociedade) encontrada nas camadas “degrais” da escada. Em específico, usamos a metodologia para qualidade de interfaces definida por Schimiguel, que instancia critérios de qualidade para cada uma das camadas da escada semiótica de Stamper. Com base nos resultados dessa investigação foi possível sugerir algumas recomendações nas seis camadas analisadas, sinalizando ainda para cada uma dessas recomendações um enfoque CTS, uma descrição que tem como propósito facilitar a implementação e otimização de AVAS
Exact Effective action for (1+1)-dimensional fermions in an Abelian background at finite temperature and chemical potential
In this paper we study the effects of a nonzero chemical potential in the
effective action for massless fermions in (1+1) dimensions in an abelian gauge
field background at finite temperature. We calculate the n-point function and
show that the structure of the amplitudes corresponds to a generalization of
the structure noted earlier in a calculation without a chemical potential (the
associated integrals carry the dependence on the chemical potential). Our
calculation shows that the chiral anomaly is unaffected by the presence of a
chemical potential at finite temperature. However, unlike the earlier
calculation (in the absence of a chemical potential) odd point functions do not
vanish. We trace this to the fact that in the presence of a chemical potential
the generalized charge conjugation symmetry of the theory allows for such
amplitudes. In fact, we find that all the even point functions are even
functions of the chemical potential while the odd point functions are odd
functions of it which is consistent with this generalized charge conjugation
symmetry. We show that the origin of the structure of the amplitudes is best
seen from a formulation of the theory in terms of left and right handed
spinors. The calculations are also much simpler in this formulation and it
clarifies many other aspects of the theory
Sulfur, Chlorine, & Argon Abundances in Planetary Nebulae. I: Observations and Abundances in a Northern Sample
This paper is the first of a series specifically studying the abundances of
sulfur, chlorine, and argon in Type II planetary nebulae (PNe) in the Galactic
disk. Ratios of S/O, Cl/O, and Ar/O constitute important tests of differential
nucleosynthesis of these elements and serve as strict constraints on massive
star yield predictions. We present new ground-based optical spectra extending
from 3600-9600 Angstroms for a sample of 19 Type II northern PNe. This range
includes the strong near infrared lines of [S III] 9069,9532, which allows us
to test extensively their effectiveness as sulfur abundance indicators. We also
introduce a new, model-tested ionization correction factor for sulfur. For the
present sample, we find average values of S/O=1.2E-2(+/- 0.71E-2),
Cl/O=3.3E-4(+/- 1.6E-4), and Ar/O=5.0E-3(+/- 1.9E-3).Comment: 44 pages, 6 figures. Accepted for publication in the Astrophysical
Journa
Efeito de espaçamentos entre emissores e densidades de plantio no rendimento do melão irrigado por gotejamento superficial em vertissolo.
O objetivo do trabalho foi estudar o efeito de espaçamentos entre emissores (G) com diferentes densidades de plantio (P), no rendimento do melão irrigado por gotejamento superficial em Vertissolo
Planetary Nebula Abundances and Morphology: Probing the Chemical Evolution of the Milky Way
This paper presents a homogeneous study of abundances in a sample of 79
northern galactic planetary nebulae whose morphological classes have been
uniformly determined. Ionic abundances and plasma diagnostics were derived from
selected optical line strengths in the literature, and elemental abundances
were estimated with the Ionization Correction Factor developed by Kingsbourgh &
Barlow (1994). We compare the elemental abundances to the final yields obtained
from stellar evolution models of low-and intermediate-mass stars, and we
confirm that most Bipolar planetary nebulae have high nitrogen and helium
abundance, and are the likely progeny of stars with main-sequence mass larger
than 3 solar masses. We derive =0.27, and discuss the implication of such
a high ratio in connection with the solar neon abundance. We determine the
galactic gradients of oxygen and neon, and found Delta log (O/H)/Delta R=-0.01
dex/kpc$ and Delta log (Ne/H)/Delta R=-0.01 dex/kpc. These flat PN gradients do
not reconcile with galactic metallicity gradients flattening with time.Comment: The Astrophysical Journal, in pres
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