5,653 research outputs found
UV spectral analysis of very hot H-deficient [WCE]-type central stars of planetary nebulae: NGC 2867, NGC 5189, NGC 6905, Pb 6, and Sand 3
We analysed UV FUSE, IUE, and HST/STIS spectra of five of the hottest
[WCE]-type central stars of planetary nebulae: NGC 2867, NGC 5189, NGC 6905, Pb
6, and Sand 3. The analysis leveraged on our grid of CMFGEN synthetic spectra,
which covers the parameter regime of hydrogen deficient central stars of
planetary nebulae and allows a uniform and systematic study of the stellar
spectra. The stellar atmosphere models calculated by us include many elements
and ionic species neglected in previous analyses, which allowed us to improve
the fits to the observed spectra considerably and provided an additional
diagnostic line: the Ne VII 973 , which had not been
modelled in [WCE] spectra and which presents, in these stars, a strong P-Cygni
profile. We report newly derived photospheric and wind parameters and elemental
abundances. The central stars of NGC 2867, NGC 5189, and Pb 6 had their
temperatures revised upward in comparison with previous investigations and we
found the carbon to helium mass ratio of the sample objects to span a wide
range of values, 0.42C:He1.96. Modelling of the Ne VII
973 P-Cygni profile indicated strong neon overabundances for the
central stars of NGC 2867, NGC 5189, NGC 6905, and Pb 6, with Ne mass fractions
between 0.01 and 0.04. Nitrogen abundances derived by us for the central stars
of NGC 5189, Pb 6, and Sand 3 are higher than previous determinations by
factors of 3, 10, and 14, respectively.Comment: Accepted on MNRA
Planetary nebulae in the inner Milky Way: new abundances
The study of planetary nebulae in the inner-disk and bulge gives important
information on the chemical abundances of elements such as He, N, O, Ar, Ne,
and on the evolution of these abundances, which is associated with the
evolution of intermediate-mass stars and the chemical evolution of the Galaxy.
We present accurate abundances of the elements He, N, S, O, Ar, and Ne for a
sample of 54 planetary nebulae located towards the bulge of the Galaxy, for
which 33 have the abundances derived for the first time. The abundances are
derived based on observations in the optical domain made at the National
Laboratory for Astrophysics (LNA, Brazil). The data show a good agreement with
other results in the literature, in the sense that the distribution of the
abundances is similar to those works.Comment: Accepted for publication in RevMexAA (29 pages, 15 figures, 7 tables,
uses rmaa.cls
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
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