207 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
The metallicity distribution of G dwarfs in the solar neighbourhood
We derive a new metallicity distribution of G dwarfs in the solar
neighbourhood, using uvby photometry and up-to-date parallaxes. Our
distribution comprises 287 G dwarfs within 25 pc from the Sun, and differs
considerably from the classic solar neighbourhood distribution of Pagel &
Patchett and Pagel by having a prominent single peak around [Fe/H] = -0.20 dex.
The raw data are corrected for observational errors and cosmic scatter assuming
a deviation of 0.1. In order to obtain the true abundance distribution, we use
the correction factors given by Sommer-Larsen, which take into account the
stellar scale heights. The distribution confirms the G dwarf problem, that is,
the paucity of metal-poor stars relative to the predictions of the simple model
of chemical evolution. Another feature of this distribution, which was already
apparent in previous ones, is the small number of metal-rich stars again in
comparison with the simple model. Our results indicate that it is very
difficult to fit the simple model to this distribution, even with the
definition of an `effective yield'. A comparison with several models from the
literature is made. We find that models with infall are the most appropriate to
explain the new metallicity distribution. We also show that the metallicity
distribution is compatible with a major era of star formation occurring 5 to 8
Gyr ago, similar to results found by several authors.Comment: Tex, uses mn.tex v1.6, 13 pages, 8 figures available upon request,
accepted for publication in Monthly Notices of Roy. Astr. So
Planetary nebulae in the inner Milky Way
New abundances of planetary nebulae located towards the bulge of the Galaxy
are derived based on observations made at LNA (Brazil). We present accurate
abundances of the elements He, N, S, O, Ar, and Ne for 56 PNe located towards
the galactic bulge. The data shows a good agreement with other results in the
literature, in the sense that the distribution of the abundances is similar to
those works. From the statistical analysis performed, we can suggest a
bulge-disk interface at 2.2 kpc for the intermediate mass population, marking
therefore the outer border of the bulge and inner border of the disk.Comment: 2 pages, 1 figure, uses iaus.cls, in press, IAU Symp. 265, Chemical
abundances in the Universe: Connecting the first Stars to Planets, Ed. K.
Cunha, M. Spite, B. Barbu
Planetary nebulae as probes for galactic chemical evolution
The role of planetary nebulae as probes for the galactic chemical evolution
is reviewed. Their abundances throughout the Galaxy are discussed for key
elements, in particular oxygen and other alpha elements. The abundance
distribution derived from planetary nebulae leads to the establishment of
radial abundance gradients in the galactic disk that are important constraints
to model the chemical evolution of the Galaxy. The radial gradient, well
determined for the solar neighborhood, is examined for distinct regions. For
the galactic anticenter in particular, the observational data confirm results
from galactic evolution models that point to a decreasing in the gradient slope
at large galactocentric distances. The possible time evolution of the radial
gradient is also examined comparing samples of planetary nebulae of different
ages, and the results indicate that a flattening in the gradient occurred,
which is confirmed by some galactic evolution models. The galactic bulge is
another important region whose modeling can be constrained by observational
results obtained from planetary nebulae. Results derived in the last few years
indicate that bulge nebulae have an abundance distribution similar to that of
disk objects, however with a larger dispersion.Comment: 8 pages, 6 figures, LaTeX, To be published in the Proceedings of the
IAU Symposium 234: Planetary Nebulae in Our Galaxy and Beyon
Planetary nebulae and the chemical evolution of the galactic bulge
Electron temperatures, densities, ionic and elemental abundances of helium,
nitrogen, oxygen, argon, sulfur and neon were derived for a sample of bulge
planetary nebulae, representative of its intermediate mass population. Using
these results as constraints, a model for the chemical evolution of the
galactic bulge was developed. The results indicate that the best fit is
achieved using a double-infall model, where the first one is a fast collapse of
primordial gas and the second is slower and enriched by material ejected by the
bulge itself during the first episode.Comment: 4 pages, 4 figures, to appear in the proceedings of the conference
"Planetary Nebulae as astronomical tools" held in Gdansk, Poland, jun 28/jul
02, 200
Planetary nebulae and the chemical evolution of the galactic bulge: new abundances of older objects
In view of their nature, planetary nebulae have very short lifetimes, and the
chemical abundances derived so far have a natural bias favoring younger
objects. In this work, we report physical parameters and abundances for a
sample of old PNe located in the galactic bulge, based on low dispersion
spectroscopy secured at the SOAR telescope using the Goodman Spectrograph. The
new data allow us to extend our database including older, weaker objects that
are at the faint end of the planetary nebula luminosity function (PNLF). The
results show that the abundances of our sample are lower than those from our
previous work. Additionally, the average abundances of the galactic bulge do
not follow the observed trend of the radial abundance gradient in the disk.
These results are in agreement with a chemical evolution model for the Galaxy
recently developed by our group.Comment: 2 pages, 2 figures, to appear in proceedings of the IAU Symposium
283: "Planetary Nebulae: An Eye to the Future", Eds.: A. Manchado, L.
Stanghellini and D. Schoenberne
- âŠ