439 research outputs found
Heavy element abundances in planetary nebulae: A theorist's perspective
The determination of heavy element abundances from planetary nebula (PN)
spectra provides an exciting opportunity to study the nucleosynthesis occurring
in the progenitor asymptotic giant branch (AGB) star. We perform
post-processing calculations on AGB models of a large range of mass and
metallicity to obtain predictions for the production of neutron-capture
elements up to the first s-process peak at strontium. We find that solar
metallicity intermediate-mass AGB models provide a reasonable match to the
heavy element composition of Type I PNe. Likewise, many of the Se and Kr
enriched PNe are well fitted by lower mass models with solar or close-to-solar
metallicities. However the most Kr-enriched objects, and the PN with sub-solar
Se/O ratios are difficult to explain with AGB nucleosynthesis models.
Furthermore, we compute s-process abundance predictions for low-mass AGB models
of very low metallicity ([Fe/H] =-2.3) using both scaled solar and an
alpha-enhanced initial composition. For these models, O is dredged to the
surface, which means that abundance ratios measured relative to this element
(e.g., [X/O]) do not provide a reliable measure of initial abundance ratios, or
of production within the star owing to internal nucleosynthesis.Comment: 5 pages, presentation at the workshop on the Legacies of the
Macquarie/AAO/Strasbourg H-alpha Planetary Nebula project, accepted for
publication in PAS
Absolute Spectrophotometry of Northern Compact Planetary Nebulae
We present medium-dispersion spectra and narrowband images of six northern
compact planetary nebulae (PNe): BoBn 1, DdDm 1, IC 5117, M 1-5, M 1-71, and
NGC 6833. From broad-slit spectra, total absolute fluxes and equivalent widths
were measured for all observable emission lines. High signal-to noise emission
line fluxes of H-alpha, H-beta, [OIII], [NII], and HeI may serve as emission
line flux standards for northern hemisphere observers. From narrow-slit
spectra, we derive systemic radial velocities. For four PNe, available emission
line fluxes were measured with sufficient signal-to-noise to probe the physical
properties of their electron densities, temperatures, and chemical abundances.
BoBn 1 and DdDm 1, both type IV PNe, have an H-beta flux over three sigma away
from previous measurements. We report the first abundance measurements of M
1-71. NGC 6833 measured radial velocity and galactic coordinates suggest that
it is associated with the outer arm or possibly the galactic halo, and its low
abundance ([O/H]=1.3x10E-4) may be indicative of low metallicity within that
region.Comment: 9 pages, 1 figure, accepted in A&A (03/14/2005
New candidate planetary nebulae in M81
A 34 x 34 arcmin field centred on the spiral galaxy M81 has been searched for
emission-line objects using the prime focus wide field camera (WFC) of the 2.54
m Isaac Newton Telescope (La Palma, Spain). A total of 171 candidate planetary
nebulae (PNe) are found, 54 of which are in common with the ones detected by
Jacoby et al. (1989). The behaviour of PNe excitation as a function of
galactocentric distance is examined, and no significant variations are found.
The PNe luminosity function is built for the disk and bulge of M81, separately.
A distance modulus of 27.92+-0.23 mag is found for disk PNe, in good agreement
with previous distance measurements for M81 (Jacoby et al. 1989; Huterer et al.
1995).Comment: 7 pages including 2 tables. A&A accepted; also available at
http://www.iac.es/publicaciones/preprints.htm
When Shape Matters: correcting the ICFs to derive the chemical abundances of bipolar and elliptical PNe
The extraction of chemical abundances of ionised nebulae from a limited
spectral range is usually hampered by the lack of emission lines corresponding
to certain ionic stages. So far, the missing emission lines have been accounted
for by the ionisation correction factors (ICFs), constructed under simplistic
assumptions like spherical geometry by using 1-D photoionisation modelling.
In this contribution we discuss the results (Goncalves et al. 2011, in prep.)
of our ongoing project to find a new set of ICFs to determine total abundances
of N, O, Ne, Ar, and S, with optical spectra, in the case of non-spherical PNe.
These results are based on a grid of 3-D photoionisation modelling of round,
elliptical and bipolar shaped PNe, spanning the typical PN luminosities,
effective temperatures and densities.
We show that the additional corrections --to the widely used Kingsburgh and
Barlow (1994) ICFs-- are always higher for bipolars than for ellipticals.
Moreover, these additional corrections are, for bipolars, up to: 17% for
oxygen, 33% for nitrogen, 40% for neon, 28% for argon and 50% for sulphur.
Finally, on top of the fact that corrections change greatly with shape, they
vary also greatly with the central star temperature, while the luminosity is a
less important parameter.Comment: Oral contribution (4 pages, 2 figures) to IAU Symposium 283:
"Planetary Nebulae: An Eye to the Future" held in Puerto de la Cruz,
Tenerife, Spain in July 25th-29th 201
The chemistry of planetary nebulae and HII regions in the dwarf galaxies Sextans A and B from deep VLT spectra
Spectroscopic observations obtained with the VLT of one planetary nebula (PN)
in Sextans A and of five PNe in Sextans B and of several HII regions (HII) in
these two dwarf irregular galaxies are presented. The extended spectral
coverage, from 320.0 to 1000.0nm, and the large telescope aperture allowed us
to detect a number of emission lines, covering more than one ionization stage
for several elements (He, O, S, Ar). The electron temperature (Te) diagnostic
[OIII] line at 436.3 nm was measured in all six PNe and in several HII allowing
for an accurate determination of the ionic and total chemical abundances by
means of the Ionization Correction Factors method. For the time being, these
PNe are the farthest ones where such a direct measurement of the Te is
obtained. In addition, all PNe and HII were also modelled using the
photoionization code CLOUDY. The physico-chemical properties of PNe and HII are
presented and discussed. A small dispersion in the oxygen abundance of HII was
found in both galaxies: 12 + (O/H)=7.60.2 in SextansA, and
7.80.2 in SextansB. For the five PNe of SextansA, we find that 12 +
(O/H)=8.00.3, with a mean abundance consistent with that of HII. The
only PN known in SextansA appears to have been produced by a quite massive
progenitor, and has a significant nitrogen overabundance. In addition, its
oxygen abundance is 0.4 dex larger than the mean abundance of HII, possibly
indicating an efficient third dredge-up for massive, low-metallicity PN
progenitors. The metal enrichment of both galaxies is analyzed using these new
data.Comment: 19 pages, 11 figures, Accepted by A&
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