676 research outputs found
Planetary nebulae abundances and stellar evolution
A summary is given of planetary nebulae abundances from ISO measurements. It
is shown that these nebulae show abundance gradients (with galactocentric
distance), which in the case of neon, argon, sulfur and oxygen (with four
exceptions) are the same as HII regions and early type star abundance
gradients. The abundance of these elements predicted from these gradients at
the distance of the Sun from the center are exactly the solar abundance. Sulfur
is the exception to this; the reason for this is discussed. The higher solar
neon abundance is confirmed; this is discussed in terms of the results of
helioseismology. Evidence is presented for oxygen destruction via ON cycling
having occurred in the progenitors of four planetary nebulae with bilobal
structure. These progenitor stars had a high mass, probably greater than 5
solar masses. This is deduced from the high values of He/H and N/H found in
these nebulae. Formation of nitrogen, helium and carbon are discussed. The high
mass progenitors which showed oxygen destruction are shown to have probably
destroyed carbon as well. This is probably the result of hot bottom burning.Comment: 9 pages, 2 Tables, 8 figures, A&A in pres
Neon, sulphur and argon abundances of planetary nebulae in the sub-solar metallicity Galactic anti-centre
Context: Spectra of planetary nebulae show numerous fine structure emission lines from ionic species, enabling us to study the overall abundances of the nebular material that is ejected into the interstellar medium. The abundances derived from planetary nebula emission show the presence of a metallicity gradient within the disk of the Milky Way up to Galactocentric distances of ∼ 10 kpc, which are consistent with findings from studies of different types of sources, including H II regions and young B-type stars. The radial dependence of these abundances further from the Galactic centre is in dispute.
Aims: We aim to derive the abundances of neon, sulphur and argon from a sample of planetary nebulae towards the Galactic anti- centre, which represent the abundances of the clouds from which they were formed, as they remain unchanged throughout the course of stellar evolution. We then aim to compare these values with similarly analysed data from elsewhere in the Milky Way in order to observe whether the abundance gradient continues in the outskirts of our Galaxy.
Methods: We have observed 23 planetary nebulae at Galactocentric distances of 8–21 kpc with Spitzer IRS. The abundances were calculated from infrared emission lines, for which we observed the main ionisation states of neon, sulphur, and argon, which are little affected by extinction and uncertainties in temperature measurements or fluctuations within the planetary nebula. We have complemented these observations with others from optical studies in the literature, in order to reduce or avoid the need for ionisation correction factors in abundance calculations.
Results: The overall abundances of our sample of planetary nebulae in the Galactic anti-centre are lower than those in the solar neighbourhood. The abundances of neon, sulphur, and argon from these stars are consistent with a metallicity gradient from the solar neighbourhood up to Galactocentric distances of ∼ 20 kpc, albeit with varying degrees of dispersion within the data
Advanced optimal extraction for the Spitzer/IRS
We present new advances in the spectral extraction of point-like sources
adapted to the Infrared Spectrograph onboard the Spitzer Space Telescope. For
the first time, we created a super-sampled point spread function of the
low-resolution modules. We describe how to use the point spread function to
perform optimal extraction of a single source and of multiple sources within
the slit. We also examine the case of the optimal extraction of one or several
sources with a complex background. The new algorithms are gathered in a plugin
called Adopt which is part of the SMART data analysis software.Comment: Accepted for publication in PAS
PAH Strength and the Interstellar Radiation Field around the Massive Young Cluster NGC3603
We present spatial distribution of polycyclic aromatic hydrocarbons and
ionized gas within the Galactic giant HII region NGC3603. Using the IRS
instrument on board the Spitzer Space Telescope, we study in particular the PAH
emission features at ~5.7, 6.2, 7.7, 8.6, and 11.3um, and the [ArII] 6.99um,
[NeII] 12.81um, [ArIII] 8.99um, and [SIV] 10.51um forbidden emission lines. The
observations probe both ionized regions and photodissociation regions. Silicate
emission is detected close to the central cluster while silicate absorption is
seen further away. We find no significant variation of the PAH ionization
fraction across the whole region. The emission of very small grains lies closer
to the central stellar cluster than emission of PAHs. The PAH/VSG ratio
anticorrelates with the hardness of the interstellar radiation field suggesting
a destruction mechanism of the molecules within the ionized gas, as shown for
low-metallicity galaxies by Madden et al. (2006).Comment: Accepted for publication in ApJ. Corrected typo
Planetary nebulae abundances and stellar evolution II
Context: In recent years mid- and far infrared spectra of planetary nebulae
have been analysed and lead to more accurate abundances. It may be expected
that these better abundances lead to a better understanding of the evolution of
these objects. Aims: The observed abundances in planetary nebulae are compared
to those predicted by the models of Karakas (2003) in order to predict the
progenitor masses of the various PNe used. The morphology of the PNe is
included in the comparison. Since the central stars play an important role in
the evolution, it is expected that this comparison will yield additional
information about them. Methods: First the nitrogen/oxygen ratio is discussed
with relation to the helium/hydrogen ratio. The progenitor mass for each PNe
can be found by a comparison with the models of Karakas. Then the present
luminosity of the central stars is determined in two ways: first by computing
the central star effective temperature and radius, and second by computing the
nebular luminosity from the hydrogen and helium lines. This luminosity is also
a function of the initial mass so that these two values of initial mass can be
compared. Results: Six of the seven bipolar nebulae can be identified as
descendants of high mass stars (4Msun - 6Msun) while the seventh is ambiguous.
Most of the elliptical PNe have central stars which descend from low initial
mass stars, although there are a few caveats which are discussed. There is no
observational evidence for a higher mass for central stars which have a high
carbon/oxygen ratio. The evidence provided by the abundance comparison with the
models of Karakas is consistent with the HR diagram to which it is compared. In
the course of this discussion it is shown how `optically thin' nebulae can be
separated from those which are 'optically thick'.Comment: 12 pages, 4 tables, 4 figures. Accepted for publication in A&
Abundances of Planetary Nebula M1-42
The spectra of the planetary nebula M1-42 is reanalysed using spectral
measurements made in the mid-infrared with the Spitzer Space Telescope. The aim
is to determine the chemical composition of this object. We also make use of
ISO, IUE and ground based spectra. Abundances determined from the mid- and
far-infrared lines, which are insensitive to electron temperature, are used as
the basis for the determination of the composition, which are found to
substantially differ from earlier results. High values of neon, argon and
sulfur are found. They are higher than in other PN, with the exception of
NGC6153, a nebula of very similar abundances. The high values of helium and
nitrogen found indicate that the second dredge-up and hot bottom burning has
occurred in the course of evolution and that the central star was originally
more massive than 4Msun. The present temperature and luminosity of the central
star is determined and at first sight may be inconsistent with such a high
mass.Comment: 9 pages, 8 tables, 1 figure. Accepted for publication in A&
Abundances of planetary nebulae NGC 3242 and NGC 6369
The spectra of the planetary nebulae NGC3242 and NGC6369 are reanalysed using spectral measurements made in the mid-infrared with the Spitzer Space Telescope and the Infrared Space Observatory (ISO). The aim is to determine the chemical composition of these objects. We also make use of International Ultraviolet Explorer (IUE) and ground based spectra. These elliptical PNe are interesting because they are well-studied, nearby, bright objects and therefore allow a reasonably complete comparison of this type of nebulae. Abundances determined from the mid-infrared lines, which are insensitive to electron temperature, are used as the basis for the determination of the composition, which are found to differ somewhat from earlier results. The abundances found, especially the low value of helium and oxygen, indicate that the central star was originally of rather low mass. The abundance of phosphorus has been determined for the first time in NGC 3242. The electron temperature in both of these nebulae is roughly constant unlike NGC 6302 and NGC 2392 where a strong temperature gradient is found. The temperature of the central star is discussed for both nebulae. Finally a comparison of the element abundances in these nebulae with the solar abundance is made. The low abundance of Fe and P is noted and it is suggested that these elements are an important constituent of the nebular dust.</p
Photoionization modeling of the Galactic planetary nebulae Abell 39 and NGC 7027
We estimate distances to the spherical planetary nebula Abell 39 and the
bipolar planetary nebula NGC 7027 by interpolating from a wide grid of
photoionization models using the 3-D code, MOCASSIN. We find preliminary
distances of 1.5 kpc and 0.9 kpc respectively, with uncertainties of about 30%.Comment: 2 pages, 1 figure, presented at the IAU Symposium 283 "Planetary
Nebulae: an Eye to the Future", Puerto de la Cruz, Tenerife, Spain, 201
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