Stuart R. Pottasch

The Lower Solar Corona: Interpretation of the Ultraviolet Spectrum.

An analysis of the resonance lines of nine elements (27 ions) formed in the chromosphere and corona of the sun yields the following results: the chemical composition of this region of the sun can be determined, without any knowledge of the detailed temperature-density structure in this region; a further clue concerning the detailed structure of this region may be obtained. A prediction of the expected emission of the sun in radio frequencies can be used both as a check on the correctness of the analysis and as a means of obtaining the ratio of the observed elements to hydrogen. (auth)

The ISO–SWS spectrum of planetary nebula NGC 7027

We present the infrared spectrum of the planetary nebula NGC 7027 observed with the Short Wavelength Spectrometer (SWS), on board the Infrared Space Observatory (ISO). These data allow us to derive the electron density and, together with the IUE and optical spectra, the electron temperature for several ions. The nebular composition has been determined, the evolutionary status of the central star is discussed and the element depletion in the nebula is given. We conclude that the progenitor was a C{rich star with a mass between 3 and 4 M.

Probing AGB nucleosynthesis via accurate Planetary Nebula abundances

The elemental abundances of ten planetary nebulae, derived with high accuracy including ISO and IUE spectra, are analysed with the aid of synthetic evolutionary models for the TP-AGB phase. The accuracy on the observed abundances is essential in order to make a reliable comparison with the models. The advantages of the infrared spectra in achieving this accuracy are discussed. Model prescriptions are varied until we achieve the simultaneous reproduction of all elemental features, which allows placing important constraints on the characteristic masses and nucleosynthetic processes experienced by the stellar progenitors. First of all, it is possible to separate the sample into two groups of PNe, one indicating the occurrence of only the third dredge-up during the TP-AGB phase, and the other showing also the chemical signature of hot-bottom burning. The former group is reproduced by stellar models with variable molecular opacities (see Marigo 2002), adopting initial solar metallicity, and typical efficiency of the third dredge-up, λ ∼ 0.3−0.4. The latter group of PNe, with extremely high He content (0.15 ≤ He/H ≤ 0.20) and marked oxygen deficiency, is consistent with original sub-solar metallicity (i.e. LMC composition). Moreover, we are able to explain quantitatively both the N/H-He/H correlation and the N/H-C/H anti-correlation, thus solving the discrepancy pointed out long ago by Becker & Iben (1980). This is obtained only under the hypothesis that intermediate- mass TP-AGB progenitors (M > 4.5−5.0 M� ) with LMC composition have suffered a number of very efficient, carbon-poor, dredge-up events. Finally, the neon abundances of the He-rich PNe can be recovered by invoking a significant production of 22 Ne during thermal pulses, which would imply a reduced role of the 22 Ne(α, n) 25 Mg reaction as neutron source to the Planetary Nebulae (PNe) are assumed to consist of the gas ejected via stellar winds by low- and intermediate-mass stars (having initial masses 0.9 ≤ M/M� ≤ Mup, with Mup ∼ 5−8 Mdepending on model details) during their last evo- lutionary stages, the so-called Thermally Pulsing Asymptotic Giant Branch (TP-AGB) phase. PNe offer potentially a good possibility to test the results of stellar nucleosynthesis. This can be done in a reliable way by comparing the predicted abundances of the gas ejected close to the end of the AGB phase with the observed abundances because the expelled hot gas remains unaffected by interac- tion with the ISM or with previous shell ejection. Furthermore the ionized gas surrounding the central star shows lines of many elements from which accurate abundances can be de- rived. Also by the ejection of the outer layers PNe contribute to the enrichment of the interstellar medium (ISM) and therefore,

Planetary nebulae abundances and stellar evolution

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 tothose predicted by the models of Karakas (2003, Thesis, Monash Univ. Melbourne) 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 (4−6 M� ) 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”.

CHEMICAL ABUNDANCES AND DUST IN PLANETARY NEBULAE IN THE GALACTIC BULGE

We present mid-infrared Spitzer spectra of 11 planetary nebulae in the Galactic bulge. We derive argon, neon, sulfur, and oxygen abundances for them using mainly infrared line fluxes combined with some optical line fluxes from the literature. Due to the high extinction toward the bulge, the infrared spectra allow us to determine abundances for certain elements more accurately than previously possible with optical data alone. Abundances of argon and sulfur (and in most cases neon and oxygen) in planetary nebulae in the bulge give the abundances of the interstellar medium at the time their progenitor stars formed; thus, these abundances give information about the formation and evolution of the bulge. The abundances of bulge planetary nebulae tend to be slightly higher than those in the disk on average, but they do not follow the trend of the disk planetary nebulae, thus confirming the difference between bulge and disk evolution. In addition, the bulge planetary nebulae show peculiar dust properties compared to the disk nebulae. Oxygen-rich dust features (crystalline silicates) dominate the spectra of all of the bulge planetary nebulae; such features are more scarce in disk nebulae. In addition, carbon-rich dust features (polycyclic aromatic hydrocarbons) appear in roughly half of the bulge planetary nebulae in our sample, which is interesting in light of the fact that this dual chemistry is comparatively rare in the Milky Way as a whole.

Neon and Sulfur Abundances of Planetary Nebulae in the Magellanic Clouds

The chemical abundances of neon and sulfur for 25 planetary nebulae (PNe) in the Magellanic Clouds are presented. These abundances have been derived using mainly infrared data from the Spitzer Space Telescope. The implications for the chemical evolution of these elements are discussed. A comparison with similarly obtained abundances of Galactic PNe and H II regions and Magellanic Cloud H II regions is also given. The average neon abundances are 6.0 X 10^(-5) and 2.7 X 10^(-5) for the PNe in the Large and Small Magellanic Clouds, respectively. These are ~1/3 and 1/6 of the average abundances of Galactic planetary nebulae to which we compare. The average sulfur abundances for the LMC and SMC are, respectively, 2.7 X 10^(-6) and 1.0 X 10^(-6). The Ne/S ratio (23.5) is on average higher than the ratio found in Galactic PNe (16), but the range of values in both data sets is similar for most of the objects. The neon abundances found in PNe and H II regions agree with each other. It is possible that a few (3-4) of the PNe in the sample have experienced some neon enrichment, but for two of these objects the high Ne/S ratio can be explained by their very low sulfur abundances. The neon and sulfur abundances derived in this paper are also compared to previously published abundances using optical data and photoionization models.

Abundances of Planetary Nebulae IC 418, IC 2165 and NGC 5882

The ISO and IUE spectra of the elliptical nebulae NGC 5882, IC 418 and IC 2165 are presented. These spectra are combined with the spectra in the visual wavelength region to obtain a complete, extinction corrected, spectrum. The chemical composition of the nebulae is then calculated and compared to previous determinations. A discussion is given of: (1) the recombination line abundances; (2) the exciting stars of the nebulae; and (3) possible evolutionary effects.

Evolution of planetary nebulae

A second argument in this direction was given by several astronomers in the 1950s. It had long been known from the double peaked emission lines observed in the nebulae, that they were expanding. The velocity of expansion is on the average 15 to 20 km s -t, with only exceptional cases differing by more than a factor 2 from this value. Since this nebular material presumably escaped from the central star, the question was asked: which stars have an escape velocity of the order of 20 km s-t? The answer is Red Giants. On considering this answer, other reasons for its correctness became apparent. A red giant actually has the characteristics of an early stage of a PN. Much of its mass is present in a high density core which could easily be the forerunner of the PN central star. Some of its mass is present in the enormous giant atmosphere which could easily become the nebula. It only remained to set the giant atmosphere in motion at the escape velocity. The solution the star has chosen to solve this problem is not yet completely understood. It is clear however that the mass lost by the star to form a PN occurs at a stage prior to its being observed as a PN. Thus the previous stage, whether OH/IR star, Mira variables and other types of red giants will also be discussed.

Abundances of planetary nebula NGC 5315

The spectra of the planetary nebula NGC 2392 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 IUE and ground based spectra. 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. Abundances of phosphorus, iron, silicon and chlorine have been determined for the first time in this nebula. The variation of electron temperature in this nebula is very clear, reaching quite high values close to the center. The temperature of the central star is discussed in the light of the observed high stages of ionization. The nebular information indicates that the spectrum of the star deviates considerably from a blackbody.

Abundances of Planetary Nebulae BD+30 3639 and NGC 6543

Infrared spectra taken with the Short Wavelength Spectrometer on board ISO and UV observations with IUE of planetary nebulae BD+30 3639 and NGC 6543 are presented. The extinction derived using infrared lines for BD+30 3639 is EB−V = 0.34, slightly higher than previous determinations. For NGC 6543 the extinction found from the hydrogen lines is EB−V = 0.07. Infrared, optical and ultraviolet data have been used to derive the physical parameters of the nebula. BD+30 3639 has an average Te = 8500 K and Ne = 11 000 cm −3 . In the case of NGC 6543 a Te = 8200 K and Ne = 5000 cm −3 have been found, in agreement with previous determinations. The element abundances have been derived and compared to those found in the Sun and O, B stars. This comparison gives a hint of the mass of the progenitor stars from which they evolved.