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It has been suggested that the ratio of total-to-selective extinction RV in dust in the interstellar medium differs in the Galactic bulge from its value in the local neighborhood. We attempt to test this suggestion. The mid-infrared hydrogen lines in 16 Galactic bulge PNe measured by the Spitzer Space Telescope are used to determine the extinction corrected H{\beta} flux. This is compared to the observed H{\beta} flux to obtain the total extinction at H{\beta}. The selective extinction is obtained from the observed Balmer decrement in these nebulae. The value of RV can then be found. The ratio of total-to-selective extinction in the Galactic bulge is consistent with the value RV =3.1, which is the same as has been found in the local neighborhood. We conclude that the suggestion that RV is different in the Galactic bulge is incorrect. The reasons for this are discussed., Comment: 6 pages, 2 figures, 3 tables, accepted for publication in A&A

The aim of the paper is to determine abundances in a group of PNe with uniform morphology. The PNe discussed are circular excited by rather low-temperature central stars. The relation between abundance and evolution is discussed. The mid-infrared spectra of NGC1535, NGC6629, He2-108 and Tc1 taken with the Spitzer Space Telescope are presented. These spectra are combined with IUE and visual spectra to obtain complete extinction-corrected spectra from which the abundances are determined. These abundances are more accurate for several reasons, the most important is that the inclusion of the far infrared spectra increases the number of observed ions and makes it possible to include the nebular temperature gradient in the abundance calculation. The abundances of these PNe are compared to those found in five other PNe of similar properties and are further compared with predictions of evolutionary models. From this comparison we conclude that these PNe originated from low mass stars, probably between 1 and 2.5 solar masses and at present have core masses between 0.56 and 0.63 solar masses. A consistent description of the evolution of this class of PNe is found that agrees with the predictions of the present nebular abundances, the individual masses and the luminosities of these PNe. The distances to these nebulae can be found as well., Comment: 17 pages, 18 tables, 1 figure, Accepted for publication in A&A

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&A

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 NGC3242. The electron temperature in both of these nebulae is roughly constant unlike NGC6302 and NGC2392 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., Comment: 11 pages, 2 figures, 15 tables. Accepted for publication in A&A

We present mid-infrared Spitzer spectra of eleven 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 that 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. Additionally, the Bulge planetary nebulae show peculiar dust properties compared to the Disk nebulae. Oxygen-rich dust feature (crystalline silicates) dominate the spectra of all of the Bulge planetary nebulae; such features are more scarce in Disk nebulae. Additionally, 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., Comment: 16 pages, 5 figures, accepted to ApJ

The spectra of the planetary nebula NGC2392 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 high observed stages of ionization. The nebular information indicates the spectrum of the star deviates considerably from a blackbody., Comment: 9 pages, 7 tables, 3 figures. Accepted for publication in A&A

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 HII regions and Magellanic Clouds HII regions is also given. The average neon abundances are 6.0x10(-5) and 2.7x10(-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 2.7x10(-6) and 1.0x10(-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 HII 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 photo-ionization models., Comment: 13 pages, 4 tables, 5 figures. Accepted for publication in ApJ

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&A

We present the mid-infrared spectrum of the planetary nebula IC 2448. In order to determine the chemical composition of the nebula, we use the infrared line fluxes from the Spitzer spectrum along with optical line fluxes from the literature and ultraviolet line fluxes from archival IUE spectra. We determine an extinction of C(H-beta) = 0.27 from hydrogen recombination lines and the radio to H-beta ratio. Forbidden line ratios give an electron density of 1860 cm-3 and an average electron temperature of 12700 K. The use of infrared lines allows us to determine more accurate abundances than previously possible because abundances derived from infrared lines do not vary greatly with the adopted electron temperature and extinction, and additional ionization stages are observed. Elements left mostly unchanged by stellar evolution (Ar, Ne, S, and O) all have subsolar values in IC 2448, indicating that the progenitor star formed out of moderately metal deficient material. Evidence from the Spitzer spectrum of IC 2448 supports previous claims that IC 2448 is an old nebula formed from a low mass progenitor star., Comment: 7 pages, 2 figures, accepted for publication in The Astrophysical Journal

Aims: We study the optical spectral properties of a sample of stars showing far infrared colours similar to those of well-known planetary nebulae. The large majority of them were unidentified sources or poorly known in the literature at the time when this spectroscopic survey started, some 15 years ago. Methods: We present low-resolution optical spectroscopy, finding charts and improved astrometric coordinates of a sample of 253 IRAS sources. Results: We have identified 103 sources as post-AGB stars, 21 as ``transition sources'', and 36 as planetary nebulae, some of them strongly reddened. Among the rest of sources in the sample, we were also able to identify 38 young stellar objects, 5 peculiar stars, and 2 Seyfert galaxies. Up to 49 sources in our spectroscopic sample do not show any optical counterpart, and most of them are suggested to be heavily obscured post-AGB stars, rapidly evolving on their way to becoming planetary nebulae. Conclusions: An analysis of the galactic distribution of the sources identified as evolved stars in the sample is presented together with a study of the distribution of these stars in the IRAS two-colour diagram. Finally, the spectral type distribution and other properties of the sources identified as post-AGB in this spectroscopic survey are discussed in the framework of stellar evolution., Comment: 69 pages, 413 figures. Accepted by Astronomy and Astrophysics

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 press

ISO and IUE spectra of the round planetary nebula Me 2-1 are combined with visual spectra taken from the literature to obtain for the first time a complete extinction-corrected spectrum. With this, the physico-chemical characteristics of the nebula and its central star are determined by various methods including photoionization modeling using Cloudy. The results of the modeling are compared to those derived from a more classical, simple abundance determination approach. A discussion is presented on the validity of the different methods used and assumptions made. Finally, the main results are interpreted in terms of the evolutionary stage of Me 2-1 and its central star., Comment: Preprint

The first observations of the infrared spectrum of the LMC planetary nebula SMP83 as observed by the recently launched Spitzer Space Telescope are presented. The high resolution R~600 spectrum shows strong emission lines but no significant continuum. The infrared fine structure lines are used, together with published optical spectra, to derive the electron temperature of the ionized gas for several ions. A correlation between the electron temperature with ionization potential is found. Ionic abundances for the observed infrared ions have been derived and the total neon and sulfur abundances have been determined. These abundances are compared to average LMC abundances of HII regions to better understand the chemical evolution of these elements. The nature of the progenitor star is also discussed., Comment: Accepted in ApJSup (Spitzer Special Issue), 4 pages, 2 figures

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. 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 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 Mo) 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 22Ne during thermal pulses, which would imply a reduced role of the 22Ne(alpha,n)25Mg reaction as neutron source to the s-process nucleosynthesis in these stars., Comment: 22 pages, accepted for publication in Astronomy & Astrophysics

In this paper we study new infrared spectra of the evolved planetary nebula NGC 6445 obtained with ISO. These data show that the thermal emission from the grains is very cool and has a low flux compared to H beta. A model of the ionized region is constructed, using the photo-ionization code CLOUDY 90.05. Based on this model, we show from depletions in the gas phase elements that little grain destruction can have occurred in the ionized region of NGC 6445. We also argue that dust-gas separation in the nebula is not plausible. The most likely conclusion is that grains are residing inside the ionized region of NGC 6445 and that the low temperature and flux of the grain emission are caused by the low luminosity of the central star and the low optical depth of the grains. This implies that the bulk of the silicon-bearing grains in this nebula were able to survive exposure to hard UV photons for at least several thousands of years, contradicting previously published results. A comparison between optical and infrared diagnostic line ratios gives a marginal indication for the presence of a t^2-effect in the nebula. However, the evidence is not convincing and other explanations for the discrepancy are also plausible. The off-source spectrum taken with ISO-LWS clearly shows the presence of a warm cirrus component with a temperature of 24 K as well as a very cold component with a temperature of 7 K. Since our observation encompasses only a small region of the sky, it is not clear how extended the 7 K component is and whether it contributed significantly to the FIRAS spectrum taken by COBE. Because our line of sight is in the galactic plane, the very cold component could be a starless core., Comment: 36 pages, 8 tables, 7 figures, accepted for publication in ApJ

From an analysis of the spectrum (4000\AA to 8800\AA) of HD~101584 it is found that most of the neutral and single ionized metallic lines are in emission. The forbidden emission lines of [OI] 6300\AA and 6363\AA and [CI] 8727\AA are detected, which indicate the presence of a very low excitation nebula. The H$\alpha$, FeII 6383\AA, NaI D$_{1}$, D$_{2}$ lines and the CaII IR triplet lines show P-Cygni profiles indicating a mass outflow. The H$\alpha$ line shows many velocity components in the profile. The FeII 6383\AA also has almost the same line profile as the H$\alpha$ line indicating that they are formed in the same region. From the spectrum synthesis analysis we find the atmospheric parameters to be T$_{eff}$=8500K, log g=1.5, V$_{turb}$=13km~s$^{-1}$ and [Fe/H]=0.0. From an analysis of the absorption lines the photospheric abundances of some of the elements are derived. Carbon and nitrogen are found to be overabundant. From the analysis of Fe emission lines we derived T$_{exi}$=6100K$\pm$200 for the emission line region., Comment: To appear in A&A, 15 pages, 11 figures

Infrared spectra of NGC 7027 and NGC 6543 ranging from 2.4 to 45 micron were obtained with the Short Wavelength Spectrometer on board the Infrared Space Observatory. A first analysis of these spectra, with the aid of photo-ionization models, is presented. We report the first detection of the Ar VI 4.53 micron and Ne VI 7.65 micron lines in the spectrum of NGC 7027. When compared with older observations it is clear that the Ar VI line and possibly also other lines have increased in strength since 1981. We argue that a likely explanation for this variability is a change in the spectral energy distribution of the central star, possibly an increase in effective temperature. However, this result needs to be confirmed by further observations. We also report a non-detection of the O IV 25.9 micron line and the first detection of the Na III 7.32 micron line in the spectrum of NGC 6543. The non-detection is not expected based on a blackbody approximation for the spectrum of the central star. The ionization threshold for O(3+) is just beyond the He II limit, and the absence of this line shows that the stellar flux drops at least by a factor 350 at the He II limit. Modeling the O IV line may prove to be a valuable test for atmosphere models., Comment: 4 pages, 1 figure, published in A&A letters

We present ISO-SWS spectroscopy of the cool dusty envelopes surrounding two Planetary Nebulae with [WC] central stars, BD+30~3639 and He~2-113. The lambda < 15micron region is dominated by a rising continuum with prominent emission from C-rich dust (PAHs), while the long wavelength part shows narrow solid state features from crystalline silicates. This demonstrates that the chemical composition of both stars changed very recently (less than 1000 years ago). The most likely explanation is a thermal pulse at the very end of the AGB or shortly after the AGB. The H-rich nature of the C-rich dust suggests that the change to C-rich chemistry did not remove all H. The present-day H-poor [WC] nature of the central star may be due to extensive mass loss and mixing following the late thermal pulse., Comment: Four pages, LaTex using laa style files For preprints contact Albert Zijlstra at aaz@iapetus.phy.umist.ac.uk

The infrared and ultraviolet spectra of planetary nebula NGC 2440 is presented. The observations were made respectively by the Infrared Space Observatory (ISO) and International Ultraviolet Explorer (IUE) These data, in conjunction with published optical observations have been used to derive electron temperature and density. A trend of electron temperature with ionization potential is found. In particular the electron temperature increases from 11000 to 18000 K with increasing IBM. The electron density has a constant value of 4500/cu cm in agreement with previous determination. The chemical abundance has been derived for the following elements; helium, carbon, nitrogen, oxygen, neon, sulfur and argon. The ionization correction factor turns out to be very small (almost unnecessary) for all species except sulfur.

The ISO and IUE spectra of the elliptical nebula NGC 5315 is presented. These spectra axe 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. The HST Nicmos observations of the nebula in 3 emission lines are also presented. These observations are used to determine the helium abundance as a function of position in the nebula. A discussion is given of possible evolutionary effects.

The ISO and IUE spectra of the elliptical nebulae NGC 7662 and NGC 6741 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. The abundances found are compared to determinations made in other nebulae using ISO data. A discussion is given to see if possible evolutionary effects can be found from the abundance differences.