Karen B. Kwitter

ABUNDANCES OF GALACTIC ANTICENTER PLANETARY NEBULAE AND THE OXYGEN ABUNDANCE GRADIENT IN THE GALACTIC DISK

We have obtained spectrophotometric observations of 41 anticenter planetary nebulae (PNe) located in the disk of the Milky Way. Electron temperatures and densities, as well as chemical abundances for He, N, O, Ne, S, Cl, and Ar were determined. Incorporating these results into our existing database of PN abundances yielded a sample of 124 well-observed objects with homogeneously determined abundances extending from 0.9 to 21 kpc in galactocentric distance. We performed a detailed regression analysis which accounted for uncertainties in both oxygen abundances and radial distances in order to establish the metallicity gradient across the disk to be 12 + log(O/H) = (9.09 ± 0.05) – (0.058 ± 0.006) × Rg , with Rg in kpc. While we see some evidence that the gradient steepens at large galactocentric distances, more objects toward the anticenter need to be observed in order to confidently establish the true form of the metallicity gradient. We find no compelling evidence that the gradient differs between Peimbert Types I and II, nor is oxygen abundance related to the vertical distance from the galactic plane. Our gradient agrees well with analogous results for H II regions but is steeper than the one recently published by Stanghellini & Haywood over a similar range in galactocentric distance. A second analysis using PN distances from a different source implied a flatter gradient, and we suggest that we have reached a confusion limit which can only be resolved with greatly improved distance measurements and an understanding of the natural scatter in oxygen abundances.

Sulfur, Chlorine, and Argon in Planetary Nebulae. I. Observations and Abundances in a Northern Sample

This paper is the first of a series specifically studying the abundances of sulfur, chlorine, and argon in type II planetary nebulae (PNe) in the Galactic disk. Ratios of S/O, Cl/O, and Ar/O constitute important tests of differential nucleosynthesis of these elements and serve as strict constraints on massive star yield predictions. We present new ground-based optical spectra extending from 3600-9600 A for a sample of 19 type II northern PNe. This range includes the strong near-infrared lines of [S III] λλ9069,9532, which allows us to test extensively their effectiveness as sulfur abundance indicators. We also introduce a new, model-tested ionization correction factor for sulfur. For the present sample, we find average values of S/O = 1.2 × 10-2 ± 0.71 × 10-2, Cl/O = 3.3 × 10-4 ± 1.6 × 10-4, and Ar/O = 5.0 × 10-3 ± 1.9 × 10-3.

Large planetary nebulae and their significance to the late stages of stellar evolution

Spectrophotometry of 75 large PNe with Shklovsky radii greater than 0.15 pc is presented and used to calculate nebular parameters and compositions, stellar Zanstra temperatures and luminosities, and core masses. Nine new Peimbert type I nebulae are identified. About 40 percent of the stars that are on cooling tracks are above 0.7 solar mass, and over 15 percent are above 0.8 solar mass. The large planetaries demonstrate a clear positive correlation between nitrogen enrichment and core mass. N/O is anticorrelated with O/H. The radii of the nebulae whose stars lie along specific cooling tracks increase monotonically with decreasing central star temperature. For a given central temperature, the nebular radii also increase with increasing core mass, showing that in this part of the log L-log T plane the higher mass cores evolve more slowly in agreement with theoretical prediction. However, theoretical evolutionary rates for the large nebulae stars appear to be much too slow. 103 refs.

A New Look at Carbon Abundances in Planetary Nebulae. IV. Implications for Stellar Nucleosynthesis

This paper is the fourth and final report on a project designed to study carbon abundances in a sample of planetary nebulae representing a broad range in progenitor mass and metallicity. We present newly acquired optical spectrophotometric data for three Galactic planetary nebulae, IC 418, NGC 2392, and NGC 3242, and combine them with UV data from the IUE Final Archive for identical positions in each nebula to determine accurate abundances of He, C, N, O, and Ne at one or more locations in each object. We then collect abundances of these elements for the entire sample and compare them with theoretical predictions of planetary nebula abundances from a grid of intermediate-mass star models. We find some consistency between observations and theory, lending modest support to our current understanding of nucleosynthesis in stars below 8 M☉ in birth mass. Overall, we believe that observed abundances agree with theoretical predictions to well within an order of magnitude but probably not better than to within a factor of 2 or 3. However, even this level of consistency between observation and theory enhances the validity of published intermediate-mass stellar yields of carbon and nitrogen in the study of the abundance evolution of these elements.

Morphology and composition of the helix nebula

We present new narrowband filter imagery in Hα and [N II] λ6584, along with UV and optical spectrophotometry measurements from 1200 to 9600 A of NGC 7293, the Helix Nebula, a nearby, photogenic planetary nebula of large diameter and low surface brightness. Detailed models of the observable ionized nebula support the recent claim that the Helix is actually a flattened disk whose thickness is roughly one-third its diameter, with an inner region containing hot, highly ionized gas that is generally invisible in narrowband images. The outer visible ring structure is of lower ionization and temperature and is brighter because of a thickening in the disk. We also confirm a central star effective temperature and luminosity of 120,000 K and 100 L☉, and we estimate a lower limit to the nebular mass to be 0.30 M☉. Abundance measurements indicate the following values: He/H=0.12 (± 0.017), O/H=4.60×10-4 (± 0.18), C/O=0.87 (± 0.12), N/O=0.54 (± 0.14), Ne/O=0.33 (± 0.04), S/O=3.22 × 10-3 (± 0.26), and Ar/O=6.74 × 10-3 (± 0.76). Our carbon abundance measurements represent the first of their kind for the Helix Nebula. The S/O ratio that we derive is anomalously low; such values are found in only a few other planetary nebulae. The central star properties, the supersolar values of He/H and N/O, and a solar level of C/O are consistent with a 6.5 M☉ progenitor that underwent three phases of dredge-up and hot bottom burning before forming the planetary nebula.

A New Look at Carbon Abundances in Planetary Nebulae. III. DDDM1, IC 3568, IC 4593, NGC 6210, NGC 6720, NGC 6826, and NGC 7009

This paper is the third in a series reporting on a study of carbon abundances in a carefully chosen sample of planetary nebulae representing a large range in progenitor mass and metallicity. We make use of the IUE Final Archive database containing consistently reduced spectra to measure line strengths of C III] λ1909 along with numerous other UV lines for the planetary nebulae DDDM1, IC 3568, IC 4593, NGC 6210, NGC 6720, NGC 6826, and NGC 7009. We combine the IUE data with line strengths from optical spectra obtained specifically to match the IUE slit positions as closely as possible, in order to determine values for the abundance ratios He/H, O/H, C/O, N/O, and Ne/O. The ratio of C III] λ1909/C II λ4267 is found to be effective for merging UV and optical spectra when He II λ1640/λ4686 is unavailable. Our abundance determination method includes a five-level program whose results are fine-tuned by corrections derived from detailed photoionization models constrained by the same set of emission lines. All objects appear to have subsolar levels of O/H, and all but one show N/O levels above solar. In addition, the seven planetary nebulae span a broad range in C/O values. We infer that many of our objects are matter-bounded, and thus the standard ionization correction factor for N/O may be inappropriate for these PNs. Finally, we estimate C/O using both collisionally excited and recombination lines associated with C+2 and find the well-established result that abundances from recombination lines usually exceed those from collisionally excited lines by several times.

ABUNDANCES OF PLANETARY NEBULAE IN THE OUTER DISK OF M31

We present spectroscopic observations and chemical abundances of 16 planetary nebulae (PNe) in the outer disk of M31. The [O III] ?4363 line is detected in all objects, allowing a direct measurement of the nebular temperature essential for accurate abundance determinations. Our results show that the abundances in these M31 PNe display the same correlations and general behaviors as Type?II PNe in the Milky Way. We also calculate photoionization models to derive estimates of central star properties. From these we infer that our sample PNe, all near the bright-end cutoff of the planetary nebula luminosity function, originated from stars near 2 M ?. Finally, under the assumption that these PNe are located in M31s disk, we plot the oxygen abundance gradient, which appears shallower than the gradient in the Milky Way.

ALPHA ELEMENT ABUNDANCES IN A LARGE SAMPLE OF GALACTIC PLANETARY NEBULAE

In this paper, we present emission line strengths, abundances, and element ratios (X/O for Ne, S, Cl, and Ar) for a sample of 38 Galactic disk planetary nebulae (PNe) consisting primarily of Peimbert classification Type I. Spectrophotometry for these PNe incorporates an extended optical/near-IR range of λλ3600-9600 A including the [S III] lines at 9069 A and 9532 A, setting this relatively large sample apart from typical spectral coverage. We have utilized Emission Line Spectrum Analyzer, a five-level atom abundance routine, to determine Te , Ne , ionization correction factors, and total element abundances, thereby continuing our work toward a uniformly processed set of data. With a compilation of data from >120 Milky Way PNe, we present results from our most recent analysis of abundance patterns in Galactic disk PNe. With a wide range of metallicities, galactocentric distances, and both Type I and non-Type I objects, we have examined the alpha elements against H II regions and blue compact galaxies (H2BCGs) to discern signatures of depletion or enhancement in PNe progenitor stars, particularly the destruction or production of O and Ne. We present evidence that many PNe have higher Ne/O and lower Ar/Ne ratios compared to H2BCGs within the range of 8.5-9.0 for 12 + log(O/H). This suggests that Ne is being synthesized in the low- and intermediate-mass progenitors. Sulfur abundances in PNe continue to show great scatter and are systematically lower than those found in H2BCG at a given metallicity. Although we find that PNe do show some distinction in alpha elements when compared to H2BCG, within the Peimbert classification types studied, PNe do not show significant differences in alpha elements amongst themselves, at least to an extent that would distinguish in situ nucleosynthesis from the observed dispersion in abundance ratios.

Sulfur, Chlorine, and Argon Abundances in Planetary Nebulae. III. Observations and Results for a Final Sample

This paper is the fourth in a series whose purpose is to study the interstellar abundances of sulfur, chlorine, and argon in the Galaxy using a sample of 86 planetary nebulae. Here we present new high-quality spectrophotometric observations of 20 Galactic planetary nebulae with spectral coverage from 3700 to 9600 A. A major feature of our observations throughout the entire study has been the inclusion of the near-infrared lines of [S iii] λλ9069, 9532, which allows us to calculate accurate S+2 abundances and to either improve upon or convincingly confirm results of earlier sulfur abundance studies. For each of the 20 objects here, we calculate ratios of S/O, Cl/O, and Ar/O and find average values of S/O = (1.1 ± 1.1) × 10-2, Cl/O = (4.2 ± 5.3) × 10-4, and Ar/O = (5.7 ± 4.3) × 10-3. For six objects, we are able to compare abundances of S+3 calculated directly from available [S iv] 10.5 μm measurements with those inferred indirectly from the values of the ionization correction factors for sulfur. In the final paper of the series, we will compile results from all 86 objects, search for and evaluate trends, and use chemical evolution models to interpret our results.

The shaping of planetary nebula Sh 2–188 through interaction with the interstellar medium

Sh2-188 is an example of strong interaction between a planetary nebula (PN) and the interstellar medium (ISM). It shows a single arc-like structure, consisting of several filaments, which is postulated to be the result of motion through the ISM. We present new Hα images from the Isaac Newton Telescope Photometric Hα Survey of the Northern Galactic Plane (IPHAS) which reveal structure behind the filamentary limb. A faint, thin arc is seen opposite the bright limb, in combination forming a closed ring. Behind the faint arc a long wide tail is detected, doubling the size of the nebula. The nebula extends 15 arcmin on the sky in total. We have developed a ‘triplewind’ hydrodynamical model, comprising of the initial ‘slow’ asymptotic giant branch (AGB) wind and the later ‘fast’ stellar wind (the interacting stellar wind model), plus a third wind reflecting the motion through the ISM. Simulations at various velocities of the central star relative to the ISM indicate that a high velocity of 125 kms 1 is required to reproduce the observed structure. We find that the bright limb and the tail already formed during the AGB phase, prior to the formation of the PN. The closure of the ring arises from the slow–fast wind interaction. Most of the mass lost on the AGB has been swept downstream, providing a potential explanation of the missing mass problem in PNe. We report a proper motion for the central star of 30± 10 masyr 1 in the direction of the bright limb. Assuming the central star is moving at 125± 25 kms 1 , the distance to the nebula is estimated to be 850 +500 420 pc, consistent with a spectroscopic distance to the star. Expansion velocities measured from spectroscopic data of the bright filaments are consistent with velocities measured from the simulation. Sh2-188 is one of the largest PNe known, with an extent of 2.8 pc. The model shows that this size was already set during the AGB phase.