Walter A. Feibelman

The spectrum of NGC 7027

IUE and echelle spectrograph observations of the planetary nebular NGC 7027 are analyzed to obtain nebular plasma diagnostics and chemical abundances. It is found that most of the nebular plasma has a density of about 60,000/cu cm and a temperature of about 14,000 K. The results suggest that the stellar radiation field is greatly enhanced shortward of 130 A because of a dense non-LTE wind. It is found that the chemical composition is that of a normal C-rich object, rather than that of a N-rich object.

The spectrum of the variable planetary nebula IC 4997

The compact, dusty, presumably young planetary nebula (PN) IC 4997 has been studied extensively since the variability of the lambda 4363/lambda 4340 ratio was established in 1956. Since 1938, other nebular lines have shown changes. IC 4997 is also unique because of the great density range revealed by its spectrum which goes in excitation from Mg I to (Ar IV). We present a detailed listing of spectral lines from 360 to 1005 nm. The diagnostic diagram shows that the spectrum can be interpreted only in terms of strata with a huge density gamut. Essential spectral features can be reproduced approximately by a model consisting of a geometrically thin shell of density around 10(exp 7) atoms cm(exp -3), surrounded by a much larger shell with a density of about 10(exp 4) atoms cm(exp -3). The actual, certainly more complex structure can be evaluated only when high resolution spatial imaging is at hand. The usual method of getting abundances from N(ion)/N(H(+)) and ionization correction factors (ICFs) cannot be applied here. It is argued that a reasonable theoretical model that represents the spectrum provides a valid initial approximation to nebular abundances. We propose that the chemical composition of IC 4997 does not differ greatly from that of the Sun. The finally adopted model suggests that the ejection of the material destined to form the inner shell occurred between 1900 and 1960, but observational evidence of such an ejection event is lacking. Perhaps the shell was accelerated. A need for further study is emphasized, especially the role of dust which appears to contribute 2% of the total mass. More attention to this object is recommended. An accurate measurement of its distance is especially desirable.

Optical Spectrum of the Compact Planetary Nebula IC 5117

High-resolution spectroscopic data of the very compact planetary nebula IC 5117 were obtained in optical wavelengths of 3700-10050 A with the Hamilton Echelle Spectrograph at the Lick Observatory and have been analyzed along with the International Ultraviolet Explorer UV archive data. Although a diagnostic diagram shows significant density and temperature fluctuations, our analysis indicates that the nebular gas may be represented by a homogeneous shell of extremely high-density gas, N ~ 90,000 cm-3. The average electron temperatures, e.g., indicated by the [O III] diagnostics, are around 12,000 K. We construct a photoionization model to represent most of the observed line intensities and the physical condition of this compact nebulosity. Based on the semiempirical ionization correction approach and model indications, we derived the elemental abundances; He, C, N, O, Ne, and Ar appear to be normal or marginally depleted compared to the average planetary nebula, while the remaining elements S, Cl, and K appear to be enhanced. IC 5117 is perhaps a very young compact planetary nebula, slightly more evolved than the other well-known compact planetary nebula IC 4997. The central stellar temperature is likely to be around 120,000 K, evolved from a C-rich asymptotic giant branch progenitor.

The Optical Spectral Line List of RR Telescopii

The symbiotic nova RR Telescopii has been observed with the 1.5 m telescope of the Cerro Tololo Inter-American Observatory (CTIO), using the 1.5 m bench-mounted echelle spectrograph in conjunction with a Tektronix CCD. It displays a rich emission-line spectrum, ranging in excitation from O I to [Ni VIII]. We present a list of 491 measured lines, with their suggested identifications, covering a wavelength range from 3430 to 9320 A. Of these, only nine are unidentified, and 70 lines are cataloged that were not given in the original line list of Thackeray. Absolute line intensities are also given, which have been derived by comparing the high-resolution data with a flux-calibrated low-resolution spectrum taken with the Cassegrain spectrograph on the 1.0 m telescope at CTIO.

Ultraviolet spectra of the central stars of large planetary nebulae

The ultraviolet spectra of 32 planetary nebula nuclei have been examined with the IUE. Ultraviolet and UV-to-optical flux ratios are derived, from which color temperatures are derived. The distribution of the latter ranges from rough agreement with the Zanstra temperatures upward to infinite values. The high-luminosity stars have generally lower, determinable color temperatures; the infinite or indeterminable color temperatures are contained by the set of lower luminosity stars. About half the stars exhibit a mixture of line features, most commonly He II 1640 A and C IV 1550 A in emission or absorption. Three display clear P Cygni lines, while a fourth appears to display a remarkably broad C IV 1550 A P Cygni profile. The other half of the stars have no perceptible line features.

The binary central star of the planetary nebula LT-5

Data from the IUE shows that the true central star of the giant planetary Longmore-Tritton 5 (339+88.1 deg) is a binary companion to the G5 star SAO 82570. The slope of the energy distribution curve indicates that the star may be one of the hottest known. The Mg II doublet associated with the G star is very strong, consistent with the existence of a close binary companion.

MOTION OF THE ULTRAVIOLET R AQUARII JET

We present evidence for subarcsecond changes in the ultraviolet (~2550 A) morphology of the inner 5 arcseconds of the R Aqr jet over a 2 yr period. These data were taken with the Hubble Space Telescope (HST) Faint Object Camera (FOC) when the primary mirror flow was still affecting observations. Images of the R Aqr stellar jet were successfully restored to the original design resolution by completely characterizing the telescope-camera point spread function (PSF) with the aid of phase-retrieval techniques. Thus, a noise-free PSF was employed in the final restorations which utilized the maximum entropy method (MEM). We also present recent imagery obtained with the HST/FOC system after the COSTAR correction mission that provides confirmation of the validity of our restoration methodology. The restored results clearly show that the jet is flowing along the northeast (NE)-southwest (SW) axis with a prominent helical-like structure evident on the stronger NE side of the jet. Transverse velocities increase with increasing distance from the central source, providing a velocity range of 36-235 km s-1. From an analysis of proper motions of the two major ultraviolet jet components, we detect an ~40.2 yr event separation of this apparent enhanced material ejection occurring probably at periastron which is consistent with the suspected ~44 yr binary period; this same analysis shows that the jet is undergoing nonlinear acceleration which suggests that the propulsive force probably results from magnetohydromagnetic effects. The restoration computations and the algorithms employed demonstrate that mining of flawed HST data can be scientifically worthwhile.

Spectrum of IC 2149 and its central star

Although the asymmetric optical image of IC 2149 does not fall into any standard morphologies (Balick et al. 1993), the overall shape and its radio frequency image show a bilateral symmetry. The central star of spectral class O7.5 is a remarkable object with a rich spectrum showing many Fe ions. We discuss this spectrum in detail. The nebula has been studied utilizing our theoretical photoionization models. Detailed analyses of spectral data are done in both the UV region (lambda less than 3000 A; IUE) and the optical region (3500 A approximately 10500 A; the Hamilton Echelle at Lick Observatory). The plasma diagnostics suggest an electron temperature of 9000 approximately 10,000 K and an electron density near 5600 cm. Our model calculations were carried out with due regard to the above plasma diagnostics. The observations can be interpreted by a model of IC 2149 which consists of two components (an equatorial ring and a polar cone), but the total emission is dominated by the relatively denser equatorial shell. Carbon, nitrogen, and oxygen appear to be depleted by a factor of about 3; ratios of other elements are also lower than in the Sun.

On a suspected ring external to the visible rings of Saturn

Abstract Reexamination of a photograph of Saturn taken on 15 November 1966, when the earth was nearly in the ring plane, indicates that ring material may exist outside the visible rings, extending to more than 6 Saturnian radii. Although the suspected feature on the photograph appears to be real, the possibility of its being a developed pressure mark or a chance alignment of grains cannot be ruled out. The observed brightness in blue light was estimated to be mB = 19.5 ± 0.5 per linear arcsecond, implying a normal optical thickness, τ ⋍ 10 −7 , for ice-covered particles. For spacecraft passing through this region, the hazards are found to be minimal.

Optical and IUE spectra of the planetary nebula NGC 7026

We investigated spectroscopic data of the extended planetary nebula NGC 7026 in the wavelengths 3700-10,050 A, secured with the Hamilton Echelle Spectrograph at Lick Observatory. This optical wavelength spectrum has been analyzed along with the International Ultraviolet Explorer (IUE) UV spectral data. The diagnostic diagram indicates that the planetary nebula has very complex electron densities of N ~ 3000-10,000 cm-3. The electron temperatures are relatively low: around T = 8000-9500 K, probably as a result of an enhanced heavy elemental cooling. The electron temperature variation also indicates that the low-excitation line region is slightly higher than the high-excitation line regions perhaps as a result of the hardening of escaped UV ionizing photons into the outer shell part of low-excitation lines. We construct a photoionization model, with the central star of the planetary nebula at a temperature of Teff = 80,000 K, to fit most of the relatively strong line intensities and the observed physical conditions. With this photoionization model construction and with a semiempirical ionization correction method, we derived the elemental abundances of the nebula. Compared to the average or normal planetary nebula, most elemental abundances of He, C, N, O, Ne, S, Ar, and Cl appear to be enhanced.