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We re-analyzed OJ287 in 120 Very Long Baseline Array (VLBA, MOJAVE) observations (at 15 GHz) covering the time span between Apr. 1995 and Apr. 2017. We find that the radio jet motion over the sky is consistent with a precessing and nutating jet source. The variability of the radio flux-density can be explained by Doppler beaming due to a change in the viewing angle. We suggest that part of the optical emission is due to synchrotron emission related to the jet radiation. We find a strikingly similar scaling for the timescales for precession and nutation as indicated for SS433 with a factor of roughly 50 times longer in OJ287.

We report on a multiwavelength campaign on the TeV gamma-ray blazar Mrk 421 performed during 2002 December and 2003 January. These target of opportunity observations were initiated by the detection of X-ray and TeV gamma-ray flares with the All Sky Monitor (ASM) on board the Rossi X-Ray Timing Explorer (RXTE) and the 10 m Whipple gamma-ray telescope. The campaign included observational coverage in the radio (University of Michigan Radio Astronomy Observatory), optical (Boltwood, La Palma KVA 0.6 m; WIYN 0.9 m), X-ray (RXTE pointed telescopes), and TeV gamma-ray (Whipple and HEGRA) bands. At TeV energies, the observations revealed several flares at intermediate flux levels, peaking between 1 and 1.5 times the flux from the Crab Nebula. While the time-averaged spectrum can be fitted with a single power law of photon index Gamma = 2.8 from dN gamma/dE proportional to E-Gamma, we find some evidence for spectral variability. Confirming earlier results, the campaign reveals a rather loose correlation between the X-ray and TeV gamma-ray fluxes. In one case, a very strong X-ray flare is not accompanied by a comparable TeV gamma-ray flare. Although the source flux was variable in the optical and radio bands, the sparse sampling of the optical and radio light curves does not allow us to study the correlation properties in detail. We present a simple analysis of the data with a synchrotron self-Compton model, emphasizing that models with very high Doppler factors and low magnetic fields can describe the data.

We present model atmosphere analyses of high resolution Keck and VLT optical spectra for three evolved stars in globular clusters, viz. ZNG-1 in M 10, ZNG-1 in M 15 and ZNG-1 in NGC 6712. The derived atmospheric parameters and chemical compositions confirm the programme stars to be in the post-Asymptotic Giant Branch (post-AGB) evolutionary phase. Differential abundance analyses reveal CNO abundance patterns in M 10 ZNG-1, and possibly M 15 ZNG-1, which suggest that both objects may have evolved off the AGB before the third dredge-up occurred. The abundance pattern of these stars is similar to the third class of optically, bright post-AGB objects discussed by van Winckel (1997). Furthermore, M 10 ZNG-1 exhibits a large C underabundance (with � (C/O) ∼− 1.6 dex), typical of other hot post-AGB objects. Differential � (α/Fe) abundance ratios in both M 10 ZNG-1 and NGC 6712 ZNG-1 are found to be approximately 0.0 dex, with the Fe abundance of the former being in disagreement with the cluster metallicity of M 10. Given that the Fe absorption features in both M 10 ZNG-1 and NGC 6712 ZNG-1 are well observed and reliably modelled, we believe that these differential Fe abundance estimates to be secure. However, our Fe abundance is difficult to explain in terms of previous evolutionary processes that occur on both the Horizontal Branch and the AGB.

The ISO and IUE spectra of the "elliptical" nebula Hu 1-2 are presented. These spectra are combined with new, high resolution spectra in the visual wavelength region to obtain a complete, extinction corrected, spectrum. The chemical composition of the nebula is then calculated and compared to previous determinations. The abundances determined are the lowest yet found in the analysis of ISO data. The evolutionary significance is discussed.

Recent R-matrix calculations of electron impact excitation rates among the 3s23p4 levels of Cl II are used to derive the nebular emission-line intensity ratios R1 = I(6161.8 A)/I(8578.7 A) and R2 = I(6161.8 A)/I(9123.6 A) as a function of electron temperature (Te) and density (Ne). The ratios are found to be very sensitive to changes in Te but not Ne for densities lower than 105 cm-3. Hence, they should, in principle, provide excellent optical Te diagnostics for planetary nebulae. The observed values of R1 and R2 for the planetary nebulae NGC 6741 and IC 5117, measured from spectra obtained with the Hamilton echelle spectrograph on the 3 m Shane Telescope, imply temperatures in excellent agreement with those derived from other diagnostic lines formed in the same region of the nebula as [Cl II]. This provides some observational support for the accuracy of the [Cl II] line ratio calculations and hence the atomic data on which they are based. The [Cl II] 8578.7 and 9123.6 A lines are identified for the first time (to our knowledge) in a high-resolution spectrum of the symbiotic star RR Telescopii, obtained with the University College London Echelle Spectrograph on the 3.9 m Anglo-Australian Telescope. However, the 6161.8 A feature is unfortunately too weak to be identified in the RR Telescopii observations, consistent with its predicted line strength.

To record the spectral lines of stars and nebulae, photographic photometry was routinely used in the early 1900s, to be later replaced by methods employing more advanced detectors. We review the progress of planetary nebular spectroscopic studies, i.e. from the early photographic photometry, through to the 1950s photoelectric scanner observations, and finally to recent charge-coupled-device spectroscopy.

Absolute and differential abundance analyses have been performed from high-resolution, high signal-to-noise ratio optical (Keck I) spectra for three evolved Galactic halo stars, namely PG 1704 + 222, HD 341617 and LS IV -04 01. Their derived atmospheric parameters indicate that all three objects are undergoing a post-asymptotic giant branch (post-AGB) phase of evolution. A differential abundance analysis reveals HD 341617 as having a mild carbon deficiency of 0.74 dex, possibly due to the star having evolved off the AGB before the onset of the third dredge-up. Although such carbon underabundances are typical of hot post-AGB objects, the same trend is not observed in PG 1704 + 222, where the carbon abundance is found to be consistent with those derived for nitrogen and oxygen. Hence, a dredge-up scenario need not be invoked to explain the chemical composition of PG 1704 + 222. For LS IV -04 01 no iron deficiency is apparent relative to magnesium and silicon, and hence a gas-dust separation event in the AGB progenitor need not be invoked for this star.

Recent R-matrix calculations of electron impact excitation rates in K v are used to derive the nebular emission line ratio R = I (4122.6 Å)/ I (4163.3 Å) as a function of electron density ( N e ). This ratio is found to be very sensitive to changes in N e over the density range 10 3 to 10 6 cm −3 , but does not vary significantly with electron temperature, and hence in principle should provide an excellent optical N e diagnostic for the high-excitation zones of nebulae. The observed value of R for the planetary nebula NGC 7027, measured from a spectrum obtained with the Hamilton Echelle spectrograph on the 3-m Shane Telescope, implies a density in excellent agreement with that derived from [Ne iv ], formed in the same region of the nebula as [K v ]. This observation provides observational support for the accuracy of the theoretical [K v ] line ratios, and hence the atomic data on which they are based. However, the analysis of a high-resolution spectrum of the symbiotic star RR Telescopii, obtained with the University College London Echelle Spectrograph on the 3.9-m Anglo–Australian Telescope, reveals that the [K v ] 4122.6 Å line in this object is badly blended with Fe ii 4122.6 Å. Hence, the [K v ] diagnostic may not be used for astrophysical sources that show a strong Fe ii emission line spectrum.

A high-dispersion optical (3700-10100 A) spectrum was secured with the Hamilton echelle spectrograph at the coude focus of the 120 inch (3 m) Shane telescope at Lick Observatory, with the goal of deriving chemical abundances for the compact planetary nebula IC 4846. We also remeasured the UV spectra from the IUE archive. Diagnostics indicate that the entire planetary nebula may be represented by an electron density N 9000 cm–3 and electron temperature T 10,500 K. However, diagnostics seem to imply that the [O ii] and [S ii] zone electron temperatures are higher than the [O iii] radiating strata. The electron density of the [O ii] and [S ii] zone might be higher as well (for which N 20,000 cm–3). The photoionization model, which represents most of the observed line intensities, seems to confirm this physical condition, although the actual situation may be much more complicated. On the basis of the semiempirical ionization correction method and the photoionization model calculation, we derived the chemical abundances in this planetary nebula. Our optical data were compared with the image-tube scanner data from Aller & Czyzak. Apparently, there is a long-term spectral line variation in IC 4846. Chemical abundances derived in the present study agree, within 30%, with those previously estimated in Aller & Czyzak, except for carbon; the C, N, O, and Ne ratios are lower than both the average planetary nebula and the solar abundance. IC 4846 is a metal-deficient planetary nebula that may have evolved from a Population II progenitor.

R-matrix calculations of electron impact excitation rates among the 2s22p2 3P, 1D, 1S, and 2s2p3 5S levels of N II are presented. These results are used in conjunction with other recent calculations of electron impact excitation rates and Einstein A-coefficients for N II to derive the emission-line ratio: ratio diagrams (R1, R2) and (R1, R3), where R1 = I(5756.2 A)/I(6549.9 + 6585.2 A), R2 = I(2143.5 A)/I(6549.9 + 6585.2 A), and R3 = I(2139.7 A)/I(6549.9 + 6585.2 A), for a range of electron temperatures (Te = 5000-20,000 K) and electron densities (Ne = 102-107 cm-3) appropriate to gaseous nebulae. These diagrams should, in principle, allow the simultaneous determination of Te and Ne from measurements of the [N II] lines in a spectrum. Plasma parameters deduced for a sample of gaseous nebulae, using observational data obtained from ground-based telescopes plus the International Ultraviolet Explorer and Hubble Space Telescope satellites, are found to show generally excellent internal consistency and to be in good agreement with the values of Te and Ne estimated from other line ratios. These results provide observational support for the accuracy of the theoretical ratios and hence the atomic data adopted in their derivation. Theoretical ratios are also presented for the infrared line pair R4 = I(122 μm)/I(205 μm), and the usefulness of R4 as an electron density diagnostic is briefly discussed.

We investigate the electron temperature of the inner halo and nebular core regions of NGC 6543, using archival Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) images taken through narrow band (O iii) filters. Balick et al. (2001) showed that the inner halo consists of a number of spherical shells. We find the temperature of this inner halo to be much higher (�15000 K) than that of the bright core nebula (�8500 K). Photo-ionization models indicate that hardening of the UV radiation from the central star cannot be the main source of the higher temperature in the halo region. Using a radiation hydrodynamic simulation, we show that mass loss and velocity variations in the AGB wind can explain the observed shells, as well as the higher electron temperature.

We have measured the spectral line intensities of the metal poor planetary nebula IC 4634. Using a photo-ionization model calculation, we try to fit the the optical and UV region spectra, i.e., Hamilton Echelle and IUE observations. From direct images, one expects complicated density variations, but the model predicts a range in densities that may be smaller than actually exist. We find N(sub epsilon) approximates 5000 /cubic meter. In spite of the geometrical complexity of the S shaped double-lobed structure, the simple photoionization model with a spherical symmetry can fit most emission lines, fairly well. The derived chemical composition has been compared with previous estimates and also with the Sun - The metallicity in IC 4634 appears to be lower than in the Sun or the average planetary nebula. The most likely temperature of the central ionizing source of IC 4634 appears to be about 55,000 K. We find a central star mass of about 0.55 Solar Mass from comparison with theoretical evolutionary tracks.

The symbiotic nova RR Telescopii has been ob- served with the 3.9 m telescope at the Anglo-Australian Observatory (AAO), using the University College London Echelle Spectrograph (UCLES) in conjunction with a Tek CCD. It displays a rich emission line spectrum, ranging in excitation from N i to (Ni viii). We present a list of 811 measured lines, with their suggested identications and absolute line intensities, covering a wavelength range from 3180 A to 9455 A 1 . The absolute line intensities have been derived by comparing the high resolution data with a flux-calibrated low resolution spectrum taken with the Australian National University 2.3 m telescope. All of the lines have been successfully identied. Comparing our re- sults with those of previous studies indicates that the RR Tel system is advancing towards higher degrees of ionisation.

Recent R-matrix calculations of electron impact excitation rates in 0 III are used to calculate electron temperature and density-dependent emission line ratios R (sub 1) = I(4363 Angstroms)/ I(4960 Angstroms + 5007 Angstroms), R (sub 2) = I(1661 Angstroms + 1667 Angstroms)/ I(4960 Angstroms + 5007 Angstroms) and R (sub 3)= I(2322 Angstroms)/ I(1661 Angstroms + 1667 Angstroms), for a range of electron temperatures (7500 less than or equal to Te less than or equal to 30 000 K) and densities (10 (exp 4) less than or equal to N (sub e) less than or equal to 10 (exp 7) per cubic centimeters) applicable to gaseous nebulae. The ratio-ratio diagrams (R (sub 1), R (sub 2)) and (R (sub 1), R (sub 3)) should, in principle, allow the simultaneous determination of T (sub e) and N (sub e) from measurements of the 0 III features in a spectrum. Plasma parameters derived for a sample of high-excitation planetary nebulae from (R (sub 1), R (sub 2)) and (R (sub 1), R (sub 3)) measurements, produced using a combination of ultraviolet spectra obtained with the International Ultraviolet Explorer (IUE) and optical data from a number of observing runs, are found to show excellent internal consistency. They also show, in general, good agreement with the values of Te and Ne estimated from other line ratios in the nebulae, therefore providing observational support for the accuracy of the theoretical ratios and hence the atomic data adopted in their derivation.

Recent R-matrix calculations of electron impact excitation rates in Ne IV are used to calculate emission line ratio–ratio diagrams involving both the ultraviolet (1602, 2422 and 2424 A) and optical (4714, 4716, 4724 and 4726 A) [Ne IV] transitions, for a range of electron temperatures (Te=10 000–30 000 K) and electron densities (Ne=102–106.5 cm−3) appropriate to gaseous nebulae. These diagrams should, in principle, allow the simultaneous determination of Te and Ne from measurements of the [Ne IV] lines in a spectrum. Plasma parameters deduced for a sample of high-excitation planetary nebulae, using a combination of observational data obtained with the IUE satellite and the Hamilton Echelle Spectrograph (HES) on the 3-m Shane Telescope at the Lick Observatory, are found to show generally excellent internal consistency. In addition, they are in good agreement with the values of Te and Ne estimated from other high-excitation line ratios in the HES spectra, and by previous authors using infrared and ultraviolet transitions in [O IV] and [Ne V]. These results provide observational support for the accuracy of the theoretical [Ne IV] ratios, and hence the atomic data adopted in their derivation. An inspection of IUE and GHRS/HST spectra of the symbiotic stars Z And and RR Tel reveals asymmetries in the line profile of the [Ne IV] 2s22p34S–2s22p32P1/2, 3/2 doublet at 1602 A, hence allowing the measurement of the wavelength separation of the 4S–2P1/2 and 4S–2P3/2 components. The separation is found to be 0.21±0.02 A, in good agreement with the theoretical estimate of 0.16±0.03 A; this is the first time (to our knowledge) that this quantity has been experimentally determined.

Wavelengths and identifications have been provided for approximately 300 lines between 3660 and 10125 A in the spectrum of the bipolar planetary nebula NGC 2440. These lines measured with the Hamilton echelle spectrograph at Lick Observatory and supplemented by published IUE results are used to construct diagnostic diagrams and derive ionic concentrations. The electron temperature indicated by [O iii] is ~14,200 K; the electron density is ~5000 cm–1. With the aid of a standard model and an ionization correction factor method, we obtain a set of abundances that are compared with the Sun and with results of previous studies. As with other Peimbert Type I's, He and N are enriched as compared with the Sun; C is likewise overabundant. NGC 2440 evolved from a progenitor star that was more massive than the Sun. The central star temperature seems close to 180,000 K.

Wavelengths and identifications have been provided for approximately 300 lines between 3660 and 10125 A in the spectrum of the archetypal bright, high-excitation planetary nebula NGC 7662. These lines are measured with the Hamilton echelle spectrograph at Lick Observatory and are supplemented by image-tube data. Published results are used to construct diagnostic diagrams and derive ionic concentrations. The electron temperature indicated by [O III] is about 12,500 K; the density regimes consist of N = 5000-17,000 cm-1. Derivation of precise abundances will require appropriate model calculations, but with the aid of homogeneous spherical model procedures we find C to be enhanced, N marginally so if at all, and heavier elements depleted with respect to the Sun. These conclusions are in harmony with those of Barker.

Recent R-matrix calculations of electron impact excitation rates in Ar IV are used to calculate the emission-line ratio: ratio diagrams (R1, R2), (R1, R3), and (R1, R4), where R1 = I(4711 ?)/I(4740 ?), R2 = I(7238 ?)/I(4711 + 4740 ?), R3 = I(7263 ?)/I(4711 + 4740 ?), and R4 = I(7171 ?)/I(4711 + 4740 ?), for a range of electron temperatures (Te = 5000-20,000 K) and electron densities (Ne = 10-106 cm-3) appropriate to gaseous nebulae. These diagrams should, in principle, allow the simultaneous determination of Te and Ne from measurements of the [Ar IV] lines in a spectrum. Plasma parameters deduced for a sample of planetary nebulae from (R1, R3) and (R1, R4), using observational date obtained with the Hamilton echelle spectrograph on the 3 m Shane Telescope at the Lick Observatory, are found to show excellent internal consistency and to be in generally good agreement with the values of Te and Ne estimated from other line ratios in the echelle spectra. These results provide observational support for the accuracy of the theoretical ratios and, hence, the atomic data adopted in their derivation. In addition, they imply that the 7171 ? line is not as seriously affected by telluric absorption as previously thought. However, the observed values of R2 are mostly larger than the theoretical high-temperature and density limit, which is due to blending of the Ar IV 7237.54 ? line with the strong C II transition at 7236 ?.

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.

The bright, compact, moderately high-excitation planetary nebula, NGC 6884 (Perek-Kohoutek 82 +07°1) has a rich spectrum promising good diagnostics and offering a good target for the Hamilton Echelle Spectrograph at Lick Observatory. We combine International Ultraviolet Explorer (IUE), near-UV, visual, and IR data to obtain N(ion)/N(H+) for more than 30 species from H to [Fe VI]. These diagnostics suggest a rather large density range and an electron temperature near 10,000 K. The relative structural simplicity of this planetary nebula makes it worthwhile to attempt a plausible photoionization model and deduce atomic abundances, T*, and other properties implied for the central star. It is believed substantially improved chemical compositions are found.

The spectra of gaseous nebulae differed strikingly from those of stars which were well understood since 1922 thanks to the work of Saha. Gaseous nebulae exhibited exotic bright line spectra characterized by strange emissions of unknown origin as well as familiar lines of hydrogen and helium. The strongest lines in most nebulae fell at 4959A and 5007A. They were originally attributed to an unknown element. First came the interpretation of the lines of H.

We present multiwavelength observations of the ultraluminous blazar-type radio loud quasar PKS 0528+134 in quiescence during the period July to December 2009. Significant flux variability on a time scale of several hours was found in the optical regime, accompanied by a weak trend of spectral softening with increasing flux. We suggest that this might be the signature of a contribution from the accretion disk at the blue end of the optical spectrum. The optical flux is weakly polarized with rapid variations of the degree and direction of polarization, while the polarization of the 43 GHz radio core remains steady. Optical spectropolarimetry suggests a trend of increasing degree of polarization with increasing wavelength, providing additional evidence for an accretion disc contribution towards the blue end of the optical spectrum. We constructed four SEDs indicating that even in the quiescent state, the bolometric luminosity of PKS 0528+134 is dominated by its gamma-ray emission. A leptonic single-zone jet model produced acceptable fits to the SEDs with contributions to the high-energy emission from synchrotron self-Compton radiation and Comptonization of direct accretion disk emission. Fit parameters close to equipartition were obtained. The moderate variability on long time scales implies the existence of on-going particle acceleration, while the observed optical polarization variability seems to point towards a turbulent acceleration process. Turbulent particle acceleration at stationary features along the jet therefore appears to be a viable possibility for the quiescent state of PKS 0528+134., Comment: Accepted for Publication in The Astrophysical Journal. - Acknowledgement added

Theoretical O V electron-density-sensitive emission line ratios for R = I(2s21S0 - 2s2p3P2)/I(2s21S0 - 2s2p3P1) are presented. Inspection of Goddard High Resolution Spectrograph Hubble Space Telescope spectra of RR Tel reveals the presence of the [O V] 2s21S - 2s2p3P2 line at 1213.80 A, which is 4.62 ± 0.12 A away from the 2s21S - 2s2p3P2 intercombination transition at 1218.42 A, in good agreement with the theoretical prediction of Δλ = 4.54 A. The resultant value of R = 0.82 ± 0.11 implies a logarithmic electron density, log Ne, of 5.2 ± 0.2 cm-3, in good agreement with that found from other ions with high electron temperature, such as Ne VI, which also provides support for the identification.

Of the blazars detected by EGRET in GeV gamma rays, 3C 279 is not only the best-observed by EGRET, but also one of the best-monitored at lower frequencies. We have assembled eleven spectra, from GHz radio through GeV gamma rays, from the time intervals of EGRET observations. Although some of the data have appeared in previous publications, most are new, including data taken during the high states in early 1999 and early 2000. All of the spectra show substantial gamma-ray contribution to the total luminosity of the object; in a high state, the gamma-ray luminosity dominates over that at all other frequencies by a factor of more than 10. There is no clear pattern of time correlation; different bands do not always rise and fall together, even in the optical, X-ray, and gamma-ray bands. The spectra are modeled using a leptonic jet, with combined synchrotron self-Compton + external Compton gamma-ray production. Spectral variability of 3C 279 is consistent with variations of the bulk Lorentz factor of the jet, accompanied by changes in the spectral shape of the electron distribution. Our modeling results are consistent with the UV spectrum of 3C 279 being dominated by accretion disk radiation during times of low gamma-ray intensity., Comment: 39 pages including 13 figures; data tables not included (see ApJ web version or contact author)

Electron impact excitation rates in Cl III, recently determined with the R-matrix code, are used to calculate electron temperature ( T e ) and density ( N e ) emission line ratios involving both the nebular (5517.7, 5537.9 Å) and auroral (8433.9, 8480.9, 8500.0 Å) transitions. A comparison of these results with observational data for a sample of planetary nebulae, obtained with the Hamilton Echelle Spectrograph on the 3-m Shane Telescope, reveals that the R 1 = I (5518 Å)/ I (5538 Å) intensity ratio provides estimates of N e in excellent agreement with the values derived from other line ratios in the echelle spectra. This agreement indicates that R 1 is a reliable density diagnostic for planetary nebulae, and it also provides observational support for the accuracy of the atomic data adopted in the line ratio calculations. However the [Cl iii ] 8433.9 Å line is found to be frequently blended with a weak telluric emission feature, although in those instances when the [Cl iii ] intensity may be reliably measured, it provides accurate determinations of T e when ratioed against the sum of the 5518 and 5538 Å line fluxes. Similarly, the 8500.0 Å line, previously believed to be free of contamination by the Earth's atmosphere, is also shown to be generally blended with a weak telluric emission feature. The [Cl iii ] transition at 8480.9 Å is found to be blended with the He i 8480.7 Å line, except in planetary nebulae that show a relatively weak He i spectrum, where it also provides reliable estimates of T e when ratioed against the nebular lines. Finally, the diagnostic potential of the near-UV [Cl iii ] lines at 3344 and 3354 Å is briefly discussed.

NGC 6537 is an unusually high excitation point symmetric planetary nebula with a rich spectrum. Its kinematical structures are of special interest. We are here primarily concerned with the high resolution spectrum as revealed by the Hamilton echelle Spectrograph at Lick Observatory (resolution ≈ 0.2 Å) and supplemented by UV and near-UV data. These extensive data permit a determination of interstellar extinction, plasma diagnostics, and ionic concentrations. The photoionization models that have been used successfully for many planetary nebulae are not entirely satisfactory here. The plasma electron temperature of a photoionization model cannot much exceed 20,000 K, but plasma diagnostics show that regions emitting radiation of highly ionized atoms such as [Ne iv ] and [Ne v ] are much hotter, showing that shock excitation must be important, as suggested by the remarkable kinematics of this object. Hence, instead of employing a strict photoionization model, we are guided by the nebular diagnostics, which reveal how electron temperature varies with ionization potential and accommodates density effects. The predictions of the photoionization model may be useful in estimating ionization correction factor. In effect, we have estimated the chemical composition by using both photoionization and shock considerations.

From the 1950’s to 1962, the Observatory of the University of Michigan in Ann Arbor was graced by the presence of Edith Muller who worked with Leo Goldberg and me in a study of the chemical composition of the sun. (*) Before this investigation was undertaken, various attempts had been made to modernize determinations of the abundances of the most abundant elements in the solar atmosphere, but there had been no attempt to update Henry Norris Russell’s classical work of the late twenties taking advantage of the progress that had been made both in the study of stellar atmospheres and atomic data. A few primitive attempts had been made to analyze stellar spectra (e.g. of B stars) trying to take into account the variation of temperature and pressure with depth. The standard mode of procedure for the sun and stars was to use the curve of growth technique for an isothermal atmosphere at a constant pressure. At Michigan, we introduced the refinement of trying to take the effect of the solar atmospheric structure into account via the method of weighting functions (Unsold, Minnaert, Pecker). The model atmosphere adopted was based on the work of Aller and Pierce (1953) and extended by Elste to deeper and higher layers. Equivalent widths were taken from investigations by C.W. Allen, the Utrecht work and measurements made at McMath-Hulbert Observatory.

We present the results of the multi-wavelength campaigns on 3C 273 in 1993-1995. During the observations in late 1993 this quasar showed an increase of its flux for energies greater equal 100 MeV from about 2.1 x 10^-7 photons/cm^2/s to approximately 5.6 x 10^-7 photons/cm^2/s during a radio outburst at 14.5, 22 and 37 GHz. However, no one-to-one correlation of the gamma-ray radiation with any frequency could be found. The photon spectral index of the high energy spectrum changed from (3.20 +- 0.54) to (2.20 +- 0.22) in the sense that the spectrum flattened when the gamma-ray flux increased. Fits of the three most prominent models (synchrotron self-Comptonization, external inverse Comptonization and the proton initiated cascade model) for the explanation of the high gamma-ray emission of active galactic nuclei were performed to the multi-wavelength spectrum of 3C 273. All three models are able to represent the basic features of the multi-wavelength spectrum. Although there are some differences the data are still not decisive enough to discriminate between the models., 63 pages including 9 eps figures and 9 tables, LateX, uses aaspp4.sty and tighten.sty, accepted for publication in ApJ

The 2s2 1S0 – 2s2p 3P1 intercombination line at 1218.34 A of Be-like O V has been observed in IUE spectra of gaseous nebulae such as RR Tel (Doschek & Feibelman 1993). However, the forbidden line at 1213 A has not been detected to date in any astrophysical object, with the possible exception of the Sun, where Sandlin, Brueckner & Tousey (1977) very tentatively identify the line at 1213.90 A in an off-limb spectrum.

Emission lines arising from transitions in S II have been detected in a wide variety of astronomical sources, including planetary nebulae (Hyung, Keyes & Aller 1995). These transitions are used to derive information on emitting plasmas parameters (Te, Ne) through diagnostic line ratios, although to calculate these quantities reliably, accurate atomic data must be employed, especially for electron impact excitation rates.

Determinations of the plasma diagnostics and chemical compositions of planetary nebulae require ultimately high dispersion spectra. For objects of high surface brightness the Hamilton Echelle Spectrograph at Lick Observatory is satisfactory for the region 3650–10100A which involves the 168th to 56th echelle orders. For a slit width of 640 μm amounting to 1.15 arcsec at the Coude focus, the actual spectral resolution (FWHM) is about 0.2A at 8850A. The length of the slit is chosen as 4.0 arcsec. Hyung (1994) & Aller (1994) describe the observing procedure. Since the Hamilton echelle was designed primarily for star-like sources, it is not useful for extended low surface brightness PN. The earlier observations were obtained with an 800 × 800 chip that did not cover the echelle field, so several settings were needed. Later, we used a slower 2048 × 2048 chip which covered the whole field and was somewhat more efficient at longer wavelengths. The program has been completed and definitive measurements have obtained for NGC 2440, NGC 6543, NGC 6741, NGC 6818, NGC 7026, NGC 7662, and Hu 1–2. All of these PN display particularly rich, interesting spectra. Previously observed and published objects include IC 351, IC 418, IC 2149, IC 4997, NGC 6567, NGC 6572, NGC 6790, NGC 6886, NGC 7009, BD +30 3639, & Hubble 12. NGC 6884 is in press. Additional nebulae which are yet to be discussed are IC 4634, IC 4846, IC 5117, NGC 6210, & NGC 6803.