High-excitation Planetary Nebula Research Articles

ITAL]HUBBLE SPACE TELESCOPE[/ITAL][ITAL]Hubble Space Telescope[/ITAL] Imaging of Central Stars of High-Excitation Planetary Nebulae with WFC and WFPC2

We present observations of four high excitation planetary nebulae (IC 2165, Me 2-1, NGC 2440, NGC 7027) taken with the Wide Field Camera and the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. The high angular resolution of the instruments allows for the unambiguous identi—ca- tion of the central star, which was not found to be the case in most previous studies. In addition, the panchromatic nature of the data, which include several ultraviolet bandpasses, permits us to directly determine the amount of dust extinction present along the sight line to the central star. The combination of these two characteristics make it possible to delineate the spectral energy distribution of the central stars. Assuming a Planck function spectrum and a standard interstellar extinction law with the R V 3.1, adopted values are 0.40 (IC 2165), 0.15 (Me 2-1), 0.15 (NGC 2440), and 1.10 (NGC 7027). The E B~V wavelength coverage of our measurements is not particularly sensitive to temperature values in the range 150,000¨400,000 K. However, we do —nd that 200,000 K produces the best —ts. Again using a Planck function to represent the wavelength dependence of the stellar —ux, we determine V magnitudes of 17.47 ^ 0.06 (IC 2165), 18.40 ^ 0.05 (Me 2-1), 17.63 ^ 0.16 (NGC 2440), and 16.04 ^ 0.04 (NGC 7027). Combining these results with literature values for the emission-line —uxes and distances, we derive the Zanstra temperature, crossover magnitude, radius, and luminosities for each object. The implications of these measurements for the mass and the evolutionary state of each object is brie—y discussed.

The remarkably high excitation planetary nebula GC 6537.

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 approximately 0.2 A) 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 [NeIV] and [NeV] 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.

Emission lines of [O II] in the optical and ultraviolet spectra of planetary nebulae

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.

Emission lines of [Ne iv] in the optical and ultraviolet spectra of gaseous nebulae

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.

The High‐Excitation Planetary Nebula NGC 7662

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.

The high-excitation planetary nebula NGC 6741

The compact, bright Peimbert type I planetary nebula NGC 6741 shows an unusually rich spectrum which is studied here from 3700 to 10 370 Å with the Hamilton Echelle spectrograph at the Lick Observatory. By combining Hamilton Echelle observations with archive UV data secured with the International Ultraviolet Explorer (IUE), and with available IR data, we can obtain improved diagnostics and chemical composition of this high-excitation planetary nebula. The diagnostic diagram gives Tϵ = 12 500 K, Nϵ= 6300 cm−3 for most ions. An improved ‘composite’ theoretical nebular model is employed. The stellar temperature quoted in the earlier literature seems too high; the highest likely temperature appears to be 140 000 K. Elemental abundances are estimated from ionic concentrations, using an ionization correction factor in the usual way, and also from a theoretical model. These different methods show large discordances, implying that one must use great caution in choosing Tϵ for the determination of the ionic concentrations. For every 10 000 H atoms, there appear to be 8.0 atoms of C, 2.4 of N, 5.4 of 0,1.3 of Ne, 0.068 of S, 0.0023 of Cl and 0.035 of Ar. NGC 6741 is rich in He, C and N, compared with the solar abundance. O is probably depleted by ‘hot bottom’ burning. NGC 6741 may be also depleted in metals like Ca, Mg, Si and Fe, which may be tied up in grains. The progenitor star may have been an object in which C, N and Ne had essentially solar abundances, while heavier elements were less plentiful than in the Sun.

Chemical Abundances of Planetary Nebulae in the Sagittarius Dwarf Elliptical Galaxy

Spectrophotometry and imaging of the two planetary nebulae He2-436 and Wray16-423, recently discovered to be in the Sagittarius dwarf elliptical galaxy, are presented. Wray16-423 is a high excitation planetary nebula (PN) with a hot central star. In contrast He2-436 is a high density PN with a cooler central star and evidence of local dust, the extinction exceeding that for Wray16-423 by E(B-V)=0.28. The extinction to Wray16-423, (E(B-V)=0.14), is consistent with the extinction to the Sagittarius (Sgr) Dwarf. Both PN show Wolf-Rayet features in their spectra, although the lines are weak in Wray16-423. Images in [O III] and H-alpha+[N II], although affected by poor seeing, yield a diameter of 1.2'' for Wray16-423 after deconvolution; He~2-436 was unresolved. He2-436 has a luminosity about twice that of Wray16-423 and its size and high density suggest a younger PN. In order to reconcile the differing luminosity and nebular properties of the two PN with similar age progenitor stars, it is suggested that they are on He burning tracks The abundance pattern is very similar in both nebulae and shows an oxygen depletion of -0.4 dex with respect to the mean O abundance of Galactic PN and [O/H] = -0.6. The Sgr PN progenitor stars are representative of the higher metallicity tail of the Sgr population. The pattern of abundance depletion is similar to that in the only other PN in a dwarf galaxy companion of the Milky Way, that in Fornax, for which new spectra are presented. However the abundances are larger than for Galactic halo PN suggesting a later formation age. The O abundance of the Sgr galaxy deduced from its PN, shows similarities with that of dwarf ellipticals around M31, suggesting that this galaxy was a dwarf elliptical before its interaction with the Milky Way.

The Spectrum of the Planetary Nebula NGC 6884

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.

Ca and dust in Planetary Nebulae

Under nebular conditions it is expected that the [CaII] lines at 7291.470 and 7323.880 Å should be among the strongest emission lines in the red part of the optical spectrum unless the nebula is matter bounded. The lines are observed in novae, supernovae and some symbiotic systems but the only observation of these lines in planetary nebulae (PN) is for AFGL 618 where the lines are thought to be shock-excited. While higher ionization states of Ca do produce some lines observed in a few high-excitation PN, from which gas-phase Ca abundances averaging to about 5% of solar are found, the CaII lines are not observed even in PN with massive neutral envelopes such as NGC 7027. If the Ca is depleted onto dust grains it would explain the absence of the CaII lines. However it is thought that the dust is destroyed with time in the ionized region. Any observation of the CaII lines would produce better abundance estimates and would test the extent of dust destruction that is taking place in the outer parts of the PN.

AN O VIII SEQUENCE FOR PLANETARY NEBULA NUCLEI

ABSTRACT The detection of O VIII emission lines in ultraviolet and optical spectra of high-excitation planetary nebula central stars suggests an "O VIII Sequence" for these object. Emission lines of O VII are also detected. Seven out of ten central stars for these planetaries are known to be nonradial g-mode pulsators. The stars form a narrow strip when plotted on the log L*/Lsun - log Teff H-R diagram.

The spectrum of the planetary nebula IC 351

International Ultraviolet Explorer (WE) and optical region spectroscopic observations are presented for the moderately high-excitation planetary nebula IC 351. Most of the atoms are distributed over a narrow range of ionization. Thus [N ii] and [S ii] are weak; [Ne iv] is prominent, but [Ne v] is missing. It appears that the metal/hydrogen ratio is smaller than in the Sun. No Si lines are seen either in optical or UV regions. It is suggested that virtually all Si atoms are locked in grains. The ionization correction factor method fails utterly for N, for example. Hence IUE data are essential for chemical composition estimates. Until high spatial resolution images are available, construction of a new theoretical model does not seem warranted.

Hopkins Ultraviolet Telescope observations of H2 toward the planetary nebula NGC 1535

We have observed the far-ultraviolet spectrum (912-1860 A) of the bright high-excitation planetary nebula NGC 1535 with approximately 3 A resolution using the Hopkins Ultraviolet Telescope (HUT) aboard the Astro-1 space shuttle pmission in 1990 December. We see strong continuum emission down to the Lyman limit and strong P Cygni profiles from high-excitation lines such as C IV wavelength 1549, N V wavelength 1240, O V wavelength 1371, and O VI wavelength 1035. Below 1150 A strong absorption bands of H2 are seen, which were unanticipated by us because of the low reddening and high galactic latitude of the object and the absence of detected H2 emission in the infrared. We construct model H2 spectra and convolve them to the HUT resolution for comparison with the NGC 1535 data. We find good agreement with a population distribution characterized by a single temperature (T = 300 K) or a two-temperature model (T = 144/500 K), and determine limits on the H2 column density. While both inter-stellar and circumstellar origins for the observed H2 absorption are plausible, we ascribe the material to the planetary nebula in order to estimate the conditions of excitation and place upper limits on the mass of both H2 and H1 in this system. Because the UV transitions are ground-state connected, we determine a stringent upper limit of 0.03 d(sup 2)(sub 1.6) solar mass on the mass of H2, where d(sub 1.6) is the distance relative to an assumed distance of 1.6 kpc. This value is less model-dependent than IR estimates. Along with the central star and nebular masses, these estimates allow us to limit the main-sequence mass of the progenitor star to less than 1.8 solar mass. This upper limit is consistent with a relatively low-mass extended thick disk or Population II progenitor, as expected for an object approximately 1 kpc off the galactic plane.

Observations of the Bowen fluorescence mechanism and charge transfer in planetary nebulae - II

Deep, medium resolution, long-slit spectrophotometric data have been taken for a number of high-excitation planetary nebulae, covering a wavelength range from 3100 A to 7200 A with some selected regions observed at higher resolution. For about half the objects, the whole optical region has been observed, from 3100 A to 11600 A. Accurate flux calibration is achieved over this whole wavelength range. These data allow a detailed quantitative study of the Bowen fluorescence mechanis-m and the charge transfer reaction \({{O}^{{3 + }}}{{(}^{2}}{{P}_{{3/2,1/2}}}) + {{H}^{0}} - {{O}^{{2 + }}}\left( {^{{2{{{s'}}_{a}} + }}{{{L'}}_{{a {{{j'}}_{a}}}}}} \right) + {{H}^{ + }} \) in planetary nebulae, the primary goal of this program. In this paper measurements of O2+ Bowen fluorescence and charge transfer lines are presented. We show that LS coupling fails for the O2+ permitted transitions studied in this program and intermediate coupling may be a better assumption. Efficiencies of the Bowen fluorescence mechanism are derived for 15 objects, and a wide range of possible values is apparent. There is a remarkable linear positive correlation between the Bowen efficiency and the fractional abundance of oxygen in the ionization stage of O2+ and the fractional abundance of helium in the form of He+. Evidence that the Bowen efficiency is anticorrelated with the electron temperature, as first noted by Likkel and Aller, is established. The Bowen efficiency drops substantially when the nebular expansion velocity 2V exp (O2+) > 55 km/sec. For lower expansion velocities there is no detectable correlation between these two quantities. There are no observable differences in Bowen efficiency among objects of different morphological type nor between objects excited by stars of different spectral types as suggested by Likkel and Aller.

Observations of the Bowen Fluorescence Mechanism and Charge Transfer in Planetary Nebulae I

Deep, medium resolution, long-slit spectrophotometric data have been taken for a number of high-excitation planetary nebulae, covering a wavelength range from 3100 Å to 7200 Å with some selected regions observed at higher resolution. For about half the objects, the whole optical region has been observed, from 3100 Å to 11600 Å. Accurate flux calibration is achieved over this whole wavelength range. These data allow a detailed quantitative study of the Bowen fluorescence mechanism and the charge transfer reaction in planetary nebulae, the primary goal of this program. In this paper measurements of O2+ Bowen fluorescence and charge transfer lines are presented. We show that LS coupling fails for the O2+ permitted transitions studied in this program and intermediate coupling may be a better assumption. Efficiencies of the Bowen fluorescence mechanism are derived for 15 objects, and a wide range of possible values is apparent. There is a remarkable linear positive correlation between the Bowen efficiency and the fractional abundance of oxygen in the ionization stage of O2+ and the fractional abundance of helium in the form of He+. Evidence that the Bowen efficiency is anticorrelated with the electron temperature, as first noted by Likkel and Aller, is established. The Bowen efficiency drops substantially when the nebular expansion velocity 2Vexp(O2+) > 55 km/sec. For lower expansion velocities there is no detectable correlation between these two quantities. There are no observable differences in Bowen efficiency among objects of different morphological type nor between objects excited by stars of different spectral types as suggested by Likkel and Aller.

High-Dispersion Spectroscopy of IC 351 and NGC 3242, Planetaries by High Internal Motion

We made high-dispersion spectroscopic observations of IC 351 = PK 159 − 15°1, a compact high-excitation planetary nebula. We found the expansion velocity from Hα, [OIII] and HeII line, of 45.1kms−1, 38.7kms−1, and 33.3kms−1, respectively. Moreover, for the Hα line we detected an additional faint blue-shifted component escaping from the center of expansion with high velocity of 120kms−1. This value is larger than the usual expansion velocity (< 50kms−1) but considerably smaller than that of stellar wind. This faint blue-shifted component of Hα is also coincidently identified as the helium line, HeIIλ6560 (Pickering 6). Using the emission coefficients of recombination lines (Case B, T= 10,000K; Osterbrock 1989), and the intensity ratio, I(HeIIλ4686)/I(Hβ)=0.56±0.05 (Aller&Czyzak 1979), we can estimate the expected line intensity ratio, I(HeIIλ6560)/I(Hα)=0.026±0.002. On the other hand, the intensity ratio of faint component to Hα in the present observations is estimated as Ifaintcomp./I(Hα)=0.064±0.013, based up the Hα profile in 1988. Comparing these two estimated intensity ratios it is clearly unable to emit the all intensity of the observed faint component by HeIIλ6560 line only. Therefore, even if this component is partly polluted by HeIIλ6560 line, we can consider the rest part as an emission from high velocity component of Hα. We can also recognize the bumped feature of [OIII]λ5007 line around the same velocity to the blue-shifted component of Hα. This fact supports the idea that these components can be attributed to the high velocity flow. We discuss the three ideas for the interpretation of such component; (a) colliding winds, (b) unresolved bipolar flow, and (c) secondary formation of an expanding shell. We have investigated whether the colliding winds model gives such high velocity or not. As the results of the calculation, if we use the usual mass-loss rate, the model can not give such high velocity. Therefore, it is doubtful that we can take this model as an explanation of the high velocity components. The high velocity component of IC 351 is not uncommon; such large internal motions are reported for other planetaries by Weinberger(1989) and compiled by Grewing(1989). The article on the results of IC 351 is submitted to PASP (Yadoumaru & Tamura 1992). We also report the other analysis on NGC 3242.

HIGH-DISPERSION SPECTROSCOPY OF IC 351--A CASE STUDY OF A HIGH-EXCITATION PLANETARY NEBULA

ABSTRACT In 1988 and 1991 we made high-dispersion spectroscopic observations of IC 351 = PK 159-15 degrees 1, a compact high-excitation planetary nebula. The H-alpha and [O III] lines obtained in 1988 are decomposed into two components. The resultant expansion velocities (the difference between these components), VDelta rho, are 25.3 ± 5.8 km s-1 and 25.1 ± 8.0 km s-1 for H-alpha (P.A. = -21.4) and [O III] (P.A. = +30.8). We found the expansion velocity, 2Vexp, from H-alpha(P.A. = -21.4), [O III] (P.A. = +30.8) and He II (P.A. = +60.1) lines, of 45.1 km s-1, 38.7 km s-1, and 33.3 km s-1, respectively. Moreover, for the H-alpha line we detected an additional faint component at the velocity of -120 km s-1. Although the intensity of this apparently blueshifted component of H-alpha is estimated as two and a half times the expected intensity of the coincident helium line, He II lambda-6560 (Pickering 6), it might be due to the helium line itself.

The Spatial Distribution of Line Ratio O III/O II in High Excitation Planetary Nebulae

Previous attempts to model the integrated line ratio [O III] 5007 A/[O II] 3727 A for high excitation planetary nebulae (Che and Köppen 1983) have suggested that the charge exchange coefficient (k) for the reaction could be 10 times smaller than the predicted theoretical value. The influence of other factors that may contribute to differences in the O III/O II ratio, such as the ionizing spectrum and the nebular gas density distribution, seem to be relatively small in the high excitation nebulae.

Molecular Hydrogen Emission from Cold Condensations in NGC 2440

We report observations of the v = 1-0 S(1) line of molecular hydrogen in the high excitation Planetary Nebula NGC 2440. The emission is particularly strong at the positions of the two bright condensations which lie well within the H II region and close to the position of the very hot T = 350,000 K central star. The emission is consistent with an excited molecular hydrogen mass of 2–4 × 10−5 M⊙ in the condensations, and we estimate the total mass of excited molecular hydrogen associated with the H II region to be 6 × 10−3 M⊙. We show that the radiation pressure from the central star is insufficient to excite the S(1) line emission. We also show that a stellar wind driven shock would imply a mass loss rate of 3 × 10−7 M⊙ yr−1 if we adopt a wind velocity of 2000 km s−1.

Molecular hydrogen emission from cold condensations in NGC 2440

We report observations of the |$\upsilon\, = 1-0\,S(1)$| line of molecular hydrogen in the high-excitation planetary nebula NGC 2440. The emission is particularly strong at the positions of the two bright condensations which lie well within the Hrr region and close to the position of the very hot T ≈ 350 000 K central star. The emission is consistent with an excited molecular hydrogen mass of |$\approx 2-4\,\times\,{10}^{-5}\,{M}_{\odot}$| in the condensations, and we estimate the total mass of excited molecular hydrogen associated with the H II region to be |$\approx 6.1\,\times\,{10}^{-3}\,{M}_{\odot}$|⁠. We show that radiation pressure from the central star is insufficient to excite the S(1) line emission. We also show that a stellar-wind-driven shock would imply a mass loss rate of |$3\,\times\,{10}^{-7}\,{M}_{\odot}\,\text{yr}^{-1}$| if we adopt a wind velocity of 2000 km s–1.

IUE and Optical Observations of HE 2-104

AbstractHe 2-104 has been classified as both a planetary nebula and as a symbiotic star. Optical and ultraviolet spectra and CCD images have been obtained in order to learn more about the evolutionary state of this object. The spectra of the object indicate that it could be either a symbiotic star or a very high excitation planetary nebula. The CCD images show a nebulosity with a radius of about 4 seconds of arc and faint bipolar outer structures extending out approximately 25 seconds of arc from the center of the nebula.