---
abstract: |
    We present medium-resolution (R $\sim$ 700) near-infrared ($\lambda =
    1 - 2.5$ ) spectra of a sample of planetary nebulae (PNe). A narrow slit was used which sampled discrete locations within the nebulae; observations were obtained at one or more positions in the 41 objects included in the survey. The PN spectra fall into one of four general categories: emission line-dominated PNe, and 2 emission line PNe, 2 emission line-dominated PNe, and continuum-dominated PNe. These categories correlate with morphological type, with the elliptical PNe falling into the first group, and the bipolar PNe primarily in the 2 and continuum emission groups. The categories also correlate with C/O ratio, with the O-rich objects generally falling into the first group and the C-rich objects in the other groups. Other spectral features were observed in all categories, such as continuum emission from the central star, C$_2$, CN, and CO emission, and warm dust continuum emission towards the long wavelength end of the spectra.

    Molecular hydrogen was detected for the first time in four PNe. An excitation analysis was performed using the 2 line ratios for all of the PN spectra in the survey where a sufficient number of lines were observed. From the near-infrared spectrum, we determined an ortho-to-para ratio, the rotational and vibrational excitation temperatures, and the dominant excitation mechanism of the 2 for many objects surveyed. One unexpected result from this analysis is that the 2 is excited by absorption of ultraviolet photons in most of the PNe surveyed, although for several PNe in our survey collisional excitation in moderate velocity shocks plays an important role. The correlation between bipolar morphology and 2emission has been strengthened with the new detections of 2 in this survey. We discuss the role of winds and photons to the excitation of 2 in PNe, and consider some implications to the utility of 2 as a nebular diagnostic and to our understanding of PNe structure and evolution.
author:
- 'Joseph L. Hora'
- 'William B. Latter'
- 'Lynne K. Deutsch'
title: |
    Investigating the Near-Infrared Properties of Planetary Nebulae\
    II. Medium Resolution Spectra
---

2[H$_2$]{} 1[$v = 1 \to 0$ S(1)]{} [*H*]{} 30[BD+30$^{\circ}$3639]{} 10420[IRC+10$^{\circ}$420]{} 3[cm$^{-3}$]{} 3[[cm]{}\^3 [s]{}\^[-1]{}]{} 3[[cm]{}\^[-3]{}]{}

Introduction
============

This is the second of two papers describing the results of surveys examining the properties of planetary nebulae (PNe) as observed in the near-infrared ($\lambda = 1 - 2.5$ ). The first paper (Latter et al. 1995; hereafter Paper I) presented an infrared imaging survey; here we present the results of a near-infrared spectral survey.

There are several reasons why knowledge of the near-infrared (near-IR) characteristics of PNe is important, as described in Paper I. In order to interpret the imaging results and to learn more about the physical conditions in the nebulae, the spectra of these objects must be examined to understand the processes responsible for the emission. There are many emission lines present in the 1 – 2.5  spectral region, most notably those due to recombination lines of , and lines of vibrationally excited 2. Also present are atomic lines of and \[\], and emission from other molecular species such as CO and C$_2$. These lines can act as diagnostic tools to probe the physical conditions inside the nebula, sampling different regions and ranges of temperature, density, and excitation than is seen by observing the optical line emission. Some PNe also exhibit in the near-IR strong continuum emission from hot dust. This emission becomes significant longward of $\lambda = 2$  in many of the PNe, requiring near-IR spectroscopy to detect it and to differentiate between line and continuum emission sources. Finally, the lower optical depth of the PNe in the IR as compared to optical wavelengths allows us to potentially see into regions of the nebula that are obscured by dust.

There have been several previous surveys that have explored the properties of PNe in the infrared. Early spectroscopic and photometric surveys (e.g., Gillett, Merrill, & Stein 1972; Cohen & Barlow 1974) determined that there was an excess of IR emission over what was expected from reflected continuum emission from the central star. Other photometric surveys in the following years (Whitelock 1985; Persi et al. 1987) determined the primary sources of IR emission to be stellar continuum, thermal dust emission, and thermal & line emission from the nebula itself. The near-IR color characteristics of most PNe are unique and can be used to identify new PNe and post-AGB objects (Garcia-Lario et al. 1990).

Recently there have been more detailed spectral observations of PNe in the near-IR. Hrivnak, Kwok, & Geballe (1994) surveyed a set of proto-PNe in the H and K bands. In these objects the Brackett lines were observed in absorption, and most objects had CO absorption or emission, indicating recent mass loss events. Rudy et al. (1992, and references therein) and Kelly & Latter (1995) have surveyed several PNe and proto-PNe in the $\lambda = 0.5 - 1.3$  range. Dinerstein & Crawford (1998) have completed a survey of a set of PNe in the K-band, focusing on excitation of molecular hydrogen.

There are several unique aspects of the survey results presented here that were made possible by the KSPEC spectrograph (see the instrument description below). First, because of KSPEC’s high sensitivity and simultaneous sampling of the full near-infrared spectral range, we were able to obtain data on a comparatively large number of objects (41) in a short period of time. Using the relatively narrow and short slit of the spectrograph, we sampled different regions of the PNe to examine the emission throughout the nebula. In most of the other surveys described above, larger beams were used that included much or all of the object. Another aspect of the data presented here is because of the cross-dispersed design of the instrument, the entire $\lambda = 1.1 - 2.5$  range is obtained at once, eliminating the possibility of telescope pointing errors or other fluctuations affecting the relative line strengths in the spectra. Finally, the slit-viewing detector allowed precise positioning and guiding, so the region of the PN being observed was well known for each spectrum. The PNe observed in this survey were chosen to overlap with the near-IR imaging survey (Paper I), along with several other optically-bright PNe and unresolved objects that were not included in the imaging survey.

Observations and Data Reduction
===============================

The observations were performed on several runs during the period 1992 October through 1994 September at the University of Hawaii 2.2m telescope on Mauna Kea, using the near-IR KSPEC spectrograph (Hodapp et al. 1994). KSPEC is a $\lambda = 1 - 2.5$  cross-dispersed spectrograph that has a separate slit-viewing IR array for acquiring the source and guiding. The full spectral range is obtained in a single exposure, resulting in accurate relative line measurements and highly efficient data acquisition. The diffraction orders are well-matched to the atmospheric transmission windows, with the K band in 3rd order (1.9 – 2.5 ), H in 4th order (1.45 – 1.8 ) and J in 5th order (1.15 – 1.32 ), with a resolution R $\sim$ 700. The 1$\times$6 arcsecond slit provides a small aperture that was used to sample different spatial regions of the nebula to search for spectral variations.

Table 1 lists the nebulae observed and details of the observations. Integration times for each exposure ranged from a few seconds for the extremely bright sources to 5 minutes for faint sources. The off-axis guider was used to keep a consistent on-source slit position. Multiple “Fowler” sampling was used to reduce the read noise. The number of samples for a particular integration ranged from 4 to 16, with more samples used for the longer integration times. The spectra were reduced using IRAF; the extraction and processing of the spectral data were done using the functions in the noao.twodspec and noao.onedspec packages. Alternating source and sky integrations of the same length were taken and differenced to remove sky and telescope background flux. Dome flats were used to correct for pixel-to-pixel gain variations in the array. Stars of known spectral type (either G0 or A0) were observed at the same airmass as the nebulae immediately before and after the PNe observations and were used for correction of the instrumental response and sky transmission. Individual lines were removed from the stellar spectra, and then normalized using a blackbody function of T = 5920 K for the G0 stars and T = 10800 K for the A0 standards. The spectra were wavelength-calibrated using observations of an Argon reference lamp. The wavelength values used for lines greater than 1.1  are from Rao et al. (1966); for lines less than 1.1 , the wavelengths were taken from Wiese, Smith, & Miles (1969) and corrected to vacuum wavelengths. The average 1$\sigma$ uncertainty in the measured wavelengths of the lines is about 5 Å.

Infrared photometric standard stars were observed in the same way as the PNe and used to flux calibrate the spectra. Absolute calibration is difficult with these spectra because not all of the flux from the star enters the narrow slit during a single integration, and the amount differs for each integration depending on how well the star is centered on the slit (the seeing at 2  during typical observations was 05 – 10). The amount of light lost was estimated by the following method: the full width at half maximum (FWHM) brightness of the standard star was measured along the spatial direction of the slit, in the spectrum with maximum flux for that star. It was then assumed that the point spread function (PSF) is well represented by a two-dimensional Gaussian distribution with the measured FWHM, and the amount of flux falling outside of the slit was then calculated. This was typically 20 – 30% of the total light for a single integration. The calibration for each star was corrected by this factor, along with corrections for airmass. Comparing results from different standard stars taken throughout the night indicated that this method is accurate to approximately 20%. For the observations of the PNe, no correction was applied for the slit width or length. The length along the slit of the extracted regions for the three bands were 20 (J), 35 (H), and 40 (K).

Results 
========

The spectra are presented in Figures 1 – 32. One to three PNe spectra are plotted in each figure. Tables 2 through 7 list the line identifications and extracted fluxes with uncertainties for the spectra shown in the Figures. The PN in this section are listed in Tables 2, 3, and 4. There are a number of features not identified (indicated by question marks in the tables). These features tend to appear above the $\sim3\sigma$ level and must be considered real, though confirming spectra would be valuable. Our search for possible identifications for these lines has been careful, but perhaps not exhaustive. In addition to the relatively low S/N of these lines, the moderate wavelength resolution is not sufficient to differentiate between the several possible identifications for each line.

The PNe spectra are separated into four groups that share common characteristics. These are recombination line-dominated, recombination line + 2 emission, 2 – dominated, and continuum-dominated. A fifth group of objects is included that contains two objects that were at one time classified as PNe, but are now generally regarded as being regions (M 1–78 and K 4–45; Acker et al.1992). We do not discuss these objects further, but include them for comparison. Within each group, the NGC objects are listed first, followed by the remaining PNe in alphanumeric order. The morphological classifications are given according to Balick (1987), unless otherwise noted.

 - line dominated
-----------------

The line emission in these PNe is dominated by lines of and . In the J-band, the Paschen $\beta$ () line is the most intense, with contributions from lines of , \[\], and . In the H band, the Brackett series of dominate, with emission at 1.7002  and \[\] emission at 1.6440   present in some PNe. In the K band, the brightest line is usually Brackett $\gamma$ (), with strong lines of at 2.058 and 2.112 . When the lines are strong enough, one begins to see the Pfund series lines starting near 2.35  where they are just beginning to be separated at this resolution. There are also two unidentified lines at 2.199 and 2.287  (Geballe et al. 1991) that appear in several PNe in this category. A few of the spectra shown here have contributions from central star continuum flux that is larger towards shorter wavelengths, or warm dust continuum which is stronger at longer wavelengths.

### NGC 1535

NGC 1535 is classified as early round, and its near-IR spectrum is dominated by emission lines of . It has a bright ionized shell of emission which is surrounded by a fainter halo (e.g., see Schwarz, Corradi, & Melnick 1992). Near-IR images were presented in Paper I. This PN has previously been observed to have 2 lines in absorption in the far-UV (Bowers et al. 1995). Recent observations by Luhman et al. (1997) failed to detect 2 in emission in the 1 line. They attributed the earlier detection of absorption at shorter wavelengths to the interstellar medium, or a region in the PN itself of a much smaller size than the ionized zone.

The spectrum of NGC 1535 shown in Figure 1 was obtained at a position centered on the brightest part of the ring directly west (W) of the central star. We also fail to detect the 2 emission in the 1 line, at a 1 $\sigma$ level of about $5\times10^{-17}$ ergs cm$^{-2}$ s$^{-1}$ Å$^{-1}$. There is some indication of emission from 2 in the $v
= 1\to 0$ Q(1) and $v=1\to 0$ Q(3) lines at the long wavelength end of the spectrum. However, the spectrum is noisier in this region and there is confusion with the Pfund-series lines, therefore the 2lines are not detected above the 3 $\sigma$ level.

### NGC 2022

NGC 2022 is an early elliptical PN, but is morphologically very similar to NGC 1535 in optical images (e.g., Schwarz et al.1992). The main difference between the two is a different relative outer halo size as compared to the inner ring (the halo is relatively smaller in NGC 2022). Zhang & Kwok (1998) also find similar parameters with their morphological fits of these two PNe. Near-IR images of this PN were presented in Paper I.

NGC 2022 is also spectrally similar to NGC 1535, as seen in Figure 1. The spectrum of NGC 2022 was taken centered on the ring directly east (E) of the central star. The dominant emission lines are those of atomic hydrogen.

### NGC 2392

The PN NGC 2392 (the “Eskimo nebula”) is another double-shell nebula; however, this PN has a significant amount of structure in the inner ring and outer shell. Spectrophotometric (Barker 1991) and kinematic studies (Reay, Atherton, & Taylor 1983; O’Dell, Weiner, & Chu 1990) that have been carried out with optical imaging and spectroscopy have revealed the abundances and ionization states and velocities of the various components. Near-IR images of this PN were presented in Paper I.

The spectrum of NGC 2392 in Figure 1 was taken centered on the brightest part of the ring directly E of the central star. This third early-type round PN differs from the other two in Figure 1 primarily from the bright line at 2.058 , and the \[\] lines in the J and H bands of the spectrum.

### NGC 3242

NGC 3242 is an early elliptical with several interesting morphological features. In addition to the bright elliptical ring, there are several filaments and knots of emission in the central region, and two ansae that are placed roughly along the major axis of the elliptical emission. Also, there is a larger faint halo that envelopes the inner structure.

Spectra acquired at three different positions on the nebula are shown in Figure 2, on the SE knot (NGC 3242SE), on the E section of the bright ring (NGC 3242E), and on the SW halo (NGC 3242H). The spectra are similar in all locations; the bright lines are present in all positions, along with stellar continuum at the shorter wavelengths. One difference is that the line at 2.189  are much brighter in the E ring than in the SE knot or halo position.

### NGC 6210

NGC 6210 is a fairly compact PN with a core – halo morphology similar to other ellipticals. Phillips & Cuesta (1996) performed a visible wavelength spectroscopic study that revealed a complex velocity structure which suggests multiple shells and possibly “jets” at various position angles. The near-IR image presented in Paper I does not reveal much of the structure. This PN is one of several in which Geballe et al. (1991) detected unidentified emission at 2.286  (but not at 2.199 ).

Three spectra are presented for this PN, and shown in Figure 3. They were acquired with the slit centered on the core (Core), 1 east (E1), and 3 east (E3). The core position shows a contribution from stellar continuum in the $\lambda = 1 - 1.8$  region that decreases successively in the E1 and E3 positions. The unidentified feature at 2.287  is detected at the E1 position, but not the 2.199 line, similar to the findings of Geballe et al. (1991).

### NGC 6543

NGC 6543 (the “Cat’s Eye”) has a complex morphology, with a high degree of symmetry. Recent HST imaging (Harrington & Borkowski 1994) has shown more clearly the structure of rings, shock fronts, jets, and fast, low-ionization emission-line regions (FLIERS) present in this PN.

The spectrum shown in Figure 4 was taken at the position S of the central star and slightly E, where the two emission arcs cross and create a local emission maximum (see Paper I, Figure 6a). The spectra is similar to the other PN in the / - dominated class, and also show the unidentified lines at 2.199 and 2.286 .

### NGC 6572

This young PN has a bipolar morphology in the near-IR, with its major axis in the N-S direction, and a bright ring structure closer to the central star (Paper I). The near-IR spectrum from 0.77 to 1.33  was measured by Rudy et al. (1991), and the UV and optical spectrum was recently obtained by Hyung, Aller, & Feibelman (1994), who found evidence of variability.

Figure 5 shows two slit positions on the PN, one on the nebula center, and one on the brightest location in the E lobe of the PN. Both the core and E lobe spectra show a strong contribution from stellar continuum flux. There is strong and line emission, and relatively strong unidentified line emission at 2.199 and 2.286 .

### NGC 6790

This PN has been shown by radio continuum observations to be an elliptically-shaped nebula with a diameter of roughly an arcsecond (Aaquist & Kwok 1990). Aller, Hyung, & Feibelman (1996) obtained UV and optical spectra and suggest that NGC 6790 is a relatively young object, slightly more evolved than Hb 12. Kelly & Latter (1995) obtained a 0.9 – 1.3  spectrum and find similarities to Hb 12 and AFGL 618.

The spectrum of NGC 6790 in Figure 4 shows a stellar contribution because the slit was centered on the object, and includes the star as well as the nebular emission that is typical of this class of PN.

### NGC 6803

This compact elliptical PN has a uniformly bright disk with no apparent structure in the optical, and is surrounded by a fainter halo about twice the size of the bright shell (Schwarz et al. 1992). In the KSPEC imaging channel, however, the PN was seen to be double-lobed, with the lobes on the minor axis of the bright elliptical region. Spectra were obtained at two positions, one centered on the E lobe, and the other position 5 arcseconds NW in the halo region. The lobe spectrum shows bright and lines, with a stellar continuum contribution out to about 1.5 . There is also unidentified line emission at 2.286 . The halo spectrum is quite different, with weak  and continuum emission, and is probably reflected star and nebular emission.

### NGC 6826

This elliptical PN is morphologically similar to NGC 3242, with a bright elliptical inner ionized ring, ansae along the major axis, and a fainter halo that envelopes the system (Balick 1987). The I-band image in Paper I shows evidence for a shell between the inner bright ring and the outer halo, and at the same radial distance as the ansae.

The spectra are shown in Figure 7. The bright core is dominated by stellar continuum emission. The other positions show strong line emission. The spectrum labeled SW Lobe was taken on the inner bright ring directly SW of the central star. The SW Halo position was taken in the halo midway between the bright ring and the outer edge of the PN. The lobe and halo emission is similar, except for relatively brighter lines of at 1.7002  in the lobe. There is also some continuum emission at the short wavelength end of the nebular positions, which is probably scattered light from the central star.

### NGC 7009

NGC 7009 (the “Saturn” nebula) is an elliptical with an interesting twisted symmetry in its shell and in the various filaments and knots of emission. Balick et al. (1998) recently published HST images that show the “microstructures” in this PN. The images show that the inner knots are actually groups of FLIERs, and jets in \[\] are seen that terminate at the tips of the nebula.

Two positions were sampled, one in the halo region on the W edge of the PN, and one in the N part of the nebula. Both positions show a stellar continuum emission contribution from the central star. The halo emission is similar to the spectrum taken on the north edge of the PN.

### NGC 7662

NGC 7662 is a triple-shell elliptical, with a bright inner ring, a fainter outer shell, and a very faint nearly circular halo (Hyung & Aller 1997). This PN also has a large number of complex microstructures, recently examined using HST imaging by Balick et al. (1998). They suggest that there is a prolate elliptical bubble around the central star aligned perpendicular to the bright ring. The bright ring is interpreted as a torus seen at roughly 30$^\circ$ inclination.

The spectrum shown in Figure 9 was taken on the bright ring directly SE of the central star. The N end of the slit was near the central star, so some stellar continuum is seen in the spectrum, which is otherwise dominated by and lines.

### IC 351

This compact PN has a double-lobed structure with a round halo (Hua & Grundseth 1986; Aaquist & Kwok 1990; Manchado et al.1996). Feibelman, Hyung, & Aller (1996) obtained UV and visible light spectra of IC 351 that show it to be a high excitation nebula, but without the presence of the usual silicon lines, and suggest that the silicon atoms could be locked up in grains.

Our spectrum (Figure 9) taken centered on the PN suffers somewhat from an incomplete subtraction of OH airglow lines, due to the sky frames not being taken properly for the on-source images. The OH lines show up in emission mainly in the H and K-band portions of the spectrum. However, the major features of and emission lines can be seen.

### IC 418

The spectrum of this well-studied young, low-excitation PN was previously shown to be dominated by lines of and in the near-IR, with a hot dust continuum (Willner et al. 1979; Zhang & Kwok 1992; Hodapp et al. 1994). Hora et al. (1993) and Paper I showed broad- and narrow-band near-IR images of the PN, showing the elliptical, double-lobed structure in the IR.

Three positions in the nebula were observed to determine the spectral variations across the object. The positions observed were on the central star, the peak of the E lobe, and in the E halo region outside of the bright ring (Figure 10). In the central position, stellar continuum is visible, rising towards shorter wavelengths. The nebular lines are similar in the central and lobe positions. The halo emission is almost devoid of lines; there is some faint  present as well as . This is possibly reflected from the bright lobes. The main component of the halo emission is a weak continuum that rises toward longer wavelengths.

### IC 2149

This peculiar PN has a bright core and a roughly bipolar nebula extending approximately E-W, but does not show the usual 2 signature of bipolar PN. The two spectra shown in Figure 11 were taken centered on the bright central star, and on the E lobe. The core spectrum shows strong stellar continuum, with the nebular lines superimposed. The E lobe emission is primarily from lines of and , in addition to weak continuum emission which is probably reflected from the central star.

### IC 3568

This round PN consists of spherical shells, an inner bright one and a outer halo. Balick et al. (1987) showed that the structure was consistent with simple hydrodynamic models of PN that are shaped by interior stellar winds. The spectrum taken on the N edge of the PN is shown in Figure 12. The predominant features are and emission lines, and low level continuum emission which is probably reflected from the central star.

### IC 4593

Bohigas & Olguin (1996) obtained spectroscopy and imaging of this PN which has two inner shells surrounded by an outer highly excited halo. IC 4593 is unusual in that the condensations outside of the inner region are located asymmetrically in the SW region.

Two spectra were taken on this PN, one positioned on the central star, and the other at a position 3 E (Figure 12). The core shows bright stellar continuum, with nebular lines superimposed, and  absorption. The E spectrum has the typical and emission lines. The sky subtraction was not of high quality for this spectrum, which resulted in OH airglow lines showing up in emission in the H and K spectral regions.

### J 320

Images of J 320 (Balick 1987) show it to have a central region that is elongated roughly E-W, but the low level flux has a N-S elongation indicating a shell or streamer extending in this direction. Three spectra for J 320 are shown in Figure 13, centered on the core (C), offset 1 N, and offset 15 N. The core has a stronger contribution from stellar continuum, but otherwise the spectra are similar. We therefore detect no spectral differences between the N extension and the nebula near the core.

### M 4–18

This young, low-excitation PN was recently imaged with HST by Dayal et al. (1997) and shows a toroidal shell surrounding the central star. There is strong mid-IR emission from warm ($\sim$ 200 K) dust that has a similar morphology but the position angle of the dust emission maxima is orthogonal to those shown in H$\alpha$ emission. The spectrum of the core of this PN shown in Figure 14 has strong stellar continuum, as well as and line emission. The slit was positioned N-S across the compact ring of the PN, so the ionized regions of the nebula are included in this spectrum.

 - line and 2 emission
----------------------

The emission lines present in the spectra of this PNe group contain those mentioned in the previous section plus lines of molecular hydrogen. The 2 lines are strongest in the K band, although in some objects there are lines visible in the H and J bands as well. In the objects where more than one slit position was measured, there is often a large change in the relative line strength of versus 2 emission, indicating that the emission is being produced in different regions of the nebula. In general, the 2 emission is more likely to be in the outer regions of the PNe, whereas the emission lines more closely trace the ionized regions and has similar morphology to the visible appearance. The PNe in this group all have some bipolar symmetry in their shape, most being of the “butterfly” morphology characterized by narrow equatorial regions and large bipolar lobes (e.g., M 2–9, Hb 12). However, some are classified as elliptical based on the shape of the brightest components. Tables 5 and 6 list the line identifications and extracted fluxes for the PNe in this section.

### Molecular hydrogen excitation in PNe

A near-infrared 2 emission line spectrum can occur through slow electric quadrupole vibration-rotation transitions in the ground electronic state. Because the allowed transitions are such that $T_{\rm
ex} \gtrsim 1000$ K is required to produce a detectable near-IR 2 spectrum, special excitation conditions must exist when the near-IR spectrum is present. In Paper I we discussed mechanisms of 2excitation in PNe; see also Kastner et al. (1996). If detected in sufficient number, the observed 2 line ratios are an excellent diagnostic for determining the relative importance of shocks and UV photons in a photodissociation region (PDR) for the excitation of the 2emission. Even if the excitation mechanism cannot be determined, the presence of 2 emission is important to understanding the conditions in PNe and how they evolve through wind interactions and photodissociation.

An analysis of near-IR 2 emission can determine an ortho-to-para (O/P) ratio, the rotational excitation temperature $T_{ex}(J)$, and the vibrational excitation temperature $T_{ex}(v)$ of the molecules. If the rotational and vibrational excitation temperatures differ, then UV excitation is indicated. This is most readily determined by comparing the column densities in the upper state vibration-rotation levels with the upper state energy (in temperature units; see Hora & Latter 1994, 1996 for a full discussion; see also Black & van Dishoeck 1987; Sternberg & Dalgarno 1989). When many 2 lines are detected, especially those from highly excited levels that fall in the J band, a much stronger case can be made for the importance of UV excitation than does the traditional $v = 2\to 1$ S(1) to 1 line ratio (e.g. Hora & Latter 1994, 1996). The comparison of the column densities to the upper state energy levels has been done for the PNe with detected 2 emission and the results are summarized in Table 8. Only the rotational excitation temperatures are listed. For collisionally- (shock-) excited spectra, the rotational and vibrational excitation temperatures are coupled and the same. For UV excited spectra, the vibrational temperature is the result of a cascade through levels and not a thermal process. The rotational levels are easily thermalized by collisions. The observed O/P ratio is in general rather uncertain, especially if only $v = 1$ lines are detected. We did not attempt to determine the O/P ratio for objects without sufficient line detections. An observed O/P ratio lower than 3 indicates that the 2 emission is not thermally excited. A subthermal O/P ratio as determined from near-IR spectra is not caused by the UV excitation process itself, but is a function of chemistry and density in the PDR (Hora & Latter 1996; Black & van Dishoeck 1987), and might not be indicative of the “true” O/P abundance ratio of the 2 (see Sternberg & Neufeld 1999). Selected excitation diagrams for several objects that are discussed below are shown in Figure 33.

### NGC 40

The morphological classification of NGC 40 is middle elliptical (Balick 1987), which seems to contradict the previously observed strong correlation between bipolar morphology and 2 detection. However, if one examines the low-level emission in the N and S regions of this PN (see Paper I), one can see material that has broken through and expanded beyond the elliptical shell defined by the E and W bright lobes. Mellema (1995) has found the morphology consistent with models of “barrel”-shaped PNe, which have roughly cylindrical emission regions slightly bowed outwards at the equatorial plane, and less dense polar regions. Higher resolution and more sensitive optical imaging has recently been carried out by Meaburn et al. (1996) show gas escaping from the polar regions of the PN, with other filamentary structure in the outer halo.

This is the first reported detection of 2 in NGC 40. The spectrum was taken centered on the W lobe, and the 2 lines are relatively weak compared to the and lines from the ionized gas in this region. The 2 emission was not detected in the narrowband imaging surveys of Paper I or Kastner et al. (1996), so the molecular emission must be confined to a region near the bright ionized gas that dominates the spectrum. The data suggest that the 2 is shock-excited. However, insufficient line detections make this result less than firm.

### NGC 2440

NGC 2440 is a bipolar PN with complex morphological and spectral structure. In the optical, the nebula is bipolar with the major axis in roughly the E-W direction for the large outer lobes (Balick 1987; Schwartz 1992). However, there are two bright lobes near the core that are positioned along an axis roughly perpendicular to the major axis of the outer lobes. There are two fainter knots that are also along a roughly E-W axis, but not aligned with the outer lobes. There are filaments and knots throughout the lobes. Lopez et al. (1998) finds up to three outflowing bipolar structures in the lobes, and find from their kinematic study that the inner bright lobes (their lobes “A” and “B”) are the emission maxima from a radially-expanding toroid viewed nearly in the plane of the sky.

In the near-IR, the inner pairs of lobes are also prominent, but the large E-W lobes are not visible (see Paper I). Instead, there is a circular outer halo visible in 2 that is not quite centered on the inner lobe structure. Also visible are faint 2 “spikes” that extend from the center to the circular outer halo, roughly in the equatorial plane of the large optical E-W lobes (Latter & Hora 1998).

The spectra shown in Figures 15 and 16 were taken at three different positions in the PN: on the N lobe (of the innermost bright pair of lobes), on the fainter E knot, and on a clump of 2 emission located on the NE edge of the outer circular halo (see Paper I, Figure 4a; it is the clump visible at the upper left corner of the “2 sub” image). The N lobe exhibits and lines from the ionized gas in this region, but also has significant 2 emission. There is also strong \[\] emission at 1.64 and 1.257 . The E knot also displays similar , , \[\], and 2 emission, although fainter. In contrast to the inner regions, the NE clump spectrum in Figure 16 is dominated by 2 emission, with the only lines detected being  and . There is strong \[\] emission at 1.64 and 1.257  in this region as well. The excitation analysis for the three positions observed showed that they are UV-excited, except for the E knot position for which there is insufficient data. The low value of the observed O/P ratio is suggestive of the 2 emission arising from a PDR at this location as well.

Since the inner region of NGC 2440 is morphologically complex and any line of sight through the PN is likely to intersect several distinct regions, it is probably the case that the ionized and molecular zones are not mixed as the spectra might seem to indicate, but that the slit simply includes several nebular components, or is looking through a PDR and is sampling both the molecular and the recently ionized gas.

### NGC 6720

NGC 6720 (the “Ring Nebula”) is probably the best-known PN, and is the archetype for the ring or elliptical morphology that characterizes the brightest part of the nebula. The emission is not consistent with a uniform prolate shell, however, since the ratio of flux between the edge and center of the ring is higher than expected from a limb-brightened shell (Lame & Pogge 1994). Balick et al. (1992) have suggested that NGC 6720 is actually a bipolar PN viewed along the polar axis, based on narrow-band imaging and high-resolution spectroscopic observations. This view is supported by the presence of 2 in the nebula and halo, which correlates strongly with bipolar morphology. Guerrero, Manchado, & Chu (1997) draw different conclusions, however, based on their chemical abundance and kinematic study of the nebula. They argue that the Ring has a prolate ellipsoid structure, with a halo of remnant red giant wind.

Our spectra of the Ring (Figure 17) were obtained at two positions, one on the bright ring directly N of the central star, and the second position several arcseconds further north, off the bright ring but on a moderately bright (in 2) position in the halo. Both positions show bright 2emission, with the lobe position also showing contributions from emission lines of and from the ionized gas, as one would expect based on the visible wavelength and IR images showing the distribution of the line emission. The ring spectrum is strongly UV excited, indicating it is the PDR interface to the outer molecular shell.

### NGC 7026

The late elliptical PN NGC 7026 has two bright lobes on either side (E-W) of the central star, with fainter bipolar emission extending roughly N-S from the core. Cuesta, Phillips, & Mampaso (1996) obtained optical spectra and imaging of this object and found kinematically complex structure, with several separate outflows at the outer edge of an inner spherical shell, and suggested that the primary shell may be undergoing breakup in transition to a more typical bipolar outflow structure.

Two positions were sampled in NGC 7026, shown in Figure 18, centered on the E and W bright lobes near the central star. The lobe spectra are nearly identical, as one might expect from the symmetry in this PN. The 2 emission in this PN is fairly weak at these positions. This might be due to the 2 being concentrated in other regions of the PN, and not in the bright ionized lobes that were sampled by the spectra presented here. We are unable to determine the excitation mechanism.

### NGC 7027

NGC 7027 is one of the most highly studied PN at all wavelengths, particularly in the infrared because of its brightness and wealth of spectral features. Treffers et al. (1976) obtained a spectrum for $\lambda = 0.9 - 2.7$  with a beam that included the entire nebula. They identified the major near-IR spectral components, including the first detection of 2 lines in a PN, and the first detection of the unidentified line at 2.29 . Since then, several near-IR spectra have been published, including Scrimger et al. (1978), Smith, Larson, & Fink (1981), Rudy et al. (1992), and Kelly & Latter (1995).

The spectra shown in Figure 19, one taken centered on the W bright lobe, and the other at the brightest position in 2 of the NW lobe (see Paper I). Both show and 2 emission; the 2 is relatively stronger in the NW position than in the bright lobe. Narrowband imaging has shown that the and emission is primarily in the bright inner ring of the nebula, and the 2 emission is in what appears to be bipolar lobes outside of this shell (Graham et al. 1993a,b; Paper I; Latter et al. 1998). It has been argued before based on morphology that the 2 is in a PDR (Graham et al. 1993a). Our data clearly demonstrate this to be the case, with the 2 showing a strongly UV excited spectrum in a relatively high density medium (see Figure 33b).

### 30

The young PN 30 is well-studied in the infrared, and is remarkable primarily because of its large IR emission excess. It has many similarities to NGC 7027, including its IR morphology and the presence of 2 in the near-IR and unidentified IR (UIR) emission features in the mid-IR spectrum, which are usually attributed to polycyclic aromatic hydrocarbons (PAHs). Rudy et al. (1991) obtained a $\lambda = 0.46 -
1.3$  spectrum of 30; high-resolution visible and near-IR images were recently obtained by Harrington et al. (1997) and Latter et al. (1998), and ground-based near- and mid-IR images have been presented by Hora et al. (1993), Paper I, and Shupe et al. (1998).

Three positions in 30 were sampled in the spectra presented in Figure 20; the emission peak on the N lobe of the ring, the E side of the ring, and on the 2 emission region located approximately 3 E of the ring. These spectra show a steady progression of decreasing emission from the ionized gas and increasing molecular emission as one moves east. As for NGC 7027, the 2 emission in 30 is UV excited (Figure 33a) and defines the PDR (see also Shupe 1998).

### Hubble 12

Hubble 12 (Hb 12) has been notable primarily because it represents one of the clearest cases known of UV excited near-IR fluorescent 2 emission (Dinerstein et al. 1988; Ramsay et al. 1993). Our Hb 12 results from this survey and our imaging survey were presented in a previous paper (Hora & Latter 1996); the spectra are reproduced here for comparison with the rest of the survey.

Dinerstein et al. had mapped the inner structure and found it to be elliptical surrounding the central star; our deep 2 images showed the emission to be tracing the edges of a cylindrical shell around the star, with faint bipolar lobes extending N-S. We also detected \[\] line emission at 1.64  in a position along the edge of the shell. The 2 line ratios observed were in excellent agreement with predictions by theoretical 2 fluorescence calculations, and no significant differences were found between the excitation in the two positions of the nebula that were sampled (see also Luhman & Rieke 1996).

### IC 2003

IC 2003 is a round, high-excitation PN that has a ring of emission, with a bright knot on the S edge (Manchado et al. 1996; Zhang & Kwok 1998). Feibelman (1997) obtained IUE spectra of this PN that shows a wealth of nebular and stellar lines. The IR spectra presented in Figure 22 taken in the center of the PN shows that there is little continuum from the nebula; the emission is primarily from lines of in the J, H, and K bands. There is strong unidentified emission at 2.286  but none detected at 2.199 . 2 emission is tentatively detected in the K-band, in the 1, $v=3\to 2$ S(1), and $v=1\to 0$ Q(1) lines. Each of the lines are detected at roughly a 2$\sigma$ level. The line fluxes are not reliable or numerous enough to allow fitting of the line ratios.

### IRAS 21282+5050

The young, carbon-rich PN IRAS 21282+5050 has been identified as having an 07(f)-\[WC11\] nucleus (Cohen & Jones 1987) with possibly a binary at its center. Strong $^{12}$CO has been detected in a clumpy expanding shell (Likkel et al. 1988) with elongated emission N-S. Shibata et al. (1989) believe the elongated emission suggests the presence of a dust torus in the E-W direction; however, Meixner et al.(1993) was evidence for a clumpy, expanding elliptical envelope. The elongated structure is also seen in the visible (Kwok et al. 1993). Weak continuum flux at 2 and 6 cm suggests a young PN just beginning to be ionized (Likkel et al. 1994; Meixner et al. 1993). Kwok et al. (1993) believe there has been a recent sharp drop in luminosity based on the measured CO/FIR ratio. Weak HCO$^+$ and $^{13}$CO are present (Likkel et al. 1988).

Two positions were sampled on IRAS 21282+5050, centered on the bright core, and offset approximately 3 N and 3 W. The spectrum of the offset position is shown in Figure 22. The core is dominated by continuum emission from the central star. Also present are both emission lines from the ionized gas, and 2 features in the K-band. The nebula is compact, about 4 in diameter at K (Paper I). The slit therefore samples a slice through the entire nebula, and as a result this spectrum does not necessarily imply that the molecular and ionized gas is mixed. The Lobe spectrum shows primarily lines of 2 (with the OH night sky lines showing up in absorption because of imperfect sky subtraction in this spectrum). The lack of emission lines due to and in the Lobe spectrum indicates that the 2 emission is predominantly in the outer regions of the PN. The data are suggestive of shock excitation, but this should be considered tentative.

### M 1–16

M 1–16 is a PN with a near-IR bright central region and bipolar lobes with fast winds extending at least 35 from the core (Schwartz et al.1992; Aspin et al. 1993; Sahai et al. 1994). Several spectra were obtained in this PN scanning across the central region; the two positions shown in Figure 23 are on the core position and 1 S of the core. Both positions show 2 emission; the S position is slightly brighter in both 2 and the ionized nebular lines. Our data reveal that the 2 is UV excited in both regions observed. This had be suggested earlier by Aspin (1993). The core shows a slight rise towards long wavelengths indicating emission from warm dust continuum.

### M 1–92

M 1–92 (“Minkowski’s Footprint”) is a bipolar proto-planetary nebula similar in near-IR appearance to AFGL 618, and has evidence of highly collimated outflows along the bipolar axis (Paper I; Trammell & Goodrich 1996 and references therein). Two positions were sampled in M 1–92, one in the core and one on the NW bipolar lobe. There are problems with the sky background subtraction in both spectra, which are most prominent in the $\lambda = 1.9 - 2.1$  region of the spectrum, but also contribute to a lower signal to noise ratio (S/N) over the whole dataset. Nevertheless, the primary characteristics are apparent. The core region is dominated by strong warm dust continuum emission. There is also weak  and  emission, but the other most other and lines are too weak to be detected. The lobe position shows weak 2 emission. The emission appears to be shock-excited, but the low excitation suggested by our data is suggestive of UV excitation. Data of higher S/N are required to discern the dominant excitation mechanism. There might also be 2 emission near the core that is being masked by the strong continuum emission. In both positions, there also seems to be CO bandhead emission at $\lambda = 2.3 - 2.5$ although the S/N is not high in these regions.

### M 2–9

M 2–9 (the “Butterfly”) is a highly symmetric bipolar nebula, with lobes extending from opposite sides of a bright central core, nearly in the plane of the sky. Bright knots of emission are visible in the lobes at the N and S ends. Our results for M 2–9 from this survey were previously presented in Hora & Latter (1994), and some of the spectra are reproduced in Figures 25 and 26 for comparison. High-resolution imaging in several near-IR lines indicated that the lobes had a double-shell structure, with the inner shell dominated by and line emission from ionized gas and continuum emission scattered from the central source, and the outer shell (the “Lobe O” spectrum in Figure 25) of the lobes showing strong 2 emission which exhibit a spectrum consistent with UV excitation in a PDR. The core region shows a strong dust continuum component, as well as emission lines of , , , \[\] and . The N knot has strong \[\] emission, with relatively weaker , , and 2 emission.

### Vy 2–2

Vy 2–2 is a compact PN, so very little is known about its morphology. The spectrum obtained in this survey was taken centered on the bright core and the slit sampled most or all of the emission from this object. The spectrum contains stellar continuum, lines of and emission from the nebula, and weak 2 emission. This detection confirms the indication of 2 emission as reported by Dinerstein et al. (1986). The spectrum shown in Figure 26 is similar to others in this category, such as 30 and NGC 2440, where several nebular components are superimposed because of the position and size of the slit. As for those objects, the 2 spectrum in Vy 2–2 is also UV excited.

2 dominated
-----------

The PNe in this group have spectra that primarily contain emission lines of 2. These objects all have bipolar morphology, and most are young or proto-PNe (PPNe). The PPNe also have warm continuum dust emission or stellar continuum that is strongest in the core. Table 7 lists the line identifications and fluxes for the PNe in this section.

### NGC 2346

NGC 2346 is a PN with faint bipolar lobes seen clearly in 2 emission (Paper I). The brightest part of the nebula is in the “equatorial” region near the central star where the bipolar lobes meet. Walsh, Meaburn, & Whitehead (1991) performed deep imaging and spectroscopy that showed the full extent of the lobes, and they model the PN as two ellipsoidal shells that are joined near the central star. The distribution of the 2 emission is similar to the optical (Zuckerman & Gatley 1988; Kastner et al. 1994; Paper I).

The near-IR spectrum of NGC 2346 in Figure 27 is dominated by UV-excited 2 emission, as shown in Figure 33d. The spectrum was obtained with the slit positioned on the bright condensation to the W of the central star. There is also weak  and  emission seen, which is possibly reflected from near the central star.

### J 900

The PN J 900 is a bipolar nebula with an unusual “jet”-like structure and an outer shell structure that is seen primarily in 2 emission (Shupe et al. 1995; Paper I). The spectrum of J 900 shown in Figure 27 was obtained at a position N of the brighter lobe just NW of the central star, centered on the “jet” of emission. Problems with sky-subtraction caused the J and H-band portions of the spectrum continuum to be slightly negative. There is no detected continuum in any part of the spectrum. The 2 spectrum is shock-excited in a moderate velocity wind (Figure 33e).

### AFGL 618

AFGL 618 is a carbon-rich, bipolar reflection nebula with a relatively hot central star ($\approx 30,000$ K), similar spectra in the two lobes, and the eastern lobe is significantly brighter than the other. In this as in other ways, the object bears a great resemblance to AFGL 2688 (see below), despite the fact that their central star temperatures differ by about a factor of 5. The visible spectrum shows numerous emission lines characteristic of ionized gas (Westbrook et al. 1975; Schmidt & Cohen 1981) which are scattered by dust into the line of sight, with a small region surrounding the central object (Carsenty & Solf 1982; Kelly, Latter, & Rieke 1992). The near-IR spectrum of AFGL 618 is also dominated by rotation-vibration lines of 2 (Thronson 1981, 1983; Latter 1992; Paper I).

AFGL 618 exhibits a rich spectrum of molecular line emission (Lo and Bechis 1976; Knapp et al. 1982; Cernicharo et al. 1989; Kahane et al.1992; Martin-Pintado & Bachiller 1992; Bachiller et al. 1997; Young 1997). The lines detected include $^{12}$CO, $^{13}$CO, C$^{17}$O, C$^{18}$O, CS, NH$_3$, HCN, HCO$^+$, CN, and .

Two of the positions sampled are presented here in Figure 28 – the core spectrum and one taken 24 E of the core. Both spectra show strong 2 emission, along with \[\] and weak  and . In addition, the core has a warm dust continuum that is apparent throughout the spectrum, and clear CO bandhead features in the 2.3 – 2.4 region. The CO features are also present but at lower levels in the 24 E spectrum position. Our analysis of the 2 spectrum confirms the earlier results by Latter (1992) – the spectrum is dominated by a shock-heated component, but a UV excited component is clearly present as well (Figure 33c).

### AFGL 2688

AFGL 2688 (the “Egg Nebula”) is a bipolar reflection nebula (Ney et al.1975) at visible and near-infrared wavelengths. It has a central star that exhibits the spectrum of a carbon-rich supergiant (Crampton 1975; Lo & Bechis 1976). Similar in visible appearance to AFGL 915, each lobe shows two “jets” or “horns” extending away from the central region (Crampton et al. 1975; Latter 1993; Sahai 1998a). The lobes have identical spectra at visible wavelengths, but their brightness differs significantly (Cohen & Kuhi 1977). The near-IR spectrum is dominated by 2 rotation-vibration lines (Thronson 1982; Beckwith 1984; Latter et al. 1993). A central source is seen in the mid-IR and longer wavelengths, with fainter extended emission along the axis of the nebula (Hora et al. 1996). There is an enigmatic equatorial region seen in 2emission and might be traced by other molecular species, such as HCN (Latter 1993; Bieging & Ngyuen-Quang-Rieu 1996; Sahai 1998b).

Similar to AFGL 618, this object also has a rich molecular content. SiC$_2$ is seen in absorption (Cohen & Kuhi 1977); this feature is usually found in stars of the highest carbon abundance. Strong absorption features of C$_3$ and emission in C$_2$ (Crampton et al. 1975) are present, while C$_2$ is also seen in absorption in reflected light from the lobes (Bakker et al. 1997). The CO $J = 1 \to 0$ line shows three distinct velocity structures (Kawabe et al. 1987; Young et al. 1992).

Our results for this object from this survey were previously presented in Hora & Latter (1994, 1995) and our narrowband imaging in Latter et al.(1993). The spectra are reproduced here for comparison with the rest of the survey. Spectra were obtained at several positions in the nebula, including positions along the N lobe, and in the equatorial region (see Hora & Latter 1994 for details). The emission is segregated; the core is dominated by continuum emission, there are emission lines of C$_2$ and CN further from the core along the lobes, and the 2 emission is confined to the ends of the lobes and in the equatorial region in what appears to be a ring or toroidal structure (Latter 1993; Sahai 1998b). Our analysis of the 2 line ratios showed that the emission is collisionally excited in shocks, with no discernible difference between the emission in the lobes and the equatorial region.

Continuum - dominated
---------------------

These are young PNe or PPNe that have strong warm dust continuum and little line emission. The strongest component is in general the core, with most of the emission from an unresolved point source. In some of the nebulae, emission structure extends a few arcseconds from the core region. Also, in objects such as AFGL 915, they are associated with larger optical nebulae that extend arcminutes from the core. In this survey, only the regions near the core were sampled.

### AFGL 915

AFGL 915 (the “Red Rectangle”) is a carbon-rich biconical reflection nebula with a metal-depleted spectroscopic binary at its center (Cohen et al. 1975). The nebula appears axially symmetric and shows spikes running tangent to the edge of the bicone. Surrounding the post-AGB star at its center is a circumbinary disk viewed edge-on (Jura, Balm, & Kahane 1995) which could be oxygen-rich (Waters et al. 1998).

C$_2$ and CN are not detected near the binary, though C$_2$ is present in emission in the reflection lobes. CH$^+$ (0,0) and (1,0) are detected in emission (Bakker et al. 1997; Balm & Jura 1992). CO is underabundant, with relatively weak emission and broad wings detected (Dayal & Bieging 1996; Greaves & Holland 1997; Loup et al. 1993; Bujarrabal et al.1992). Glinski et al. (1997) found CO and in the UV in both absorption and emission. They expect strong CO overtone emission in the IR based on their observations of hot CO emission and absorption in the UV. The object shows ERE (extended red emission) from about $\lambda =
5400$ to 7200 Å and a set of emission bands around 5800 Å (Schmidt, Cohen, & Margon 1980) whose carriers might be the same material as the carriers of the DIBs (diffuse interstellar bands). This object also shows strong emission in the PAH bands at 3.3, 7.7, and 11.2  (Cohen et al. 1975), which are located predominantly in the lobes and spikes of emission (Bregman et al. 1993; Hora et al. 1996).

Spectra taken at two different positions are shown, one centered on the core, and the other at 4 S of the core. Both show strong warm dust continuum, and the core also has strong CO bandhead emission features in the $\lambda = 2.3 - 2.4$  range.

### 10420

10420 is a highly evolved, OH/IR star that is thought to be in a post-red supergiant phase (Jones et al. 1993). The central star seems to have changed spectral type, transitioning recently to an early A type (Oudmaijer 1998). Oudmaijer et al. (1996) detected several of the hydrogen lines in absorption and emission in the near-IR, and Oudmaijer (1998) presented a high-resolution 0.38 – 1  spectrum showing a large number of emission and absorption lines. Recent HST imaging by Humphreys et al. (1997) shows that the circumstellar environment around this star is extremely complex, with spherical outer shells that extend to a diameter of 6 arcseconds, and several inner condensations. In the near- and mid-infrared, bipolar lobes are visible that extend $\sim 2$ arcseconds from the core.

The spectrum of 10420 shown in Figure 31 was taken centered on the object. The slit length includes the inner few arcseconds of the object, although it is dominated by the bright core. The observed spectrum shows a bright and relatively featureless continuum. Some lines, e.g., , are seen in absorption.

### M 2–56

The PPN M 2–56 is a bipolar nebula with a bright central core. It is similar in morphology to AFGL 618, although it seems to be at an earlier evolutionary stage since it does not appear to have an region (Trammell, Dinerstein, & Goodrich 1993; Goodrich 1991). The spectrum of M 2–56 shown in Figure 31 was taken centered on the core of this PPN. The dominant feature is a hot dust continuum that is most prominent in the K band region of the spectrum. There are some residual features in the spectrum from imperfect sky subtraction, mostly in the K band.

Discussion and Summary
======================

Spectral Categories
-------------------

The PNe spectra presented in this paper were grouped according to spectral characteristics as described above. The groups are an efficient way to present the data, but also can be seen to correlate strongly with other characteristics of the PN.

### Morphology

The group of - line dominated PNe is composed of primarily elliptical or round PNe, along with the peculiar or irregular nebulae of the sample. In general these PNe are well-known from optical studies, identified either by their morphology or their optical spectra. Many of the PNe in this group of the sample, however, do have IR “excess” continuum emission from warm dust, which in some cases prompted their inclusion in this sample.

The spectral groups with molecular and/or dust continuum emission are primarily bipolar. This classification includes objects such as NGC 6720 which have a ring morphology but are thought to be bipolar viewed pole-on; Hb 12 which is brightest in 2 in the equatorial region and along the outer edges of the lobes; M 2–9 which is brightest in 2 at the edges of the lobes with no equatorial emission other than at the core; and AFGL 2688 which is brightest along the axis of the bipolar lobes, with 2emission in the equatorial plane. Clearly this is a heterogeneous group with a wide range of emission and morphological differences that imply a range of evolutionary tracks and states.

### The Carbon-to-Oxygen Ratio

Carbon stars, although a small fraction of all AGB stars, return about half of the total mass injected into the ISM by all AGB stars, since they have on average much higher mass loss rates ($>10^{-4}\ {\rm
M}_{\sun}\ {\rm yr}^{-1}$) than do O-rich objects. The carbon-to-oxygen (C/O) abundance ratio in PNe has previously been shown to correlate with morphology (Zuckerman & Aller 1986), with bipolar PNe tending to be carbon-rich. It is therefore expected that the C/O ratio also correlates with the spectral classifications presented here. This is in general the case, with the - line dominated PNe having C/O ratios less than or about 1, whereas the remaining categories which are dominated by the bipolar PNe have C/O ratios $>$ 1, as reported by Zuckerman & Aller (1986) and Rola & Stasińska (1994). Rola & Stasińska discuss problems with previous determinations of the C/O ratio, and use different criteria that result in a slightly lower percentage of carbon-rich PNe (35%) than others. Their ratios are used in the discussion below.

The morphology of PN also has been shown to depend on the progenitor mass (see Corradi & Schwarz 1995), with the bipolar PN being more massive than other morphological types. This relationship, along with the link between carbon abundance and morphology, suggests that carbon stars are the progenitors of bipolar PN and those with a large amount of molecular material. The mechanisms that cause massive carbon-rich stars to preferentially form bipolar PN are still not understood.

There are some exceptions to the correlation of morphological type to C/O ratio; in particular, NGC 6543 has a much higher value (9.55) than the others in the class. In the other extreme, NGC 2346 stands out as having a low C/O ratio (0.35) compared to other bipolar PNe in the 2 - dominated group. This object is a much more evolved object than the others in its group (e.g., AFGL 2688), and exhibits weak  emission, showing that the ionized gas is present although weak relative to the molecular emission in the nebula.

Spectral Sampling of Morphological Features
-------------------------------------------

This survey has differed from many previous investigations in that a short, narrow slit was used to obtain the data, rather than a large beam that could include most or all of the nebula. Because of this, one cannot easily use the spectra presented here to model the PNe in a global sense, if that requires a measurement of the total flux from the object. Also, if a complete census of emission lines were required, some might be missed if there were variations of emission characteristics across the nebula and certain regions were not sampled.

The spatial selectivity that prevents viewing the entire PN at once, however, has proven to be an advantage when trying to examine various aspects of the PN, including variations across the nebula, as a function of distance from the central star, or in examining certain morphological features. For example, in M 2–9 and NGC 2440, the emission of the lobe walls and emission knots were separately sampled, which showed the large spectral differences in these regions. This information is important for modeling the structure and formation of the PN. Another reason why the small aperture is useful is that if the emission from a group of lines such as 2 is to be modeled, it is important to compare the emission from a clump of material where the conditions do not vary greatly over its size. For example, the emission from 2 present very close to the central star in a strong UV field could be quite different from 2emission from the outer parts of the halo. Also, the small slit has aided in detecting weak 2 emission from several PNe such as NGC 40, where detection would have been difficult if the central star and the rest of the nebula could not be excluded from the measurement.

Summary of Molecular Hydrogen Emission in PNe
---------------------------------------------

A long-standing problem in the interpretation of 2 emission from interstellar and circumstellar environments is understanding the excitation mechanism. Three fundamental mechanisms are possible. One is excitation of a near-infrared fluorescence spectrum resulting from a rotational-vibrational cascade in the ground electronic state following electronic excitation by the absorption a UV photon in the Lyman and Werner bands (Black & van Dishoeck 1987). A second excitation mechanism is collisional excitation in a warm gas ($T_{\rm K} \gtrsim 1600$ K). While UV excitation in a low density gas produces an easily identifiable spectrum, the level populations can be driven to produce thermal line ratios when the UV flux is large and densities begin to exceed $10^4$ 3 (Sternberg & Dalgarno 1989). Detailed spectral and morphological analysis are often required to determine an origin of the near-IR spectrum. A third excitation mechanism is formation of 2 on the surfaces of dust grains and in the gas phase. While potentially important in isolated regions of certain objects, we do not consider this to be generally important in PNe and PPNe relative to the other two processes. This is because molecular formation in PNe is relatively slow compared to dissociation rates.

In PNe and PPNe, the situation can be complicated by both dominant excitation mechanisms being present simultaneously, and in different forms. Several ways of exciting near-IR 2 emission have been identified as possible: direct thermal excitation in warm gas created behind moderate velocity shocks, direct excitation by UV photons from the hot central star, somewhat indirectly by collisional excitation in warm gas created by rapid grain streaming (e.g. Jura & Kroto 1990), and excitation through absorption of photons (by an accidental resonances with the B$^1\Sigma_u^+ - {\rm X}^1\Sigma_g^+$ $v = 1 - 2$ P(5) and R(6) transitions of 2) which can be generated in a nearby strong shock (Black & van Dishoeck 1987).

The first two mechanisms have been identified in several PNe, such as thermal excitation in AFGL 2688 (Hora & Latter 1994; Sahai 1998a), pure UV excitation in a low density gas around Hb 12 (Dinerstein 1988; Hora & Latter 1996; Luhman & Rieke 1996), and UV excitation in a high density gas in M 2–9 (Hora & Latter 1994) and NGC 7027 (Graham 1993b; this paper). A combined spectrum was found from a detailed analysis of AFGL 618 (Latter 1992; this paper). While the form of the excitation might be apparent for these and other objects, it is not always evident what is the source of the warm gas or UV photons. Winds are present in AFGL 2688 which could directly heat the gas through shocks, but considerable grain streaming is likely taking place as well (see Jura & Kroto 1990).

Very fast winds and dissociating shocks are present in AFGL 618, M 2–9 (Kelly, Latter, & Hora 1998), and M 1–16 (Sahai 1994; Schwarz 1992), and all show clear evidence of UV excitation. In addition, the photon path to the 2 emitting regions is not in a direct line-of-sight to the central star, which for photons coming from the central star suggests scattering in what is a fairly low density medium. Alternatively, we are seeing in each of these objects excitation of 2 at the bipolar lobe walls by UV photons generated within strong shocks produced by the fast winds. This hypothesis was explored through detailed modeling by Latter (1992) of AFGL 618, but the high relative intensity of the thermally excited emission and poor spatial resolution limited this analysis. The presence of very fast winds in the lobes of each of these objects, and the presence of UV excited 2 emission at the lobe walls strongly suggests that indirect excitation of the 2 is occurring by interactions with photons generated by wind-produced shocks. Detailed modeling of sensitive, high spatial resolution spectra is required. It is also evident, in general, that without detailed spectra, 2 is a rather poor diagnostic of overall conditions in PNe and PPNe.

If we conclude that all of the ways to excite 2 in PNe and PPNe listed above are present and important, what does this imply for our understanding of these objects and the utility of 2 as a diagnostic? It is now well understood that the presence of molecular emission from PNe and PPNe is tied to the morphology of the objects such that if molecular emission is present, the object has a bipolar morphology (Zuckerman & Gatley 1988; Latter 1996; Kastner 1996, and references therein). We have argued that 2 emission is excited in multiple ways in PNe and PPNe. While special conditions are required for 2 emission to be seen in near-IR spectra, the conditions that drive the excitation are common in all PNe and PPNe and are not clearly dependent on morphological type. A conclusion that can be drawn from this argument alone is that molecular material is present in nebulae with a bipolar morphology and a significant amount of molecular material is $not$ present in other morphological types. Therefore, objects that have a bipolar morphology must have a dense, high mass envelope in which the molecular material can be shielded and survive dissociation for relatively long times – suggesting a high mass loss rate and a high mass progenitor star. A correlation between bipolar morphology and high mass progenitor stars has been found by others (Corradi & Schwarz 1995). It is apparent that the presence of 2 emission in a PN is not tied to directly to the morphology, but that the bipolar morphology is intimately related to the density and mass of the circumstellar envelope, and therefore the mass of the progenitor star. Why high mass, high mass loss rate asymptotic giant branch stars shed material in an axisymmetric, not spherical, way remains a mystery.

We thank Xander Tielens and David Hollenbach for useful discussions and encouragement. We acknowledge support from NASA grant 399-20-61 from the Long Term Space Astrophysics Program. WBL was supported during part of this study by a National Research Council Research Associateship.

Aaquist, O. B., & Kwok, S. 1990, , 84, 229 Acker, A., Marcout, J., Ochsenbein, F., Scholn, C., Stenholm, B., & Tylenda, R. 1992, The Strasbourg - ESO Catalogue of Galactic Planetary Nebulae (München: ESO) Aspin, C. et al. 1993, , 278, 255 Bachiller, R., Fuente, A., Bujarrabal, V., Colomer, F., Loup, C., Omont, A., & de Jong, T. 1997, , 319, 235 Bakker, E. J., van Dishoeck, E. F., Waters, L. B. F. M., & Shoenmaker, T. 1997, , 323, 469 Balick, B. 1987, AJ, 94, 671 Balick, B., Bignell, C. R., Hjellming, R. M., & Owen, R. 1987, , 94, 948 Balick, B., Gonzalez, G., Frank, A., & Jacoby, G. 1992, , 392, 582 Balick, B., Alexander, J., Hajian, A. R., Terzian, Y., Perinotto, M., & Patriarchi, P. 1998, , 116, 360 Barker, T. 1991, , 371, 217 Beckwith, S., Beck, S. C., & Gatley, I. 1984, , 280, 648 Bieging, J. H. & Ngyuen-Quang-Rieu 1996, , 112, 706 Black, J. H., & van Dishoeck, E. F. 1987, , 322, 412 Bohigas, J., & Olguin, L. 1996, RMxAA, 32, 47 Bowers, C. W., Blair, W. P., Long, K. S., & Davidsen, A. F. 1995, , 444, 748 Bregman, J. D., Rank, D., Temi, P., Hudgins, D., & Kay, L. 1993, , 411, 794 Bujarrabal, V., Alcolea, J., & Planesas, P. 1992, , 257, 701 Carsenty, U., & Solf, J. 1982, , 106, 307 Cernicharo, J., Guelin, M., Martin-Pintado, J., Denalver, J.,& Maversberger, R. 1989, , 222, L1 Cohen, M., & Barlow, M. J. 1974, , 193, 401 Cohen, M. & Jones, B. F. 1987, , 321, L151 Cohen, M. & Kuhi, L. V. 1977, , 213, 79 Cohen, M., et al. 1975, , 196, 179 Corradi, R. L. M., & Schwarz, H. E. 1995, , 293, 871 Crampton, D., Cowley, A.P., & Humphreys, R. M. 1975, , 198, L135. Cuesta, L., Phillips, J. P., & Mampaso, A. 1996, , 313, 243 Dayal, A. & Bieging, J. H. 1996, , 472, 703 Dayal, A., Sahai, R., Trauger, J., Hora, J. L., Fazio, G. G., Hoffmann, W. F., Bieging, J. H., Deutsch, L. K., & Latter, W. B. 1997, , 191, 1509 Dinerstein, H. L., Carr, J. S., Harvey, P. M., & Lester, D. F. 1986, in Summer School on Interstellar Processes (Abstract book), p. 43 Dinerstein, H. L., Lester, D. F., Carr, J. S., & Harvey, P. M. 1988, , 327, L27 Dinerstein, H. L., & Crawford, J. 1998, in Proc. IAU Symp. 180, [*Planetary Nebulae*]{}, eds. H. J. Habing & H. J. G. L. M. Lamers (Dordrecht: Kluwer), 221 Feibelman, W. A., Hyung, S., & Aller, L. H. 1996, , 278, 625 Feibelman, W. A. 1997, , 109, 481 Garcia-Lario, P., Manchado, A., Suso, S. R., Pottasch, S. R., & Olling, R. 1990, , 82, 497 Geballe, T. R., Burton, M. G., & Isaacman, I. 1991, , 253, 75 Gillett, F. C., Merrill, K. M., & Stein, W. A. 1972, , 172, 367 Glinski, R. J., Lauroesch, J. T., Reese, M. D., & Sitko, M. L. 1997, , 490, 826 Goodrich, R. W. 1991, , 376, 654 Graham, J. R., Serabyn, E., Herbst, T. M., Matthews, K., Neugebauer, G., Soifer, B. T., Wilson, T. D., & Beckwith, S. 1993a, , 105, 250 Graham, J. R., Herbst, T. M., Matthews, K., Neugebauer, G., Soifer, T., Serabyn, E., & Beckwith, S. 1993b, , 408, L105 Greaves, J. S. & Holland, W. S. 1997, , 327, 342 Guerrero, M. A., Manchado, A., & Chu, Y.-H. 1997, , 487, 328 Harrington, J. P., & Borkowski, K. J. 1994, , 26, 1469 Harrington, J. P., Lame, N. J., White, S. M., & Borkowski, K. J. 1997, , 113, 2147 Hodapp, K.-W., Hora, J. L., Irwin, E., & Young, T. 1994, , 106, 87 Hora, J. L., Deutsch, L. K., Hoffmann, W. F., Fazio, G. G., & Shivanandan, K. 1993, , 413, 304 Hora, J. L., & Latter, W. B. 1994, , 437, 281 Hora, J. L., & Latter, W. B. 1995, , 449, 397 Hora, J. L., & Latter, W. B. 1996, , 461, 288 Hora, J. L., Deutsch, L. K., Hoffmann, W. F., & Fazio, G. G. 1996, , 112, 2064 Hrivnak, Kwok, S., & Geballe, T. R. 1994, , 420, 783 Hua, C. T., & Grundseth, B. 1986, , 92, 853 Humphreys, R. M., et al. 1997, , 114, 2778 Hyung, S., Aller, L. H., & Feibelman, W. A. 1994, , 269, 975 Hyung, S. & Aller, L. H. 1997, , 491, 242 Jones, T. J., et al. 1993, , 411, 323 Jura, M., & Kroto, H. 1991, , 351, 222 Jura, M., Balm, S. P., & Kahane, C. 1995, , 453, 721 Kahane, C., Cernicharo, J., Gomez-Gonzalez, J., & Guelin, M. 1992, , 256, 235 Kastner, J. H., Weintraub, D. A., Gatley, I., Merrill, K. M., & Probst, R. 1996, , 462, 777 Kawabe, R., Ishiguro, M., Kasuga, T., Morita, K. I., Ukita, N., Kobayashi, H., Okumura, S., Fomalont, E., & Kaifu, N. 1987, , 314, 322 Kelly, D. M., & Latter, W. B., & Rieke, G. H. 1992, , 395, 174 Kelly, D. M., & Latter, W. B. 1995, , 109, 1320 Kelly, D. M., Latter, W. B., & Hora 1998, in preparation Knapp, G. R., Phillips, T. G., Leighton, R. B., Lo, K. Y., Wannier, P. G., & Wooten, H. A. 1982, , 252, 616 Kwok, S., Hrivnak, B. J., & Langill, P. P. 1993, , 408, 586 Lame, N. J., & Pogge, R. W. 1994, , 108, 1860 Latter, W. B., Hora, J. L., Kelly, D. M., Deutsch, L. K., & Maloney, P. R. 1993, , 106, 260 Latter, W.B., Kelly, D. M., Hora, J. L., & Deutsch, L. K. 1995, , 100, 159 (Paper I) Latter, W. B., Maloney, P. R., Kelly, D. M., Black, J. H., Rieke, G. H., & Rieke, M. J. 1992, , 389, 347 Latter, W. B., Hora, J. L., Kelly, D. M., Dayal, A., Bieging, J. H., Tielens, A. G. G. M., & Trammell, S., 1998, , 191, 1510 Latter, W. B., & Hora, J. L 1997, in IAU Symp. 180, [*Planetary Nebulae*]{}, eds. H. J. Habing & H. J. G. L. M. Lamers (Dordrecht: Kluwer), 254 Likkel, L., Forveille, T., Omont, A., & Morris, M. 1988, , 198, L1 Likkel, L., Morris, M., Kastner, J. H., & Forveille, T. 1994, , 282, 190. Lo, K. Y. & Bechis, K. P. 1976, , 205, L21 Lopez, J. A., Meaburn, J., Bryce, M., & Holloway, A. J. 1998, , 493, 803 Loup, C., Forveille, T., Omont, A., & Paul, J. F. 1993, , 99, 291. Luhman, K., & Rieke, G. H. 1996, , 461, 298 Luhman, M. L., Luhman, K. L., Benedict, T., Jaffe, D. T., & Fischer, J. 1997, , 480, L133 Manchado, A., Guerrero, M. A., Stanghellini, L., & Serra- Ricart, M. 1996, The IAC Morphological Catalog of Northern Galactic Planetary Nebulae (Tenerife: Instituto de Astrofisica de Canarias). Martin-Pintado, J. & Bachiller, R. 1992, , 391, L93 Meaburn, J., Lopez, J. A., Bryce, M., & Mellema, G. 1996, , 307, 579 Meixner, M., Skinner, C. J., Temi, P., Rank, D., Bregman, J., Ball, J. R., Keto, E., Arens, J. F., & Jernigan, J. G. 1993, , 411, 266 Mellema, G. 1995, , 277, 173 Ney, E. P., Merrill, K. M., Becklin, E. E., Neugebauer, G., & Wynn-Williams, C. G. 1975, , 198, L129 O’Dell, C. R., Weiner, L. D., & Chu, Y.-H. 1990, , 362, 226 Oudmaijer, R. D., Groenewegen, M., Matthews, N., Blommaert, J., & Sahu, K. 1996, , 280, 1062 Oudmaijer, R. D. 1998, , 129, 541 Persi, P., Preite Martinez, A., Ferrari-Toniolo, M., & Spinoglio, L. 1987, in Planetary and proto-planetary nebulae: From IRAS to ISO (Dordrecht, D. Reidel), 221 Phillips, J. P., & Cuesta, L. 1996, , 111, 1227 Ramsay, S. K., Chrysostomou, A., Geballe, T. R., Brand, P. W. J. L., & Mountain, M. 1993, , 263, 695 Rao, K. N., Humphreys, C. J., & Rank, D. H. 1966, Vacuum Wavelengths in the Infrared (New York: Academic Press, Inc.) Reay, N. K., Atherton, P. D., & Taylor, K. 1983, , 203, 1087 Rieke, G. H., & Lebofsky, M. J. 1985, , 288, 618 Rola, C., & Stasińska, G. 1994, , 282, 199 Rudy, R. J., Rossano, G. S., Erwin, P. & Puetter, R. C. 1991, , 368, 468 Rudy, R. J., Erwin, P., Rossano, G. S., & Puetter, R. C. 1992, , 384, 536 Sahai, R., Wootten, A., Schwarz, H. E., & Wild, W. 1994, , 428, 237 Sahai, R., 1998a, , 493, 301 Sahai, R., 1998b, , 492, 163 Schmidt, G. D., Cohen, M., & Margon, B. 1980, , 239, L133 Schmidt, G. D., & Cohen, M. 1981, , 246, 444 Schwarz, H. E., Corradi, R. L. M., & Melnick, J. 1992, , 96, 23 Scrimger, M. J., Lowe, R. P., Moorhead, J. M., & Wehlau, W. H. 1978, , 90, 257 Shibata, K. M., Tamura, S., Deguchi, S., Hirano, N., Kameya, O., & Kasuga, T. 1989, , 345, L55 Shupe, D. L., Armus, L., Matthews, K., & Soifer, B. T. 1995, , 109, 1173 Shupe, D. L., Larkin, J. E., Knop, R. A., Armus, L., Matthews, K, & Soifer, B. T. 1998, , 498, 267 Smith, H. A., Larson, H. P., & Fink, U. 1981, , 244, 835 Sternberg, A., & Dalgarno, A. 1989, , 338, 197 Sternberg, A., & Neufeld, D. A. 1999, , in press Thronson, H. A. 1981, , 248, 984 Thronson, H. A. 1982, , 87, 1207 Thronson, H. A. 1983, , 264, 599 Trammell, S. R., & Goodrich, R. W. 1996, , 468, L107 Trammell, S. R., Dinerstein, H. L., & Goodrich, R. W. 1993, , 402, 249 Treffers, R. R., Fink, U., Larson, H. P., & Gautier III, T. N. 1976, , 209, 793 Walsh, J. R., Meaburn, J., & Whitehead, M. J. 1991, , 248, 613 Waters, L. B. F. M., et al. 1998, Nature 391, 868 Wiese, W. L., Smith, M. W., & Miles, B. M. 1969, Nat. Stand. Ref. Data Ser., Nat. Bur. Stand. (US), 22, 189 Willner, S. P., Jones, B., Puetter, R. C., Russell, R. W., & Soifer, B. T. 1979, , 234, 496 Westbrook, W. E., Becklin, E. E., Merrill, K. M., Neugebauer, G., Schmidt, M., Willner, S. P., & Wynn-Williams, C. G. 1975, , 202, 407 Whitelock, P. A. 1985, , 213, 59 Young, K., Serabyn, G., Phillips, T. G., Knapp, G. R., Gusten, R., & Schultz, A. 1992, , 385, 265 Young K. 1997, , 488, L157 Zhang, C. Y., & Kwok, S. 1992, , 385, 255 Zhang, C. Y., & Kwok, S. 1998, , 117, 341 Zuckerman, B., & Gatley, I. 1988, , 324, 501
