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Astron. Astrophys. 355, 69-78 (2000)

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4. Results

4.1. PNe with 3He measurements

The results of the observations are given in Table 1 where we list the PN name, the intensities [FORMULA] of the 12CO and 13CO [FORMULA]-1 lines and the derived isotopic ratio. The sample of the six PNe studied in 3He shows little emission in CO: with the exception of NGC 6720, the values of I21 represent upper limits to the intensity and have been estimated from the line widths deduced from the expansion velocities listed in the catalog of Acker et al. (1992).


[TABLE]

Table 1. CO results for PNe observed in 3He


Bachiller et al. (1989) detected an extended molecular envelope in NGC 6720, the Ring nebula. The CO emission reveals a clumpy ring, resembling to that of the ionized nebula. We observed the [FORMULA]-1 and 1-0 lines of 13CO in three positions and detected emission in all cases. Fig. 1 shows the spectra of the four CO and 13CO lines observed toward the strongest peak in the molecular envelope (offset [-40", -20"] from the central position). The measured isotopic ratio toward this position, where the S/N is the highest, is 12C/13[FORMULA]. This value is in agreement with a previous estimate of Bachiller et al. (1997) and with the values obtained toward the two positions with lower S/N spectra. Thus, our data provide no evidence for variations of the isotopic ratio across the nebula.

[FIGURE] Fig. 1. CO emission in PNe. Two examples are shown from the sample of PNe with (left ) and without (right ) 3He measurements. The offset in arcsec from the central PN is also indicated. The stronger [FORMULA]-1 transitions are shown in the upper panels, while the [FORMULA]-0 transitions in the bottom panels. The isotopic lines are detected in both cases.

We have also detected a line around the 12CO [FORMULA]-0 frequency in the central position of NGC 6543, but we failed to detect the [FORMULA]-1 line at relatively low levels. This indicates that the line near the [FORMULA]-0 frequency is probably not due to CO. It is interesting to recall that the H38[FORMULA] recombination line is only separated by 3 MHz (7.8 km s-1) from the 12CO [FORMULA]-0 line. As discussed in Bachiller et al. (1992), the H38[FORMULA] line can dominate the emission around the 12CO [FORMULA]-0 frequency in some nebulae with little or no molecular gas. We believe that this is the case in the central position of NGC 6543.

4.2. PNe without 3He measurements

The parameters of the detected sources and the derived isotopic ratios are listed in Table 2. We have also included in this table data for NGC 2346, NGC 7293, NGC 6781, M 4-9, and CRL 618 from previous observations by Bachiller et al. (1989, 1997). The spectra of the remaining PNe are shown in Fig. 1 (for M 1-16) and in Fig. 2. In four cases (NGC 7008, NGC 6853, M 1-13, BD[FORMULA]) we obtained tentative detections in the 13CO lines, or only crude upper limits could be derived.

[FIGURE] Fig. 2. 13CO [FORMULA]-1 and 1-0 spectra observed toward the central position of the PN identified in the upper left corner of each panel.


[TABLE]

Table 2. CO results for PNe not observed in 3He


The assumption of optically thin emission used to determine the isotopic ratio is likely to be accurate in the case of evolved nebulae like the Ring, the Helix, NGC 2346, etc. In fact, Large-Velocity-Gradient (LVG) models confirm that the 12CO and 13CO line emission is optically thin in these cases. On the other hand, the same assumption is not appropriate for the CO lines in young objects such as CRL 2688, CRL 618, NGC 7027, and M 1-16. In such cases, the derived CO column densities and 12CO/13CO column density ratios represent only approximate lower limits.

One could also have weak emission arising from small optically thick clumps very diluted within the 12CO and 13CO beams. This could be the case in some compact CO envelopes such as the Butterfly nebula, M 2-9. The CO in M 2-9 is concentrated in an expanding clumpy ring which has a mean diameter of 6 arcsec (Zweigle et al. 1997). Individual clumps in this ring have sizes [FORMULA] arcsec. The weakness of the 12CO and 13CO lines we observe could be due to the important dilution of such small clumps within the 30-m beam. The clumps could be optically thick in CO, and the reported 12CO/13CO intensity ratio would just represent a lower limit to the abundance ratio in these compact PNe.

In the case of extended PNe (e.g. NGC 6720, NGC 2346, NGC 7293), the assumption of optically thin CO 2-1 emission is based on detailed studies of individual nebulae which show that the intrinsic 2-1/1-0 line intensity ratio is typically [FORMULA]2 (Bachiller et al. 1989, 1993; Huggins et al. 1996). Such high ratios imply that the emission is optically thin, and that the excitation temperature is [FORMULA]10 K. In this approximation, and assuming homogeneous excitation conditions along the line of sight, the column density in the J=2 level is proportional to the observed 2-1 line intensity. Moreover, the relative population of the J=2 level is quite insensitive to the value of the excitation temperature for the typical conditions of PNe. The total CO column densities determined in this way are correct for Tex in the range from 7 to 77 K, and are within a factor of 2 over the range from 5 to 150 K, which should cover most PNe. In any event, because of the similar dipole moments, the excitation temperature should be the same for both CO and 13CO. Then, if the excitation conditions do not vary along the line of sight, the isotopic ratios are independent of Tex.

The major source of uncertainty for these extended nebulae is thus related to the observational procedures. The calibration of our observations is accurate within 20%, but small differences in the filling factor for the different spectral lines could increase the uncertainty of the measured isotopic ratios up to a factor [FORMULA]1.5. As Table 2 shows, in the extended PNe where 13CO is well detected (NGC 6720, NGC 2346, NGC 7293, NGC 6781, M 4-9, and M 2-51) the values of the isotopic ratios are in the range 9 to 23, to within a factor of [FORMULA] 1.5. Such values are thus significantly lower than the Solar System value of 89, and appear to be consistent with those derived in AGB stars, as we shall discuss in Sect. 6.1.

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© European Southern Observatory (ESO) 2000

Online publication: March 17, 2000
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