Astron. Astrophys. 332, 721-731 (1998)

3. Physical conditions

The logarithmic extinction constant , the electron density ([SII]) and temperatures and , have been computed in each position from the Balmer decrementum, the [SII] nebular doublet and the standard auroral to nebular line ratios. The values are listed in Tables 4-16, while densities and temperatures are presented in Tables 17-29.

The computed densities and temperatures are also visualized in Figs. 2 - 5. The face values of are typically larger than those of , at least in the most central regions of the nebulae, by some 2000-3000 K, the largest difference (about 6000 K) occurring in He 2-111. Various objects result to be quite excited, having rather above 13000 K throughout the nebulae. They are NGC 2440, NGC 2818, NGC 2899, NGC 6537, He 2-111, and M 3-2. Of these, the latter five reach 16000 K, usually in their central regions. The electron density, as measured from the [SII] doublet, ranges from 50 cm^-3 (our low density limit) up to 7000 cm^-3 in the central region of NGC 6537. For the latter nebula, see also the discussion in the following section.

Fig. 2a-d. Density, temperature, and abundance profiles for NGC 2440, NGC 2818, NGC 2899, and NGC 6072. and are indicated by filled and open circles, respectively. Symbols are plotted slightly displaced in d in order to avoid overlapping. In the lower panels, ionic and total abundance profiles for He, O, N, Ne, Ar, and S are shown. The explanation of the symbols used is given in the Legenda at the bottom of the figure. Above the uppermost right box, the limits of the regions into which the slit was divided are indicated by horizontal "errorbars".

Fig. 3a-d. As for Fig. 2, but for NGC 6537, He 2-36, He 2-84, and He 2-111.

Fig. 4a-d. As for Fig. 2, but for He 2-114, M 1-13, M 1-16, and M 3-2.

Fig. 5. As for Fig. 2, but for Mz 1.

The innermost regions of NGC 6537 has remarkably high electron densities and temperatures. Since data in Table 23 refer to a relatively extended "central" region (the inner 9.8 arcsec), we also extracted a 1-D spectrum corresponding to the innermost .0, which matches adequately the seeing during observations () and some imprecision in the manual guiding. In the core, the [SII] line ratio approaches its high density limit, implying 10000 cm^-3, and the density for the high ionization regions computed from the [ClIII]5518,5538 doublet is of the order of 20000 cm^-3 or more. Assuming densities in the range 10000-20000 cm^-3, both and are computed to be between K and K. High densities and temperatures were also found by Rowlands et al. (1994), Cuesta et al. (1995), and McKenna et al. (1997). In these works, the spread in both parameters is large, ranging from 17000 cm^-3 to 30000 cm^-3 for densities, and from K to K for temperature. At least part of these discrepancies are due to the different diagnostic lines used, which are likely to origin in regions with different physical conditions within the core for NGC 6537. In addition, the extent of the central region analysed in the various works is not the same, so that the computed densities and temperatures are the "averages" within different volumes. As for other PNe with cores with very high densities, see Corradi (1995).

NGC 6537 is known to be a very high excitation object ([SiVI] was detected in NGC 6537 by Ashley and Hyland 1988), with a very hot central star ( K, Jacoby & Kaler 1989, Kaler & Jacoby 1989). Shock excitation is suspected to affect the inner volume of the nebula (Rowlands et al. 1994). Conditions in the high density core of NGC 6537 are therefore quite extreme, and due to the uncertain combination of photo- and shock-ionization the computed physical and chemical parameters in the c region have to be taken with caution.

© European Southern Observatory (ESO) 1998

Online publication: March 23, 1998
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