6. Ionic abundances
The ionic abundances have been derived with the usual assumption that the line emission originates in a layer characterized by single values for the density (given by ) and the temperature. The values of have been used +, N+, S+, to determine the abundances of those ions with low ionization degree: O Cl + and Ni ++ . The values of have been used (when available, otherwise) to determine the rest of the ionic abundances: O++, S++, Cl ++, Ar ++, Ar 3+, He + and C++ . The intensity ratios used to determine the ionic abundances relative to H+ are listed in Table 4.
Table 4. Line ratios used in the abundance determination.
HI emissivities have been taken from Hummer & Storey (1987), the emissivities for the forbidden lines (excluding [ClII ] and the Ni lines) have been calculated with the IRAF nebular package, with the atomic parameters referenced therein (version of 1997 May).
The atomic data for Cl + (not considered in IRAF) have been taken from the compilation by Mendoza (1983).
The lines [NiII ] 7378, 7412 and [NiIII ] 7890 could be measured in some objects, but the determination of the Ni abundance is hampered by several problems. Both [NiII ] lines are severely affected by fluorescence effects (Lucy 1995). On the other hand, there are no calculations in the literature for the collision strengths of Ni ++, although Osterbrock et al. (1992) present some estimates based on the collision strengths of Fe 6+ . Furthermore, the [NiIII ] 7890 line is affected by sky emission, and ionization models - needed to estimate the contribution of Ni 3+ to the total abundance - do not usually consider the Ni ions. Anyway, the abundance ratio has been calculated with the approximations used by Osterbrock et al. (1992), but these values must be considered with caution and no attempt will be made to determine the total Ni abundance.
The abundance ratio has been obtained from the intensity of the recombination line CII , using the relationship derived by Peimbert et al. (1995), which is based on the recombination coefficient calculated by Péquignot et al. (1991):
The HeI emissivities have been taken from Smits (1991). [Smits (1996) presents more accurate calculations, but the results are very similar to those of Smits (1991) for the lines and physical conditions considered here.] The values derived for must be corrected for the effects of collisional excitation and self-absorption. The collisional corrections have been obtained from the empirical formulas given by Clegg (1987); the correction factors are below 17% for the lines used in the abundance determination (4471, 5876 and 6678). The effective optical depth for self-absorption processes and the corresponding correction factors have been estimated from the calculations of Almog & Netzer (1989). The optical depth adopted for each area is the mean value of those implied by and ; the correction factors for self-absorption are %.
The values obtained for the ionic abundances are listed in Table 5. Note that the values of the abundance ratio are not presented for the regions studied in M43. The spectra of this nebula - the only object in the sample which is excited by a B star (NU Ori) - are greatly affected by dust-scattered stellar light, in such a way that some HeI lines, like 4471, 4713 or 4922, are completely swallowed up by the stellar absorption lines. The HI lines (mainly H and H) are also affected, but to a much lower degree. A lower limit to the He + abundance in M43 has been obtained from the least affected HeI lines, 5876 and 6678: . On the other hand, the CII line could not be measured in M43. An upper limit to its intensity implies , but this value is too low (even for this low-excitation object) and CII is probably affected by the corresponding absorption feature in the dust-scattered stellar spectrum.
Table 5. Ionic abundances .
© European Southern Observatory (ESO) 1999
Online publication: November 16, 1999