In contrast to planetary nebulae, H II regions usually have lower
surface brightness and ionization, resulting in fewer emission line
diagnostics being available for calculating T
, N, and abundances. Most
Galactic H II regions are of low excitation and are highly reddened,
making observations of UV emission lines difficult at best. By contrast,
H II regions in metal-poor, gas-rich, irregular galaxies generally have
higher ionization and temperatures, along with lower extinction, which makes
observations of UV emission lines more feasible and rewarding.
Therefore, we chose the small, high surface-brightness H II region SMC N88A
in the Small Magellanic Cloud (Testor & Pakull 1985, hereafter TP85) to
illustrate the use of the nebular tasks applied to this type of
nebula. One of us (RJD) recently obtained HST FOS spectra of this
object, covering the 1150-6800 ÅÅ wavelength range. The measured
emission line fluxes are presented in Table 9 . The fluxes were
corrected for extinction using the SMC curve of
Prévot et al. (1984) for 
Å and of Seaton (1979) longward,
with 
. Most of the line intensities in
this table come from measurements of the HST FOS spectra; however the
line intensities longward of 6800 Å come from TP85, and the C III]
1907 Å/1909 Å ratio comes from measurement of an archival IUE
high dispersion spectrum (SWP 24701).
We used the ntplot task to construct a nebular diagnostic diagram
of T vs. N
for SMC N88A, similar to that shown in Figure 5 .
For this nebula we have good agreement between the temperature-sensitive
nebular/auroral ratios of [O III] and [Ne III], but a spread in
the densities indicated by the nebular/nebular line ratios of C III],
[S II], Si III], and [Ar IV]. In addition, the
nebular/auroral ratios of [O II] and [S II], which are sensitive
to both T
and N
, suggest lower values for T and
N than most of the
primary diagnostics above. We used the zones
task to derive average T and
N for the low- and medium-ionization
zones (since there were no high-ionization zone diagnostics in this H II
region). The results were T=11,643 K and N=4085 cm
for the low
ionization zone, and T=13,831 K and
N=3478 cm for the medium
ionization zone. Inspection of Figure 5 suggests that these
derived zone values are reasonable given the intersections of the various
line ratio diagnostics.
We use the output of the zones task as input to the abund task
to derive the ionic abundances. Table 10 presents the ionic
abundances and compares them to three other studies: Dufour & Harlow (1975,
hearafter DH75), TP85, and Mathis' (1987, hereafter M87) unpublished model
calculation based on IUE and DH75 data. While some ionic abundances,
such as O
and Ne, agree between the various studies, most
others do not. This is due to a combination of aperture sizes and extinction
values used in the different investigations, rather than any aspects of
the nebular software. For example, DH75 used a 10"x78" slit,
TP85 used a 4" square aperture, and the UV spectra were taken with the
10
x20 oval large aperture on IUE. By contrast, the HST
FOS spectra were taken with a 1 square aperture and sampled the same
central region of the nebula over the entire wavelength range. In addition,
the HST spectra go deeper and have better wavelength resolution than
the previous IUE and ground-based observations-thus permitting the
first two-zone analysis of ionic abundances in this rather compact object
(about 5 FWHM in H
).
This demonstration is intended to show the straightforward use of the
``automated" analysis techniques in nebular to obtain what are probably
improved results (given the better HST spectrophotometric data!) over
previous ground-based and IUE studies using much larger (and different)
apertures. The empirically derived T, N, and ionic abundances can then
be evaluated to derive ionization correction factors to get estimates of
total elemental abundances, or as input parameters to a photoionization
nebular model code. How well this automated approach works depends primarily
upon the accuracy of the spectrophotometric data, particularly for the
usually weak auroral T diagnostic lines, and secondarily upon the nebula
having a rather well-behaved ionization stratification.
