5.3.2. S/O
Shields & Searle
(1978)
pointed out that sulfur is
an important secondary coolant in metal-rich HII regions, and that
measurements of [S III] could help constrain photoionization models
for such nebulae. Although it is well established that S/O in HII
regions is generally near the solar system value, log S/O
-1.7
(Anders & Grevesse
1989),
the question of
systematic variations with O/H remains unclear and controversial
from several recent studies.
Shaver et al. (1983),
Vílchez et
al. (1988) and
Díaz et al. (1991)
derived an apparent decline
in S/O by approximately 0.5 dex from log O/H = -3.7 to log O/H = -2.6
in the Galaxy, M33, and M51. On the other hand,
Garnett (1989) and
Torres-Peimbert et
al. (1989)
discern no systematic variation
in S/O from the solar system value over the range -4.8 < log O/H <
-3.3 in dwarf irregular galaxies and M101. Meanwhile, measurements of
IR [S III] lines
(Pipher et al. 1984,
Simpson et al. 1995)
indicate that the sulfur abundance
gradient in the Milky Way is much steeper than the gradient derived
by Shaver et
al. (1983),
and is essentially identical to the oxygen gradient. Note that
Garnett (1989)
and Vílchez et
al. (1988)
disagree by a factor of two in the S abundance for the region
NGC 588 in M33, a critical low-abundance point in the spiral
galaxy sample.
A new observation of this HII region would be very valuable.
Our new [S III] measurements for the NGC 2403 sample provide a new homogeneous set of data to address this question. Our derived S/O abundance ratios are displayed in Figure 20; the top panel shows the S/O with full ionization corrections for S (Section 3.3); the lower panel shows S+ + S+2 alone. The ionization corrections we apply for S are minor, but they do introduce a small systematic effect. Figure 20 shows that there is little variation in S/O across the disk of NGC 2403. Formally, the observed gradient in S/O is barely significant, and the total range in S/O in NGC 2403 is only about 0.2 dex between 1 and 6 kpc, essentially the same as the observational scatter.
|
Figure 20. Top: S/O vs. radius in NGC 2403. Bottom: (S+ + S+2)/O vs. radius in NGC 2403. (From Garnett et al. 1997b). |
This would seem to indicate a universal value for S/O consistent with the solar value. However, much of the evidence for systematic variation in S/O is based on measurements of HII regions more metal-rich than those in the NGC 2403 sample; this remains a critical area for future study.