Astronomy and Astrophysics – Astrophysics
Scientific paper
Feb 1981
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1981apj...244...54k&link_type=abstract
Astrophysical Journal, Part 1, vol. 244, Feb. 15, 1981, p. 54-65.
Astronomy and Astrophysics
Astrophysics
23
Astronomical Spectroscopy, Nebulae, Oxygen, Sulfur, Abundance, Magellanic Clouds, Milky Way Galaxy, Planetary Nebulae, Supernova Remnants
Scientific paper
The relationships between the intensities of the λ6723 [S II] line and those of λ3727 [O II], λ6300 [O I], and N63 12 [S in] are examined for both planetary nebulae and diffuse nebulae in the Galaxy, the Magellanic Clouds, M101, and M33, and for supernova remnants in the Galaxy and M33.
The [S II]-[O II] relation in planetaries shares the characteristics of the relations between [O I] and [O II], [N I] and [N II], and O+ and N+. First, the [S II] line strengths for planetaries with higher central star temperatures (log T* ≥ 4.65), which are mostly optically thin, are linearly related to those of [O II] over a range of two orders of magnitude. Second, the [S II]/[O II] ratio for optically thick planetaries with log T* < 4.65, which contain neutral helium, is negatively correlated with T* in the same way as is N+/O+. For these objects the [S II]/[O II] ratio can be used to estimate effective blackbody central star temperatures by the relation log T* = 4.325-0.147 log [I(λ6723)/I(λ3727)]. The [S II]/[O II] line ratios are probably also suppressed in the low-T* nebulae at least in part because of increased homogeneity, as already shown by the suppression of the [O I]/[O II] ratio. The [S II] line strengths in planetaries are similarly related to those of [O I], and are related to roughly the cube of those of [S III].
The relations suggest that for planetaries without neutral helium, S+/O+ ∝ S/O, which provides support for the reliability of N+/O+ = N/O, and thus indicates that N+/S+ ∝ N/S. These sulfur relations are also true for diffuse nebulae, providing that one makes some modifications for central star temperature, or for I(λ5007) [0 III]/I(Hβ). Because of very different conditions, the average [S II]/[O II] intensity ratio for SNR is 2.5 times that for diffuse nebulae of comparable [O III] strength, although at low [O III] strength, [S II]/[O II] in diffuse nebulae can be comparable to that in SNR.
There seems to be no detectable vertical S/O gradient within the Galaxy, nor is there a detectable radial gradient. The data indicate that the previously reported low S/O ratio in the halo planetary Ha 4-1 may be a result of the above relation between S+/O+ and T*. The S/O ratio in the Magellanic Clouds and M33 is the same as that found in the Galaxy to within about ±10%, and in M101 to within about ±25%. There is no unambiguous evidence that S/O is anything but constant among the objects studied. This constancy allows the N/S ratio, and thus the N+/S+ ratio, to be a good indicator of N/O.
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