Proc. Natl. Acad. Sci. USA Vol. 74, No. 12, pp. 5203-5206, December 1977 Astronomy Chemical compositions of Magellanic Clouds' HII regions based on photoelectric spectrophotometry (gaseous nebulae/galaxies) L. H. ALLER*, S. J. CZYZAKt, AND C. D. KEYES* Department of Astronomy, University of California, Los Angeles, California 90024; and tDepartment of Astronomy, Ohio State University, Columbus, Ohio 43210 Contributed by L. H. Aller, August 29,1977 ABSTRACT Detailed line intensity measurements secured Table 1. Summary of intensity measurements for the Small at Cerro Tololo Interamerican Observatory and corrected for Magellanic Cloud interstellar extinction are presented for 19 HII regions in the Large Magellanic Cloud and 6 in the Small Magellanic Cloud. X, Iden- log [III(HI#)] + 2.00 for nebulosities The elemental abundances derived by simple methods appear A tification NGC346 N12B N78 N83 N84 N90 to be in good accord with those found by other observers. De- tailed discussion is deferred to a later paper. 3727 [OII] 2.09 2.33 2.39 2.38 2.39 2.26 3868 [NeIII] 1.59 1.23 1.37 1.49 Diffuse gaseous nebulae, commonly called HII regions, offer 4340 Hy 1.66 1.69 1.68 a number of advantages for determinations of abundances of 4363 [0III 0.79 0.57 0.53 the light elements He, N, 0, Ne, S, and Ar in galaxies. They can 4861 HO 2.00 2.00 2.00 2.00 2.00 2.00 be seen at great distances where individual stars lie far below 5007 [01III] 2.72 2.77 2.04 2.47 2.58 2.67 the limit of detection of any possible telescope. Furthermore, 5876 Hel 1.04 1.11 1.06 they offer information on precisely those elements whose 6562 Ha 2.45 2.47 2.46 2.45 2.45 2.46 abundances are likely to be most affected by stellar nuclear 6584 [NII] 0.78 0.95 1.08 1.14 0.85 0.87 reactions. Hence they provide a means for comparing rates of 6717 [SII] 0.88 11 11 0.95 1.20 element building in one galaxy with those in another. 6730 [SII] 0.64 0.79 1.00 The Magellanic Clouds offer special advantages for such 7135 [ArIII] 0.96 0.60 0.81 0.94 1.19 investigations. They are sufficiently nearby that we can still C 0.22 (0.4) 0.24 0.3 (0.2) observe individual stars and compare compositions derived t 1.22 (1.2) (1.2) 1.22 1.085 (1.2) from them with nebular results. Furthermore, the dimensions x 0.01,0.03 (0.01) (0.01) (0.01) (0.01) (0.01) of the nebulae and luminosites of the exciting stars can be es- tablished. We can investigate the small-scale structure of the Table 1 summarizes the intensity measurements for the Small nebulae and ascertain whether individual objects are density Magellanic Cloud as corrected for interstellar extinction in bounded or radiation bounded. The former condition appears = to hold for most of the objects we have measured, although for accordance with the constant C log Ic(H(3)/Io(Hfl), in which some, notably Henize 44, significant excitation differences do I, is the H# flux corrected for extincinon, while Io is the ob- exist between Ha and [OIII] monochromatic images (1). served flux. A standard extinction law is assumed. Successive In recent years, three independent investigations of the columns in the table give the wavelength of the line, the iden- chemical compositions of HII regions in the Magellanic Clouds tification, and logarithmic values of the intensity on the scale have been undertaken at Cerro Tololo Interamerican Obser- W(H,3) = 100 for each of the six nebulosities observed in the vatory (2-5). The Peimberts used exclusively photoelectric Small Magellanic Cloud. The last two rows give values of pa- spectrophotometry with the Cerro Tololo scanner, while Dufour rameters t and x. t is related to the kinetic electron temperature and Aller et al. supplemented such measurements with pho- TE (in 'K) by t = 10-4 T; the density parameter x is related to tographic data. Here we present only our photoelectric results the electron density NE by x = 10-4 Net-/2. In some instances, for 19 HII regions in the Large Magellanic Cloud and 6 in the no measurement of T, or of x was obtainable. The adopted Small Magellanic Cloud. Bad weather caused the observing numbers are then indicated in parentheses; abundance deter- program to extend over several years. In November 1972 there minations for these nebulae are then given lower weight. was only one clear night, so we could not get started; 1973 was Table 2 gives similar data for the nebulosities observed in the our best year because we had a few nights with the 1.5-m Large Magellanic Cloud. Interstellar extinction is determined telescopes and the weather was better. In 1974 there were no largely from the Ha/H# ratio. We chose sufficiently narrow good photometric nights; in 1975 we were able to complete slots and small incremental steps to resolve X6584 from H-y and those aspects of the program that could be done with the 91-cm to deconvolve X6300 from X6312 and X6717 from X6730 (all telescope. It was not possible to measure a number of the weaker wavelengths in A). Especially with the 91-cm telescope, with diagnostic lines we deemed important because further obser- which most of our observations were made, long integration would have been times were required for weak but important lines such as X5876 vations with a telescope with larger aperture and X4363. Except for NGC346, the nebulosities in the small necessary. Magellanic Cloud were especially difficult with this tele- The costs of publication of this article were defrayed in part by the scope. payment of page charges. This article must therefore be hereby marked A number of the same nebulosities were observed also by "advertisement" in accordance with 18 U. S C. §1734 solely to indicate either the Peimberts or Dufour but detailed comparisons are this fact. not easy because the slot of the scanner was not necessarily put 5203 Downloaded by guest on September 25, 2021 5204 Astronomy: Aller et al. Proc. Natl. Acad. Sci. USA 74(1977) Table 2. Summary of intensity measurements for the Large Magellanic Cloud log [(I/I(H13)] + 2.00 for nebulosities X, Identifi- A cation N.8 NlBN11CN44BN44C N44DN51C N55 N57 N59 N79N105AN119 N120N144A N158CN159AN160AN160C 3727 [0111 2.20 2.11 2.29 2.09 2.19 2.53 2.48 2.41 2.65 2.34 2.51 2.63 2.38 2.57 2.52 2.17 1.95 2.18 2.35 3835 H9 0.43 0.73 0.81 0.53 0.89 0.83 3868 [NeIll] 1.28 1.08 1.31 1.40 1.78 0.85 1.01 0.95 0.79 1.49 0.86 0.15? 0.78 0.66 1.51 1.10 1.35 1.31 1.27 3889 H8,HeI 1.12 1.24 1.36 1.13 1.21 4340 Hzy 1.68 1.66 1.66 1.67 1.67 1.67 1.68 1.62 (1.66) 1.66 1.67 1.68 1.67 1.66 4363 [0111] 0.28 0.44 0.10 0.80 -0.37 0.21 0.30 -0.04 0.11 -0.2: 0.48 0.39 0.46 4861 Hg 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 5007 [0111] 2.57 2.55 2.56 2.38 2.87 2.25 2.16 2.42 2.11 2.55 2.16 2.46 2.11 2.07 2.56 2.33 2.69 2.65 2.57 5876 HeI 1.12 1.13 1.04 1.08 1.06 1.03 0.80 1.10 1.01 1.15 1.14 0.99 1.15 1.18 1.04 1.22 1.14 1.10 6300 [OII 0.49 0.35 01 6312 [SIll] 1060.6: 0.47 (0.810.10.23~0.48 -0.3: 0.30 { 0.18 6548 [NIl] 1.00 0.67 0.88 0.71 0.97 .0.98 0.67 0.87 0.98 1.03 1.15 0.81 0.94 0.76 (0.7) 0.92 6562 Ha 2.46 2.46 2.47 2.46 2.46 2.47 2.46 2.45 2.46 2.46 2.46 2.46 2.46 2.46 2.46 2.45 2.46 2.45 2.46 6584 [NIl] 1.30 1.22 1.23 1.37 1.27 1.36 1.33 1.20 1.47 1.12 1.37 1.44 1.50 1.45 1.29 1.36 1.18 1.19 1.39 1.14 1.35 1.34 1.27 1.39 1.18 1.21 1.06 1.02 1.72 6717 [SIIl 1.25 1.08 1.02 1.34 1.24 1.15 1 1 6730 [SIll 1.20 0.85 1.05 1.13 1.15 1.18 91.2911.28 1.27 0.99 1.29 1.19 1.06 1.28 1.03 1.24 1.05 0.95 1.47 7135 [ArnTI] 1.19 0.93 1.00 1.01 0.89 0.98 1.19 0.89 1.07 0.92 1.12 1.00 0.79 1.03 0.89 1.23 1.02 1.12 7325 [0II1 0.65 0.82 0.95 0.99 1.27 0.86 0.70 1.04 C 0.08 (0.2) 0.3 0.2 0.25 (0.2) (0.2) 0.12 (0.3) 0.3 0.15 0.4 0.15 0.2 0.27 (0.2) 0.2 0.3 (0.2) t 0.91 0.93 1.04 0.90 1.07 0.9 0.9 0.91 0.9 0.92 0.91 0.90 0.90 0.91 0.90 0.93 0.96 0.934 0.90 x 0.01 0.04 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.01 on the same position.