On Photoelectric Spectrophotometry (Gaseous Nebulae/Galaxies) L

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 October 1, 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. In our series, which stretched over three The next step is to derive the total elemental abundance from observing seasons, we prepared charts and tried to set on exactly the ionic concentrations. For low excitation nebulae, the ex- the same region if we could not complete a given nebulosity on trapolation procedures suggested by Peimbert and Costero (7) one night. In some objects, such as NGC55, there appeared to for the Orion nebulae, and since applied by many other in- be conspicuous differences from one point to another. vestigators, seem satisfactory pending the development of Table 3 gives the derived ionic concentrations for HII regions trustworthy theoretical models. observed in the Large Magellanic Cloud. The data are all ex- Table 4 gives the abundances derived for the Small Magel- pressed on a logarithmic scale, with the conventional zero point, lanic Cloud HII regions. The last column gives the mean value log N(H+) = 12.00 [N(ion) symbolizes the concentration of the of the logarithmic abundance for each element, on the scale log ion]. The first column gives the designation of the nebula in the N(H) = 12.0. The last row gives the weight that is assigned in catalog of Henize (6). The next column gives other designations taking the mean. It is based on count statistics and reflects both for these nebulae. The procedure for deriving these ionic con- effects of nebular faintness and the fact that a complete series centrations has been summarized in an earlier paper (ref. 3; see was obtained with the 1.5-im telescope only for NGCS46. Table also refs. 8-11), in which references are given to the necessary 5 gives similar data for HII regions in the Large Magellanic atomic parameters and relevant formulas and tables. It should Cloud. In both Table 4 and Table 5 the sulfur and argon be emphasized that the results depend very strongly on the abundances are extrapolated from the ionic concentrations of choice of electron temperature and whether one assumes, as did S+, and occasionally S2+, and of Ar2+ with the aid of theoretical the Peimberts, that there exist appreciable temperature fluc- nebular models. These give relative proportions of ions of ni- tuations along the line of sight through every nebulosity. trogen, oxygen, neon, sulfur, and argon and predictions of line Table 3. Derived ionic concentrations for HII regions in the Large Magellanic Cloud Nebula designation log [N(ion)/N(H+)] + 12.00 for ions Henize (6) Other He+ N+ 0+ 02+ Ne2+ S+ s2+ Ar2+ N8 NGC 1736 10.96 6.71 7.98 8.23 7.46 6.38 6.19 N11B NGC 1793 10.98 6.58 7.83 8.18 7.21 6.07 5.91 N11C NGC 1769 10.88 6.48 7.79 8.03 7.24 6.02 5.90 N44B NGC 1935 10.92 6.78 7.89 8.08 7.58 6.38 6.02 N44C NGC 1936 10.91 6.48 7.65 8.30 7.65 6.17 6.72 5.92 N44D IC 2128 10.87 6.76 8.33 7.90 7.04 6.29 5.98 N51C >10.65 6.75 8.29 7.75 7.22 6.12 N55III 10.93 6.60 8.19 8.08 7.13 6.04 N57 10.85 6.88 8.45 7.80 6.98 6.11 5.90 N59 10.99 6.50 8.08 8.20 7.63 6.18 6.04 N79 NGC 2111 10.98 6.76 8.29 7.83 7.03 6.42 5.92 N105A NGC 1858 10.83 6.75 8.44 8.15 6.40 7.05 6.13 N119 S Dor 10.99 6.91 8.19 7.80 6.97 6.31 6.01 N120 NGC 1918 7.04 8.35 7.74 6.83 6.46 5.77 N144A NGC 1966 11.02 6.69 8.32 8.26 7.69 6.24 6.03 N158C NGC 2081 10.89 6.72 7.97 7.96 7.22 6.32 5.86 N159A NGC 2079 11.06 6.52 7.60 8.39 7.42 6.12 6.76 6.17 N160A NGC 2080 11.00 6.56 7.94 8.29 7.43 6.09 6.68 5.99 N160C NGC 2086 10.93 6.81 8.15 8.27 7.45 6.48 6.74 6.13 Downloaded by guest on October 1, 2021 Astronomy: Aller et al. Proc. Natl. Acad. Sci. USA 74 (1977) 5205

Table 4. Derived abundances for HII regions in the Small rentheses are based on Pradhan's cross sections. We include Magellanic Cloud them for completeness and so that our results can be compared log [N(element)/N(H)l + 12.00 for nebulosities with those of other investigators. Note that the mean value for Element NGC346 N12B N78 N83 N84 N90 Mean the Large Magellanic Cloud is based on results for 30 Doradus obtained with the 1.5-m telescope (reported previously) and He 10.91 10.97 10.91 10.92 on the 19 HII nebulosities listed in Table 5, with relative weights N 6.52 6.42 6.31 6.55 6.42 6.57 6.52 of 1 and 2, because of the greater accuracy of the 30 Doradus 0 8.09 8.19 7.87 8.03 8.23 8.06 8.10 results. The Peimberts derived generally higher abundances Ne 7.39 7.21 7.47 7.32 7.38 for nitrogen, oxygen, and neon, and a lower abundance for (S) (6.38) (6.36) (6.37) (6.71) (6.42) helium in the Large Magellanic Cloud. A comparison with the S 6.75 6.73 6.74 7.03 6.78 Harlow for the Small Ar 5.84 5.52 5.81 6.03 6.10 5.86 results of Dufour and (12) Magellanic Cloud shows a generally satisfactory agreement. wt 10 0.25 0.25 1 1 0.25 Further discussion of chemical abundances will be deferred to a later paper where comparison with nebular models can be The sulfur values in parentheses are calculated with Pradhan's (11) cross sections, while the others are based on Krueger and Czyzak's made. Our purpose here has been to present observational (10) values. material on which such an investigation can be based.

intensities. Although a discussion of these models is deferred L.H.A. and S.J.C. were guest investigators at Cerro Tololo Inter- to a later paper, we have used predictions of models that yield american Observatory, operated by the Association of Universities for correct [OIII]/[OII] line intensity ratios to evaluate ion/element Research in Astronomy under contract with the National Science ratios, e.g. n(Ar2+)/n(Ar). A further complication arises for SI. Foundation. We thank Director Blanco and the staff of the observatory Revisions by Pradhan (11) of the cross sections for collisional for their cooperation in enabling us to obtain scanner observations in excitation of metastable levels yield ionic concentrations about 1973, 1974, and 1975. This program was supported by National Science Foundation Grants AST71-0362 and AST76-21457 to the University half those given by the Krueger and Czyzak cross sections (10), of California, Los Angeles, and National Science Foundation Grant which we have employed in these and previous calculations. AST76-15727 to the Ohio State University. The cooperation of the Table 6 compares derived nebular chemical compositions University of California, Los Angeles, campus computing network is with earlier results (3) and with those published by Dufour (4) gratefully acknowledged. Dr. Reginald Dufour kindly sent us preprints and by the Peimberts (2, 5). The sulfur values indicated in pa- of his paper in advance of publication.

Table 5. Derived abundances for HII regions in the Large Magellanic Cloud log [N(element)/N(H)] + 12.00 for nebulosites Ele- ment N8 N11B N11C N44B N44C N44D N51C N55 N57 N59 He 10.96 10.98 10.88 10.92 10.91 10.87 > 10.65 10.93 10.85 10.99 N 7.15 6.92 6.91 7.19 7.22 6.92 6.86 6.91 6.96 6.86 0 8.42 8.34 8.23 8.30 8.39 8.48 8.42 8.47 8.53 8.45 Ne 7.65 7.36 7.43 7.80 7.74 7.56 7.77 7.50 7.71 7.88 (S) (7.09) (6.93) (6.78) (7.12) (6.96) (6.75) (6.55) (7.06) (6.48) (6.89) S 7.38 7.18 7.03 7.38 7.08 7.00 6.81 7.37 6.74 7.15 Ar 6.42 6.20 6.13 6.26 5.85 6.48 6.62 7.30 N79 N105A N119 N120 N144 N158C N159A N160A N160C Mean He 10.98 10.83 10.99 11.02 10.89 11.06 11.00 10.93 10.97 N 6.89 7.03 7.06 7.14 6.97 7.02 7.37 7.07 7.18 7.04 0 8.42 8.62 8.33 8.45 8.59 8.27 8.45 8.45 8.52 8.44 Ne 7.61 7.52 7.54 8.02 7.53 7.48 7.58 7.70 7.68 (S) (6.78) (7.28) (6.72) (6.86) (6.94) (6.72) (6.90) (7.25) (8.48) (7.00) S 7.10 7.32 6.99 7.13 7.20 6.86 7.19 7.29 8.74 7.23 Ar 7.58 7.54 7.65 7.40 7.33 7.15 7.29 7.26 7.38- 6.37

Table 6. Comparison of derived chemical compositions of HII regions in the Magellanic Clouds log [N(element)/N(H)J + 12.00 Large Magellanic Cloud Small Magellanic Cloud Present Present Element results Ref. 3 Ref. 4 Ref. 2 results Ref. 3 Ref. 12 Ref. 5 He 10.94 10.99 11.01 10.92 10.92 11.00 10.90 10.89 N 6.98 6.94 6.80 7.10 6.52 6.28 6.48 6.48 0 8.43 8.46 8.43 8.58 8.10 7.97 8.02 8.05 Ne 7.71 7.80 7.64 7.94 7.38 7.40 7.29 7.30 (S) (7.00) (6.42) S 7.22 7.2 7.15 6.78 6.5 6.4 Ar 6.34 6.6 7.10 5.86 6.0 Downloaded by guest on October 1, 2021 5206 Astronomy: Aller et al. Proc. Nati. Acad. Sci. USA 74 (1977)

1. Czyzak, S. J. & Aller, L. H. (1977) Astrophys. Space Sci. 46, cubaya, 3 Bull. no. 31. 371-378. 8. Saraph, H. E., Seaton, M. J. & Shemming, J. (1969) Phil. Trans. 2. Peimbert, M. & Torres-Peimbert, S. (1974) Astrophys. J. 193, R. Soc. Ser. A 264, 77-105. 327-341. 9. Czyzak, S. J., Krueger, T. K., Martins, P. deA. P., Saraph, H. E. 3. Aller, L. H., Czyzak, S. J., Keyes, C. D. & Boeshaar, G. (1974) & Seaton, M. J. (1970) Mon. Not. R. Astron. Soc. 148, 361- Proc. Nati. Acad. Sci., USA 71, 4496-4499. 365. 4. Dufour, R. (1975) Astrophys. J. 195,315-332. 10. Krueger, T. K. & Czyzak, S. J. (1970) Proc. R. Soc. Ser. A 318, 5. Peimbert, M. & Torres-Peimbert, S. (1976) Astrophys. J. 203, 531-539. 531-586. 11. Pradhan, A. K. (1976) Mon. Not. R. Astron. Soc. 171,31-38. 6. Henize, K. G. (1956) Astrophys. J. Suppl. 2,315-344. 12. Dufour, R. J. & Harlow, W. V. (1977) Astrophys. J. 216, 706- 7. Peimbert, M. & Costero, R. (1969) Bul. Obs. Tonanzintla y Ta- 712. Downloaded by guest on October 1, 2021