N(Ion) = A,[1 + A2x]Tl/2Eo4210blti(Ne)

Proc. Nat. Acad. Sci. USA Vol. 71, No. 11, pp. 4496-4499, November 1974 Chemical Composition of Nebulosities in the Magellanic Clouds (gaseous nebulae/galaxies) L. H. ALLER*t, S. J. CZYZAKtt, C. D. KEYES*, AND G. BOESHAARt t Department of Astronomy, The Ohio State University, Columbus, Ohio; and *Department of Astronomy, University of California, Los Angeles, Calif. 90024 Contributed by L. H. Aller, August 14, 1974 ABSTRACT From photoelectric spectrophotometric quate accuracy, application of the relevant theory to observa- data secured at Cerro Tololo Interamerican Observatory tions suffers difficulties and frustrations. we have attempted to derive electron densities and tem- spectral line emission per cm3 peratures, ionic concentrations, and chemical abundances The equations express the of He, C, N, 0, Ne, S. and Ar in nebulosities in the Magel- as a function of ionic concentration, Nf, electron density, AT. lanic Clouds. Although 10 distinct nebulosities were and gas kinetic electron temperature, TE. We actually observe observed in the Small Cloud and 20 such objects in the the emission from a radiating column of gas of many parsecs Large Cloud, the most detailed observations were secured depth and cross-section within which both and can only for the brighter objects. Results for 30 Doradus are in T. N, harmony with those published previously and recent work fluctuate considerably. Peimbert (9) has calculated the in- by Peimbert and Torres-Peimbert. Nitrogen and heavier fluence of a mean square fractional temperature fluctuation elements appear to be less abundant in the Small Cloud ((T - To)/To)2 upon the emmissivity of a spectral line in a than in the Large Cloud, in accordance with the conclu- medium of average temperature To. sions of Dufour. A comparison with the Orion nebula may be observed only in suggests He, N, Ne, 0, and S may all be less abundant in Some elements such as nitrogen the Magellanic Clouds, although adequate evaluations the neutral or singly ionized stages. In a typical ionized will require construction of detailed models. For example, hydrogen (HII) region, most of the nitrogen exists as un- if we postulate that the [NIl], 10111, and [S111 radiations observable N++. Seaton (10) and Peimbert and Costero (11) originate primarily in regions with electron temperatures have proposed recipes for allowing approximately for the near 8000'K, while the [0111], [NelIl], [ArIbl, and H radi- ionization ations are produced primarily in regions with TE = 10,000° distribution of various atoms among different K, the derived chemical abundances in the clouds are stages by considering, e.g., n(O+)/n(O++), n(He++)/n(He+), enhanced. etc. ratios. We used their procedures only because there now way of calculating the nebular radiation the exists no satisfactory A basic assumption that underlies methods of establishing field and ionization equilibrium of elements such as nitrogen. cosmological distance scale is that the chemical composition Electron densities can be estimated from certain forbidden of stars and nebulosities in distant galaxies is essentially the line doublet ratios observed in ions with a p3 configuration same as in our own stellar system, exhibiting the same pre- (12-15); the important ratios are 3726/3729 [OIl] and 6717/ ponderance of light elements and possible spreads of metal-to- 6731 [SII]. Whenever possible, we calculated T, from the hydrogen ratios from one stellar population type to another. ratio. com- 4363/5007 [OIII] Three direct methods of assessing possible chemical For p2 and p4 ions, the ionic concentration N(ion) with position differences are immediately apparent: (i) quantita- respect to ionized hydrogen N(H+) is given by: tive spectral analyses of the brightest stars; (ii) studies of of composite spectral features and energy distributions N(ion) = a,[1 + a2x]tl/2Eo4210blt I(ne) [1] integrated light of a stellar system; and (iii) analysis of the spectra of emission nebulosities. and transition Inspired by Przybylskis ((1-3) pioneering efforts, several where a, and a2 depend on collision strength workers (4-6) have measured concentrations of abundant probabilities. elements in a number of Magellanic Cloud supergiants. Since x = 10-4NT/ Vt, t = 10-4T,, b = 0.540XIj [2] of these stars teeter on the brink of instability, atmospheres the or the usual assumptions of hydrostatic equilibrium, stratifica- where x1j is the excitation potential of (1D2) ('Se) level, and local thermodynamic equi- respectively. The emission per unit volume in HO3 is E(HO3) = tion in plane parallel layers, been calculated are to question. Spectra of integrated starlight N(H+)N10-25 E04,2. where E04,9 has by librium open expres- can give only crude estimates of metal/II ratios. Spectra of Clarke (16)§ and by Brocklehurst (17). Appropriate emission nebulosities can be studied even in distant galaxies, sions or tables (12-15) exist for p3 ions. and certainly abundance ratios can be reliably established, METHODS AND RESULTS as shown, e.g., by Peimbert and Spinrad (7, 8). Although physical processes occurring in thermally excited Optical and Radio-frequency Studies of Emission Nebulosities gaseous nebulae appear to be well understood, and required in the Magellanic Clouds. Finding charts, positions, and de- atomic parameters have been calculated to seemingly ade- scriptions of emission nebulosities, and also surface bright- & t Guest investigators at Cerro Tololo Interamerican Observatory, § Values are quoted by Aller and Liller (1968) in Table 6; Stars operated by the Association of Universities for Research in Stellar Systems 7, 483-574; Nebulae & Interstellar Matter, ed. Astronomy under contract with the National Science Foundation. Middlehurst, B. & Aller, L. H. (Univ. of Chicago Press, Chicago). 4496 Downloaded by guest on September 23, 2021 Proc. Nat. Acad. Sci. USA 71 (1974) Composition of Nebulosities in Magellanic Clouds 4497 TABLE 1. Adopted line intensities for 30 Doradus* TABLE 2. Derived t and x parameters for 30 Doradus* X(k) Identity I X(A) Identity I Derived (assumed) 3727 [OII] 93 6300 [0I] 5.9 parameter 3835 H8 5.5 6312 [SIII] 2.2 Corrected 3868 [NeIII] 25.0 6548 [NII] 8.4 Diagnostic ratio I ratio x t Refs. 3889 H,He 14.0 6563 Ha 405 4340 Hly 42.3 6584 [NII] 20.5 3729/26 (0+) 0.94 0.074 12, 36 4363 [GIll] 2.8 6678 HeI 9.6 6717/31 (S+) 1.23 0.09 (0.8) 13 4861 H# 100 6717 [SII] 14.2 4068/6725 (S+) 0.045 0.09 (0.8) 13, 36 4959 [0111] 164 6730 [SII] 11.5 3727/7330 (0+) 33.4 (0.074) 1.02 12 5007 [GIll] 499 7065 HeI 4.6 5007/4363 (O++) 125 (0.074) 1.04 40 5755 [NII] 0.25 7135 [ArIII] 22.7 5755/6548,84 (N+) 0.010 (0.074) 1.0 38, 39t 5876 HeI 13.8 7330 [OII] 6.0 * See Eq. (2) for definitions. Each ratio depends on t and x. * These line intensities have not been corrected for inter- The insensitive one is indicated by parentheses. For example, stellar extinction. 3729/3726 is sensitive to NE at relevant densities but insensitive to T,; the reverse is true for the 5007/4363 ratio. t A-values for [NII] are from ref. 38. The collision strengths nesses in Ha + [NII], have been given by Henize (18), and by are from ref. 39. Doherty et al. (19), respectively. Dickel et al. (20) published a photoelectric spectrophotometric survey of brighter emission a two-temperature model. We examine here two idealized nebulosities, and assigned excitation classes (21). Later, models: (I) uniform temperature, t = 1.03, x = 0.074 for all Dickel (22) measured isophotic contours and determined ions and (II) t = 0.95 for zones of O++, Ne++, S+*+, Ar++, mean electron densities and masses for HII regions of the Large Cloud. Ar+3, He+, x = 0.074; and t = 0.8 for N+, 0+, S+, x = 0.074. It is assumed that most of the hydrogen emission comes from From data secured at 73 cm in radio-frequency both clouds, the t = 0.95 zone. Mills and Aller (23) found smaller root mean square N, values For model I the values-of log N(ion)/N(H+) are: -5.63 than indicated by other studies. Filamentary structure can (N+), -5.53 (O°), -4.33 (O+), -3.87 (0++), -4.53 (Ne++), explain these differences. Extensive radio-frequency studies - 5.63 (S+), -5.46 (S++), and -5.93 (Ar++). Higher values in the Large Cloud were made by Mathewson and Healey (24) would be obtained for model II or by following Peimbert's at 73 and 21 cm, by J. N. Clarke (25), and by McGee and his procedure, for which one must know the root mean square associates (26-28) at 6 and 11 cm. temperature fluctuation. The Observations. No scanner was available in 1970 when Uncertainties in t (and to some extent in x), as well as this program was initiated. We obtained slit spectrograms difficulties in allowing for unobserved ionization stages, all with the 0.91-m and 1.5-im Cerro Tololo telescopes at carefully introduce inaccuracies. We used the same correction factor selected positions in each nebulosity. Unfavorable weather for n(S+) + n(S++) as for n(N+) to get the sulfur concentra- made it impossible to obtain the necessary calibrations by tion (11). Uncertainties are large, not only for S, but par- scanner measurements of individual lines and line pairs in ticularly for Ar. November 1972, so the program had to be carried out a year The derived logarithmic elemental abundances from model later. Although the present discussion is based primarily on I and model II, denoted by parentheses, are: He -1.09 photoelectric measurements, photographic spectral data (-1.10); N -5.05 (-5.01); 0 -3.72 (-3.48); Ne -4.40 supplied supplementary information, sometimes significant (-4.13); S -4.77 (-4.82), Ar -5.9 (-5.7).

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