Proc. Nati. Acad. Sci. USA Vol. 75, No. 1, pp. 1-3, January 1978 Astronomy Chemical composition of gaseous nebula NGC 6302 (planetary nebulae/spectrophotometry) L. H. ALLER* AND S. J. CZYZAKt * Department of Astronomy, University of California, Los Angeles, California 90024; and Physics Department, University of Queensland, Brisbane, Queensland, Australia; and t Department of Astronomy, Ohio State University, Columbus, Ohio 43210 Contributed by L. H. Aller, October 20, 1977

ABSTRACT The irregular emission nebula NGC 6302 ex- ITS slots on the nebula and then on the sky alternately. Thus, hibits a rich spectrum oflines ranging in excitation from [NI] although observations from X3800-X8500A were secured at two to [FeVII]. An assessment of available spectrosco ic data, cov- points with the ITS, we have analyzed only the data for the ering a large intensity range, indicates excess ofhelium and nitrogen as compared with average planetary nebulae, but de- bright central patch. Photoelectric scanner measurements ficiencies in iron and calcium. These metals are presumably tied yielded intensities of the stronger lines and provided a funda- up in solid grains, as suggested by Shields for iron in NGC mental calibration for the ITS data. 7027. The first 2 columns of Table 1 give the wavelengths and spectral line identifications. The third column gives the loga- It is well recognized that, in the terminal phases of their evo- rithm of the adopted nebular line intensities on the scale lution, many stars eject their outer envelopes, which become logI(H3) = 2.00, corrected for interstellar extinction. We planetary nebulae, while the compact residue of the dying star adopted an extinction correction C = log[I(Hf)/F(Hf)] = 1.0 evolves into a white dwarf. The chemical composition of this (ref. 4), in which F(Hf) is the observed flux at Hf3 and I(Hf) detached envelope thus becomes a matter of great interest. Is is the flux we would have observed, had there been no inter- it characteristic of the material from which the star was origi- stellar extinction. The table also includes estimates for some nally formed, or does it carry the signature of nuclear trans- ultraviolet lines, based on a reassessment of earlier data (3) and formations within the stellar core? measurements obtained by Walker and Aller (11) with the Most planetary nebulae probably show elemental abundances Lallemand electronic camera, recalibrated with the aid of ITS corresponding to the interstellar medium at the time of for- data. All entries followed by a colon (:) are to be considered very mation of the parent star. A few objects have been found to uncertain (the intensities may be in error by 30-50%). For the display excess amounts of helium and probably nitrogen as well. stronger lines, the errors should be of the order of 5%, increasing Such nebulae are often irregular or filamentary in appearance, to 10-15% for lines of intensity less than 5, and to 20-25% for show no visible central star, and exhibit a wide range of exci- lines of intensity of about 1.0. Near the end of the spectral range, tation. there also may be larger errors. Kinematical studies by Minkowski and Johnson (1) suggest that NGC 6302 originated in some type of explosive event. The Electron densities and temperatures spectrum has been investigated by Evans (2), by Oliver and Aller (3), and by Danziger et al. (4). At 5 GHz NGC 6302 shows Plasma diagnostics for an ionized gaseous nebula depend on a a flux density of 3.49 flux units (5); it is probably not a non- variety of measured line ratios, interpreted with the aid of ap- thermal source. Higgs (6) has summarized the earlier radio propriate Einstein A values and collision strengths within the frequency observations. Oliver and Aller found NGC 6302 to framework of a relatively straightforward astrophysical theory be helium rich, a result confirmed by Danziger et al. Densities (12-19). Complications are introduced by the nonuniformity from 103-104 electrons/cm3 and an electron temperature in of temperature, density, and level of excitation within the the neighborhood of 17,500 K were indicated (3, 4). Danziger nebula. Basic parameters are the density parameter x = et al. found an emission excess in the infrared at 3.5-20 ,um, 10-4N/VX;; and t = 10-4T,, in which NE is the electron which they attributed to dust grains heated by trapped Lya density and TE is the kinetic electron temperature. photons. They noted the similarity between NGC 7027 and The X5007/X4363 [0111] and X6583/X5755 [NII] line in- NGC 6302. tensity ratios give Te = 15,700 K and 15,600 K for o2+ and N+, respectively. The [SIT] X6718/X6730, X4068/(X6718, 6730), Observations [ClIII] X5517/X5537, and [ArIV] X4711/4740 ratios suggest x We measured NGC 6302 with the Oke photoelectric spectrum values of 2, 5.5, and 3 for these sulfur, chlorine, and argon ions, scanner attached to the 2.5-m reflector at Mt. Wilson and with respectively. Revisions in cross sections for 3p3 configurations the Robinson-Wampler image-tube scanner (ITS) attached to may change these values somewhat, but the more highly ionized the Cassegrain focus of the Lick 3-m reflector. We calibrated atoms may originate in regions with electron densities appre- the photoelectric data with the aid of comparison stars that had ciably exceeding 04 cm-3. been observed by Oke (7) and by Hayes (8) and for which Vega serves as a fundamental reference standard (9). For the ITS Ionic concentrations observations, we employed standard stars of intermediate The adopted line intensities may now be used to obtain ionic brightness that had been observed by Stone (10). Because of its concentrations. For lines of HeI and HeII we use the data of large angular size, it is necessary to observe NGC 6302 with both Brocklehurst (20, 21). For hydrogen the calculations by Clarke The costs of publication of this article were defrayed in part by the (ref. 22, quoted in ref. 16) and those of Brocklehurst (20, 21) are payment of page charges. This article must therefore be hereby marked in good agreement. For the forbidden lines, we have relied on "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate this fact. Abbreviation: ITS, image-tube scanner. Downloaded by guest on September 29, 2021 2 Astronomy: Aller and Czyzak Proc. Natl. Acad. Sci. USA 75 (1978)

Table 1. NGC 6302 A, Identi- A, Identi- A, Identi- A, Identi- A fication logI A fication logI A fication logI A fication logI 3312.3 011 1.00: 4570.9 MgI -0.51 5537.8 [ClIII] -0.05 6678.2 HeI 0.50 3341.0 011 1.21: 4606.5 [FeIII] 0.24 5577.4 [Oi] -0.62 6716.5 [SI1] 0.81 3345.9 [NeV] 1.84: 4634.5 NIII 0.76 5630.9 [FeVI] -1.05 6730.7 [SII] 1.11 3426.0 [NeV] 2.27: 4640.7 NIII 1.05 5677.0 [FeVI] -0.84 6890.8 HeII -0.25 3444.0 011 1.4: 4685.7 Hell 1.88 5721.1 [FeVII] -0.62 7005.7 ArN 1.00 3705.0 HeI 0.37 4711.4 [ArIV] 1 31 5754.6 [NII] 1.32 7065.3 HeI 0.90 3712.0 H15 0.19 4713.2 HeI 5801.4 CIV -0.60 7135.8 [ArIII] 1.41 3721.9 H,[SIII] 0.70 4725.0 [NeIV] 0.79 5812.1 CIV -0.91 7170.6 [ArIV] 0.85 3726.0 [OIl] 1.46 4740.2 [ArIVI 1.41 5875.6 HeI 1.16 7177.8 HeII 3728.74 [OIl] 1.12 4861.3 Hf3 2.00 5913.5 HeII -1.2: 7237.3 [ArIVI 0.01 3734.3 H13 0.39 4922.0 Hel 0.29 5932.2 HeIl -1.1: 7262.8 [ArIV] -0.04 3750.1 H12 0.41 4931.0 [OIII] -0.11 5945 -0.89 7281.3 HeI -0.12 3759.8 011 0.76 4959.0 [OIII] 2.70 5977.1 Hell -1.03 7319.92 [OIl] 1.17 3770.6 H11 0.60 5006.9 [OIII] 3.19 6004.8 Hell -1.0: 7330.19 3791.3 OIII -0.05 5015.6 HeI 0.43 6037.2 HeII -0.89 7530.5 [ClIVI -0.12 3797.8 H10 0.74 5041.3 Sill 0.294 6074.3 HeII -0.8: 7592.8 Hell 0.045 HeI [FeVII]1 70 -0.68 3819.6 0.30 5047.7 HeI 0.0 6086.9 Ca _0.29 7726.5[S0]-0074 3835.3 H9 0.89 5056.4 Sill 0.35 ~ [a] 765 [I 07 3868.8 [NeIl] 2.10 5112 OV -0.57 6101.8 [KIV] -0.14 7751.0 [ArIII] 0.77 3889.9 H,HeI 1.27 5158.9 [FeVII] -0.51 6118.3 HeII -0.77: 8045.6 [ClIVI 0.24 3967.5 [NeIllI] 1.55 5176.4 [FeVI] -0.72 6170.7 Hell -0.65 8196.6 -0.41 3970.0 H7 1.22 5191.8 [ArIII] -0.10 6228.4 [KVI] -0.84 8236.6 Hell 0.28 4068.6 [511] 1.15 5198.0 [NI] 0.70 6233.8 HeII -0.65 8359.0 H,P22 -0.82 4076.2 [SIll 0.81 5200.4 [NI] 0.59 6300.3 [OI] 1.29 8374.5- H,P21 -0.68 4097.3 NIII 0.71 5270.3 [FeIV] -0.77 6312.1 [SIll] 0.66 8392.4 H,P20 -0.60 4101.7 H6 1.41 5280 [FeVI]+-0.8: 6347.5 Sill -0.58 8413.3- H,P19 -0.52 4199.8 Hell 0.44 5297 -0.26 6363.8 [01] 0.78 8437.9 H,P18 -0.40 4340.6 Hy 1.67 5309.2 [CaV] -0.59 6371.3 Sill -0.25 8446 01 -0.68 4363.2 [0111] 1.57 5323.4 [CIIV] -0.93 6406.5 Hell -0.57 8467.3 H,P17 -0.37 4387.8 HeI 0.04 5335.2 [FeVI] -0.81 6435.1 [ArV] 0.66 8502.5 H,P16 -0.30 4471.4 HeI 0.80 5411.5 HeIl 0.83 6548.1 [NII] 2.15 8545.5 H,P15 -0.25 4541.4 Hell 0.56 5517.7 [ClIII] -0.54 6563.0 Ha 2.45 8577 [ClII] -0.04 6583.4 [NII] 2.62 8598.3 H,P14 -0.15

some recent calculations by Seaton and his associates (15), and N+, the deduced concentrations of ions of p2 and p4 configu- we have reassessed cross sections for a number of ions. Uncer- rations are not affected much by the choice of x in the pertinent tainties in the target area parameter are worrisome for the 3p3 range of densities. For ions of 2p3 or 3p3 configuration, we have configuration of [SII], [ClIII], and [ArIV]. Fortunately, colli- relied on x values deduced from diagnostic ratios. Unless oth- sional cross sections are now available for [FeVII] (23), and for erwise noted, the calculations are carried out for an assumed [FeIII] and [FeVI] (24). temperature of 16,000 K. It seems likely, however, that lines Table 2 gives the ionic concentrations for observed ions from of [NeIV], [NeV], [FeVI], and [FeVIII] are excited in a domain helium to iron. The permitted CIV lines X5801, X5812 may arise where the electron temperature can reach 20,000 K (19), so we from excitation in a stellar radiation field; hence they do not have also calculated ionic concentrations for Tf = 20,000 K. shed much light on the nebular carbon abundance. Except for These entries are denoted by an asterisk (*).

Table 2. Ionic concentrations in NGC 6302

NN(ion) _ion)_I N(ion)~in Ion x x N(Hon) Ion x log Ion logg~~~~~N(H+)N~o)IonIo log N(H+) Io N(H+) HeI 1A -0.95 NeV 1A -4.32 ArIV 3 -5.58 Hell 1A -1.13 NeV* 1A -4.04* ArV 1A -5.78 NII 1.0 -4.50 SII 2.1 -6.16 KIV 1A -7.24 OIl 1.0 -5.09 SIll 1A -5.70 KVI 1A -7.38 OII 1A -3.87 CliI 1A -7.51 CaV 1A -7.39 NeIlI 1A -4.59 Cliii 5.5 -7.36 FeIII 1A -7.14 NeIV 1.0 -3.87 ClIV 1A -6.85 FeVI* 1A -6.92* NeIV* 1.0 -4.40* ArIII 1A -6.08 FeVII* 1A -7.15* 1A denotes that for these ions an approximate value of x, here chosen as 1.0, is satisfactory because, for the-density range in this nebula, N(ion)/N(H+) is not sensitive to x. * T, = 20,000 K is assumed for these entries. Downloaded by guest on September 29, 2021 Astronomy: Aller and Czyzak Proc. Natl. Acad. Sci. USA 75 (1978) 3

Table 3. Adopted composition of NGC 6302 Drs. David Jenner and Harland Epps for their indispensable aid in securing observations with the image-tube scanner, and to Douglas logN Loeffler for his assistance in reductions of the Mt. Wilson photoelectric *High-excitation data. The program was supported in part by National Science Foun- dation Grant AST 76-21457 to the University of California, Los An- Element NGC6302 planetaries Sun geles, and National Science Foundation Grant AST 76-15727 to Ohio H 12.00 12.00 12.00 State University. He 11.27 11.01 10.8: N 8.5: 8.00 7.94 1. Minkowski, R. & Johnson, H. M. (1967) Astrophys. J. 148, 0 8.70 8.69 8.84 659-662. Ne 7.96 8.10 7.57 2. Evans, D. S. (1959) Mon. Not. R. Astron. Soc. 119, 150-156. S 7.4: 7.35 7.2 3. Oliver, J. P. & Aller, L. H. (1969) Astrophys. J. 157,601-605. Cl 5.33 5.26 5.5: 4. Danziger, I. J., Frogel, J. A. & Persson, S. E. (1973) Astrophys. Ar 6.71 6.4 6.0 J. 184, L29-L32. K 5.2 4.98 5.16 5. Milne, D. K. & Aller, L. H. (1975) Astron. Astrophys. 38, 183- Ca 4.6 5.24 6.35 196. Fe 6.0 7.50 6. Higgs, L. A. (1971) Publ. Astrophys. Branch. Natl. Res. Counc. Canada 1, 1. Chemical composition 7. Oke, J. B. (1964) Astrophys. J. 140, 689-693. 8. Hayes, D. S. (1970) Astrophys. J. 159, 165-172. Table 3 gives the adopted chemical composition for NGC 6302 9. Hayes, D. S. & Latham, D. W. (1975) Astrophys. J. 197,589- compared with mean values derived for 5 high-excitation 592. planetaries (25) and the sun (26). The greatest difficulty lies in 10. Stone, R. (1974) Astrophys. J. 193, 135-137. extrapolating ionic abundances to total elemental abundances. 11. Walker, M. F. & Aller, L. H. (1970) Astrophys. J. 161, 917- The Peimbert-Costero procedure (27) that seems to work well 944. 12. Aller, L. H. (1956) Gaseous Nebulae (Chapman and Hall, Lon- for low-excitation nebulae appears to fail for high-excitation, don). extremely inhomogeneous objects. Our extrapolation for ni- 13. Menzel, D. H. (ed.) (1962) Physical Processes in Ionized Plasmas trogen and sulfur is guided by results suggested by models for (Dover, New York). other inhomogeneous'nebulae (25). For chlorine and argon it 14. Seaton, M. J. (1960) Rep. Prog. Phys. 23, 313-333. is assumed that the observed ions contribute the bulk of the 15. Seaton, M. J. (1975) Mon. Not. R. Astron. Soc. 170, 475-486. atoms present. For neon and iron we have assumed that highly 16. Aller, L. H. & Liller, W. (1968) in Stars and Stellar Systems, eds. ionized atoms are excited in the TE = 20,000 K zone. Also we Middlehurst, B. & Aller, L. H. (University of Chicago Press, have used the deduced ratios of N(Ne3+) and N(Ne4.) to Chicago), Vol 7, pp. 483-574. N(Ne2+) as a guide in extrapolating the oxygen abundance from 17. Krueger, T., Czyzak, S. J. & Aller, L. H. (1970) Astrophys. J. 160, N(02+). For iron, it has been necessary to estimate the contri- 921-927. 18. Saraph, H. & Seaton, M. J. (1970) Mon. Not. R. Astron. Soc. 148, bution of unobserved ionization stages from a model for NGC 367-381. 7027 given by Shields (28). 19. Osterbrock, D. E. (1973) Astrophysics of Gaseous Nebulae, In spite of the considerable uncertainities involved, several (Freeman, San Francisco, CA). firm results seem to emerge from the analysis. The He/H ratio 20. Brocklehurst, M. (1971) Mon. Not. R. Astron. Soc. 153, 471- is greater than in normal planetaries and is in good accord with 490. the value obtained by Danziger et al. (4). Although the nitrogen 21. Brocklehurst, M. (1972) Mon. Not. R. Astron. Soc. 157, 211- abundance is uncertain because only a small fraction of its atoms 227. are in the N+ stage and the extrapolation procedure is accord- 22. Clarke, W. (1965) Ph.D. Dissertation, University of California, ingly unsatisfactory, there can be no question that nitrogen is Los Angeles. 23. Nussbaumer, H. & Osterbrock, D. (1970) Astrophys. J. 161, overabundant. Abundances of 0, Ne, S, Cl, Ar, and K are in 811-820. good accord with results obtained from other high-excitation 24. Garstang, R. H., Robb, W. D. & Rountree, S. P. (1978) Astrophys. nebulae. Neon appears to be more abundant than in the sun, J., in press. while oxygen may be slightly less abundant. The under- 25. Aller, L. H. (1977) in Planetary Nebulae, ed. Terzian, Y., Inter- abundance of iron, which is similar to that found by Shields for national Astronomical Union Symposium, no. 76 (Reidel, Dor- NGC 7027 and several other planetaries (28, 29) is striking. drecht, Netherlands). Shields suggested that iron tends to be tied up in solid grains. 26. Ross, J. E. & Aller, L. H. (1976) Science, 191, 1223-1227. Likewise, calcium is probably underabundant, as has been 27. Peimbert, M. & Costero, R. (1969) Boletin Observatorio To- found for a number of nanzintla y Tacubaya, 5,3. planetaries (25). Again, retention of this 28. Shields, G. Astrophys. J., in press. element in solid grains is likely to be the explanation. 29. Shields, G. (1975) Astrophys. J. 195, 475-478. Peimbert (30) has assigned NGC 6302 to his composition 30. Peimbert, M. (1977) in Planetary Nebulae, ed. Terzian, Y., In- Type I nebulae, which have high helium and nitrogen abun- ternational Astronomical Union Symposium, no. 76 (Reidel, dances and pronounced filamentary structure. They include Dordrecht, Netherlands). NGC 6445 (31), NGC 2440 (32) (86 -8°1) (33), and NGC 2818, 31. Aller, L. H., Czyzak, S. J., Craine, E. & Kaler, J. B. (1973) As- which has recently been investigated by Dufour and Hack (34). trophys. J. 182, 509-515 Improvements in model calculations are needed for these 32. Kaler, J. B. & Aller, L. H. (1974) Publ. Astron. Soc. Pac. 86, complex objects. 635-638 33. Kaler, J. B. (1975) Astrophys. J. 188, L15-17' We thank Director Babcock and the staff of Hale Observatories for 34. Dufour, R. & Hack, E. C. (1977) in Planetary Nebulae, ed. the opportunity to use the Mt. Wilson reflectors to secure photoelectric Terzian, Y., International Astronomical Union Symposium, no. measurements of the stronger lines in NGC 6302. We are indebted to 76 (Reidel, Dordrecht, Netherlands). Downloaded by guest on September 29, 2021