International Ultraviolet Explorer Satellite Observations of Seven High-Excitation Planetary Nebulae (Gaseous Nebulae/Chemical Compositions/Excitation) L

Proc. Natl. Acad. Sci. USA Vol. 77, No. 3, pp. 1231-1234, March 1980 Astronomy

International Ultraviolet Explorer satellite observations of seven high-excitation planetary nebulae (gaseous nebulae/chemical compositions/excitation) L. H. ALLER AND C. D. KEYES Department of Astronomy, University of California, Los Angeles, California 90024 Contributed by Lawrence H. Aller, December 7, 1979

ABSIRACT Observations of seven hig-xcitation planetary Table 1. Basic data for observed nebulae nebulae secured with the International Ultraviolet Explorer (1UE) satellite were combined with extensive ground-based data P-Kt log HeII(4686)§ to obtain electron densities, gas kinetic temperatures, and ionic Nebula* notation F(H#)t I(H/3) C1 r"'' concentrations. We then employed a network of theoretical model nebulae to estimate the factors by which observed ionic NGC2392 197 + 1701 -10.39 0.35 0.15 22.4 concentrations must be multiplied to obtain elemental abun- NGC 2440 234 + 201 -10.45 0.60 0.63 16.4 dances. Comparison with a large sample of nebulae for which NGC 2867 278- 501 -10.57 0.265 0.47 8.6 extensive ground-based observations have been obtained shows Me 2-1 342 + 2701 -11.32 0.82 0.29 3.3 nitrogen to be markedly enhanced in some of these objects. NGC 6302 349 + 101 -10.53 0.76 1.41 22.3 Possibly most, if not all, high-excitation nebulae evolve from NGC 6741 33 - 201 -11.49 0.45 1.5: 3.9 stars that have higher masses than progenitors of nebulae of low-to-moderate excitation. NGC 6886 60- 702 -11.50 0.45 1.1: 3.0 * Conventional designation of object. It has long been recognized that an exploration of the ultraviolet f Perek-Kohoutek (20). spectra of gaseous nebulae would yield rich dividends in our 1 Logarithm of H/3 flux as received at earth. understanding of these objects. Early predictions (1-3) sug- § HeII A4686/H/3. gested the importance not only of HeI and HeII recombination C = log IO(HflVF(Hf3). lines, but also of lines of CII, CIII, CIV, NIhI, NV, OIl, 0111, Radius in arc sec. OIV, OV, MgII, SiII, SiIII, and SiIV. Extensive observations (4-8) have confirmed the wealth of information obtainable tinction by comparing the HeIl X1640 (Balmer a) with A4686 from this spectral region. (Paschen a) as suggested by Seaton (22). Such a procedure was In particular, abundances of carbon and nitrogen whose possible for NGC 2867, NGC 6741, NGC 6886, and to a good nuclides are of great diagnostic importance in assessing possible approximation for NGC 2440 (where most of the radiation is element-building scenarios of precursor stars are obtainable concentrated in two knots). In NGC 2392 and NGC 6302 the reliably only with access to ultraviolet spectral data. For ex- entrance covered of In ample, CII X4267 has been interpreted regularly as a recom- aperture only a portion the nebula. NGC bination line for the purpose of deriving concentration ratios 6302 one could center the brightest portion of the image, but N(C2+)/N(H+) (9-13). The line is usually weak, and the un- in NGC 2392 it was necessary to avoid the central star. Refer- certainty is further exacerbated by the difficulty in extrapo- ence to isophotic contours proved only of limited usefulness. lating from N(C2+)/N(H+) to N(C)/N(H). The total nitrogen Hence, the ultraviolet intensities can be tied to the visual region abundance is usually extrapolated from N(N+)/N(H+), because intensity system only approximately. only [NII] emission is regularly measured. Perinotto (14) finds Studies of extinction in the ultraviolet have been made by that, even in low-excitation HII regions where conditions should Seaton (23), by Pottasch et al. (24), and by Nandy et al. (25). be most favorable for extrapolation, the uncertainty is about The finally adopted extinction function is that of Seaton, which a factor of 2. Extrapolation procedures for N appear to work agrees reasonably well with results of the other observers. well for NGC 6720 (15, 16), but poorly for NGC 7009 (17). Table 2 gives the finally adopted line intensities. Some With the International Ultraviolet Explorer (IUE) satellite emissions-e.g., NV 1239, 1241; CIV 1548, 1550; and 0111 we observed seven high-excitation planetaries in the wavelength 1661, 1666-consist of close pairs. All data are corrected for regions X1200-X1920 A and X1900-X3250 A with the "short" interstellar extinction; zero-point errors may occur for NGC camera, which gives the lower of the two possible dispersions. 2392 and NGC 6302. All intensities are provisional and await The instrument and its operation have been described by definitive calibration and reassessment by the National Aero- Boggess and his associates (18, 19). Table 1 gives some basic data nautics and Space Administration; they appear to be sufficiently for the nebulae observed. Data in columns 3 and 6 are from a accurate for our present purposes (presumably ±10-20%) ex- compilation by Milne and Aller (21). The intensity ratio cept perhaps for NGC 6741 and NGC 6886. For those objects I(4686)/I(Hf3) comes from ground-based measurements se- the effects of small total fluxes and severe interstellar extinction cured at Lick Observatory and with the Anglo-Australian meant that measured fluxes were low and reduction errors were telescope (NGC 2867 and NGC 6302). exacerbated. The extinction constant C was obtained whenever possible from a comparison of IUE and ground-based observations. F(H/) is the flux in ergs cm-2 so1 in the HO3 line received at the Plasma diagnostics and analysis of line intensities earth, corrected for atmospheric extinction but not for obscu- Extensive ground-based observations (26, 27) of line intensities, ration by interstellar smog. Io(HO) is the flux corrected for the including recently obtained further data, enable us to solve for latter. nebular diagnostics, electron density NE and temperature TE, For small nebulae in which the entire image falls in the entrance aperture of the IUE, we may estimate interstellar ex- Abbreviation: IUE, International Ultraviolet Explorer. 1231 Downloaded by guest on September 29, 2021 1232 Astronomy: Aller and Keyes Proc. Natl. Acad. Sci. USA 77 (1980) Table 2. Line intensities corrected for extinction log I/II[Hf + 2.0 NGC NGC NGC NGC NGC NGC Ion X, A 2392 2440 2867 Me 2-1 6302 6741 6886 NV 1239/41 1.28 2.11 0.74 1.57 3.18 CII 1335 1.20 1.15 1.23 SiIV 1391 1.65 1.21 0.47 1.25 1.44 OIV 1403, 1409 1 [NeV] 1575 0.54 NIV 1487 1.73 2.32 1.07 1.66 2.98 2.5: CIV 1548/50 2.14 2.74 2.36 3.09 2.92 2.96: 3.0: Hell 1640 2.37 2.61 2.31 2.75 2.72 2.48: 2.44: 0111 1661/66 1.84 1.68 1.26 1.7 2.28 NIV, SiII 1718 1.94 NIII 1747 1.93 2.13 0.92 1.40 2.79 2.35: 2.05: Neill, Sil 1817 0.87 1.67 SilIlI 1892 1.77 1.79 CIII 1906, 1909 2.35 2.90 2.80 2.88 2.82 3.13: 3.0: OIIl 2326/8 2.29 1.88 1.98 1.62 [NeIV] 2422 1.84 2.14 1.39 2.38 2.55 2.8: 2.5: OIl 2462 1.3 0.9 1.64 Hell 2508 1.29 0.91 1.29 1.10 Hell 2734 0.87 1.06 0.79 1.26 1.16 [ArV] 2781 0.87 1.18 MgII 2800 0.77 0.96 2.18: 1.2: HeI 2834 0.84 0.74 0.89 GIll 3021 0.32 0.78 0III 3046 0.66 0.77 1.05 1.21 1.13 OIV 3133 1.33 1.73 1.63 2.07 1.95 1.88 Hel 3188 0.82 Hell 3203 1.19 1.30 1.04 1.55 1.50 :, Uncertain measurement.

and ionic concentrations n(N+), n(0+), n(02+), n(Ne2+), n(2D3/2), n(2D5/2), etc., expressed in units of nf(4S,3/2), and ob- n(Ne3+), n(Ne4+), n(S+), n(S2+), n(CI+), n(Cl2+), n(CI3+), tain formulae for n(NeS+)/n(H+): n(Ar2+), n(Ar3+), and n(Ar4+) in terms of H-ion concentration, n(Ne3 ) = 8.757 X 103.844/t assessment of the leads to the con- 0-6 E04,2 It X-0.025 n(H+). Careful diagnostics n(H+) clusion that different ions originate in strata of different tem-

perature and density, an interpretation that is substantiated by {I(4724) + I(4725)} theoretical models (28-30) of high-excitation nebulae. Table X [1] 3 gives temperature and density parameters for individual for log x <-0.25 and nebulae; the vatious ions are placed in four groups. Tempera- tures are assigned on the basis of diagnostic lines of [011I], [OIl], n(H+N ) = 7.877 X 10-6 E04,2 103.844/t X0211 [NII], etc., as far as possible; otherwise, we use insights provided by the theoretical models. Uncertainties in these temperatures cause errors in the derived abundances. The the largest density X {I(4724) + I(4725)1 [2] parameter x is found from lines of [011], [SH1], and [0Ill]. In principle, one could use the [NeIVI nebular X2422 tran- for log x > -0.25. Here E04,2 is a factor of the order of unity sition ratio also, as indicated by Lutz and Seaton (8). The UCLA that depends on temperature (34); x and t are Seaton's pa- Image-Tube Scanner Program has emphasized the measure- rameters: ment the auroral transitions X4724.2 and X4725.6. of [NeIV] t = TJL10,000 x = 10-4 Ne/i/i. [3] If one employs collisional cross sections of Saraph et al. (31) and A values of Garstang (32), it is possible to solve equations of Eqs. 1 and 2 have been used to estimate concentrations of Ne3+ statistical equilibrium (33), tabulate occupation numbers that are extremely sensitive to the assumed electron tempera-

Table 3. Temperature (t) and density (x) parameters for individual nebulae NGC NGC NGC NGC NGC NGC Ion 2392 2440 2867 Me 2-1 6302 6741 6886 group t x t x t x t x t x t x t x 1 1.0; 0.3 1.0; 0.3 1.11; 0.25 1.2; 0.125 1.6; 0.56 0.95; 0.7 1.16; 0.56 2 1.31; 0.3 1.38 1.11 1.32; NeIV 1.6 1.15 1.26 3 1.4; 0.3 1.45 1.27 1.4 1.6 1.3 1.35 4 1.4; 0.3 1.45 1.35 1.47 1.8 NeIV 1.3 1.5; NeIV x is defined by Eq. 3. Groups of ions: 1 = NIl, SII, C1II; 2 = CIII, NIII, OIl, OIII, NeIII, H; 3 = CIV, NIV, OIV, NeIV; 4 = NV, NeV. Downloaded by guest on September 29, 2021 Astronomy: Aller and Keyes Proc. Natl. Acad. Sci. USA 77 (1980) 1233 Table 4. Parameters for ionic concentrations Also, Ion A log A b = 8.62 q12 10-6 2 [10] CII 1335 -6.820 4.677 p e-X12/kTE CIII 1906, 08 -7.105 3.276 where Q12 is the collision strength (see refs. 28 and 35 for nu- CIV 1548, 50 -7.683 4.032 merical values) and gi is the statistical weight of the lower NIII 1747 -6.490 3.573 level. NIV 1487 -6.975 4.203 NV 1329, 41 -7.665 5.04 N(Xi) 0III 1661/66 -6.193 3.75 N(H+) OIV 1403/09 -6.456 4.446 MgII 2800 -7.721 2.231 5.841 10-7 SiIII 1892 -7.404 3.301 ) X X; L exp(xl2/kTe)EO4,2Vr I(HN SiIV 1391 blend -8.020 4.491 Ai [11] = 10b/t E04,2 ture. The intensity of the nebular-type X2422 doublet transition Table 4 gives the necessary parameters for I(H/3.the principal can in principle be employed together with the 4724, 4726 pair lines. to get a functional relation between x and t, namely: For the forbidden lines falling in the "optical" range for ionic I(4726) + 0.886 n(2P1/2) [4] X3000-X8600 A, appropriate expressions calculating 7.86 n(3P3/2) = fn(x,t); concentrations are easily obtained but often are more compli- I(2422) n(2D3/2) + 0.1053 n(2D5/2) cated because transition probabilities are frequently low and collisional deexcitation of metastable levels becomes signifi- also cant. Once the ionic concentrations are obtained, the next step is N(Ne3+) _____ 57/tE(4 to estimate the total elemental abundances. Even when the IUE -(+) = 2.179 x lo8 4,2-%/ 102.575/t E(2442) 5 N(H+) 2.79 iH12 ) [5] ultraviolet range is included in addition to the "optical" range, not all significant ionic stages of all ions are represented. It is where necessary to extrapolate to include unobserved ionization stages. We have done this with the aid of a network of theoretical = [n(2D3/2) + 0.1053 n(2D5/2)]x'-. [6] models (29). The observed ratios, n(He+)/n(He2+), I(3727)/ These formulae need to be modified when the basic atomic I(5007), I(3426)/I(3868)-i.e., [NeV]/[NeIII]-serve to define parameters are revised. There is some indication that a seemingly valid choice among models in which the principal I(4726)/I(2422) tends to be weaker than expected. adjustable parameters include the stellar energy flux, the In our calculations we have included the Na3+, K3+, K5+, and nebular chemical composition, the nebular mass, and den- Ca4+ ions in group 2; perhaps they should have been placed in sity. group 3; consequently, our estimates of these ionic abundances Table 5 summarizes the principal results. The C and N data and derived elemental abundances may be upper limits. for NGC 6741 and 6886 are uncertain because the IUE data for For the ultraviolet lines the equations of equilibrium are very these nebulae are less accurate than for the other objects. For simple. If the lower and upper levels are denoted by groups 1 the other elements the data should be satisfactory. These data and 2, respectively, then also involve extrapolation with the aid of models. Nj(Xs)Nfq2 = N2(Xi)EA2i, [71 Discussion where the summation is taken over all lower levels. In practical For these seven high-excitation planetaries it appears that Ne, examples 2A2i = A21. The emission per unit volume in X is Cl, Ar, and the metals Na, K, and Ca have mean abundances similar to those found in earlier surveys (36, 38). Extrapolation Ex(Xi) = N2(Xi)A21hv = Ni(Xj)Nfq12hv [8] factors for the metals are sometimes large, but there is no doubt for an ion Xi and in Hf is that Ca is depleted with respect to the sun. Our sulfur results are tentative; they are based only on [SII] X6717, 6730, and [SIII] E(HO) = N(H+)NE04,2 X 10-25. [9] (usually only X6312). Table 5. Logarithm of the elemental abundances on scale log N(H) = 12.00 log abundances NGC NGC NGC NGC NGC NGC Planetary Element 2392 2440 2867 Me 2-1 6302 6741 6886 Mean mean* Solart He 10.96 11.08 11.05 11.01 11.28 11.04 11.01 11.07 11.02 1.08 C 8.35 8.52 8.84 8.91 9.22 9.1: 8.9: 8.93 9.10 8.62 N 8.41 8.80 8.09 8.12 9.04 8.7: 8.8: 8.68 7.97 7.94 0 8.74 8.72 8.65 8.86 8.79 8.84 8.72 8.77 8.66 8.84 Ne 7.97 8.08 7.95 8.36 8.05 8.27 8.11 8.14 8.02 8.1 Na 6.22 6.32 6.23 6.28 S 7.06 6.98 7.73 7.3: 7.37 7.2: 7.24 7.34 6.97 7.2 Cl 5.11 5.38 5.19 5.20 5.50 5.34 5.36 5.32 5.26 5.5 An 6.09 6.44 6.25 6.39 6.83 6.63 6.49 6.50 6.38 6.0 K 4.59 4.58 5.17 5.25 4.68 5.10 4.98 4.90 5.16 Ca 4.95 4.81 4.95 5.07 5.40 4.93 5.07 5.10 6.35 * Mean for ;30 planetaries of low-to-high excitation (36). t Solar abundances were from ref. 37, except for neon for which we chose 8.1. Downloaded by guest on September 29, 2021 1234 Astronomy: Aller and Keyes Proc. Natl. Acad. Sci. USA 77 (1980)

The high-excitation objects considered here contain a number 9. Aller, L. H. & Menzel, D. H. (1975) Astrophys. J. 102, 239- of N-rich nebulae, essentially type I in Peimbert's classification 263. (39). NGC 6302 is perhaps the most extreme example. Dufour 10. Aller, L. H. (1971) Natl. Bur. Stand. Spec. Publ. 353, 161- 168. and Talent (40) have described an emission nebula in NGC 6822 11. Kaler, J. B. (1972) Astrophys. J. 173, 601-609. that also is very rich in N. Kaler and his associates (41, 42) have 12. Grandi, S. (1976) Astrophys. J. 206, 658-671. discussed the enrichment of N and He in planetary nebulae. 13. Torres-Peimbert, S. & Peimbert, M. (1977) Rev. Mex. Astron. Previous estimates of nitrogen enhancements have depended Astrophys. 2, 181-208. on extrapolations from n(N+) with attendant inaccuracies. The 14. Perinotto, M. (1977) Mem. Soc. Astron. Ital. 48, 745-750, 843. chief uncertainty here arises from possible errors in tempera- 15. Hawley, S. A. & Miller, J. E. (1977) Astrophys. J. 212,94. tures assigned to the NIV and NV zones. For NGC 2867 and Me 16. Aller, L. H. (1976) Publ. Astron. Soc. Pac. 88,574-584. 2-1 the N abundance may be "normal"; for NGC 2392, NGC 17. Czyzak, S. J. & Aller, L. H. (1979) Mon. Not. R. Astron. Soc. 188, 6741, and NGC 6886 some N enhancement is suggested, 229-240. whereas for NGC 2440 and NGC 6302 a substantial N increase 18. Boggess, A., Carr, F. A., Evans, D. C., Fischel, D., Freeman, H. is indicated. In these latter objects N may be more abundant R., Fueschel, C. F., Klinglesmith, D. A., Krueger, V. L., Longa- necker, G. W., Moore, J. V., Pyle, E. J., Rebar, F., Sizemore, K. than 0. Carbon is strongly enhanced in NGC 6302 and prob- O., Sparks, W., Underhill, A. B., Vitagliano, H. D., West, D. K., ably is increased in the other objects (except NGC 2392 and Macchetto, F., Fitton, B., Barker, P. J., Dunford, E., Gondhalekar, NGC 2440) as well. The Peimberts (13) have concluded that P. M., Hall, J. E., Harrison, V. A. W., Oliver, M. B., Sandford, M. quite generally carbon is enhanced with respect to oxygen in C. W., Vaughan, P. A., Ward, A. K., Anderson, B. E., Boksenberg, planetaries. Oxygen seems 'to remain constant within a factor A.,.Coleman, C. I., Snijders, M. A. J. & Wilson, R. (1978) Nature (London) 275, 372-377. of about 1.6; most of the objects seem slightly enhanced with 19. Boggess, A., Bohlin, R. C., Evans, D. C., Freeman, H. R., Gull, respect to the average planetary (36). T. R., Heap, S. R., Klinglesmith, D. A., Longanecker, G. R., Becker and Iben (43) have considered asymptotic giant Sparks, W., West, D. K., Holm, A. V., Perry, P. M., Schiffer, F. branch evolution of stars of masses in the range 3-11 M>. H., III, Turnrose, B. E., Wu, C. C., Lane, A. L., Linsky, J. L., Products of nuclear processing are dredged up to the surface Savage, B. D., Benvenuti, P., Cassatella, A., Clavel, J., Heck, A., at a late evolutionary phase; 4He and 14N can be enhanced, Macchetto, F., Penston, M. 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