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

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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 en- trance 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).
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