thesis - IRS, The Infrared Spectrograph

4.6. Physical conditions 61Table 4.9–. Electron temperature in K. For BD+30 3639 a N e= 11 000 cm −3 has been used, and forNGC 6543 N e= 5000 cm −3 .Ion IP Lines BD+30 3639 NGC 6543(eV) used (nm) Ratio T e Ratio T eN II 14.5 658.3/575.4 77.8 8400 69.4 9500N II 14.5 654.8/575.4 28.8 8800S III 23.3 18 700/631.2 20.5 8600 † 48.4 7800O III 35.1 500.7/436.3 387 7800Ne III 40.9 15 500/386.9 4.0 8000Ar III 27.6 8900/519.2 531 7500 †† Large error.4.6.1 Electron densityThree ions have been used to derive the electron density (N e ) for both nebulae and are shownin Table 4.8. An electron temperature of 8500 K (see Sect. 4.6.2) was used for BD+30 3639and T e =8200 K for NGC 6543.For BD+30 3639 the S III and O II ions give density values of 10 200 and 11 300 cm −3respectively. This is in agreement with the value of Aller & Hyung (1995) (10 000 cm −3 )and somewhat higher than the one found by Pottasch (1984). Keenan et al. (1999) calculatedemission–line–ratio diagrams in [O II] using R–matrix calculations of electron impactexcitation rates and obtain a somewhat lower density of N e = 8000 cm −3 . The line ratio forCl III gives a very high value. This ion must be formed in a denser region and could indicatethat clumps are present in the nebula. In order to determine the abundances an N e (11 000cm −3 ) average of these given by the S III and O II ions has been used, except for Cl III where48 000 cm −3 was used.In the case of NGC 6543 the N e found ranges from 3400 to 9300 cm −3 , although thelatter value has a high uncertainty. These three ions have the same IP and the variation inN e could be due to the presence of clump structures in the nebula. Images of the nebulaindeed reveal some structure. For the determination of the abundances an average density of5000 cm −3 from the S III and Cl III has been used which agrees with previous determinations(Hyung et al. 2000). The O II was avoided due to its larger uncertainty.4.6.2 Electron temperatureTo compute the electron temperature (T e ) lines ratios are needed originating from energylevels differing by several electron volts. For BD+30 3639 an electron density of 11 000cm −3 was assumed to derive the temperature and for NGC 6543 N e =5000 cm −3 (seeSect. 4.6.1). These values are listed in Table 4.9.S III and N II have been used to derive T e for BD+30 3639. Both values agree and averageout to 8500 K. This is in good agreement with previous determinations by Aller & Hyung(1995), Pottasch et al. (1986) and Keenan et al. (1999) who give an T e of 8800, 8000 and