thesis - IRS, The Infrared Spectrograph

120 CHAPTER 6: Physical Conditions in PDRs around Planetary NebulaeTable 6.6–. Values of the luminosity and distances according to the solid lines in Fig. 6.3.Legend L/d 2 Luminosity Distance(10 −10 Watt/m 2 ) (L ⊙ ) (pc)1000 200a 1005000 44710 000 6321000 408b 245000 91310 000 12911000 816c 6.05000 182510 000 25811000 1195d 2.85000 267310 000 37801000 1825e 1.25000 408210 000 57741000 3162f 0.45000 707110 000 10 000lines for comparison. This is calculated by substituting F IR by L/(4πd 2 ) in Eq.6.1, with Lthe luminosity and d the distance to the object. Different combinations of L and d 2 can yieldthe same G 0 and for clarity these are given in the Table 6.6. The observed infrared fluxes canalso be translated into distances using the adopted luminosities,d ir L= ( ) 1 2 (6.2)4 π F IRThese distances are given in Table 6.7. The distances inferred this way for NGC 7027,BD+30 3639, NGC 6302, and Mz 3 are in good agreement with those assumed in Table 6.2.The distance derived from the F IR flux for NGC 3918 is much larger than that derived fromthe extinction methods (Table 6.2). While, the extinction method is highly uncertain, we thinkthat the differences between these methods mainly reflect the pointing error in the SWS observation(see Sect. 6.2). For the nebulae, NGC 6153, Hb 5 and NGC 6543, there is a largediscrepancy between the distance derived from the observed F IR flux and the stellar luminosityderived using a He II Zanstra temperature. We note that all three nebulae also show alarge difference between the He and H Zanstra luminosities. If we adopt the luminosity usingthe H-Zanstra-temperature, we would have arrived at F IR -derived distances of 2.4, 1.7, and1.4 kpc for NGC 6153, Hb 5 and NGC6543, respectively; in very good agreement with thedistances given in Table 6.2. While we realize that the He Zanstra method should, in general,provide more reliable results, perhaps the H-Zanstra method is to be preferred for thesenebulae which display complex geometries.