thesis - IRS, The Infrared Spectrograph

2.8. Conclusions 29Figure 2.4–. The logarithm of N/O ratio versus He/H. Abundance ratios by number, expected in PN’staken from the semi–analytical models of Marigo et al. (1996) (metallicity of Z=0.008) are shown. Thenumbers in the figure indicate the mass of the progenitor star in solar units. The position of NCG 7027is plotted.with IP is found, although a tendency of density to decrease at high IP ion seems to be present(Table 2.5 and Fig. 2.2). The relative abundances for 14 ions have been determined and arepresented in Table 2.6. By comparing these abundances with the semi–analytical model ofMarigo et al. (1996) at a metallicity Z=0.008 we have constrained the mass of the progenitorstar to be between 3–4 solar masses. However, the (C+N+O)/H ratio found in NGC 7027 issimilar to the solar value (indicating a solar mass progenitor). This discrepancy suggests thatit is too early to draw definite conclusions as to the progenitor mass.AcknowledgementsWe are grateful to Peter van Hoof for his help with the atomic parameter data (especially the effectivecollision strength) used in our calculations.