thesis - IRS, The Infrared Spectrograph

6.11. General discussion 135Figure 6.11–. Sum of nebular masses versus the core mass.may have been exposed for a shorter period to the FUV flux.The relatively large ionized mass in NGC 3918 (Fig. 6.10, Table 6.10) is expectedbecause from the HR diagram (Fig. 6.1) it is the most evolved object in our sample. On theother hand, NGC 6543 show also a large fraction of ionized mass yet it seems to be in anearly phase of PN evolution. Observations of the molecular gas masses for these objects maybe an interesting test since we expect a low molecular mass for NCG 3918 and a large onefor NGC 6543.Fig.6.11 represents the sum of the three masses with respect to the core mass inferredfrom the HR diagram (Fig. 6.1). For the objects lying below the evolutionary tracks in theHR diagram the lowest core mass (0.57 M ⊙ ) was adopted. The vertical arrows for NGC 3918and NGC 6543 indicate that the sum for these objects is a lower limit since no molecularmass was measured. This plot should be used with caution because the parameters plottedstrongly depend on the distance. In view of the large uncertainties associated with the quantitiesshown, there is an uncanny good relationship between the total gas mass and the coremass of the PNe. Part of the spread in this relationship may actually reflect the uncertaintiesin the adopted distances which has the tendency to spread the points perpendicular to therelationship. This is illustrated for NGC 7027 (number 1) where the arrows indicate an errorin the distance of a factor of 1.5. This nebula has been thoroughly studied in the literatureand its distance is one of the better determined (probably an error lower than 1.5), but ithelps to realize how inaccurate the other objects might be. We therefore do not venture toconclude anything firmly from it but we believe that the discussion and suggestions given areworthwhile.The core mass is related to the progenitor mass. Larger core masses imply largerprogenitor masses, and more material is then ejected. This is the trend displayed in Fig. 6.11.We note that NGC 6302, which is believed to descend from a high mass progenitor (M> 4.5