Variation of central star masses in planetary nebulae with height above the Galactic plane

There are various reasons for suspecting that the progenitor masses of planetary nebulae (PNe) decline with height z above the Galactic plane. This, if true, would also imply a similar decrease in mean central star masses <M-CS>. We report here a further way in which such gradients may be determined. It will be shown that the distribution of planetary nebulae with respect to 5 GHz brightness temperature varies strongly with Galactic latitude. This variation is likely to arise from a change in the central star mass function N(M-CS). High latitude sources appear to have a steeply varying function N(M-CS), implying the presence of relatively few nebulae with high central star masses. By contrast, the low latitude sources have a much gentler fall-off in N(M-CS), implying a larger proportion of high M-CS nebulae. This is shown to imply significant gradients of mean mass <M-CS> with latitude b. We find that d<Mcs>\d/b(LOW)\ similar or equal to 2.0 x 10(-3) M-circle dot deg(-1) for nebulae having 1.0 <log(T-B/K)< 3.6, where \b(LOW)\ represents the lower limit latitude of the sources. This corresponds to a gradient d<M-CS>\d/z(LOW)\ similar or equal to 5.6x 10(-2) kpc(-1) for nebulae with heights \Z\ > \z(LOW)\, and where one adopts the statistical distances of Phillips (2002).