The high-excitation planetary nebula NGC 6741

Abstract

The compact, bright Peimbert type I planetary nebula NGC 6741 shows an unusually rich spectrum which is studied here from 3700 to 10 370 Å with the Hamilton Echelle spectrograph at the Lick Observatory. By combining Hamilton Echelle observations with archive UV data secured with the International Ultraviolet Explorer (IUE), and with available IR data, we can obtain improved diagnostics and chemical composition of this high-excitation planetary nebula. The diagnostic diagram gives Tϵ = 12 500 K, Nϵ= 6300 cm−3 for most ions. An improved ‘composite’ theoretical nebular model is employed. The stellar temperature quoted in the earlier literature seems too high; the highest likely temperature appears to be 140 000 K. Elemental abundances are estimated from ionic concentrations, using an ionization correction factor in the usual way, and also from a theoretical model. These different methods show large discordances, implying that one must use great caution in choosing Tϵ for the determination of the ionic concentrations. For every 10 000 H atoms, there appear to be 8.0 atoms of C, 2.4 of N, 5.4 of 0,1.3 of Ne, 0.068 of S, 0.0023 of Cl and 0.035 of Ar. NGC 6741 is rich in He, C and N, compared with the solar abundance. O is probably depleted by ‘hot bottom’ burning. NGC 6741 may be also depleted in metals like Ca, Mg, Si and Fe, which may be tied up in grains. The progenitor star may have been an object in which C, N and Ne had essentially solar abundances, while heavier elements were less plentiful than in the Sun.