THE HIGH-EXCITATION PLANETARY-NEBULA NGC-6741

The compact, bright Peimbert type I planetary nebula NGC 6741 shows an unusually rich spectrum which is studied here from 3700 to 10 370 Ang strom with the Hamilton Echelle spectrograph at the Lick Observatory. By combining Hamilton Echelle observations with archive UV data secure d with the International Ultraviolet Explorer (IUE), and with availabl e IR data, we can obtain improved diagnostics and chemical composition of this high-excitation planetary nebula. The diagnostic diagram give s T-epsilon = 12 500 K, N-epsilon = 6300 cm(-3) for most ions. An impr oved 'composite' theoretical nebular model is employed. The stellar te mperature 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 fa ctor in the usual way, and also from a theoretical model. These differ ent methods show large discordances, implying that one must use great caution in choosing T-epsilon for the determination of the ionic conce ntrations. For every 10 000 H atoms, there appear to be 8.0 atoms of C , 2.4 of N, 5.4 of O, 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 abundanc e. O is probably depleted by 'hot bottom' burning. NGC 6741 may be als o depleted in metals like Ca? Mg, Si and Fe, which map be tied up in g rains. The progenitor star may have been an object in which C, N and N e had essentially solar abundances, while heavier elements were less p lentiful than in the Sun.