Spectral analyses of late-type WC central stars of planetary nebulae.

Abstract

The optical spectra of the seven late-type Wolf-Rayet central stars NGC40 ([WC8]), BD+30 3639 ([WC9]), He2-99 ([WC9]), CPD-56 8032, He2-113, M4-18 and K2-16 (all [WC11]) are analyzed by means of spherically expanding model atmospheres. The NLTE simulations account for the elements helium, carbon and oxygen. As main results effective temperature T_*_, element abundances and final velocity of the wind v_infinity_ are determined for each star. Assuming distances or luminosities for the objects, also the stellar radii and the mass-loss rates can be fixed. The values of T_*_ and v_infinity_ are found to be correlated with the spectral subtype ([WC8]: 78kK, 1000km/s; [WC9]: 47-49kK, 700-900km/s; [WC11]: 30-32kK, 200-350km/s). The obtained carbon-to-helium ratios are about unity (by mass), and the oxygen abundances are between 4 and 10% (mass fractions). Stellar hydrogen abundances are below the limit of save detection in all cases (typically <=2% by mass); however in one star (He2-113) we find a weak indication for the presence of hydrogen. Furthermore the occurrence of nitrogen lines in two of the stars (He2-113, K2-16) is discussed qualitatively. The results of these analyses put empirical constraints for the evolutionary status of WC-type central stars as post-AGB objects and provide input for modeling their planetary nebulae.