Physical conditions and elemental abundances in the symbiotic novae V 1016 Cyg, HM SGE and HBV 475.

We have obtained optical, near-infrared and UV spectra of the symbiotic stars HM Sge, V 1016 Cyg and HBV 475. We present diagnostic diagrams which indicate that physical conditions vary strongly throughout the symbiotic nebulosities. In HM Sge and V 1016 Cyg we find a steep electron-density gradient covering more than an order of magnitude from the lowest to the highest observed ionization stages.

We discuss the formation of hydrogen and helium recombination lines in dense nebulae in order to obtain He abundances. We emphasize that Balmer self-absorption and collisional excitation in He I are important processes in symbiotic nebulae. Their inclusion leads to considerably lower He abundances than previously reported. We obtain He abundances in HM Sge, which are consistent with the solar value. Also in V1016 Cyg and HBV 475 no He overabundance is found although some problems concerning the H I and He I lines remain unsolved.

We determine the abundances of C, N, O and Ne in all three objects and additionally Si, Ar and Fe in HM Sge and V1016 Cyg. Compared to solar, nitrogen is enhanced by a factor of 10 in HM Sge and HBV 475 and a factor of 3.5 in V1016 Cyg. The other elements are compatible with solar abundances.

The overall abundance pattern found in these symbiotic stars differs markedly from the one observed in nova ejecta. The H and He mass fractions in both HM Sge and V1016 Cyg are 0.72 and 0.25, in contrast to the hydrogen mass fractions ≲0.53 observed in novae. We suggest that the material presently constituting these symbiotic nebulae has not undergone nova-processing.

The C, N, O abundances in V1016 Cyg are almost identical to the mean abundances observed in M and S giants. HM Sge shows the signs of a more advanced nuclear burning stage and can be interpreted as due to a wind of a highly evolved red giant. We also find no depletion of typical dust constituents like Si and Fe in the D-type symbiotics HM Sge and V1016 Cyg. We conclude that the dust observable in the IR is located outside the ionized nebulosity.

We suggest that symbiotic stars can be used to determine elemental abundances in red giants including Miras by means of nebular diagnostic tools. This may be particularly important for poorly known elemental abundances such as He.