Evidence for binarity in the bipolar planetary nebulae A 79, He 2-428 and M 1-91 ^*

¹ Instituto Nacional de Astrofísica, Óptica y Electrónica INAOE, Apdo Postal 51 y 216, 72000 Puebla, Pue., México
² Isaac Newton Group of Telescopes, Apartado de Correos 321, 38700 Santa Cruz de La Palma, Canarias, Spain e-mail: rcorradi@ing.iac.es
³ Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Canarias, Spain e-mail: amr@ll.iac.es

Corresponding author: M. Rodríguez, mrodri@inaoep.mx

Received: 9 July 2001
Accepted: 6 August 2001

Abstract

We present low and high resolution long-slit spectra of three bipolar planetary nebulae (PNe) with bright central cores: A 79, He 2-428 and M 1-91. He 2-428 and M 1-91 have high density (from 10^3.3 to 10^6.5 cm^-3) unresolved nebular cores that indicate that strong mass loss/exchange phenomena are occurring close to their central stars. An F0 star is found at the centre of symmetry of A 79; its reddening and distance are consistent with the association of the star with the nebula. The spectrum of the core of He 2-428 shows indications of the presence of a hot star with red excess emission, probably arising in a late-type companion. A 79 is one of the richest PNe in and , the abundances of M 1-91 are at the lower end of the range spanned by bipolar PNe, and He 2-428 shows very low abundances, similar to those measured for halo PNe. The extended nebulae of A 79 and He 2-428 have inclined equatorial rings expanding at a velocity of ~15 km s^-1, with kinematical ages ≥10⁴ yrs. The association of these aged, extended nebulae with a dense nebular core (He 2-428) or a relatively late type star (A 79) is interpreted as evidence for the binarity of their nuclei.