Spectroscopy of the Eclipsing Variable AR Pavonis

Spectra of the remarkable symbiotic object AR Pavonis covering 20 years (12 eclipse periods) are analysed for radial velocities and line-intensities. The eclipsed body shows an emission O-type spectrum with H, He I (triplets, singlets) N III , He II 4686 emission having increasing concentration to the centre in that order. A mass function of 0.14 ⊙ is found. In the nebular spectrum the [O III ] (4959 + 5007)/4363 ratio is found to be 2.8 ± 0.3 indicating log Ne ∼ 7, i.e. a much higher density than planetaries and more comparable with Z And and AG Peg; the absence of [Ne V ], [Fe V ] to [Fe VII ] emission indicates a lower ionization temperature for AR Pav. The forbidden emission apparently varies with small velocity amplitude significantly out of phase with the primary source. The secondary is believed to be M3 III (possibly variable) whose TiO bands can be detected within 50 days of mid-eclipse; supporting evidence is provided by observations in J−L bands by Glass. The cF shell absorption, very strong during the 1954 egress when the variable was unusually bright, can be detected weakly through the cycle, possibly weaker with primary behind. Radial velocities suggest closer association with the primary than with the secondary. A model is proposed in which the M star is filling its Roche lobe and losing mass to the secondary through a stream which is mainly responsible for the cF absorption. Masses of order 2.5 and 1.2 ⊙ for the primary and secondary are regarded as most likely. Although the distance cannot be accurately determined the primary is estimated to have Mv ∼ −2.5 (mean) while the system reached Mv ∼ −4 at the 1954 outburst. The changing structure of the Balmer emission is explained as due to a hydrogen shell surrounding the system and expanding with velocity 22 km s −1 . The P Cyg structure seems to have changed permanently since Sahade's 1948 observations.