Measuring the orbital inclination of Z Andromedae from Rayleigh scattering

Astronomical Institute, Slovak Academy of Sciences, 059 60 Tatranská Lomnica, Slovakia
e-mail: skopal@astro.sk

Received: 2 June 2011
Accepted: 10 September 2012

Abstract

Context. The orbital inclination of the symbiotic prototype Z And has not been established yet. At present, two very different values are considered, i ~ 44° and i ≳ 73°. The correct value of i is a key parameter in, for example, modeling the highly-collimated jets of Z And.

Aims. To measure the orbital inclination of Z And.

Methods. First, we derive the hydrogen column density (n_H), which causes the Rayleigh scattering of the far-UV spectrum at the orbital phase φ = 0.961 ± 0.018. Second, we calculate n_H as a function of i and φ for the ionization structure during the quiescent phase. Third, we compare the n_H(i,φ) models with the observed value.

Results. The most probable shaping of the H i/H ii boundaries and the uncertainties in the orbital phase limit i of Z And to 59°−2°/+3°. Systematic errors given by using different wind velocity laws can increase i up to  ~74°. A high value of i is supported independently by the orbitally related variation in the far-UV continuum and the obscuration of the O i] λ1641 Å emission line around the inferior conjunction of the giant.

Conclusions. The derived value of the inclination of the Z And orbital plane allows treating satellite components of Hα and Hβ emission lines as highly-collimated jets.