Measuring the orbital inclination of Z Andromedae from Rayleigh scattering

Abstract

The orbital inclination of the symbiotic prototype Z And has not been established yet. At present, two very different values are considered, i ~ 44 degrees and i >~ 73 degrees. The correct value of i is a key parameter in, for example, modeling the highly-collimated jets of Z And. The aim of this paper is to measure the orbital inclination of Z And. First, we derive the hydrogen column density (nH), which causes the Rayleigh scattering of the far-UV spectrum at the orbital phase phi = 0.961 plus/minus 0.018. Second, we calculate nH as a function of i and phi for the ionization structure during the quiescent phase. Third, we compare the nH(i,phi) models with the observed value. The most probable shaping of the HI/HII boundaries and the uncertainties in the orbital phase limit i of Z And to 59 -2/+3 degrees. Systematic errors given by using different wind velocity laws can increase i up to ~74 degrees. A high value of i is supported independently by the orbitally related variation in the far-UV continuum and the obscuration of the OI] 1641 A emission line around the inferior conjunction of the giant. The derived value of the inclination of the Z And orbital plane allows treating satellite components of H-alpha and H-beta emission lines as highly-collimated jets.