A statistical analysis of the emission line ratios in cataclysmic variables

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Abstract
A statistical analysis of the emission lines in cataclysmic variables indicates that Hδ/Hβ and HeIλ4471/Hβ are strongly correlated with Hγ/Hβ and presumably with other high members of the Balmer series as well. The equivalent width of Hβ is independent of Hγ/Hβ, but has a weak dependence on orbital period, as indicated previously by Hessman. No correlation is found for the HeIIλ4686 emission line. A catalogue of all Hα, Hβ, Hγ, Hδ, HeIλ4471 and HeIIλ4686 emission lines, available from the literature, is presented. The Balmer and HeI correlations are compared with currently available LTE models. The results by Drake & Ulrich for a static layer of hydrogren at high density reproduce entirely the Balmer line ratio observations. LTE accretion disc models produce Hβ, Hγ and Hδ line ratios which lie along the observed relation, independently of input parameters like mass transfer, disc size, viscosity and helium abundance. The accretion disc calculations predict an anticorrelation of Hα/Hβ with Hγ/Hβ which is not supported by the observations. The opposite is true for the HeI line, i.e. the observed HeI/Hβ ratios are correlated with Hγ/Hβ, while the LTE counterparts are not.

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