Top panel: observed and predicted cumulative distribution of AM CVn stars as a function of distance. Predicted distributions are shown for Galactic disc models with three different exponential vertical scale heights: 100 pc (green), 300 pc (red) and 500 pc (blue). Given that AM CVn stars are thought to be preferentially old systems, the two larger scale heights are likely to be more appropriate for this population. All models have been normalised to the data at 100 pc. The clear discrepancy between models and data – which increases with distance – points to serious incompleteness in the observational sample. Bottom panel: space density (and 1σ error bands) as a function of distance, as estimated from the number of AM CVn stars and the effective volume contained within this distance. Effective volumes have been calculated for the same three Galactic disc models as shown in the top panel, thus providing three different space density estimates.
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