Abstract
We present constraints on the space densities of non-magnetic and magnetic cataclysmic variables, and discuss some implications for models of the evolution of CVs. The high predicted non-magnetic CV space density is only consistent with observations if the majority of these systems are extremely faint in X-rays. The data cannot rule out the very simple model where long-period IPs evolve into polars and account for the entire short-period polar population. The fraction of<br />WDs that are strongly magnetic is not significantly higher for CV primaries than for isolated WDs. Finally, the space density of IPs is high enough to explain the bright, hard X-ray source population seen in the Galactic Centre.
Highlights
There are many remaining uncertainties in the theory of cataclysmic variable (CV) formation and evolution, as well as several serious discrepancies between predictions and the properties of the observed CV population (e.g. Patterson 1998; Pretorius, Knigge & Kolb 2007a; Pretorius & Knigge 2008a,b; Knigge, Baraffe & Patterson 2011)
A fundamental parameter predicted by evolution theory, that should be more measured than most properties of the intrinsic CV population, is the space density, ρ
For the 2 classes of mCVs, we find 3+−21 × 10−7 pc−3 for IPs and 5+−32 × 10−7 pc−3 for polars
Summary
There are many remaining uncertainties in the theory of cataclysmic variable (CV) formation and evolution, as well as several serious discrepancies between predictions and the properties of the observed CV population (e.g. Patterson 1998; Pretorius, Knigge & Kolb 2007a; Pretorius & Knigge 2008a,b; Knigge, Baraffe & Patterson 2011). There are many remaining uncertainties in the theory of cataclysmic variable (CV) formation and evolution, as well as several serious discrepancies between predictions and the properties of the observed CV population In order to constrain evolution models, we require more precise observational constraints on the properties of the Galactic CV populations. A fundamental parameter predicted by evolution theory, that should be more measured than most properties of the intrinsic CV population, is the space density, ρ. (i) Is the large predicted population of non-magnetic CVs at short orbital period consistent with the current observed CV sample?. These questions can be addressed empirically, with reliable measurements of the space densities of the different populations of CVs. Uncertainty in ρ measurements is in part caused by statistical errors, resulting from uncertain distances and small number statistics.
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