The Space Density of Post-Period Minimum Cataclysmic Variables

TL;DR

This study searches for isolated white dwarfs undergoing eclipses by dark companions to estimate the space density of 'dead' cataclysmic variables, finding a very low upper limit consistent with theoretical predictions.

Contribution

It provides the first empirical upper limit on the space density of dead CVs by analyzing SDSS data and using Monte Carlo simulations to interpret null detection results.

Findings

No eclipsing white dwarf systems found in the sample.

Set an upper limit on dead CV space density at $ ho_0 \\lesssim 2 \\times 10^{-5}$ pc$^{-3}$.

Supports theoretical models predicting low prevalence of such systems.

Abstract

Binary evolution theory predicts that accreting white dwarfs with sub-stellar companions dominate the Galactic population of cataclysmic variables (CVs). In order to test these predictions, it is necessary to identify these systems, which may be difficult if the signatures of accretion become too weak to be detected. The only chance to identify such "dead" CVs is by exploiting their close binary nature. We have therefore searched the Sloan Digital Sky Survey (SDSS) Stripe 82 area for apparently isolated white dwarfs that undergo eclipses by a dark companion. We found no such eclipses in either the SDSS or Palomar Transient Factory data sets among our sample of 2264 photometrically selected white dwarf candidates within Stripe 82. This null result allows us to set a firm upper limit on the space density, $\rho_0$, of dead CVs. In order to determine this limit, we have used Monte-Carlo simulations to fold our selection criteria through a simple model of the Galactic CV distribution. Assuming a $T_{WD}=7,500$ K, the resulting 2$\sigma$ limit on the space density of dead CVs is $\rho_0 \lesssim 2 \times 10^{-5}$ pc$^{-3}$, where $T_{WD}$ is the typical effective temperature of the white dwarf in such systems.