On the late spectral types of cataclysmic variable secondaries

We investigate why the spectral type of most cataclysmic variable (CV) secondaries is significantly later than that of a zero-age main-sequence (ZAMS) star with the same mean density. Using improved stellar input physics, tested against observations of low-mass stars at the bottom of the main sequence, we calculate the secular evolution of CVs with low-mass donors. We consider sequences with different mass transfer rates and with a different degree of nuclear evolution of the donor prior to mass transfer.
Systems near the upper edge of the gap ( P ∼3–6 h) can be reproduced by models with a wide range of mass transfer rates from 1.5×10⁻⁹ M_⊙ yr⁻¹ to 10⁻⁸ M_⊙ yr⁻¹. Evolutionary sequences with a small transfer rate and donors that are substantially evolved off the ZAMS (central hydrogen content 0.05–0.5) reproduce CVs with late spectral types above P ≳6 h. Systems with the most discrepant (late) spectral type should have the smallest donor mass at any given P .
Consistency with the period gap suggests that the mass transfer rate increases with decreasing donor mass for evolved sequences above the period gap. In this case, a single-parameter family of sequences with varying X _c and increasing mass transfer rate reproduces the full range of observed spectral types. This would imply that CVs with such evolved secondaries dominate the CV population.     

Iron Kα linewidths in magnetic cataclysmic variables

Following a recent report that AO Psc has broad iron Kα emission lines we have looked at the ASCA spectra of 15 magnetic cataclysmic variables. We find that half of the systems have Kα lines broadened by ∼ 200 eV, while the remainder have narrow lines. We argue that the Doppler effect is insufficient to explain the finding and propose that the lines originate in accretion columns on the verge of optical thickness, where Compton scattering of resonantly trapped line photons broadens the profile. We suggest that the broadening is a valuable diagnostic of conditions in the accretion column.     

The influence of selection effects on the observed cataclysmic variable population: modelling and application to the Palomar–Green sample

Large differences between the properties of the known sample of cataclysmic variable stars (CVs) and the predictions of the theory of binary star evolution have long been recognized. However, because all existing CV samples suffer from strong selection effects, observational biases must be taken into account before it is possible to tell whether there is an inconsistency. In order to address this problem, we have modelled the impact of selection effects on observed CV samples using a Monte Carlo approach. By simulating the selection criteria of the Palomar–Green (PG) Survey, we show that selection effects cannot reconcile the predictions of standard CV evolution theory with the observed sample. More generally, we illustrate the effect of the biases that are introduced by magnitude limits, selection cuts in U − B and restrictions in Galactic latitude.     

The mass and radius of the M dwarf companion to GD 448

We present spectroscopy and photometry of GD 448, a detached white dwarf – M dwarf binary with a period of 2.47 h. We find that the Na  I  8200-Å feature is composed of narrow emission lines, owing to irradiation of the M dwarf by the white dwarf, within broad absorption lines that are essentially unaffected by heating. Combined with an improved spectroscopic orbit and gravitational redshift measurement from spectra of the Hα line, we are able to derive masses for the white dwarf and M dwarf directly (0.41 ± 0.01 and 0.096 ± 0.004 M_⊙, respectively). We use a simple model of the Ca II emission lines to establish the radius of the M dwarf assuming the emission from its surface to be proportional to the incident flux per unit area from the white dwarf. The radius derived is 0.125 ± 0.020 R_⊙. The M dwarf appears to be a normal main-sequence star in terms of its mass and radius, and is less than half the size of its Roche lobe. The thermal time-scale of the M dwarf is much longer than the cooling age of the white dwarf, so we conclude that the M dwarf was unaffected by the common-envelope phase. The anomalous width of the Hα emission from the M dwarf remains to be explained, but the strength of the line may be due to X-ray heating of the M dwarf owing to accretion on to the white dwarf from the M dwarf wind.     

The space density of cataclysmic variables: constraints from the ROSAT North Ecliptic Pole survey

We use the ROSAT North Ecliptic Pole (NEP) survey to construct a small, but purely X-ray flux-limited sample of cataclysmic variable stars (CVs). The sample includes only four systems, two of which (RX J1715.6+6856 and RX J1831.7+6511) are new discoveries. We present time-resolved spectroscopy of the new CVs and measure orbital periods of 1.64 ± 0.02 and 4.01 ± 0.03 h for RX J1715.6+6856 and RX J1831.7+6511, respectively. We also estimate distances for all the CVs in our sample, based mainly on their apparent brightness in the infrared. The space density of the CV population represented by our small sample is  1.1^+2.3_−0.7× 10⁻⁵ pc⁻³  . We can also place upper limits on the space density of any subpopulation of CVs too faint to be included in the NEP survey. In particular, we show that if the overall space density of CVs is as high as  2 × 10⁻⁴ pc⁻³  (as has been predicted theoretically), the vast majority of CVs must be fainter than   L _X≃ 2 × 10²⁹ erg s⁻¹  .     

Brown dwarfs and the cataclysmic variable period minimum

Using improved, up-to-date stellar input physics tested against observations of low-mass stars and brown dwarfs, we calculate the secular evolution of low-mass donor cataclysmic variables (CVs), including those that form with a brown-dwarf donor. Our models confirm the mismatch between the calculated minimum period ( P _min70 min) and the observed short-period cut-off (80 min) in the CV period histogram. We find that tidal and rotational corrections applied to the one-dimensional stellar structure equations have no significant effect on the period minimum. Theoretical period distributions synthesized from our model sequences always show an accumulation of systems at the minimum period, a feature absent from the observed distribution. We suggest that non-magnetic CVs become unobservable as they are effectively trapped in permanent quiescence before they reach P _min, and that small-number statistics may hide the period spike for magnetic CVs.     

The properties of cataclysmic variables in photometric Hα surveys

We report on the properties of 71 known cataclysmic variables (CVs) in photometric Hα emission-line surveys. Our study is motivated by the fact that the Isaac Newton Telescope (INT) Photometric Hα Survey of the northern galactic plane (IPHAS) will soon provide r ',  i ' and narrow-band Hα measurements down to   r '≃ 20  for all northern objects between  − 5° < b < +5°  . IPHAS thus provides a unique resource, both for studying the emission-line properties of known CVs and for constructing a new CV sample selected solely on the basis of Hα excess. Our goal here is to carry out the first task and prepare the way for the second. In order to achieve this, we analyse data on 19 CVs already contained in the IPHAS data base and supplement this with identical observations of 52 CVs outside the galactic plane.
Our key results are as follows: (i) the recovery rate of known CVs as Hα emitters in a survey like IPHAS is ≃70 per cent; (ii) of the ≃30 per cent of CVs which were not recovered ≃75 per cent were clearly detected but did not exhibit a significant Hα excess at the time of our observations; (iii) the recovery rate depends only weakly on CV type; (iv) the recovery rate depends only weakly on orbital period; (v) short-period dwarf novae tend to have the strongest Hα lines. These results imply that photometric emission-line searches provide an efficient way of constructing CV samples that are not biased against detection of intrinsically faint, short-period systems.     

SDSS unveils a population of intrinsically faint cataclysmic variables at the minimum orbital period

We discuss the properties of 137 cataclysmic variables (CVs) which are included in the Sloan Digital Sky Survey (SDSS) spectroscopic data base, and for which accurate orbital periods have been measured. 92 of these systems are new discoveries from SDSS and were followed-up in more detail over the past few years. 45 systems were previously identified as CVs because of the detection of optical outbursts and/or X-ray emission, and subsequently re-identified from the SDSS spectroscopy. The period distribution of the SDSS CVs differs dramatically from that of all the previously known CVs, in particular it contains a significant accumulation of systems in the orbital period range 80–86 min. We identify this feature as the elusive 'period minimum spike' predicted by CV population models, which resolves a long-standing discrepancy between compact binary evolution theory and observations. We show that this spike is almost entirely due to the large number of CVs with very low accretion activity identified by SDSS. The optical spectra of these systems are dominated by emission from the white dwarf photosphere, and display little or no spectroscopic signature from the donor stars, suggesting very low mass companion stars. We determine the average absolute magnitude of these low-luminosity CVs at the period minimum to be  〈 M_g 〉= 11.6 ± 0.7  . Comparison of the SDSS CV sample to the CVs found in the Hamburg Quasar Survey and the Palomar Green Survey suggests that the depth of SDSS is the key ingredient resulting in the discovery of a large number of intrinsically faint short-period systems.