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Register a new userBlunting the Spike: the CV Minimum Period
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The standard picture of CV secular evolution predicts a spike in the CV distribution near the observed short-period cutoff P_0 ~ 78 min, which is not observed. We show that an intrinsic spread in minimum (`bounce) periods P_b resulting from a genuine difference in some parameter controlling the evolution can remove the spike without smearing the sharpness of the cutoff. The most probable second parameter is different admixtures of magnetic stellar wind braking (at up to 5 times the GR rate) in a small tail of systems, perhaps implying that the donor magnetic field strength at formation is a second parameter specifying CV evolution. We suggest that magnetic braking resumes below the gap with a wide range, being well below the GR rate in most CVs, but significantly above it in a small tail.
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We consider a recently-proposed alternative explanation of the CV period gap in terms of a revised mass-radius relation for the lower main sequence. We show that no such thermal-equilibrium relation is likely to produce a true gap. Using population synthesis techniques we calculate a model population that obeys the claimed equilibrium mass-radius relation. A theoretical period histogram obtained from this population shows two prominent period spikes rather than a gap. We consider also recent arguments suggesting that the period gap itself may not be real. We argue that, far from demonstrating a weakness of the interrupted-braking picture, the fact that most CV subtypes prefer one side of the gap or the other is actually an expected consequence of it.
We present time-resolved spectroscopy and photometry of CSS 120422:111127+571239 (= SBS1108+574), a recently discovered SU UMa-type dwarf nova whose 55-minute orbital period is well below the CV period minimum of ~78 minutes. In contrast with most other known CVs, its spectrum features He I emission of comparable strength to the Balmer lines, implying a hydrogen abundance less than 0.1 of long period CVs---but still at least 10 times higher than than in AM CVn stars. Together, the short orbital period and remarkable helium-to-hydrogen ratio suggest that mass transfer in CSS 1204 began near the end of the donor stars main-sequence lifetime, meaning that the system is probably an AM CVn progenitor as described by Podsiadlowski, Han, and Rappaport (2003). Moreover, a Doppler tomogram of the Halpha line reveals two distinct regions of enhanced emission. While one is the result of the stream-disk impact, the other is probably attributable to spiral disk structure generated when material in the outer disk achieves a 2:1 orbital resonance with respect to the donor.
We present simultaneous spectral and photometric observations of SDSS J123813.73-033933.0. From Ha radial velocity measurements we determined the orbital period of the system to be 0.05592+/-0.00002 days (80.53 min). The spectrum shows double Balmer emission lines flanked by strong, broad absorption, indicating a dominant contribution from the white dwarf. The photometric light curve shows complex variability. The system undergoes cyclic brightening up to 0.4 mag which are semi-periodical on short time scales with periods of the order of 7-12 hours. We also detect 40.25 min variability (~0.15 mag) in the light curve, that corresponds to half the orbital period. Its amplitude increases with the cyclic brightening of the system.
We present here results of an optical spectroscopic study of a new Cataclysmic Variable SDSS J001856.93+345444.3. We demonstrate that the most probable value of the orbital period of the system is Porb = 0.6051 pm 0.022 days (=14.5226 hours), based on the measurements of radial velocity of a complex of absorption features emanating from the K2-K4V type secondary component. However, the radial velocity measurements from the emission lines are best folded with the period Pem = 0.5743day (=13.78 hours). The gamma-velocity of the emission lines varies significantly from epoch to epoch. There is an underlying broader and weaker component to the emission lines, which we could not resolve. Based on the appearance of the emission lines, the presence of very strong He II lines and the moderate polarization detected by Dillon et al. (2008), we conclude that SDSS J0018+3454 is an asynchronous magnetic CV (Polar).
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.
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