The observational properties of the ten known AM Her systems are reviewed. A multitude of components of continuum and line radiation from these objects are outlined. We discuss the important physical processes involved in the accretion flow and funnel shock, with emphasis on the properties of the emission line regions. Other topics include the properties of the white dwarf primaries and M dwarf secondaries, the maintenance of synchronous rotation, and the implications of “dumpings” in the known orbital periods (80–115 minutes) and magnetic field strengths (20-35 megagauss).

The AM Herculis Magnetic Variables

We present Spitzer photometry of six short-period polars, EF Eri, V347 Pav, VV Pup, V834 Cen, GG Leo, and MR Ser. We have combined the Spitzer IRAC (3.6-8.0 μm) data with the 2MASS JHK_s photometry to construct the SEDs of these systems from the near- to mid-IR (1.235-8 μm). We find that five out of the six polars have flux densities in the mid-IR that are substantially in excess of the values expected from the stellar components alone. We have modeled the observed SEDs with a combination of contributions from the white dwarf, secondary star, and either cyclotron emission or a cool, circumbinary dust disk to fill in the long-wavelength excess. We find that a circumbinary dust disk is the most likely cause of the 8 μm excess in all cases, but we have been unable to rule out the specific (but unlikely) case of completely optically thin cyclotron emission as the source of the observed 8 μm flux density. While both model components can generate enough flux at 8 μm, neither dust nor cyclotron emission alone can match the excess above the stellar components at all wavelengths. A model combining both cyclotron and dust contributions, possibly with some accretion-generated flux in the near-IR, is probably required, but our observed SEDs are not sufficiently well sampled to constrain such a complicated model. If the 8 μm flux density is caused by the presence of a circumbinary dust disk, then our estimates of the masses of these disks are many orders of magnitude below the mass required to affect CV evolution.

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Spitzer space telescope observations of magnetic cataclysmic variables: possibilities for the presence of dust in polars

We present VTL/ISAAC infrared spectroscopy of a sample of short-orbital-period cataclysmic variables that are candidates for harbouring substellar companions. We have detected the K I and Na I absorption lines of the companion star in VY Aqr. The overall spectral distribution in this system is best fitted with an M9.5 type dwarf spectrum, implying a distance of 100 ′ 10 pc. VY Aqr seems to fall far from the theoretical distribution of secondary star temperatures around the orbital period minimum. Fitting of the IR spectral energy distribution (SED) was performed by comparing the observed spectrum with late-type templates. The application of such a spectral fitting procedure suggests that the continuum shape in the 1.1-2.5 μm spectral region in short-orbital-period cataclysmic variables may be a useful indicator of the companion's spectral type. SED fitting for RZ Leo and CU Vel suggests M5-type dwarf companions, and distances of 340 ′ 110 and 150 ′ 50 pc, respectively. These systems may be placed in the upper evolution branch for short-period cataclysmic variables.

/pdf/a-search-for-brown-dwarf-like-secondaries-in-cataclysmic-f9b3313hon.pdf

A search for brown dwarf like secondaries in cataclysmic variables – II

We report on new simultaneous phase-resolved spectroscopic and polarimetric observations of the polar (AM Herculis star) V834 Cen during a high state of accretion. Strong emission lines and high levels of variable circular and linear polarization are observed over the orbital period. The polarization data are modelled using the Stokes imaging technique of Potter et al. The spectroscopic emission lines are investigated using the Doppler tomography technique of Marsh and Horne and the Roche tomography technique of Dhillon and Watson. Up to now, all three techniques have been used separately to investigate the geometry and accretion dynamics in cataclysmic variables. For the first time, we apply all three techniques to simultaneous data for a single system. This allows us to compare and test each of the techniques against each other and hence to derive a better understanding of the geometry, dynamics and system parameters of V834 Cen.



All three techniques are consistent with an interpretation in which a ballistic stream extends to a minimum of ∼40 degrees in azimuth around the white dwarf before becoming threaded by the magnetic field lines. Interestingly, the observed ballistic Doppler velocities do not show a reduced vy component, as found in Doppler imaging of other AM Her systems. Furthermore, the secondary star in V834 Cen shows more He ii (4686 A) emission on its leading inner face, as opposed to the trailing face like in other AM Her systems. We propose that the accretion shock preferentially illuminates the leading face of the secondary star. In addition, the ballistic stream does not obscure the leading face of the secondary from the accretion shock, and, in fact, our Doppler maps show that the ballistic stream is a strong He ii (4686 A) emission source in itself and thus adds to the illumination of the leading face of the secondary.

/pdf/stokes-imaging-doppler-mapping-and-roche-tomography-of-the-4lq0yjczfr.pdf

Stokes imaging, Doppler mapping and Roche tomography of the AM Herculis system V834 Cen

Gas accreting onto T Tauri stars should form shocks that are susceptible to the classical radiative shock instability. The instability should give rise to strong periodic modulation in the X-ray emission from the shock-heated plasma. Time series analysis of soft X-rays thought to arise predominantly in an accretion shock on the classical T Tauri star TW Hydrae reveals no periodic variations and a 99% confidence pulsed fraction limit of 5% over the frequency range 0.0001-6.81 Hz. We find no clear explanation for the absence of X-ray instability signatures, but suggest that existing one-dimensional models are too simple to explain the three-dimensional shock structure, or that preheating and deceleration of the accretion stream by the damping of magnetohydrodynamic waves excited either by the shock itself, or more deeply in the stellar envelope, could ameliorate the instability in the likely case of a sub-Alfvenic shock.

K. A. Jensen

Papers

Optical identification of the X-ray source E1405-451 - A 101.5 minute binary system with extremely rapid quasi-periodic variability

E1405-451 and E1013-477