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Astron. Astrophys. 356, 490-500 (2000)

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6. Discussion

We have, for the first time, mapped the accretion stream in a polar in the light of a high-excitation ultraviolet line with a complete 3d model of an optically thick stream. We have found three different bright regions on the stream, but no strong emission at the stagnation point of the ballistic stream. In the following we will discuss the physical processes which may lead to an emission structure like the one observed.

6.1. Emission of the ballistic stream

As mentioned in Sect. 2, single-particle trajectories with different inital directions diverge after the injection at [FORMULA], but converge again at a point approximately one third of the way between [FORMULA] and the stagnation region. This is where we find emission in the line of C IV [FORMULA] 1550. Possibly the kinetic properties of the stream lead to a compression of the accreted matter, resulting in localized heating. After the convergence point, the single-particle trajectories diverge slowly and follow a nearly straight path without any further stricture. Hydrodynamical modelling of the ballistic part of the stream is required to substantiate this hypothesis.

6.2. Absence of emission at the stagnation point

In the classical model of polars, it is assumed that the ballistically infalling matter couples onto the magnetic field in the stagnation region with associated dissipation of kinetic energy (e.g. Hameury et al. 1986). Thus, one would expect a bright region near SR. The absence of C IV [FORMULA] 1550 emision in the stagnation region could be due to the fact that there is no strong heating in the coupling region. Dissipation near SR can be avoided if the material is continuously stripped from the ballistic stream and couples softly onto the field lines, as proposed by Heerlein et al. (1999) for HU Aquarii.

Another possibility is that the matter is decelerated near SR, resulting in an increase in the density. This may result in an increase of the continuum optical depth, and, therefore, in a decrease of the C IV [FORMULA] 1550 equivalent width.

6.3. Emission of the dipole stream

On the dipole section of the stream, we find two generally different emission regions: The bright and small region above the stagnation point and the broader regions near the accretion poles of the white dwarf.

Near the accretion spots: On the magnetically funneled stream, we find one region of line emission (3) which we assume to be due to photoionization by high-energy radiation from the accretion spot. The mirror region 3b is an artifact which is created by the mapping algorithm to account for the non-zero flux level in orbit 1 in the phase interval [FORMULA] (see Sect. 5.3). Even though the distribution of CIV emission on the accretion stream does not reflect the irradiation pattern in a straightforward way, the presence of CIV emission at a certain location of the stream requires that this point is irradiated, if there is no other mechanism creating CIV line emission. In Fig. 14, we define the angles [FORMULA] and [FORMULA] and the distances r and R. Assuming a point-like emission region at the accretion spot, the absorbed energy flux per unit area caused by illumination from the accretion regions varies as [FORMULA].

[FIGURE] Fig. 14. Dipole field line configuration. See Sect. 6.3.

The structure of this equation shows that the illumination of the accretion stream is at maximum somewhere between the accretion region and the point with [FORMULA]. This corresponds to the brightness distribution which we derive from the data.

We suggest the following interpretation of the emission region above the stagnation point : Independent of whether the CIV emission in this region is due to photoionozation or collisional excitation, a higher density than in other sections of the stream is required. Matter which couples in SR onto the field lines has enough kinetic energy to initially rise northward from the orbital plane against the gravitational potential of the white dwarf. If the kinetic energy is not sufficient to overcome the potential summit on the field line, the matter will stagnate and eventually fall back towards the orbital plane, where it collides with further material flowing up. This may lead to shock heating with subsequent emission of C IV [FORMULA] 1550. Alternatively, the photoinization of the region of increased density may suffice to create the emission peak. Yet another possibility is that the matter is heated by cyclotron radiation from the accretion column which emerges preferentially in direction perpendicular to the field direction. Crude estimates show, however, that the energy of the cyclotron emission does not suffice to produce such a prominent feature as observed. A detailed understanding of the emission processes in the accretion stream involves a high level of magnetohydrodynamical simulations and radiation transfer calculations, which is, clearly, beyond the scope of this paper.

6.4. Conclusion

We have successfully applied our new 3d eclipse mapping method to UZ Fornacis. In subsequent research we will allow additional degrees of freedom in the mapping process, using data sets with higher S/N and covering a larger phase interval.

Our attempts to image the accretion stream in polars should help in understanding the physical conditions in the stream, such as density and temperature. By comparison to hydrodynamical stream simulations, we will take a step towards the complete understanding of accretion physics in polars.

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© European Southern Observatory (ESO) 2000

Online publication: April 10, 2000