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last change 2005 December 21, R. Arlt
Cataclysmic Variables

A&A cover   Astronomy & Astrophysics titles:


Resolving accretion curtains in the asynchronous magnetic cataclysmic variable BY Cam

Tomography of close binary stars

BY Camelopardalis is a close stellar pair, in which a low-mass star loses gas onto a compact white dwarf. Using phase-resolved, medium-resolution spectroscopy and the technique of Doppler tomography, we could resolve an extended mass flow in this peculiar magnetic, interacting binary. The strong magnetic field of the white dwarf of 28 MegaGauss - a thousand times stronger than that in a sunspot - dramatically changes the structure of the infalling mass flow. In contrast to non-magnetic cataclysmic variables (CV) forming accretion disks, the material is usually collimated in a focused stream and directed to one or both magnetic poles. This structure, which at micro-arcsecond scales is not observable by direct imaging, can be resolved by reconstructing the 2-dimensional velocity fields from a set of spectra taken at different aspect angles.

The target binary is the brightest member of the rare class of four known asynchronous polars, where the rotation of the magnetic white dwarf differs by a few percent from the orbital revolution period. The first tomographic observations of such a system obtained during two different occasions in 1998 and 1999, revealed line emission spread out over a large velocity range forming a crescent at negative velocities in the Doppler maps. Location and extent of the structure in the Doppler maps can be reproduced with a simple curtain model raised over a wide range in azimuth. This surprising result is in stark contrast to previous observations of non-magnetic and strongly magnetic cataclysmic variables, which preferentially accrete either through an accretion disk or a funnelled stream, respectively. Magnetic accretion via curtains may be a yet unobserved but important mode of mass transfer in other types of astrophysical objects, among them T Tauri objects or neutron stars.

At present it is not clear whether the observed behaviour is related to the asynchronous nature of the object or simply caused by a larger than average mass accretion rate. Ultimate conclusion will require an extended spectroscopic campaign providing a set of Doppler maps over the entire beat cycle of 14 days.

R. Schwarz


WZ Sge HU Aqr
As a comparision to BY Cam, two Doppler maps of a a CV clearly showing an accretion disk (left) and a magnetic CV (right) dominated by strong emission from the ballistic stream.



R. Schwarz, A.D. Schwope, A. Staude, R.A. Remillard, 2005: Doppler tomography of the asynchronous polar BY Camelopardalis
Astronomy & Astrophysics 444 (Nr. 1), p. 213

[Paper draft]

View of a cataclysmic variable
Animation of a cataclysmic variable with a magnetic white dwarf (blue circle) accreting onto two poles via extended curtains. The colour coding represents the line-of-sight velocities of the specific parts in the accretion flow (J. Vogel). [MPEG 2MB] - [zoomed-in MPEG, 900KB]

Spectral trace
Time(phase)-resolved spectra of BY Cam's Hβ emission line profile observed in 1998.

Doppler map
Doppler map of BY Cam computed from the above spectra. Overlayed are the locations of the secondary star (red loop), a ballistic trajectory (red arc), and a set of magnetic trajectories representing an extended curtain (blue).

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