Astronomy and Astrophysics – Astronomy
Scientific paper
Jun 1983
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1983msngr..32....6s&link_type=abstract
The Messenger, No. 32:JUN, P. 6, 1983
Astronomy and Astrophysics
Astronomy
3
Scientific paper
When modern technologies open the possibility of astronomical measurements in wavelength regions from y rays to the ultraviolet and from the infrared to the radio band, the visual range shrinks to a very small interval in the flux diagrams. However, the time of use of the necessarily complex and expensive instruments is very limited. Observing runs of an hour or so are of little use for the investigation of astronomical events like stellar outbursts which evolve with timescales of days. So even optical telescopes with apertures below 1m still have a relevance for long-time observational programmes. An interesting group of objects which is known to show variations in the time scale mentioned is the class of cataclysmic variables (CV). The general model for all members consists of a Roche lobe filling secondary (near the main sequence) which transfers matter via the Lagrangian point L1 to the highly evolved primary, a white dwarf or neutron star. Due to its angular momentum the mass stream does not impact but rather surrounds the small primary, building a more or less circular accretion disko A hot bright spot is produced where the initial stream collides with the already circulating material of the disko By angular momentum exchange part of the disk material finally reaches the primary. The increasing amount of information from all kind of observations has made clear that the class of CVs comprises many kinds of interesting objects like X-ray sourees, oblique magnetic rotators with synchronized rotation or with rotation with critical periods of up to 10s and systems emitting highly polarized radiation from strong magnetic fields of 107 G. Prominent properties of CVs are eruptions with a large range of amplitudes. The more violent events, nova eruptions (> 10 mag) are caused by thermonuclear runaways on the primary. This explanation is quite secure although the nova explosion of a system is so rare that until now it was observed only once for an individual object. The dwarf novae brighten up by a factor of 10 or 100 for a few days only, but every 10 to 1,000 days. In spite of the more detailed observations, there is no generally accepted explanation for their eruption.
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