Astronomy and Astrophysics – Astronomy
Scientific paper
Apr 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000newar..44..171s&link_type=abstract
New Astronomy Reviews, Volume 44, Issue 1-2, p. 171-175.
Astronomy and Astrophysics
Astronomy
17
Scientific paper
Four problems are discussed. (1) Model light curves show significant increase of the disk luminosity during quiescence, an effect which is not present in the observed light curves. It is suggested that the slope of the lower branch of the Σ-Te relation should be significantly decreased. (2) The width - Porb relation for narrow outbursts is well reproduced with model data for αhot=0.2. The bimodal distribution of outburst durations and, in particular, the origin of wide outbursts and the nature of their width - Porb relation, require explanation. (3) It is suggested that problems with the thermal-tidal instability (TTI) model for superoutbursts might be solved by a hybrid model, combining the TTI model with the irradiation-enhanced mass-transfer model. A strong argument in favour of irradiation is provided by the ratio of the irradiating flux to the intrinsic flux of the secondary component, which turns out to be very large in the case of dwarf novae showing superoutbursts, with U Gem being a borderline case. (4) Characteristic time-scales observed during dwarf nova outbursts depend on the viscous time-scale, allowing an empirical determination of /α. Three independent determinations, based on the rates of decline following outburst maximum, the UV delay observed during rising light, and the widths of outbursts, give consistently αhot~0.2. It should be added, however, that those time-scales depend also strongly on the radius of the disk. In this context it is disturbing to note that the observed disk radii appear to be smaller than those resulting from model calculations.
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