Astronomy and Astrophysics – Astrophysics
Scientific paper
Dec 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...280..525b&link_type=abstract
Astronomy and Astrophysics (ISSN 0004-6361), vol. 280, no. 2, p. 525-535
Astronomy and Astrophysics
Astrophysics
20
Accretion Disks, Binary Stars, Hydrodynamics, Stellar Envelopes, Stellar Mass Ejection, Stellar Models, Stellar Rotation, Three Dimensional Models, White Dwarf Stars, Angular Momentum, Gas Dynamics, Kinematic Equations, Velocity Measurement
Scientific paper
In the present work we have searched for the existence, as to disk formation, of upper and lower limits to the equatorial rotational speed vrot of the non-collapsed companion (secondary) in asynchronous close binary systems, and found that disk formation can occur only in limited ranges of the secondary's rotational velocity. Outside these intervals disk formation is inhibited for different reasons. The analysis has been carried out for different values of the secondary's to the primary's mass ratio M2/M1, and for different values of the separation between the companions, d12, i.e. for different values of the orbital period. Comparison with the synchronous case is also made. After investigating the kinematical conditions for disk formation, we have studied in more detail the influence of the secondary's rotational speed on morphology, structure and dynamics of accretion disks, whenever they form. This has been done for a binary system with M1 = 1 solar mass, M2 = 5 solar mass, and d12 = 7.5 x 1011 cm, for different values of the secondary's equatorial rotational speed, and considering quasi-polytropic models with gamma = 1.01. The most interesting results are: (1) the disk aspect, size and mass depend on vrot; (2) the disks show sensible deviations from the thin disk and keplerian rotation laws even in the corotational case; (3) the secondary's rotational speed affects the radial temperature distribution and the accretion rate. The results obtained here allow us to conclude that disk formation can occur only in limited ranges of the polytropic index gamma and the secondary's rotational speed vrot, that the keplerian approximation has no general validity and that the thin disk approximation is reasonably good only for a mass ratio M2/M1 less than or equal to 1. However, we do not claim that analytical models referring to Shakura-Sunyaev's are not valid. We only report our numerical results that indeed show some differences with the analytical ones, because of the fact that we study a dynamical situation where the secondary's rotation, generally not considered in standard models, plays a role.
Belvedere Gaetano
Lanzafame Giuseppe
Molteni Diego
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