[en] The steady state abundance of Tc during the s-process is calculated, and a ratio of Nsub(s)(Tc)/Nsub(s)(Ba)=0.0162+-0.025 is obtained. It is pointed out that this figure sets an upper limit to the observed Tc abundance in stars. An observational number approaching this figure would imply that most of the s-process material observed in these stars is not primordial and that this material is cooled down rapidly to temperatures T⁸<1 after synthesis. The latter is consequence of a strong temperature dependence of the ₉₉Tc half-life which is calculated for T⁸<5. (orig.)

[en] Dissipative collapse and star formation within a protogalaxy are modeled using a simple scheme of gas cloud collisions within an N-body code. The gas physics model incorporates pressure, dissipation, and star formation, with the associated ejection of metal-enriched gas back into the cloud medium. The gas and stars collapse under their self-gravity alone; adding a static potential corresponding to a massive dark halo suppresses both velocity anisotropies and mass loss in low-mass ellipticals. A set of models in general accord with the kinematic and chemical properties of individual ellipticals is found. These models are slowly rotating and flattened by anisotropic velocity dispersions, with the radial profiles of surface density having a logarithmic gradient near -2. The chemical enrichment of the gas is calculated with a local instantaneous recycling approximation. The logarithmic abundance gradient within a massive model galaxy is -0.5, and it flattens toward zero with increasing mass loss in lower mass galaxies. The projected contours of constant metallicity are usually only slightly more flattened than the constant surface density contours. The dependence of the results on the assumed description of the gas physics is fairly small for high-mass galaxies, where the gas processes are overwhelmed by the gravitational collapse. However, lower mass model galaxies have significant pressure support during collapse which makes the results more dependent on the gas physics description

[en] The nature of linear perturbations to a thin, shearing accretion disk composed of compressible material is reexamined. Modes that can carry angular momentum throughout the disk are identified, and it is shown how the subsequent dissipation of these waves through nonlinear processes can form the basis of a viscosity mechanism for realistic accretion disks. It is suggested that the impact of accreting material at the outer edge of the disk provides a natural source of excitation for waves that drive angular momentum transport within the disk. 49 refs

[en] An asymptotically uniform weak magnetic field exerts a torque on a black hole equal to the cross product of its asymptotic limit and the magnetic moment of the black hole. As this torque on the intrinsic dipole moment can be much stronger than the torque exerted on the dipole moment induced by the magnetic field, a charged black hole might influence the inner region of its accretion disk magnetically. 'Larmor' precession of test charge orbits can be much stronger than Lense-Thirring precession, but is unlikely to be dominant in the middle region of a conventional stationary accretion disk. A magnetic field that would cause a solar-mass (supermassive) black hole to precess with a period of less than a month (thousand years) could not be supported by a stationary accretion disk. (orig.)

No abstract available

[en] Protogalaxies with a range of masses and sizes are numerically collapsed with a dissipative model for the formation of elliptical galaxies discussed in a companion paper (Carlberg 1984). The rates of dissipation and star formation in the model are calculated with a simple cloud description of the gas physics. The correlations between the final mass, metal abundance, and kinematics in the models are compared with the empirical correlations of these quantities in an attempt to constrain the size-mass relation of the protogalaxies. To reproduce the variation of central velocity dispersion and metallicity with mass requires that the protogalaxies and sizes such that the average initial surface density was approximately mass independent. The second-parameter sizes such that the average initial surface density was approximately mass independent. The second-parameter effect in ellipticals-at a fixed mass the scatter in central velocity dispersion is correlated with the scatter in metallicity-is possibly explained as the consequence of an initial scatter of protogalaxy sizes. These models can be placed in a cosmological context with the results that the ratio of the initial to final half-mass radii is near 10 for the favored models, and that the gas in the models needs to be largely self-gravitating to produce strong anisotropy, suggesting formation at a redshift near 10 from sheets of gas gathered up with respect to the dark material

[en] The astrophysical site of r-process nucleosynthesis is investigated theoretically using two-dimensional expanding-vortex stellar-collapse simulations based on the Eulerian adaptive-mesh MHD code of Symbalisty (1984). The results are presented graphically, and it is found that the classical r-process can be explained as the result of the collapse of the highly rotating iron core of a 15-solar-mass star, in which the angular velocity along the rotation axis reaches a maximum of 1000 rad/s, corresponding to an angular momentum of 3.4 x 10 to the 48th erg/s for the inner 1.5 solar mass. The ejected jets are shown to yield about 0.0004 solar mass per supernova, sufficient to explain the observed abundances of r-process products. 20 references

[en] We first present a detailed optical study of A0136-0801, a 16 1/2 -mag ''spindle'' galaxy girdled by a ring of gas, dust, and young stars. The spindle is a normal S0 disk seen nearly edge-on, as shown by its photometric profile and fast rotation (v/sub rot//sigma/sub v/ = 2.2); a prolate structure seems to be ruled out. The surrounding ring runs over the poles of this S0 disk and serves as a probe of the vertical potential. The ring motions suggest that a massive halo extends far beyond the S0 disk (out to 3R₂₅) and that this halo is more nearly spherical than flat. We then list 22 related galaxies and derive that a few percent of all field S0's possess near-polar rings or disks. We suggest that these structures are due to a second event, most likely the transfer of mass from a companion galaxy during a close encounter and occasionally also the merger of a companion. Although accretion occurs presumably at random angles, polar rings are favored statistically because of their slow differential precession and consequent longevity. Alternate evolutionary schemes are also discussed. Finally, we suggest that M82 may be forming a polar ring from former M81 material, and predict that the ''tilted bulge'' of UGC 7576 is an S0 disk seen nearly edge-on

[en] Slim accretion discs have a total luminosity of the order L/L_E = m ∝ 1, where L_E is the Eddington luminosity and m = M/M_c, where M_c is a critical accretion rate, related to the Teddington one. The local stability properties of such discs are examined, in the three-dimensional parameter space spanned by the (α, μ, m) axes, where α and μ are two viscosity parameters, and m = M/M_sun the central mass. We suggest that various types of observed quasi-periodic behaviour may be connected with slim disc instabilities. If this turns out to be correct, the so-called normal and horizontal branch oscillations could be due to unstable thermal and acoustic modes, respectively. It is subsequently shown that some of the observed short-term (quasi-periodic) variability in active galactic nuclei may also originate from short-wavelength acoustic modes in the innermost region of the disc. Consequently, observational characteristics, in connection with stability theory, may yield estimates of basic accretion parameters. In the case of the Seyfert galaxy NGC 6814, this process seems to favour (α, μ, m, m) ≅ (0.5, 0, 10⁶, 10^-2). We finally conclude that this line of work may provide additional evidence for both accretion discs and black holes, in various compact sources. (orig.)

[en] Waves of star formation in a uniform, differentially rotating disk galaxy are treated analytically as a propagating detonation wave front. It is shown, that if single solitary waves could be excited, they would evolve asymptotically to one of two stable forms, each of which rotates with a fixed pattern speed. Simple numerical solutions confirm these results. However, the pattern of waves that develop naturally from an initially localized disturbance is more complex and dies out within a few rotation periods. These results suggest a conclusive observational test for deciding whether sequential star formation is an important determinant of spiral structure in some class of galaxies