Planet formation in a disk around a star: The two dense-ring model

a disk around a star is the initial configuration commonly accepted for the material that forms a planetary system such as our own. in this work, the first stage in the evolution from a disk to a set of planetesimals is studied. this stage consists in the formation ofdense regions, which are able to collapse into objects that are the seedsfor planetformation. this process is examinedfor a stationary configuration containing two dense rings, fixed in keplerian radii and with mass continually increasing with time, which was reported in a previous study as the final outcome of a hydrodynamic simulation of a cloud collapse toward a star (nagel, e., 2007, rev. mex. astron. astrof., 43, 257-270). results indicate that in such a configuration the rings will eventually acquire enough mass to be prone to gravitational instabilities, which will cause fragmentation. the existence of unstable modes allows estimating typical sizes of the collapsed objects, given by the unstable wavelengths. ring masses may be found via the mass surface density of the analysed model. the conclusion is that the unstable rings contain enough mass to result in the fragmentation and formation ofplanetesimals, which eventually accumulate in objects with masses typical of planets in the solar system.