Hi Russ,
The paper you provide a link to is interesting “Massive stars in subparsec rings around galactic centres”. The author of that paper, hypotheses that the galactic cores paradox of youth stars are created when gas accretion disks form in around the core. The problem with the accretion disk explanation is there are seven different rings of stars that move about the galaxy's core.
As the massive stars in question by standard theory, are short lived, what was the recent gas source for the accretion disks that are hypothesized to have created seven different rings of stars? Were there seven separate events and seven separate gas clouds? Are the gas clouds primordial? Why are gas clouds recently falling into the Milky Way core and other galaxy's core.
Figure 2 of this paper shows the seven different rings of stars. The massive stars in question are bunched as a string of stars that orbit the Milky Way’s black hole, on seven different orbits. Based on figure 2, how many different gas clouds would be required to explain the observations?
http://arxiv.org/abs/0704.1281v1
“The Galactic Center” By R. Genzel, V. Karas
The following is an excerpt from this review paper, that discusses the observation and the theoretical problems with its explanation.
Quote:
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Several scenarios have been proposed to account for this paradox of youth. In spite of this effort the origin of central stars (S-stars) is not well understood: models have difficulties in reconciling different aspects of the Galaxy Centre – on one side it is a low level of present activity, indicating a very small accretion rate, and on the other side it is the spectral classification that suggests these stars have been formed relatively recently; see Alexander (2005) for a detailed discussion and references. The most prominent ideas to resolve the apparent problem are in situ formation in a dense gas accretion disk that can overcome the tidal limits, re-juvenation of older stars by collisions or stripping, and rapid in-spiral of a compact, massive star cluster that formed outside the central region and various scattering a three body interaction mechanisms, including resonant relaxation (Alexander 2005). Several other mechanisms have been proposed that could set stars on highly eccentric orbits and bring them to the neighbourhood of the central black hole (e.g., Hansen & Milosavljevi´c 2003; McMillan & Portegies Zwart 2003; Alexander & Livio 2004; ˇ Subr & Karas 2005), but the problem of the S-stars remains open.
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