in reply to parejkoj's comment:
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So, you are saying that the multitude of possible explanations in section 7.2 of Alexander are all irrelevant or incorrect?
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I would say that in situ formation from a gas cloud or star migration does not address and does not attempt to address other fundamental questions connected with the observation. (For example, the authors who attempt to explain the simultaneous formation of two rings of stars that encircle the galactic core, specifically state that they do not have an explanation for why the formation of the two rings of stars, was near simultaneous.)
Paradox of Youth stars.
There are papers that specifically challenge in situ formation from a gas cloud or star migration as an explanation for the paradox of youth stars.
Two Rings of Stars Encircling Galactic Core
As the two rings of stars are further from the galactic core, it might be possible with a special very unusual external forcing event, for in situ formation from a gas cloud to explain the formation of two rings of stars that encircle the galactic core.
Hypothesis Logic
A hypothesis that must appeal to very special conditions to have occurred, the interaction of a satellite galaxy with the Milky Way, 6 million years ago, is a last resort hypothesis.
There are multiple problems with the gas cloud origin hypothesis to explain the galactic centre star clusters and rings of stars. Think of how stars are formed, the density of stars in the area, the number of massive stars in the area, and so forth...
1) The star clusters and the two rings of stars appear to have formed at the same time, for the group of stars in question. There is no explanation as to why a large group of stars would suddenly all form at the same time. The authors note they do have an explanation for the sudden formation of groups of stars.
2) There is a second problem of how to create a cluster of large stars in the same close area (30 stars in less than light year distance). The highly luminous stars will heat the gas cloud, stopping it from collapsing.
3) These problems are in addition to the problem of how to get a dense enough gas cloud in this area that will not be torn apart by the massive compact object.
The two rings of stars are at roughly the same distance from the massive compact object, have both formed roughly 6 million years ago and are at roughly 90 degrees to each other.
http://arxiv.org/abs/astro-ph/0601268v2
"The Two Young Star Disks in the Central Parsec of the Galaxy: Properties, Dynamics and Formation" by T. Paumard, Genzel, Martins, Nayakshin, Beloborodov, Levin, Trippe, Eisenhauer, Ott, Gillessen, Abuter, J. Cuadra, Alexander, Sternberg
Another explanation required than gas cloud origin required for "paradox of youth" stars
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The ‘paradox of youth’ in the central S-star cluster, with > 15 apparently normal main sequence B stars residing in tightly bound orbits in the central light month around the central black hole, probably requires yet another explanation (recently Ghez et al. 2003, 2005; Genzel et al. 2003; Hansen & Milosavljevic 2003; Gould & Quillen 2003; Alexander & Livio 2004; Eisenhauer et al. 2005; Alexander 2005; Davies & King 2005).
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Two Rings of Stars
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Almost all of the ≃ 80 massive stars now known in the central parsec (central arcsecond galactic stars excluded) reside in one of two somewhat thick (h|h|/Ri ≃ 0.14) rotating disks.
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Unusual Times Problem
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We do not have an explanation for the near simultaneous occurrence of two star formation events 6 Myr ago, followed by little since then, and preceded by little for tens of Myr (Blum et al. 2003). It is unavoidable to conclude that the epoch 4–9 Myr ago must have been a very special one for the Galactic Center. It is interesting and relevant to note in this context that the other two young, massive star clusters in the central 50 pc, the Arches and Quintuplet cluster, have comparable stellar masses (10^4Msolar mass), stellar content (WC/WN etc.), ages (2–7 Myr), and (flat) mass functions (Figer et al. 1999; Figer 2003; Stolte et al. 2005). We might speculate that star formation across the Galactic Center was triggered a few Myr ago by a global event, such as an interaction with a passing satellite galaxy that raised the pressure in the central interstellar medium and/or lead to increased cloud/cloud collisions.
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Massive Stars, late sequence stars, Orbiting Massive Compact Object. Orbital distances as short as few light days?
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The lack of OB stars in these earlier studies is puzzling. The question is whether this lack is due to a true depletion or is merely a selection effect due to veiling of the weak absorption lines in near main sequence stars by bright nebular emission. Adaptive optics (AO) spectroscopy of the center-most arcsecond around Sgr A* (mostly devoid of nebular emission) has already revealed a dozen massive stars. These stars appear to be main sequence late O and B stars and orbit the central black mass at distances as short as a few light-days (Schödel et al. 2003; Ghez et al. 2003, 2005; Eisenhauer et al. 2005).
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