Massive star formation near the Galactic center and the fate of the stellar remnants

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Abstract
Several points are made regarding massive stars, star formation, and stellar remnants in the Galactic center region, particularly the inner 1-10 parsecs. First, in light of the processes which act to inhibit or suppress star formation there, it is argued that those stars which do form tend to be formed by an externally caused compression of their parent clouds rather than by spontaneous cloud collapse. As a result of this, and because of the particular characteristics of the interstellar medium near the Galactic center, it is likely that the initial mass function (IMF) favors more massive stars than that in the Galactic disk, or at least that the lower mass cutoff of the Galactic center IMF is relatively large. However, it is not yet possible to give these claims a quantitative footing. In the inner parsec, star formation would appear to be so strongly suppressed that it is difficult to see how it is possible except under very unusual circumstances. Second, when the compact remnants of massive stars are themselves significantly more massive than the normal field stars in the Galactic center, as would be the case for black hole remnants, then they are susceptible to inward migration as a consequence of dynamical friction. The resulting mass segregation can lead to a pronounced concentration of compact objects within the central stellar core within a Hubble time. Within a wide range of assumptions about the IMF, and about the minimum stellar mass capable of producing a black hole remnant, the total mass of remnants concentrated into the inner few tenths of a parsec, 0.4-5 x 10(6) M., is comparable to the dynamically inferred mass in this region. Therefore, existing dynamical evidence is insufficient to demonstrate the presence of a single central black hole containing most of this mass, although the central cluster of remnants may have coalesced as a result of dynamical instability. Finally, we consider the observed central cluster of luminous objects. The hypothesis that they are massive young stars remaining from a recent episode of star formation is deemed unlikely. The proposed central cluster of compact stellar remnants provides for an alternative class of explanations: that the luminosity of the central objects is derived from accretion. Observational clues lead to the speculation that approximately 10 M. black holes acquire temporary, optically thick atmospheres in collisions with red giants and appear externally as luminous blue stars with winds analogous to those of normal O or WR stars. The conclusions of this paper are generalizable to all normal galactic nuclei having a mass density law in the inner few tens of parsecs comparable to that of the Milky Way.

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