Star Formation at the Galactic Center

📝 Abstract

Molecular clouds at the Galactic center (GC) have environments considerably different from their disk counterparts. The GC may therefore provide important clues about how the environment affects star formation. Interestingly, while the inner 50 parsecs of our Galaxy include a remarkable population of high-mass stars, the initial mass function (IMF) appears to be consistent with a Salpeter slope down to ~ 1 solar mass. We show here that the loss of turbulent pressure due to ambipolar diffusion and the damping of Alfven and fast MHD waves can lead to the formation of dense condensations exceeding their Jeans limit. The fragmentation and subsequent collapse of these condensations is similar to the diffusion-driven protostellar collapse mechanism expected to occur within nearby “regular” molecular clouds. As such, a Salpeter IMF at the GC is not surprising, though the short dynamical timescales associated with the GC molecular clouds may help explain the lower star formation efficiency observed from this region.

💡 Analysis

Molecular clouds at the Galactic center (GC) have environments considerably different from their disk counterparts. The GC may therefore provide important clues about how the environment affects star formation. Interestingly, while the inner 50 parsecs of our Galaxy include a remarkable population of high-mass stars, the initial mass function (IMF) appears to be consistent with a Salpeter slope down to ~ 1 solar mass. We show here that the loss of turbulent pressure due to ambipolar diffusion and the damping of Alfven and fast MHD waves can lead to the formation of dense condensations exceeding their Jeans limit. The fragmentation and subsequent collapse of these condensations is similar to the diffusion-driven protostellar collapse mechanism expected to occur within nearby “regular” molecular clouds. As such, a Salpeter IMF at the GC is not surprising, though the short dynamical timescales associated with the GC molecular clouds may help explain the lower star formation efficiency observed from this region.

📄 Content

arXiv:0906.2917v1 [astro-ph.HE] 16 Jun 2009 Star Formation at the Galactic Center Marco Fatuzzo,1 and Fulvio Melia2 1Physics Department, Xavier University, Cincinnati, OH 45207 2Department of Physics and Steward Observatory, The University of Arizona, Tucson, Arizona 85721 fatuzzo@xavier.edu; melia@physics.arizona.edu ABSTRACT Molecular clouds at the Galactic center (GC) have environments considerably different from their disk counterparts. The GC may therefore provide important clues about how the environment affects star formation. Interestingly, while the inner 50 parsecs of our Galaxy include a remarkable population of high-mass stars, the initial mass function (IMF) appears to be consistent with a Salpeter slope down to ∼1 M⊙. We show here that the loss of turbulent pressure due to ambipolar diffusion and the damping of Alfv´en and fast MHD waves can lead to the formation of dense condensations exceeding their Jeans limit. The fragmentation and subsequent collapse of these condensations is similar to the diffusion-driven protostellar collapse mechanism expected to occur within nearby “regular” molecular clouds. As such, a Salpeter IMF at the GC is not surprising, though the short dynamical timescales associated with the GC molecular clouds may help explain the lower star formation efficiency observed from this region. 1. Introduction Basic star formation theory holds that a star’s initial mass is dependent, at least in part, on the environment in which it is born (Bonnell et al. 2007). It is quite reasonable therefore to expect that the IMF may vary for different types of stellar populations. Yet the IMF appears to be quite uniform throughout our galaxy (Kroupa 2002; Massey 2003), being well described by a power-law form dN/dm ∝mα for m > 1 M⊙and a lognormal form below this value (Adams & Fatuzzo 1996; Kroupa 2001; Chabrier 2003). At present, there is only one known cluster in our Galaxy that appears to have a considerably top-heavy IMF—the GC cluster. Two other clusters that lie within 50 pc in projection from the nucleus also contain a remarkable number of high-mass stars—the Quintuplet and the Arches clusters. – 2 – The Quintuplet cluster contains more Wolf-Rayet stars than any other cluster in the galaxy. However, it is significantly more dispersed than the Arches cluster, and as such, its mass function has not been determined. In contrast to the Quintuplet, the Arches cluster is quite dense, containing at least 150 O stars within a radius of 0.6 pc. While the mass function for this cluster was initially thought to be considerably shallower than a Salpeter MF (Stolte et al. 2005; Kim et al. 2006), the peculiarities in the Arches mass function can apparently be attributed to evolution, and the Arches initial mass function may well be consistent with a Salpeter slope down to ∼1 M⊙(Portegies Zwart et al. 2007). These results are somewhat surprising given that the molecular clouds within ∼100 pc of the GC have considerably different environments than their disk counterparts (e.g., Crocker et al. 2007). While a lot of progress has been made over the past decade in the field of star formation theory, a complete understanding of how a star acquires its initial mass remains elusive. At present, two different star formation paradigms are seemingly favored by the star formation community—the “standard” paradigm (Shu et al. 1987; see also more recent works by Adams & Shu 2007; Tassis & Mouschovias 2007; Kudoh & Basu 2008; Basu et al. 2009) and the turbulent fragmentation paradigm (Padoan & Nordlund 2002; Mac Low & Klessen 2004; Bonnell et al. 2007). On the observational front, a recent survey of magnetic field strengths in nearby dark cloud cores aimed at testing these scenarios failed to yield conclusive results (Troland & Crutcher 2008). The extreme molecular cloud environments at the GC (Yusef-Zadeh et al. 2000; Melia & Falcke 2001) may therefore provide an important test case from which to consider star formation theory. Indeed, recent observations now make it possible to consider certain aspects of star formation within the context of the standard paradigm. For example, Yusef-Zadeh et al. (2007) have recently compiled evidence for an en- hancement of cosmic-ray electrons within the Galactic center region, and claim that this enhancement may be responsible for heating the gas clouds to temperatures above the dust temperature. These authors note that higher cloud temperatures increase the Jeans mass, thereby accounting for what at the time was believed to be a top-heavy IMF at the Galac- tic Center (Stolte 2005; Kim et al. 2006). In addition, the authors suggest that the high ionization, which would increase magnetic coupling to the cloud material and lengthen the ambipolar diffusion time, could be responsible for the low star formation efficiency observed in this region (Gordon et al. 1993; Lis et al. 2001). We extend this line of investigation by considering how the extreme environments asso- ciated with the dense co

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