Physical Conditions in Regions of Star Formation

Abstract

The physical conditions in molecular clouds control the nature and rate of star formation, with consequences for planet formation and galaxy evolution. The focus of this review is on the conditions that characterize regions of star formation in our Galaxy. A review of the tools and tracers for probing physical conditions includes summaries of generally applicable results. Further discussion distinguishes between the formation of low-mass stars in relative isolation and formation in a clustered environment. Evolutionary scenarios and theoretical predictions are more developed for isolated star formation, and observational tests are beginning to interact strongly with the theory. Observers have identified dense cores collapsing to form individual stars or binaries, and analysis of some of these support theoretical models of collapse. Stars of both low and high mass form in clustered environments, but massive stars form almost exclusively in clusters. The theoretical understanding of such regions is considerably less developed, but observations are providing the ground rules within which theory must operate. The most rich and massive star clusters form in massive, dense, turbulent cores, which provide models for star formation in other galaxies.Comment: 48 pages, annual review latex, 4 figures supplied, 2 not supplie

Dusty star-forming galaxies at high redshift

“…A tentative trend is evident in this series 19 As a reminder, as discussed in Evans (1999), while a density of n > n crit is usually taken to be necessary for line emission, a variety of effects associated with radiative transfer can affect the observed line strength at a given density. Following Evans, we choose to state, rather than the critical density, the effective density (n eff , which is defined as the density at which a transition will have a radiation temperature of 1 K, assuming log (N/∆v) = 13.5, and T = 10 K. A table converting n crit (which is typically 1-2 orders of magnitude higher than n eff ) and n eff for a variety of molecular transitions is given in Evans (1999) and Reiter et al (2011). 20 In keeping with the standard definitions in the star formation literature, we will define "clumps" as ∼ 1 pc entities within GMCs that may form stellar clusters, and "cores" as ∼ 0.1 pc structures that serve as the precursors of individual or binary stars (e.g.…”

“…Even the results from HCN (J=1-0) alone provide a somewhat confusing picture as some groups have found a super-linear relationship between L IR and HCN (J=1-0) in local galaxies (García-Burillo et al, 2012). A tentative trend is evident in this series 19 As a reminder, as discussed in Evans (1999), while a density of n > n crit is usually taken to be necessary for line emission, a variety of effects associated with radiative transfer can affect the observed line strength at a given density. Following Evans, we choose to state, rather than the critical density, the effective density (n eff , which is defined as the density at which a transition will have a radiation temperature of 1 K, assuming log (N/∆v) = 13.5, and T = 10 K. A table converting n crit (which is typically 1-2 orders of magnitude higher than n eff ) and n eff for a variety of molecular transitions is given in Evans (1999) and Reiter et al (2011).…”

“…A summary of critical densities for a number of commonly observed molecular and atomic line transitions is given in for T = 100 K. Oftentimes it is assumed that if a line is observed, the densities present in the emitting gas are above the critical density for the line excitation. However, as pointed out by Evans (1999), this is too simplistic of a picture. In reality, a combination of collisional excitation and line pumping of excitation levels can be a significant enough effect that the effective density (n eff ≈ n crit /τ line ) for excitation can be much lower than the critical density.…”

“…In reality, a combination of collisional excitation and line pumping of excitation levels can be a significant enough effect that the effective density (n eff ≈ n crit /τ line ) for excitation can be much lower than the critical density. The effective densities to produce a 1 K line for a variety of molecules and transitions are given in Evans (1999) and Reiter et al (2011).…”

Observations of chemical differentiation in clumpy molecular clouds

“…One may also expect that the spatial distribution of molecules should also reflect the underlying cloud dynamics. Observations of dense interstellar clouds in a variety of molecular tracers show that they contain a distribution of dense gaseous structures [4]. These structures exhibit a size distribution ranging from that of dense cores [∼ 0.05 − 0.1 pc, 5], to clumps [∼ 0.01 − 0.05 pc, 6,7] down to so-called small clumpinos [∼ 0.005 pc , 8].…”