Understanding the formation of stars in galaxies is central to much of modern astrophysics. However, a quantitative prediction of the star formation rate and the initial distribution of stellar masses remains elusive. For several decades it has been thought that the star formation process is primarily controlled by the interplay between gravity and magnetostatic support, modulated by neutral-ion drift (known as ambipolar diffusion in astrophysics). Recently, however, both observational and numerical work has begun to suggest that supersonic turbulent flows rather than static magnetic fields control star formation. To some extent, this represents a return to ideas popular before the importance of magnetic fields to the interstellar gas was fully appreciated. This review gives a historical overview of the successes and problems of both the classical dynamical theory and the standard theory of magnetostatic support, from both observational and theoretical perspectives. The outline of a new theory relying on control by driven supersonic turbulence is then presented. Numerical models demonstrate that, although supersonic turbulence can provide global support, it nevertheless produces density enhancements that allow local collapse. Inefficient, isolated star formation is a hallmark of turbulent support, while efficient, clustered star formation occurs in its absence. The consequences of this theory are then explored for both local star formation and galactic-scale star formation. It suggests that individual star-forming cores are likely not quasistatic objects, but dynamically collapsing. Accretion onto these objects varies depending on the properties of the surrounding turbulent flow; numerical models agree with observations showing decreasing rates. The initial mass distribution of stars may also be determined by the turbulent flow. Molecular clouds appear to be transient objects forming and dissolving in the larger-scale turbulent flow, or else quickly collapsing into regions of violent star formation. Global star formation in galaxies appears to be controlled by the same balance between gravity and turbulence as small-scale star formation, although modulated by cooling and differential rotation. The dominant driving mechanism in star-forming regions of galaxies appears to be supernovae, while elsewhere coupling of rotation to the gas through magnetic fields or gravity may be important.

Control of star formation by supersonic turbulence

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Control of Star Formation by Supersonic Turbulence

In this paper they conclude that turbulent fragmentation is unavoidable in super-sonically turbulent molecular clouds, and given the success of the present model to predict the observed shape of the Stellar IMF, they conclude that turbulent fragmentation is essential to the origin of the stellar IMF.

https://iopscience.iop.org/article/10.1086/341790/pdf

The Stellar Initial Mass Function from Turbulent Fragmentation

We use optical and near-infrared star counts to explore the structure and dynamics of the Orion Nebula Cluster (ONC). This very young (<1 Myr) cluster is not circularly symmetric in projection but is elongated north-south in a manner similar to the molecular gas distribution in the region, suggesting that the stellar system may still reflect the geometry of the protocluster cloud. Azimuthally averaged stellar source counts compare well with simple spherically symmetric, single-mass King cluster models. The model fits suggest that the inner Trapezium region should be regarded as the core of the ONC, not as a distinct entity as sometimes advocated. We estimate that the core radius of the cluster is 0.16-0.21 pc and that the central stellar density approaches 2 × 10^4 stars pc^(-3). Adopting the stellar velocity dispersion from published proper-motion studies, virial equilibrium would require a total mass within about 2 pc of the Trapezium of ~4500 M_☉, slightly more than twice the mass of the known stellar population and comparable to the estimated mass in molecular gas projected onto the same region of the sky. If ≳ 20% of the remaining molecular gas is converted into stars, thus adding to the binding mass, given that the present stellar population alone has a total energy close to zero, the ONC is likely to produce a gravitationally bound cluster. The ONC also exhibits mass segregation, with the most massive (Trapezium) stars clearly concentrated toward the center of the cluster and some evidence for the degree of central concentration to decrease with decreasing mass down to 1-2 M_☉, as would be expected for general mass segregation. Given the extreme youth of the stars compared with the estimated range of collisional relaxation times, the mass segregation is unlikely to be the result of cluster relaxation. Instead, we suggest that the mass segregation reflects a preference for higher mass stars to form in dense, central cluster regions.

/pdf/a-preliminary-study-of-the-orion-nebula-cluster-structure-3ji4d8nplw.pdf

A Preliminary Study of the Orion Nebula Cluster Structure and Dynamics

▪ 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...

Physical conditions in regions of star formation

We have determined the abundances of HCN and HNC toward 19 nearby dark cloud cores by observations of optically thin H13CN (J = 1-0) and HN13C (J = 1-0) lines. The column density of HCN is found to be correlated with that of HNC. The abundance ratio of [HNC]/[HCN] is determined to be 0.54-4.5 in the observed dark cloud cores. These results are consistent with the idea that HCN and HNC are produced mainly by a recombination reaction of HCNH+ with electrons in dark cloud cores. Furthermore, the [HNC]/[HCN] ratio does not show any significant differences between star-forming cores and starless cores. The HCN and HNC abundances are compared with those for the OMC-1 cores previously reported. Although the abundances of HCN in the OMC-1 cores are comparable to those in the dark cloud cores, the abundances of HNC in OMC-1 are 1-2 orders of magnitude less than those in dark cloud cores. It is suggested that HNC is destroyed by neutral-neutral reactions in high kinetic temperature regions.

https://iopscience.iop.org/article/10.1086/306032/pdf

Abundances of HCN and HNC in dark cloud cores

A first high-resolution survey of molecular cloud cores in the Orion A giant molecular cloud is reported. We identified 125 molecular cloud cores from an analysis of the spatial and velocity distribution of the CS (1-0) emission. The cores are generally elongated along the filamentary molecular cloud, and the axial ratio is about 0.5. The mass spectrum index of the cores is -1.6 for M≥50 M ○ .. The physical properties of the cores identified in Orion are compared with those of cores in dark clouds reported in the literature. The average radius of the cores in the Orion A cloud, 0.16 pc, is comparable to that of the cores in dark clouds

Molecular Cloud Cores in the Orion A Cloud. I. Nobeyama CS (1--0) Survey

Intense thermal emission lines of SiO (v = 0, J = 1-0 and 2-1) are detected in a low-mass star-forming region, L1157. The emission is confined to a compact region toward the blue lobe of the CO outflow, where shock heating due to interaction between the outflow and dense gas has been found. In contrast, the emission of SiO is not detected toward the IRAS source and the red lobe. The distribution of SiO is different from that of CS; the intense CS line is observed in the vicinity of the IRAS source. The peak fractional abundance of SiO is estimated to be about 10 -7

Detection of SiO Emission in the L1157 Dark Cloud

In the L1157 dark cloud, a well-collimated bipolar CO outflow associated with a cold IRAS source 20386+6751 have been discovered. The gas kinetic temperature toward the blue lobe of the outflow rises to-30 K from the temperature of the surrounding gas (≤10 K); this high temperature region is very localized with the blue lobe. The HCO + , HCN, and NH3 lines show blueshifted and broad-line profiles toward the blue CO lobe. Furthermore, their distributions are similar to that of the blue lobe

The outflow in the L1157 dark cloud : evidence for shock heating of the interacting gas

We have measured the ortho-to-para ratio of ammonia in the blueshifted gas of the L1157 outflow by observing the six metastable inversion lines from pJ,Kp=p1,1p to (6, 6). The highly excited (5, 5) and (6, 6) lines were first detected in the low-mass star-forming regions. The rotational temperature derived from the ratio of four transition lines from (3, 3) to (6, 6) is 130-140 K, suggesting that the blueshifted gas is heated by a factor of approximately 10 as compared to the quiescent gas. The ortho-to-para ratio of the NH3 molecules in the blueshifted gas is estimated to be 1.3-1.7, which is higher than the statistical equilibrium value. This ratio provides us with evidence that the NH3 molecules have been evaporated from dust grains with the formation temperature between 18 and 25 K. It is most likely that the NH3 molecules on dust grains have been released into the gas phase through the passage of strong shock waves produced by the outflow. Such a scenario is supported by the fact that the ammonia abundance in the blueshifted gas is enhanced by a factor of approximately 5 with respect to the dense quiescent gas.

Hitomi Mikami

Papers

Abundances of HCN and HNC in dark cloud cores

Molecular Cloud Cores in the Orion A Cloud. I. Nobeyama CS (1--0) Survey

Detection of SiO Emission in the L1157 Dark Cloud

The outflow in the L1157 dark cloud : evidence for shock heating of the interacting gas

The Ortho-to-Para Ratio of Ammonia in the L1157 Outflow.