Larson's Law Research Articles

Kolmogorov‐Burgers Model for Star‐forming Turbulence

The process of star formation in interstellar molecular clouds is believed to be controlled by driven supersonic magnetohydrodynamic turbulence. We suggest that in the inertial range such turbulence obeys the Kolmogorov law, while in the dissipative range it behaves as Burgers turbulence developing shock singularities. On the base of the She-Leveque analytical model we then predict the velocity power spectrum in the inertial range to be E_k ~ k^{-1.74}. This result reproduces the observational Larson law, ~ l^{0.74...0.76}, [Larson, MNRAS 194 (1981) 809] and agrees well with recent numerical findings by Padoan and Nordlund [astro-ph/0011465]. The application of the model to more general dissipative structures, with higher fractal dimensionality, leads to better agreement with recent observational results.

33.8 GHz CCS Survey of Molecular Cores in Dark Clouds

We have conducted a survey of the CCS JN = 32-21 line toward 11 dark clouds and star-forming regions at 30'' spatial resolution and 0.054 km s-1 velocity resolution. CCS was detected only in quiescent clouds, not in active star-forming regions. The CCS distribution shows remarkable clumpy structure, and 25 clumps are identified in seven clouds. Seven clumps with extremely narrow nonthermal line widths (<0.1 km s-1) are among the most quiescent clumps ever found. The CCS clumps tend to exist around the higher density regions traced by NH3 emission or submillimeter continuum sources, and the distribution is not spherically symmetric. Variation of the CCS abundance was suggested as an indicator of the evolutionary status of star formation. However, we can only find a weak correlation between N(CCS) and n. The velocity distributions of CCS clouds reveal that a systematic velocity pattern generally exists. The most striking feature in our data is a ring structure in the position-velocity diagram of L1544 with an well-resolved inner hole of 0.04 pc × 0.13 km s-1 and an outer boundary of 0.16 pc × 0.55 km s-1. This position-velocity structure clearly indicates an edge-on disk or ring geometry, and it can be interpreted as a collapsing disk with an infall velocity ≳0.1 km s-1 and a rotational velocity less than our velocity resolution. Nonthermal line width distribution is generally coherent in CCS clouds, which could be evidence for the termination of Larson's Law at small scales, ∼0.1 pc.

Low‐Mass Clumps in TMC‐1: Scaling Laws in the Small‐Scale Regime

We present new observational data on the small-scale structure of the Taurus molecular cloud 1 (TMC-1) in the regime of 0.02-0.04 pc and 0.04-0.6 M☉. Our analysis is based on high-resolution, high-S/N, observations of an 8' × 8' area centered on the cyanopolyyne peak in the southeastern part of the TMC-1 ridge. The observations were made in the CCS 22 and 45 GHz transitions using NASA's Deep Space Network 70 m and 34 m telescopes at the Goldstone facility. The CCS emission in this region originates in three narrow components centered on LSR velocities of ~5.7, 5.9, and 6.1 km s-1. These components each represent a separate cylindrical feature elongated along the ridge. Among the three velocity components we identified a total of 45 clumps with a typical CCS column density of ~a few × 1013 cm-2, an H2 density of ~a few × 104 cm-3, and a mass in the range of 0.04-0.6 M☉. The statistical properties of these small-scale clumps are compared with those of the larger NH3 in cold clouds and CS in the hotter Orion region. The CCS clumps in TMC-1 are found to conform to Larson's scaling laws (relating observed line width to clump size) derived from the larger cores down to the small-scale regime (0.02 pc and 0.04 M☉). These clumps represent a regime in which microturbulence is small, amounting to ~10% of the thermal pressure inside a clump. Of the 45 clumps, only five appear to be gravitationally unstable to collapse. All unbound clumps have masses less than 0.2 M☉, while bound clumps have masses in the range 0.15-0.6 M☉. The 6.1 km s-1 velocity feature contains all the gravitationally unstable clumps and is the most likely site for future star formation.

Pressure-confined clumps in magnetized molecular clouds

A substantial fraction of the mass of a giant molecular cloud (GMC) in the Galaxy is confined to clumps which occupy a small fraction of the volume of the cloud. A majority of the clumps in several well-studied GMCs (Ophiuchus, Orion G, Rosette, Cepheus OB3) are not in gravitational virial equilibrium, but instead are confined by the pressure of the surrounding medium. These clumps thus violate one of 'Larson's (1981) laws'. Generalizing the standard virial analysis for spherical clouds to spheroidal clouds, we determine the Jeans mass and the magnetic critical mass for the clumps in these clouds. The Alfven Mach number, which is proportional to the internal velocity dispersion of the clumps divided by the Alfven velocity, is estimated to be of order unity for all the clumps. The more massive clumps, which are in gravitational virial equilibrium, are too massive to be supported by magnetic fields alone (i.e., they are magnetically supercritical). Internally generated turbulence must play a key role in supporting these clumps.