A Super‐Alfvenic Model of Dark Clouds

Abstract

Supersonic random motions are observed in dark clouds and are traditionally interpreted as Alfve n waves, but the possibility that these motions are has not been ruled out. In this work we super-Alfve nic report the results of numerical experiments in two opposite regimes : and whereMach numberÈthe ratio of the rms velocity to the speed. Our results show Alfve nicAlfve n that models with are consistent with the observed properties of molecular clouds that we have M A ? 1 tested (statistics of extinction measurements, distribution of integrated antenna temperature, Zeemansplitting measurements of magnetic Ðeld strength, line width versus integrated antenna temperature of molecular emission-line spectra, statistical B-n relation, and scatter in that relation), while models with have properties that are in conÑict with the observations. We Ðnd that both the density and the M A D 1 magnetic Ðeld in molecular clouds may be very intermittent. The statistical distributions of the magnetic Ðeld and gas density are related by a power law, with an index that decreases with time in experiments with decaying turbulence. After about one dynamical time it stabilizes at B P n0.4. Magnetically dominated cores form early in the evolution, while later on the intermittency in the density Ðeld wins out, and also cores with a weak Ðeld can be generated by mass accretion along magnetic Ðeld lines.

Probing the initial conditions of high-mass star formation

“…Even so, it is interesting to note that the limits on magnetic fields derived by us are not grossly inconsistent with the proposed relation. Specifically, our results for field strengths are not inconsistent with the compilation by Padoan & Nordlund (1999), examined at densities ∼10 5 cm −3 . However, a preliminary analysis of our own Zeeman and polarization observations towards high-mass cores are inconsistent with a high magnetic field, which would be relatively easy to detect (Pillai et al, work in progress).…”

“…Such field strengths may seem high and unrealistic, in light of recent measurements for low mass dense cores. However, observations spanning a large range in densities have revealed a field-density relation, such that B ∝ n 1/2 (Troland & Heiles 1986;Padoan & Nordlund 1999;Vlemmings 2008). With a lot of upper limits, particularly at the cold core densities, this relation has a significant amount of scatter.…”

Turbulent Dissipation in the Interstellar Medium: The Coexistence of Forced and Decaying Regimes and Implications for Galaxy Formation and Evolution

“…Moreover, since our interest in this paper focuses on the large-scale ISM, we have taken parameters of the Ñow corresponding to the solar neighborhood, as done in previous papers modeling the ISM at large (Chiang & Bregman 1988 ;Rosen & Bregman 1995 ;, 1996Passot Vazquez-Semadeni et al 1995), and have not assumed the Ñow to be isothermal. All of this contrasts with recent studies of turbulence dissipation in molecular clouds (Mac Low et al 1998 ;Stone et al 1998 ;Padoan & Nordlund 1999 ;Mac Low 1999a), which have considered isothermal Ñows with large-scale, ubiquitous forcing. On the other hand, those works have been based on three-dimensional simulations, while here we have used two-dimensional simulations.…”

“…In the case of the large-scale ISM, turbulence involves supersonic MHD compressible nonisothermal Ñows, which most likely are dominated by shocks. High-resolution three-dimensional simulations have been used recently to investigate dissipation in compressible MHD isothermal Ñows with parameters corresponding to Galactic molecular clouds (Mac Low et al 1998 ;Stone, Ostriker, & Gammie 1998 ;Padoan & Nordlund 1999 ;Mac Low 1999a). For the decaying regime, Mac Low et al (1998) and Stone et al (1998) have found to decay as t~a, with a D 0.8È1.0.…”

Submillimetre Studies of Prestellar and Starless Cores in the Ophiuchus, Taurus and Cepheus Molecular Clouds

“…The turbulence-dominated star-formation paradigm has been modelled by, e.g. Padoan & Nordlund (1999); Mac Low & Klessen (2004). In the weak-field model, magnetic fields cannot stop collapse, but can contribute more to the support of a collapsing region than turbulent pressure in the later stages of core collapse.…”