The Structure and Evolution of Star Forming Clouds

 

  • The Different Structures of the Two Classes of Starless Cores

Abstract Previous observations and theoretical models of starless cores suggest that the different temperature and density structures observed in individual cores and their different dynamical behaviors can be described by a consistent set of physics. Here we develop a model for the thermal and dynamical equilibrium of starless cores that includes the radiative transfer of the gas and dust, and simple CO chemistry. The model shows that the behavior of the cores is significantly different depending on whether the central density is either above or below about $10^5$ cm$^{-3}$. This is close to the critical density for both gas cooling by gas-dust collisions and also for dynamical stability, given the typical properties of the starless cores. Thus the model divides the starless cores into two classes that we refer to as thermally super-critical and thermally sub-critical. This simple model allows an improved interpretation of observational data to better understand the evolution of starless cores, their dust opacity, and the rate of cosmic-ray ionization that heats them.

Eric Keto & Paola Caselli, 2008, arXiv 0804.0822

  • Oscillating Starless Cores: The Nonlinear Regime

Abstract In a previous paper, we modeled the oscillations of a thermally-supported (Bonnor-Ebert) sphere as non-radial, linear perturbations following a standard analysis developed for stellar pulsations. The predicted column density variations and molecular spectral line profiles are similar to those observed in the Bok globule B68 suggesting that the motions in some starless cores may be oscillating perturbations on a thermally supported equilibrium structure. However, the linear analysis is unable to address several questions, among them the stability, and lifetime of the perturbations. In this paper we simulate the oscillations using a three-dimensional numerical hydrodynamic code. We find that the oscillations are damped predominantly by non-linear mode-coupling, and the damping time scale is typically many oscillation periods, corresponding to millions of years, and persisting over the inferred lifetime of gobules.

Avery E. Broderick, Eric Keto, Charles J. Lada & Ramesh Narayan, 2007, ApJ, 671, 1832

  • Oscillations of Starless Cores

Abstract If the split, asymmetric molecular spectral line profiles that are seen in many starless cores are interpreted as indicative of global collapse or expansion of the core then one possible implication is that most starless cores have short lifetimes on the order of the collapse or sound crossing time scale. An alternative interpretation of the line profiles as indicative of perturbations on an underlying equilibrium structure leads to the opposite implication, that many cores have long lifetimes. While evidence suggests that some cores are collapsing on a free-fall time scale, we show that observations of some other starless cores can be reproduced by a model of non-radial oscillations about the equilibrium configuration of a pressure-bounded, thermally-supported sphere (Bonnor-Ebert sphere). We model the oscillations as linear perturbations following a standard analysis developed for stellar pulsations and compare the column densities and molecular spectral line profiles predicted from a particular model to observations of the Bok globule B68.

Eric Keto, Avery E. Broderick, Charles J. Lada & Ramesh Narayan, 2007, ApJ, 652, 1366

  • Dark Cloud Cores and Gravitational Decoupling from Turbulent Flow

Abstract We test the hypothesis that the starless cores may be gravitationally bound clouds supported largely by thermal pressure by comparing observed molecular line spectra to theoretical spectra produced by a simulation that includes hydrodynamics, radiative cooling, variable molecular abundance, and radiative transfer in a simple one-dimensional model. The results suggest that the starless cores can be divided into two categories: stable starless cores that are in approximate equilibrium and will not evolve to form protostars, and unstable pre-stellar cores that are proceeding toward gravitational collapse and the formation of protostars. The starless cores might be formed from the interstellar medium as objects at the lower end of the inertial cascade of interstellar turbulence. Additionally, we identify a thermal instability in the starless cores. Under particular conditions of density and mass, a core may be unstable to expansion if the density is just above the critical density for the collisional coupling of the gas and dust so that as the core expands the gas-dust coupling that cools the gas is reduced and the gas warms, further driving the expansion.

Eric Keto & George B. Field, 2005, ApJ, 635, 1151