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Register a new userThe Shapes of Molecular Cloud Cores in Simulations and Observations
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S. S. R. Offner
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In this study, we investigate the shapes of starless and protostellar cores using hydrodynamic, self-gravitating adaptive mesh refinement simulations of turbulent molecular clouds. We simulate observations of these cores in dust emission, including realistic noise and telescope resolution, and compare to the observed core shapes measured in Orion by Nutter & Ward-Thompson (2007). The simulations and the observations have generally high statistical similarity, with particularly good agreement between simulations and Orion B. Although protostellar cores tend to have semi-major axis to semi-minor axis ratios closer to one, the distribution of axis ratios for starless and protostellar cores are not significantly different for either the actual observations of Orion or the simulated observations. Because of the high level of agreement between the non-magnetic hydrodynamic simulations and observation, contrary to a number of previous authors, one cannot infer the presence of magnetic fields from core shape distributions.
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We discuss the lifetimes and evolution of clumps and cores formed as turbulent density fluctuations in nearly isothermal molecular clouds. In the non-magnetic case, clumps are unlikely to reach a hydrostatic state, and instead are expected to either proceed directly to collapse, or else ``rebound towards the mean pressure and density of the parent cloud. Rebounding clumps are delayed in their re-expansion by their self-gravity. From a simple virial calculation, we find re-expansion times of a few free-fall times. In the magnetic case, we present a series of driven-turbulence, ideal-MHD isothermal numerical simulations in which we follow the evolution of clumps and cores in relation to the magnetic criticality of their ``parent clouds (the numerical boxes). In subcritical boxes, magnetostatic clumps do not form. A few moderately-gravitationally bound clumps form which however are dispersed by the turbulence in < 1.3 Myr. An estimate of the ambipolar diffusion (AD) time scale t_AD in these cores gives t_AD > 1.3 Myr, only slightly longer than the dynamical times. In supercritical boxes, some cores become locally supercritical and collapse in typical times ~ 1 Myr. We also observe longer-lived supercritical cores that however do not collapse because they are smaller than the local Jeans length. Fewer clumps and cores form in these simulations than in their non-magnetic counterpart. Our results suggest that a) A fraction of the cores may not form stars, and may correspond to some of the observed starless cores. b) Cores may be out-of-equilibrium structures, rather than quasi-magnetostatic ones. c) The magnetic field may help reduce the star formation efficiency by reducing the probability of core formation, rather than by significantly delaying the collapse of individual cores.
In this paper, we review some of the properties of dense molecular cloud cores. The results presented here rely on three-dimensional numerical simulations of isothermal, magnetized, turbulent, and self-gravitating molecular clouds (MCs) in which dense core form as a consequence of the gravo-turbulent fragmentation of the clouds. In particular we discuss issues related to the mass spectrum of the cores, their lifetimes and their virial balance.
A brief summary is presented of our current knowledge of the structure of cold molecular cloud cores that do not contain protostars, sometimes known as starless cores. The most centrally condensed starless cores are known as pre-stellar cores. These cores probably represent observationally the initial conditions for protostellar collapse that must be input into all models of star formation. The current debate over the nature of core density profiles is summarised. A cautionary note is sounded over the use of such profiles to ascertain the equilibrium status of cores. The magnetic field structure of pre-stellar cores is also discussed.
In this study we investigate the formation and properties of prestellar and protostellar cores using hydrodynamic, self-gravitating Adaptive Mesh Refinement simulations, comparing the cases where turbulence is continually driven and where it is allowed to decay. We model observations of these cores in the C$^{18}$O$(2to 1)$, NH$_3(1,1)$, and N$_2$H$^+(1to 0)$ lines, and from the simulated observations we measure the linewidths of individual cores, the linewidths of the surrounding gas, and the motions of the cores relative to one another. Some of these distributions are significantly different in the driven and decaying runs, making them potential diagnostics for determining whether the turbulence in observed star-forming clouds is driven or decaying. Comparing our simulations with observed cores in the Perseus and $rho$ Ophiuchus clouds shows reasonably good agreement between the observed and simulated core-to-core velocity dispersions for both the driven and decaying cases. However, we find that the linewidths through protostellar cores in both simulations are too large compared to the observations. The disagreement is noticably worse for the decaying simulation, in which cores show highly supersonic infall signatures in their centers that decrease toward their edges, a pattern not seen in the observed regions. This result gives some support to the use of driven turbulence for modeling regions of star formation, but reaching a firm conclusion on the relative merits of driven or decaying turbulence will require more complete data on a larger sample of clouds as well as simulations that include magnetic fields, outflows, and thermal feedback from the protostars.
We investigate the uncertainties affecting the temperature profiles of dense cores of interstellar clouds. In regions shielded from external ultraviolet radiation, the problem is reduced to the balance between cosmic ray heating, line cooling, and the coupling between gas and dust. We show that variations in the gas phase abundances, the grain size distribution, and the velocity field can each change the predicted core temperatures by one or two degrees. We emphasize the role of non-local radiative transfer effects that often are not taken into account, for example, when modelling the core chemistry. These include the radiative coupling between regions of different temperature and the enhanced line cooling near the cloud surface. The uncertainty of the temperature profiles does not necessarily translate to a significant error in the column density derived from observations. However, depletion processes are very temperature sensitive and a two degree difference can mean that a given molecule no longer traces the physical conditions in the core centre.
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