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Ongoing Research

The Dynamics of Starless Cores

Papers on this topic:

          Broderick, A.E., Narayan, R., Keto, E. & Lada, C.J., 2008, ApJ, 682, 1095
The Damping Rates of Embedded Oscillating Starless Cores

          Broderick, A.E., Keto, E., Lada, C.J. & Narayan, R., 2007, ApJ, 671, 1832
Oscillating Starless Cores: The Nonlinear Regime

          Broderick, A.E. & Rathore, Y., 2006, MNRAS, 372, 923
A multidimensional, adiabatic hydrodynamics code for studying tidal excitation

          Keto, E., Broderick, A.E., Lada, C.J., Narayan, R., 2006, ApJ, 652, 1366
Oscillations of starless cores


             The story of star formation begins with the gravitational collapse of over densities in turbulent molecular clouds.   Gravitationally bound dark, starless cores (e.g., Barnard 68) are observed in molecular clouds, and are thought to be related to the stellar progenitors.  However,  whether or not these are in hydrodynamic equilibrium is less clear.  Answering this question will have significant implications for the efficiency of star formation.

             The Bok globule Barnard 68 exhibits a column density profile in nearly exact agreement with that one would expect in the case of a gravitationally bound cloud in hydrostatic equilibrium.  However, velocity maps produced observations of the self-absorbed CS lines imply that the outer layers of the cloud are undergoing complex motions, and the core is strongly elliptical.  With collaborators Eric Keto, Charlie Lada and Ramesh Narayan, I have been investigating the possibility of modeling these observations using a pulsating, isothermal cloud.  We have found using a linear perturbation analysis that all of these features may be accounted for with a quadrupolar oscillation!

             Because the oscillation required to reproduce the observations is uncomfortably large for the linear analysis.  We are also investigating the non-linear evolution of a deformed, isothermal gas sphere using an adiabatic, Eulerian hydrodynamic code that I had originally developed with Yasser Rathore to study white dwarf oscillations.  Generally we have found that even extremely large amplitude oscillations persist for millions of years, longer than the inferred lifetimes of starless cores.

             Of considerable importance for the interpretation of observations, we have found that many of the spectral-line signatures attributed to collapse or expansion can be reproduced by stable oscillating globules, suggesting that observational attempts to characterize the dynamical states of the progenitors of stars may be confounded by long-lived pulsations.  As a consequence additional data may be required to determine which starless cores are collapsing to form stars, which are blowing apart due to turbulence in the surrounding molecular gas, and which are simply pulsating!

H2 column densities of a perturbed isothermal gas sphere as seen at various orientations.   Note the high ellipticity!

The velocities from the self-absorbed CS line for a perturbed isothermal gas sphere as seen at various orientations. 

The equatorial density and velocity distribution of a cloud undergoing a dipole oscillation.  Also shown are the CS line-shapes, which are color coded to show lines for which the outer cloud material is moving towards (blue) and away (red) from the observer.  (Click to enlarge!)

Line-shapes of the self-absorbed CS line overlaid upon the column densities of a cloud undergoing a quadrupole oscillation.  The lines are color coded to show which lines of sight appear to be moving towards (blue) and away (red) from the observer.  This view reproduces observations of Barnard 68!   (Click to enlarge!)