The Collision Origin of Cohen's Stream

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Session 12 -- Interstellar medium
Display presentation, Monday, 9, 1995, 9:20am - 6:30pm

[12.13] The Collision Origin of Cohen's Stream

C.~Tamanaha (UC Berkeley)

High spacial and spectral resolution H I observations of the high-velocity cloud in Cetus called ``Cohen's Stream'' reveal 21 cm features at velocities of $-280$, $-120$, $-37$, and $-3$ km s$^{-1}$. These new observations show in detail that the high- and very-high-velocity gas are anticorrelated in position on the sky. These observations cover only a small portion of H I clouds involved, but the anticorrelation persists on the larger scales mapped by Cohen (1981, 1982). The obvious explanation is that the high-velocity gas is the remnant of a collision between the very-high-velocity gas and a low-velocity cloud. A simple mass and momentum conserving simulation demonstrates the elegance of this model. The asymmetric velocity distribution of the individual pixels composing the remnant is a natural consequence of the different sizes of the parent clouds. Reasonable column density fluctuations added to the simulated parent clouds result in a remnant whose velocity distribution is much broader than observed. Some form of damping or momentum redistribution is needed to reduce the large velocity dispersion. The redistribution of momentum by the enhanced magnetic field in the compressed postshock gas appears to be the most promising damping mechanism. The magnetic tension in the field lines accelerates slower moving components while decelerating the faster moving ones. Recently shocked remnants are expected to have strong asymmetries in their velocity distributions. Older remnants will have more symmetric distributions, owing to the longer time over which the enhanced magnetic field has been allowed to damp the disparate velocities produced in the collision.

This research has been supported in part by NSF grant FD91-23362 to Carl Heiles.

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