The gas density gradient for three dark interstellar clouds

1984 ◽  
Vol 287 ◽  
pp. 723 ◽  
Author(s):  
S. A. Fulkerson ◽  
F. O. Clark
1967 ◽  
Vol 31 ◽  
pp. 131
Author(s):  
Donat G. Wentzel

Petschek's solution for the hydromagnetic flow at a magnetic neutral plane is applied to the interior of interstellar clouds. The stationary flow between the opposed magnetic fields involves a streaming speed of about 3 km/s and a gas density of roughly 102atoms/cm3. At most one-tenth of all the gas resides in these streams, but their low temperatures should make them observable in absorption at 21 cm; they may even dominate the interstellar absorption lines of Ca+and of Na, since the abundance of these absorbing ions, proportional to the recombination rate of the dominant ions with electrons, depends on the square of the gas density. Optical and 21-cm lines may, therefore, represent somewhat different samples of the interstellar velocity and density distributions.In analogy to the geomagnetic field, which is separated from the interplanetary field, interstellar clouds may be separated from the general galactic magnetic field. Such magnetic bubbles can move through the galactic field without twisting it, but they can maintain their identity only if they contain a twisted magnetic field. This field leads to the operation of Petschek's Mechanism and makes these clouds prominent in the optical absorption lines. A full evaluation of Petschek's Mechanism must involve some plasma instability to reconnect the lines of force.Details of this work have appeared elsewhere (Wentzel 1966).


1997 ◽  
Vol 166 ◽  
pp. 409-412
Author(s):  
Ch. Theis ◽  
S. Ehlerová ◽  
J. Palouš ◽  
G. Hensler

AbstractWe investigate the gravitational fragmentation in expanding shells by applying an instability ’thermometer’ similar to the Toomre parameter for instabilities in self-gravitating disks. For Sedov–like evolving systems the onset of instability is mainly depending on the density of the ambient medium and the sound speed of the shell matter, whereas the energy injection rate is less important. Shells evolve towards gravitational instability, if the density gradient of the ambient medium is shallower than an isothermal profile, otherwise they become more stable. For density gradients flatter than ∝ r−1, the fragmentation becomes non-linear on the same time scale as the gravitational instability needs to start. In a homogeneous ambient medium the typical size of gravitationally unstable shells is 1 kpc for a gas density of n = 1 cm−3 and decreases to 10 pc for n = 104 cm−3.


1999 ◽  
Vol 173 ◽  
pp. 45-50
Author(s):  
L. Neslušan

AbstractComets are created in the cool, dense regions of interstellar clouds. These macroscopic bodies take place in the collapse of protostar cloud as mechanically moving bodies in contrast to the gas and miscroscopic dust holding the laws of hydrodynamics. In the presented contribution, there is given an evidence concerning the Solar system comets: if the velocity distribution of comets before the collapse was similar to that in the Oort cloud at the present, then the comets remained at large cloud-centric distances. Hence, the comets in the solar Oort cloud represent a relict of the nebular stage of the Solar system.


1979 ◽  
Vol 44 ◽  
pp. 307-313
Author(s):  
D.S. Spicer

A possible relationship between the hot prominence transition sheath, increased internal turbulent and/or helical motion prior to prominence eruption and the prominence eruption (“disparition brusque”) is discussed. The associated darkening of the filament or brightening of the prominence is interpreted as a change in the prominence’s internal pressure gradient which, if of the correct sign, can lead to short wavelength turbulent convection within the prominence. Associated with such a pressure gradient change may be the alteration of the current density gradient within the prominence. Such a change in the current density gradient may also be due to the relative motion of the neighbouring plages thereby increasing the magnetic shear within the prominence, i.e., steepening the current density gradient. Depending on the magnitude of the current density gradient, i.e., magnetic shear, disruption of the prominence can occur by either a long wavelength ideal MHD helical (“kink”) convective instability and/or a long wavelength resistive helical (“kink”) convective instability (tearing mode). The long wavelength ideal MHD helical instability will lead to helical rotation and thus unwinding due to diamagnetic effects and plasma ejections due to convection. The long wavelength resistive helical instability will lead to both unwinding and plasma ejections, but also to accelerated plasma flow, long wavelength magnetic field filamentation, accelerated particles and long wavelength heating internal to the prominence.


Author(s):  
Lee F. Ellis ◽  
Richard M. Van Frank ◽  
Walter J. Kleinschmidt

The extract from Penicillum stoliniferum, known as statolon, has been purified by density gradient centrifugation. These centrifuge fractions contained virus particles that are an interferon inducer in mice or in tissue culture. Highly purified preparations of these particles are difficult to enumerate by electron microscopy because of aggregation. Therefore a study of staining methods was undertaken.


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