The Dynamics of the Interstellar Medium. II. Radiation Pressure.
Abstract
Radiation pressure will repel dust particles from a late-type star of high luminosity in the manner discussed by Schalén, while the atoms will be relatively unaffected by the encounter. Near a star of early type, however, the hydrogen is ionized, and the large interaction between dust and protons will force the interstellar medium to behave as a uniform homogeneous gas, provided that the velocity of the star is not much greater than io km/sec. The radiation pressure on dust near such a star will therefore tend to repel all the constituents of the medium, though the effect is considerably less than would appear if only dust were present. The pressure of radiation on atoms is less important; the radiation pressure per unit mass on interstellar atoms other than hydrogen is shown to be less, on the average, than the corresponding force per unit mass on dust. In the case of hydrogen the scattering of La radiation is so great that a quantum of La will diffuse slowly through space and will be absorbed by a dust particle before it has gone more than a fifth of a parsec; radiation pressure on hydrogen atoms is therefore negligible. It follows that for any one star the radiation pressure on the dust particles in a unit volume is much greater than that on all the atoms in the same volume. The direct effects of the general galactic radiation field are small. The radiative force on dust particles in the galactic plane will produce a slight difference between the medium as a whole and the stars in the net attraction of each toward the galactic center, the difference in equilibrium rotational velocities amounting to not more than a few tenths of a kilometer per second in the neighborhood of the sun. If the short time scale is assumed, interaction between dust and atoms will prevent any sorting-out of the different constituents of the medium under the pressure of radiation either parallel or perpendicular to the galactic plane. The galactic radiation field is indirectly responsible for a force between two dust particles. The absorption of radiation by two particles will reduce the energy density between them; radiation pres- sure will therefore push the particles toward each other with a force which varies as the inverse square of the distance between them. For Fe particles with a radius of less than io5 cm. this force is more than two hundred times the mutual gravitational attraction of the two particles
- Publication:
-
The Astrophysical Journal
- Pub Date:
- September 1941
- DOI:
- 10.1086/144328
- Bibcode:
- 1941ApJ....94..232S