2. Hydrogen line wing broadening mechanisms
Hydrogen lines are broadened by a number of different mechanisms. In the wings of the lines the collisional processes dominate the line profile shape. The more important of the known broadening mechanisms for hydrogen line wings in stellar atmospheres are
The first three of these are expected to dominate with relative contributions depending on the effective temperature and metallicity of the star, and the particular line under study.
The quasistatic ion field splits the line up into Stark components which are broadened by collisions with fast moving electrons, hydrogen atoms and helium atoms. As they carry an overall electric charge, electrons are substantially more effective at broadening than hydrogen atoms. However, in cool stars like the sun, with a solar composition, hydrogen atoms outnumber electrons by typically four orders of magnitude and by perhaps six orders of magnitude if the star is metal deficient. This results in collisions with hydrogen atoms being very important. In cool stars, helium atoms usually have a number density about an order of magnitude less than hydrogen atoms, they have no resonance interaction with hydrogen and have only half the polarisability and speed of hydrogen atoms so their contribution to the broadening is relatively unimportant. Broadening by collisions with hydrogen molecules will only become important in very cool stars which are not considered here.
© European Southern Observatory (ESO) 2000
Online publication: December 5, 2000