Doppler parameter
The Doppler parameter, or Doppler broadening parameter, usually denoted as, is a parameter commonly used in astrophysics to characterize the width of observed spectral lines of astronomical objects. It is defined as
where is the one-dimensional velocity dispersion. Given this parameter, the velocity distribution of the line-emitting/absorbing atoms and ions proximated by a Gaussian can be rewritten as
where is the probability of the velocity along the line of sight being in the interval.
The line width is also often specified in terms of the FWHM, which is