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Doppler parameter

From Wikipedia, the free encyclopedia

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 (Draine 2011, p. 58). 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 (full width at half maximum), which is

.

Distribution

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The Doppler parameters of Lyman-alpha forest absorption lines are in the range 10–100 km s−1, with a median value around that decrease with redshift (Kim et al. 1997). Analyses of the HST/COS dataset of low-redshift quasars gives a median parameter of around (Danforth et al. 2016, Gaikwad et al. 2017).

See also

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References

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  • Danforth, Charles W.; Keeney, Brian A.; Tilton, Evan M.; Shull, Michael (2016). "An HST/COS Survey of the Low-redshift Intergalactic Medium. I. Survey, Methodology, and Overall Results". Astrophysical Journal. 817 (2): 111. arXiv:1402.2655. Bibcode:2016ApJ...817..111D. doi:10.3847/0004-637X/817/2/111. S2CID 118631947.