Empirical decoding of the shapes of spectral-line bisectors

Abstract

The great variety of shapes found for bisectors of spectral lines seen across the cool half of the Hertzsprung-Russell diagram are explained and empirically decoded using a novel method. The velocity gradient through the photosphere, as delineated by the star's third-signature plot, sets the basic shape of the bisector, which is then modified by the asymmetry in the amount of light coming from rising versus falling material. With this concept in place, one can then derive the apparent deficit of flux from downward flowing material in the granulation structure, and this is done for nine stars. The deficits peak between 3 and 4kms-1 for dwarfs and between 4 and 5kms-1 for giants, and they span a range from ∼1 to 10kms-1 for all stars. The amplitudes of the flux deficits increase with effective temperature and with luminosity and have values ∼7%-10% for dwarfs and ∼15%-20% for giants. Such measurements of the contrast between granules and dark lanes are independent of models of granulation. © 2010. The American Astronomical Society. All rights reserved.