Stellar Granulation - Part Four - Line Formation in Inhomogeneous Stellar Photospheres

Details Stellar Granulation - Part Four - Line Formation in Inhomogeneous Stellar Photospheres Stellar Granulation - Part Four - Line Formation in Inhomogeneous Stellar Photospheres

Feb 1990

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990a%26a...228..184d&link_type=abstract

Astronomy and Astrophysics, Vol. 228, NO. 1/FEB(I), P. 184, 1990

Astronomy and Astrophysics

Astrophysics

63

Convection, Hydrodynamics, Lines: Formation, Lines: Profile, Radiation Transfer, Stars: Atmospheres Of, Stars: Late-Type

Scientific paper

Synthetic images of stellar granulation and photospheric Fe line profiles are computed in model atmospheres obtained from detailed numerical simulations of stellar surface convection. Models corresponding to Procyon (F5 IV-V), α Cen A (G2V), β Hyi (G2IV), and β Cen B (K1V) are studied (5200 ≤Teff≤6600 K). The broadening, wavelength shift and asymmetry of spatially and temporally resolved line profiles follows from radiative transfer in explicitly computed three- dimensional and time-variable velocity fields, and no adjustable - fitting parameters (such as e. g. "turbulence") are used. Synthetic white-light and monochromatic images illustrate the intensity contrast on stellar surfaces, its center-to-limb variation and the morphology of line formation. Spatially resolved and spatially averaged profiles illustrate line broadening through the Doppler effect in photospheric velocity fields. An increase in the velocity spread of spatially resolved lines near the stellar limbs reflects the larger amplitudes of horizontal velocities in line-forming layers. Time variability of spatially averaged line profiles and of their continuum flux levels reflects time evolution of convective patterns larger than individual granules. Spatially and temporally averaged data identify how different shapes, asymmetries and shifts among lines of different strength, excitation potential, ionization level, and wavelength region, map the detailed physical properties throughout the photo sphere. The properties of averaged profiles (in particular their asymmetries) are not at all typical for individual points on the stellar surface, but rather reflect the statistical distribution of photospheric inhomogeneities. Only very strong lines have sufficiently extended depths of formation for their asymmetry to be significantly influenced also by the depth-variation of photospheric flow velocities. Effects of the (non-LTE) radiative ionization of iron are not large but visible as a weakening of blueshifted Fe I line components above especially hot and bright granules. Convective blueshifts, originating from correlations between local brightness and local Doppler shift, vary between ˜=200 and 1000 ms-1 at disk center in different stars. Since such correlations change throughout the atmosphere, already small differences in line formation conditions for lines of different strength or excitation potential may result in different asymmetries and wavelength shifts. For example, the lower surface gravity on the solar near-twin α Cen A permits larger velocity amplitudes in the high photosphere, causing noticeable differences to the Sun in the asymmetries of its stronger photospheric lines.

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