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| Paper: |
Stellar Surface Convection, Line Asymmetries, and Wavelength Shifts |
| Volume: |
185, Precise Stellar Radial Velocities, IAU Colloquium 170 |
| Page: |
268 |
| Authors: |
Dravins, D. |
| Abstract: |
When observed under sufficient resolution, practically all stellar spectral lines prove to be slightly asymmetric. Absorption lines in cooler stars form in inhomogeneous atmospheres, affected by surface convection (stellar granulation). Asymmetries and wavelength shifts originate from correlated velocity and brightness patterns: rising ( = blueshifted) elements are hot (=bright), and convective blueshifts result from a larger contribution of such blueshifted photons than of redshifted ones from the sinking and cooler (=darker) gas. For the Sun, the effect is around 300 m/s. High-excitation lines form predominantly in the hottest elements and show a more pronounced blueshift. The effects are predicted to be greater in F-type stars, and in giants. In the presence of magnetic fields, convection is disturbed and granules do not develop to equally large size or velocity amplitude, resulting in smaller blueshifts (by perhaps 10% or 30 m/s) during the years around activity maximum in the 11-year solar cycle. Such activity-cycle induced lineshift variations must of course be segregated from stellar velocity signals in searches for exoplanets with comparable periods. While line asymmetries and shifts may appear as a noise source in determining stellar motions, they are an important observational signature for constraining three-dimensional (magneto-) hydrodynamic models of stellar atmospheres. These are capable of predicting not only line-widths and shapes, but also second-order quantities such as asymmetries and shifts. A high measuring precision reveals properties of the stellar surface structure also through the temporal variability of stellar line wavelengths. On the visible solar disk, there are on the order of 10**6 granules, each with a velocity amplitude of some 2 km/s, evolving over some 10 min. In integrated sunlight, this amplitude is reduced by a factor of about sqrt(10**6) to perhaps 2 m/s. Stars with larger velocity amplitudes and/or fewer granules will show correspondingly greater fluctuations, observable already with current techniques. Until the present, wavelength-shift observations have generally been for unresolved (i.e. spatially averaged) stellar disks. A major future development will be the study of wavelength variations across spatially resolved stars, e.g. the center-to-limb changes along the equatorial and polar diameters, and their spatially resolved time variability. Adaptive optics on very large telescopes, long-baseline optical interferometry, and optical aperture synthesis will soon open up new vistas of stellar atmospheric physics through radial-velocity observations. |
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