J.D. Landstreet: Observing Atmospheric Convection in Stars (Review, Tue, Aug. 22)
This review talk opened Session B of the Convection Symposium: "Observational Probes of Convection". It gave an overview of the classical ways to detect convection in observations.
Convection reaches the photosphere in most stars of Teff < 10000 K, perhaps also in hotter stars. Convection cells are directly visible in the Sun as granulation. In stars, convection can be detected indirectly as velocity fields (microturbulence, macroturbulence, bisector curvature, etc.).
Microturbulence is excess line broadening over thermal broadening, required to fit weak and strong lines. The microturbulence parameter characterizes a velocity field. For example, for Sirius, fitting weak and strong lines requires different abundances. Add a Gaussian velocity field of 2.2 km/s and all lines are fit with one abundance. Microturbulence is required for most stars with Teff < 10000 K and corresponds to convective instability, at least in cooler stars It is detectable even in broad-lined stars. Since it is only one number, one can characterize the variation of the amplitude of velocity fields across the HR diagram, but no further information can be derived.
Spectral line profiles show asymmetries due to asymmetric flows. The distortion should depend on where in the granule the line is formed. Different areal coverage of rising and falling plumes cause asymmetries and shifts in the line profiles.
Macroturbulence is required to model line profiles of most main sequence stars. Line profiles of giants and supergiants are more "pointed", with broad shallow wings. Again, one parameter (zeta) represents a characteristic velocity. Zeta varies systematically across the cool part of the HR diagram (F0 to K5). For hotter stars, rotation masks macroturbulent velocity fields. Macroturbulence drops to zero above A0V. Among hotter stars (Teff > 10000), the situation is confusing (see Lyubimkov et al. 2004 and Przybilla et al. 2006).
Bisector curvature (line profile asymmetry) is another way to detect convection. Gray and Nagel (1989) showed that bisectors are reversed for cool (K) vs. hotter stars (F), with the reversion taking place at about G0. A stars have reversed bisectors, late B stars have no curvature at all.
The use of 3D models is limited - if one disagrees with observation, testing changes is time consuming. On the other hand, MLT and other convection models can be used for testing.
- Stellar atmosphere velocity fields are clearly detectable in the spectrum.
- The behaviour over the HR diagram is varied, the largest velocities are found in supergiants
- Modelling is making progress at connecting convection theory with observations.