Friday, August 25, 2006

Convection in Astrophysics, Session B, Invited Talks (Tue, Aug. 22)

A.G. Kosovichev: Helioseismic inferences on subsurface solar convection

This talk was given at quite high speed, so it was difficult to take notes. Here is an attempt of a summary.

The idea of helioseismology is to measure travel times of resonant frequencies. Global helioseismology estimates frequencies of normal modes from oscillation power spectra. Time-distance helioseismology measures travel times of acoustic or surface gravity waves.

The depth of the solar convection zone can be measured with helioseismology (about 0.29 solar radii). It is more shallow in polar regions.

Differential rotation produces a "tachocline" - a rotational shear layer mostly below convection zone (see Kosovichev 1996, ApJ 469, L61 - analysis of BBSO data).

There are differences between the standard solar model and seismic models, which result in a difference in the solar radius of about 300 km. This could be caused by convective overshoot at the top of the convection zone.

New local helioseismology provides maps of the solar surface (synoptic maps of subsurface flows). Supergranulation can be observed by time-distance observations, vertical flows are difficult to measure. The observations show that the supergranulation pattern moves faster than the plasma.

Magnetoconvection in sunspots as well as solar cycle variations because of meridional circulations were also discussed.

  • Local and global helioseismology provide important constraints for convection in the Sun.
  • Large scale numerical simulations are needed to interpret the data.
  • Helioseismology can be used to verify simulations.

M. Asplund: Convection and the solar elemental abundances - does the Sun have a sub-solar metallicity?

Solar system abundances can be measured in meteorites (very high accuracy, but depleted in volatile elements) or the solar atmosphere (modelling dependent, very little depletion). The solar atmosphere is dynamic and three dimensional (3D) due to convection.

1D solar atmosphere models make various simplifying assumptions, but have the advantage that radiative transfer can be treated in detail. 3D solar atmosphere models are more realistic but have simplified radiative transfer. They are essentially parameter free.

The temperature structure in 3D is very different from 1D - it is very steep in upflows and there are significant inhomogeneities. The mean 3D structure is similar to 1D MARCS models but cooler than the semi-empirical Holweger-Müller model.

3D line profiles differ from 1D profiles. The spectrum formation is highly non-local and non-linear and strongly biased towards upflows. Profiles of an observed solar Fe line were shown and compared with 1D and averaged 3D line profiles. The 3D profile agrees without the need for micro- and macroturbulence. Line asymmetries and shifts are very well reproduced.

Solar C, N, O abundances have been derived using a 3D solar atmosphere model, non-LTE line formation when necessary, and atomic and molecular lines with improved data. Details are in Asplund et al. (2000-2006).

Oxygen diagnostics: In 1D, atomic and molecular lines give discordant results (log O = 8.6-8.9). In 3D, there is excellent agreement. The [O I] 630 nm line is blended with Ni, which gives a correction of -0.13 dex (not noticed in 1D), actually larger than the difference 3D minus 1D (-0.08 dex). The O I 777 nm feature has been calculated with full 3D non-LTE line formation, and the main difference is non-LTE minus LTE (-0.2 dex). This is most significant at the limb. OH vibration-rotation lines in the infrared have been calculated with 3D LTE line formation. Molecular lines are extremely sensitive to temperature.

Carbon diagnostics: Again, 1D there are discordant results (8.4-8.7), while in 3D there is excellent agreement. CO vibration-rotation lines as well as weak CO lines give an abundance of 8.39+-0.05. There are still problems with the strongest CO lines. C and O isotopic ratios can be derived, which agree with terrestial values.

There are also preliminary 3D results for all elements from Li to Ni.

The implications are a siginificantly lower solar metal mass fraction Z - from 0.0194 (Anders & Grevesse 1989) to 0.0122 (Asplund et al. 2005). This alters the cosmic yardstick, and makes the Sun normal compared with its surroundings (e.g. OB stars). The problem is that solar interior models with the new abundances are in conflict with helioseismology (there is no solution yet).

  • 3D + non-LTE + new atomic/molecular data give significantly revised solar CNO abundances
  • The new abundances solve a lot of problems but are terrible for solar modelling
  • Coming soon: New 3D models with improved radiative transfer treatment


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