Friday, August 25, 2006

Convection in Astrophysics, Session B, Oral Contributions (Tue, Aug. 22)

H. Kjeldsen: What Can We Learn About Convection From Asteroseismology?

This talk presented work done together with Tim Bedding and focussed on solar type stars. First, schematic movies of non-radial oscillation modes were shown and the classification of modes by degrees l and radial order n reviewed. The construction of power spectra and echelle diagrams was described. From the echelle diagram, the density and age of a star can be inferred.

For the solar like stars alpha Cen A and B, data have been obtained with UCLES at AAT and UVES at VLT. The precision is 50-70 cm/s with UVES for alf Cen A, i.e. almost as high as with a solar satellite. The echelle diagrams show l = 2, 0, 3, 1 modes, with a much smaller oscillation amplitude for alf Cen B. alf Cen B has a much higher density than alf Cen A and Sun. alf Cen A models do not fit well. alf Cen B models fit better, but small deviations remain.

beta Hydri is a G2 subgiant (an "old Sun"). For this star, CORALIE, UCLES and HARPS data have been obtained (one can "see the star oscillating on screen"). The echelle diagram shows normal l = 2, 0 modes and a crazy l = 1 mode, which could actually be mixed modes (due to so-called avoided crossing). Models by Di Mauro et al. (2003) seem to show indication for avoided crossing. Mixed modes are extremly sensitive to convection in the cores of these stars.

What we can learn about convection from asteroseismology: Mixed modes tell us about core convection (bet Hyrdi), structure the in echelle diagram gives information about the outer convection zone, and the surface can be studied with p-mode lifetimes and the "noise" background.

G. Cauzzi: Solar High Resolution Spectral Observations Compared with Numerical Simulations

In this talk, spatially resolved spectral observations were presented, which can be used to examine the models presented before. A movie of 80 arcsec of the solar surface at 7200Å during 1 hr with a time step of 20 s was shown. The diffraction limit is 0.24 arcsec = 160 km.

The observations are imaging spectroscopy (rapidly tunable narrow band filters, mostly IR with high transparency of 15-20%) using short exposure times (20-50 ms) and a large 2D field of view 60 arcsec squared. Narrow passbands (R > 200000) and sequential spectral sampling (10-20 points per line) is used. The instrument is called IBIS and the system works with adaptive optics. As an example, quiet Sun data from June 2004 were shown - the Fe I line at 709 nm at 16 spectral positions with R = 250000.

Simulations and LTE spectral synthesis for 6 snapshots were provided by M. Asplund, with a horizontal extent of 6x6 Mm (120 km step), matching the observations, and 1 Mm vertical extent (+800 to -200 km, step 10 km). The synthetic data were smeared with the telescope and atmospheric PSF and the instrumental resolution.

The spatial power spectra agree between observations and simulations. The line vs. continuum intensities at +-60 mÅ agree. The intensity distribution with wavelength and the velocity distribution were found to fit as well.

Reverse granulation is seen in the simulation and the observations. When plotting equivalent widths vs. continuum intensity, the simulation lies a little bit higher. Average profiles are fit by the simulations when using the higher Fe abundance (7.67).

  • Excellent agreement of high resolution observations with simulations
  • Validates both realism of simulations and reliability of instrument
  • Useful tool additional to abundance analysis


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