Convection in Astrophysics, Session D, Oral Contributions (Wed, Aug. 23)
H. Shibahashi: The DB gap of white dwarfs and semiconvection
This talk was given by Mike Thompson on behalf of Shibahashi.
First, a general overview of the classification and properties of white dwarfs (WDs) was given. The classification of WDs reflects their surface composition, but not their temperature. The spectra of DA WDs show only hydrogen lines (their atmospheres consist of pure H), those make up about 80% of all WDs. DB WDs show only He I lines (pure He). There are also DO (showing He II lines), DC, and other classes. DAs are found from hottest to coolest temperatures, DOs for Teff > 45000 K, DBs for Teff < 30000 K. No He-rich WDs are found between 45000 and 30000 K, this constitutes the DB gap.
A plot of the number of DA WDs vs. Teff from McCook & Sion (1987) catalog was shown. At about 11000 K, one can see a step in the ratio of DA/non-DA WDs.
Why does the DB gap exist? The simple picture of parallel sequences of H and He-rich objects doesn't work. There is a theory of spectral evolution (by Fontaine and Wesemael), which proposes a common origin for WDs (PNN). At 30000 K, He ionization creates convection, and He is mixed to the surface. In the spectral evolution model, a wide range of H layers is expected (10^-4 to 10^-13 solar masses).
In observations, there has been much progress recently, mainly by large surveys.
The new working hypothesis is that all WDs have some H. Only about 10^-15 (solar masses?) is needed to produce an optically thick H layer at the surface. For Teff > 45000 K, the He II/III zone creates turbulence which mixes H with He and leads to He stars. For gap stars, the He II/III zone is too deep to mix up H, and gravitational settling leads to the He envelope. At 30000 K, He I/II creates turbulence which mixes H with He and leads to He stars.
One can make a prediction of semiconvection based on this scenario. At 30000 K, the He ionization zone turns into a convectively stable layer, which is nontheless superadiabatic. This is a plane-parallel, gravitationally stratified layer of fluid in hydrostatic and radiative equilibrium, with a steep chemical gradient. The equations for this situation were shown, employing the Boussinesq approximation. The physical cause of the overstability is that radiative heat exchange brings about an assymmetry in the oscillary motion, and this leads to overshooting.
There are some open issues.
In summary, there are two groups of WDs, and a DB gap. Convective mixing and/or chemical separation might be responsible for the gap. A new type of WD variables is predicted near the red edge of the DB gap.
M. Spite: Extra-mixing in Extremely Metal-Poor red giants
This talk presented some results of the "First Stars" project. The aim of this project is an analysis of the chemical composition of the galactic matter in the early times. This works only if there is no mixing. 18 dwarf and 33 giant stars (not C-rich) with Fe/H <= -3.0 were selected, and high resolution spectra (R ~ 45000) obtained.
As an example, spectra of the Mg b lines and the NH band were shown. An LTE abundance analysis with OSMARCS models was performed.
For the discussion of Mg in turnoff stars (dwarfs and giants), a plot of [Mg/Fe] vs. [Fe/H] was shown. One could see that the abundance ratio is constant, with a scatter of about 0.1 dex. This is more or less the same for all elements. Exceptions are C and N in giant stars - [C/Fe] shows an extremely large scatter, and N is even worse.
What is the reason for the large scatter for C and N in giants? In a primordial scenario, it would mean that there was a real scatter in the ISM of the early Galaxy. In the in situ scenario, the original C and N abundances in the atmospheres of the giant stars have been altered.
Indicators of mixing in giants are:
- An anticorrelation of C and N: In a plot of [N/Fe] vs. [C/Fe], we see two groups of stars: mixed stars with [N/Fe] of about 1 and [C/Fe] <>
- A very low abundance of Li in mixed stars is expected. The Li abundance decreases as carbon 13 increases.
- The phenomenon must appear at a specific location in the HR diagram.
Next, comparison to the results of Gratton et al. (2000) was made. The mean metallicity of the Gratton et al. stars is -1.5 dex, whereas the mean metallicity of the "first stars" is -3.1 dex. Mixing appears at a higher luminosity for the "first stars", but in both cases at the location of the bump.
A discussion of the abundances of Na and Al in these stars showed that some mixed stars are Na or Al rich (none of the unmixed stars). This could be due to deep mixing. Maybe some mixed stars are AGB stars (no effect is seen in oxygen, maybe the effect is too small).
As soon as an extremely metal poor star reaches the luminosity of the bump, its atmosphere is mixed with the H burning layer and the abundances of the light elements are altered.
P.P. Eggleton: Two Instances of Convection and Mixing in Red Giant Interiors
The actual title of the talk was "Formation and destruction of 3He in low-mass stars - Big Bang nucleosynthesis rescued".
The discovery of a very important mixing process taking place on the AGB was presented. The mechanism is a Rayleigh-Taylor instability, driven by an unusual nuclear reaction of 3He. He is produced in the interior on the main sequence, then mixed into the convection zone.
The reaction equation is: 3He + 3He -> 4He + p + p
As an example, the evolution of an 0.8 solar masses population II star was shown in the HR diagram. The element distribution in the star at the end of the main sequence is as follows. There is a small exhausted core, and a lot of 3He is produced and mixed into the surface layers. There is a maximum in molecular weight at the bottom of the H burning shell at 3 million years after the turnoff (?). This leads to a little 3He burning shell.
For the simulations, the 3D hydrodynamics code "Djehuti" was used, which is described in Dearborn, Lattanzio and Eggleton (2006, ApJ 639, 405), a paper on the He flash. It implements explicit hydrodynamics, is run on 351 processors, and the timestep is Courant limited.
The Rayleigh-Taylor instability does not remove the molecular weight gradient that drives it. It is constantly replenished. Mixing advects fresh 3He in at the same rate as it advects products out.
The results were presented as a movie of the global stellar surface.