1. The A-star puzzle Poprad, Slovakia A-type stars as physics laboratories John D. Landstreet Department of Physics & Astronomy University of Western Ontario London, Canada

2. The A-star puzzle Poprad, Slovakia What are the A-type stars? On the main sequence, 7000 < T_e < 10000 K, and 1.6 < M/M_o < 3 Note that during MS life, T_e decreases by about 30% A-stars cover transition from stars that evolve to giant branch like sun, to stars that evolve to giants at almost constant luminosity Surface convective zone depth increases rapidly as T_e decreases Progenitors are Herbig AeBe stars, very active, found singly and in star-forming regions

3. The A-star puzzle Poprad, Slovakia Why are A stars useful as laboratories? Microscopic diffusion is interesting, and useful a probe of other physical processes, via resulting atmospheric abundances To be useful, diffusion velocities should be comparable to velocities of other flows In early B stars, diffusion is overwhelmed by stellar wind so abundances are only a weak probe of interior processes In solar-type stars, convective envelope is deep, so abundance changes due to diffusion are strongly diluted in massive envelope, and furnish only small clues about interior processes In A stars, no other processes overwhelm diffusion, so atmospheric abundances furnish many obvious clues. This is the main reason A stars are such good laboratories Furthermore, we can study pre-main sequence precursors of main sequence A stars in the Herbig AeBe stars

4. The A-star puzzle Poprad, Slovakia Chemical peculiarities – diffusion as a process Under gravity alone, all elements would sink in H medium Radiative acceleration counters this for elements of low abundance and rich spectra Net acceleration varies with ion and abundance, leading to time variable stratification Diffusion competes with other velocities and is affected by outer boundary conditions such as winds Magnetic fields are observed to have very strong effects on visible results of diffusion in atmosphere Ions may diffuse through convective zones, although these are well mixed

5. The A-star puzzle Poprad, Slovakia Chemical peculiarities – diffusion as a probe Diffusion calculations have not yet explained many observed abundance patterns, probably due to competition from winds and mixing processes Sensitivity to competing processes is precisely what makes diffusion useful as a probe Better identifications of cluster members (Hipparcos, Tycho-2) help us to study time variations of results of diffusion Because depth of convection zone varies strongly through A star region, we can use bottom of zone as a sliding probe

6. The A-star puzzle Poprad, Slovakia Diffusion as a probe (continued) Nice example of diffusion as a probe: S. Vauclair’s theory of He- rich magnetic stars: stellar wind levitates He into the atmosphere, where it becomes neutral and decouples from wind, thus accumulating in visible layers Neat aspect of this theory is that it reveals an otherwise invisible stellar wind Another example: Richer, Michaud & Turcotte explain Am spectra by assuming turbulent mixing of envelope down to about 10^-4 M_o. Again this theory reveals a process not directly visible

7. The A-star puzzle Poprad, Slovakia Atmospheric velocity fields Mixing length theory (MLT) of convection is essentially an order- of-magnitude estimate that gives inexact results for inefficient convection Numerical 3D hydrodynamical computations provide a complement to MLT, but have limitations and are very costly New models of convection are being developed (Canuto; Kupka), and these may be tested against observations of atmospheric velocity fields Velocities visible in spectral lines that simple microturbulent line theory cannot fit, such as lines of HD 108642 (Landstreet) Much better fit can be obtained with xi that varies with height, rapid downflow in small area, slow general upflow – but this model disagrees with the current 3D computations!

8. The A-star puzzle Poprad, Slovakia Magnetic fields Many A (and B) stars have large-scale, roughly dipolar fields of kG strength which affect both atmospheres and global properties Fields lead to low (to extremely low) specific angular momentum, and to non-radial pulsations in roAp stars Fields also cause distinctive atmospheric abundance anomalies and to very inhomogeneous abundances Ap stars provide us with lab to observe interactions of strong field with large-scale plasma, combined with range of convective instabilities Two advantages of Ap stars for such studies: substantial range of fields (0.3 – 30 kG) observed, and fields are relatively homogeneous, making interpretation easier than for very inhomogeneous solar-type stars

9. The A-star puzzle Poprad, Slovakia Magnetic fields (continued) List of unsolved problems is impressive: Uncertain how fields cause such loss of angular momentum (but Stepien has plausible semi-empirical model) Not yet clear how fields achieve their structure, or how they evolve with time, in spite of extensive computations by Moss The Ap fields may become the MG fields of white dwarfs after the red giant stage, but no detailed calculations support this yet Detailed diffusion computations (e.g. Babel & Michaud) do not yet reproduce observed abundances – nature of upper boundary is a substantial uncertainty here What are the effects of the field on the atmospheric structure?

10. The A-star puzzle Poprad, Slovakia Rotation and braking Herbig Ae stars and their A star descendents provide a lab for the study of the evolution of angular momentum in a variety of settings, and for a large range of specific angular momentum One can study the effects of rotation on structure: for example Charbonneau & Michaud have studied how the meridional circulation caused by rotation can produce mixing that competes with diffusion One can also study the processes that affect rotation: e.g. Stepien’s study of angular momentum loss, or Stepien & Landstreet’s explanation of the recent discovery (Landstreet & Mathys) that the slowest rotators have field axes roughly aligned with the rotation axes, while more rapid rotators mostly have highly oblique field axes

11. The A-star puzzle Poprad, Slovakia Pulsation Some cool A stars show delta Scuti (low overtone p-mode) pulsations with numerous periods (e.g. Breger) of order 1 hour (roughly, this is an extension of the Cepheid strip to the MS) Some cool Ap’s show roAp behaviour (high overtone p-mode) pulsations, again with numerous periods of ~10 min; these pulsations are now detected in spectral lines, especially of doubly ionized rare earths (Ryabchikova, Kochukhov) A new group is the gamma Doradus stars (multi-periodic g- mode oscillations) with periods around 1 day Since all these classes show multiple periods, the amount of information they offer about interior structure is substantial (in principle, when modes have been identified!), but interpretation is still very incomplete

12. The A-star puzzle Poprad, Slovakia Conclusions The A-type stars that are the subject of this meeting offer a wide range of possibilities for use as laboratories to study stellar physics