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Observations of Convection in A-type Stars

Observations of Convection in A-type Stars. Barry Smalley Keele University Staffordshire United Kingdom. Studies of convection from an observers perspective What effects can we see? What do observations tell us about convection? Theoretical predictions

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Observations of Convection in A-type Stars

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  1. Observations of Convectionin A-type Stars Barry Smalley Keele University Staffordshire United Kingdom

  2. Studies of convection from an observers perspective What effects can we see? What do observations tell us about convection? Theoretical predictions Can we give observers a convection prescription? Introduction

  3. Mixing-Length Theory • A single bubble of rising gas • Rises a certain length before dispersing • Problems: • Too simple! • No prescription for mixing-length • pick your own value!

  4. Turbulent Convection • Canuto & Mazzitelli Model • Using full range of bubble sizes and dispersion lengths • No free parameters! • Implemented in ATLAS9 by Kupka (1996, ASP 44, 356)

  5. Convective Overshooting • Bubbles rise above the convections zone into the stable regions • overshooting • should be present in our models

  6. Approximate Overshooting • “[Kurucz] convective models use an overshooting approximation that moves flux higher in the atmosphere above the top of the nominal convection zone. Many people do not like this approximation and want a pure unphysical mixing-length convection instead of an impure unphysical mixing-length convection.” (http://kurucz.harvard.edu)

  7. Atmospheric Structure OV MLT CM Heiter et al., 2002, A&A 392, 619 • At Teff = 8000K CM gives essentially radiative temperature gradient • less convective flux than MLT • Overshooting introduces flux in higher layers

  8. Realistic Convection Models • None of the current 1d models of convection are totally satisfactory • 2d and 3d numerical simulations (Freytag) • Improved analytical 1d treatments (Kupka) How good are 1d models?

  9. Observational Diagnostics • I will discuss the following: • Photometric colours • Flux distributions • Balmer lines • other line profiles • Mostly based on comparison with Kurucz ATLAS9 models • extensively used • computationally cheap

  10. Photometry • Fast and efficient method for determining atmospheric parameters • many calibration grids • especially uvby system • Indices sensitive to Teff, log g and [M/H], as well as convection and microturbulence

  11. uvby photometry • Comparison with fundamental stars is in good agreement • uvby photometry is good Teff and log g indicator • CM and MLT are good, but no overshooting BUT... Smalley & Kupka, 1997, A&A 293, 446

  12. Bump around 6500K • Bump in difference between log g from uvby and that from evolutionary models for Hyades • related to onset of strong surface convection? Smalley & Kupka (1997)

  13. The m0 index • Traditionally m0 index is poorly fitted • combination of varying mixing-length, microturbulence and overshooting might work?

  14. Stellar Fluxes • Emergent flux influenced by convection’s effect on atmospheric structure • subtle but measurable effects in optical spectrophotometry • In ultraviolet effects more significant • but severe problems with metal line blanketing • Infrared fluxes less sensitive • Infrared Flux Method (IRFM)

  15. Effects on Fluxes @ 8000K • CM and MLT 0.5 similar to no convection • MLT with and without overshooting identical

  16. Effects on Fluxes @ 7000K • Flux highly sensitive to value of mixing-length • Overshooting is radically different

  17. Spectrophotometry • Current spectrophotometry has insufficient resolution and precision to be really useful • The ASTRA robotic spectrophotometer will provide a huge volume of useful stellar fluxes (see Adelman et al. Poster JP2)

  18. Balmer line profiles • Useful diagnostic • strong in A and F stars • sensitive to Teff • insensitive to log g for late-A and cooler • formed at different depths within atmosphere • probe differing parts of atmospheric structure

  19. Balmer profile variations • Changing the efficiency of convection, by increasing mixing length, has significant effect on computed profile Teff = 7000 K, log g = 4.0

  20. Balmer profile sensitivities • H insensitive to mixing-length • H sensitive to mixing-length • Both lines affected by overshooting • sensitive to temperature and metallicity • surface gravity sensitivity for hotter stars Van’t Veer & Megessier, 1996, A&A 309, 879

  21. Fundamental Stars • H and H are in good agreement with fundamental stars • Both CM and MLT (l/H ~ 0.5 preferred) • no overshooting Smalley et al., 2002, A&A 395, 601

  22. H - H Gardiner et al., 1999, A&A 347, 876 • Balmer profiles prefer l/H = 0.5 hotter than 7000K and l/H = 1.25 for cooler stars

  23. What is Microturbulence? • A free parameter introduced to allow abundances from weak and strong lines to agree? • Small-scale motions within atmosphere added to thermal broadening? • Figment of our imagination caused by incomplete physics in 1d atmospheres? • Intimately related to convective motions within the atmosphere?

  24. Microturbulence Variations Based on Gray et al. 2001, AJ 121, 2159 • Microturbulence varies with Teff • increases with increasing temperature • peaks around mid-A type

  25. Line Asymmetries • Line Bisectors • Velocity fields in atmosphere • Rising elements blue shifted • Falling elements red shifted • A-type Stars • small rising columns of hot gas • larger cooler downdrafts • velocities consistent with microturbulence Landstreet, 1998, A&A 338, 1041 Gray’s (1992) Book

  26. No need for microturbulence? • Numerical simulations avoid the need for such a free parameter (Asplund et al., 2000, A&A 359, 729) • de-saturating effects • not turbulent motions • but velocity gradients • No longer a free parameter, but should be constrained when using 1d models

  27. Transition Region • Changing from weak subsurface convection to fully convective. • Observational signatures • e.g. uvby “bump” • Sudden or gradual changes in atmosphere? • Böhm-Vitensse Gap • related to internal structure changes Gray’s Book (1992)

  28. Competing Processes • We cannot treat convection and turbulence in isolation • Diffusion • Rotation • Magnetic fields • Metallicity

  29. Fundamental Stars • Stars with known properties • reduces number of free parameters when comparing observations to models • Need to extend the number and quality of such stars in the A-F star region • including peculiar stars • Need high-quality fluxes, Balmer-line profiles and high-resolution spectra of these fundamental stars. (see Posters BP2, IP1, JP2, JP3 and JP6)

  30. A Prescription for Observers? overshooting? • Schematic variation of microturbulence and mixing length with Teff. • The two appear to be intimately linked

  31. The Surface of an A Star? Thank you!

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