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Observations of Convectionin A-type Stars

Barry SmalleyKeele University

Staffordshire

United Kingdom

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Introduction

• 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?

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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!

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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)

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Convective Overshooting• Bubbles rise above

the convections zone into the stable regions– overshooting– should be present in

our models

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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)

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• At Teff = 8000K CM gives essentially radiative temperature gradient– less convective flux than MLT

• Overshooting introduces flux in higher layers

Atmospheric Structure

Heiter et al., 2002,A&A 392, 619

CM

MLTOV

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Realistic Convection Models

How good are 1d models?

• None of the current 1d models of convection are totally satisfactory– 2d and 3d numerical

simulations(Freytag)

– Improved analytical 1d treatments

(Kupka)

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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

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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

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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

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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)

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• Traditionally m0 index is poorly fitted– combination of varying mixing-length,

microturbulence and overshooting might work?

The m0 index

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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)

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Effects on Fluxes @ 8000K

• CM and MLT 0.5 similar to no convection• MLT with and without overshooting identical

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Effects on Fluxes @ 7000K

• Flux highly sensitive to value of mixing-length• Overshooting is radically different

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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)

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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

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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

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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

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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

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H - H

• Balmer profiles prefer l/H = 0.5 hotter than 7000K and l/H = 1.25 for cooler stars

Gardiner et al., 1999, A&A 347, 876

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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?

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Microturbulence Variations

• Microturbulence varies with Teff – increases with increasing temperature– peaks around mid-A type

Based on Gray et al.2001, AJ 121, 2159

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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

microturbulenceLandstreet, 1998, A&A 338, 1041

Gray’s (1992) Book

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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

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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)

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Competing Processes• We cannot treat convection and

turbulence in isolation– Diffusion– Rotation– Magnetic fields– Metallicity

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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)

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A Prescription for Observers?

• Schematic variation of microturbulence and mixing length with Teff.– The two appear to be intimately linked

overshooting?

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The Surface of an A Star?

Thank you!