Download - Observations of Convection in A-type Stars
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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!