august 22, 2006iau symposium 239 observing convection in stellar atmospheres john landstreet london,...

August 22, 2006August 22, 2006 IAU Symposium 239IAU Symposium 239

Observing Convection in Observing Convection in Stellar AtmospheresStellar Atmospheres

John LandstreetJohn Landstreet

London, CanadaLondon, Canada


IntroductionIntroduction

• Convection reaches photosphere in most Convection reaches photosphere in most stars of Tstars of Tee < 10 < 1044 K, perhaps also in hotter K, perhaps also in hotter starsstars

• Directly visible in Sun as granulationDirectly visible in Sun as granulation

• Detected in stars as microturbulence, Detected in stars as microturbulence, macroturbulence, bisector curvature, etcmacroturbulence, bisector curvature, etc

• Comparison of convection models with Comparison of convection models with observed spectra provides observed spectra provides interpretation interpretation of observations and tests of modelsof observations and tests of models


Solar granulationSolar granulation

• Appearance of sun Appearance of sun with good seeing with good seeing reveals granulationreveals granulation

• Sequences of images Sequences of images suggest coherent suggest coherent overturning flowoverturning flow

• Granulation ~ visible Granulation ~ visible convection cellsconvection cells

• => Study convection => Study convection observationallyobservationally


Indirect detection of velocity Indirect detection of velocity fieldsfields

• Granulation not directly visible on Granulation not directly visible on (unresolved) stellar surfaces(unresolved) stellar surfaces

• But velocity fields in photosphere affect But velocity fields in photosphere affect spectral line profiles & energy distribution, spectral line profiles & energy distribution, so we may still study convection so we may still study convection observationallyobservationally

• Simplest example of velocity field: stellar Simplest example of velocity field: stellar rotationrotation

• Small for “cool” stars, large for “hot” starsSmall for “cool” stars, large for “hot” stars


MicroturbulenceMicroturbulence

• Abundance analysis allows indirect detection of Abundance analysis allows indirect detection of small-scale velocity field (excess line broadening small-scale velocity field (excess line broadening over thermal), required to fit weak and strong linesover thermal), required to fit weak and strong lines

• Microturbulence parameter Microturbulence parameter characterizes velocity characterizes velocity

• Required for most stars with TRequired for most stars with Tee < 10000 K, < 10000 K, corresponds to convective instabilitycorresponds to convective instability

• => Microturbulence ~ convection, at least in => Microturbulence ~ convection, at least in cooler starscooler stars

• Detectable even in broad-line stars – much dataDetectable even in broad-line stars – much data


Convection effects on line Convection effects on line profilesprofiles

• In Sun-like flow, expect rising and descending gas In Sun-like flow, expect rising and descending gas to have different velocities along line of sightto have different velocities along line of sight

• Different areal coverage (filling factors) and Different areal coverage (filling factors) and brightness lead to different contributions to total brightness lead to different contributions to total fluxflux

• Result: spectral lines are shifted and asymmetricResult: spectral lines are shifted and asymmetric

• Importance of these effects depends on where in Importance of these effects depends on where in atmosphere the lines are formed – weak lines will atmosphere the lines are formed – weak lines will be different from strong linesbe different from strong lines


MacroturbulenceMacroturbulence

• Most main sequence stellar line profiles can be Most main sequence stellar line profiles can be roughly modelled with Voigt profile + rotationroughly modelled with Voigt profile + rotation

• Line profiles of giants & supergiants more Line profiles of giants & supergiants more “pointed”, with broad shallow wings“pointed”, with broad shallow wings

• Successful model: Successful model: radial-tangential radial-tangential macroturbulencemacroturbulence. On half of surface, lines . On half of surface, lines have Gaussian spread radially, on other half have Gaussian spread radially, on other half lines have Gaussian spread tangentially. lines have Gaussian spread tangentially.

• One parameter: macroturbulence One parameter: macroturbulence RTRT (velocity) (velocity)


Macroturbulence - 2Macroturbulence - 2

• If If RTRT > 0, we conclude that large-scale > 0, we conclude that large-scale velocity field exists velocity field exists withinwithin stellar stellar atmosphereatmosphere

• => Velocity field may be studied by => Velocity field may be studied by modelling spectral line shapesmodelling spectral line shapes

• Values vary systematically over cooler part Values vary systematically over cooler part of HR diagramof HR diagram

• Large values of macroturbulence found Large values of macroturbulence found among (low v sin i) main sequence A stars among (low v sin i) main sequence A stars near Tnear Tee ~ 8000K ~ 8000K

• Macroturbulence drops to zero above A0VMacroturbulence drops to zero above A0V


Macroturbulence - 3Macroturbulence - 3

• Among hotter stars (TAmong hotter stars (Tee > 10000) situation is > 10000) situation is quite confusingquite confusing

• For hot main sequence and giant stars, For hot main sequence and giant stars, microturbulence takes various values microturbulence takes various values between 0 and several km/s, but not between 0 and several km/s, but not systematically – are these values really > 0 systematically – are these values really > 0 (e.g. Lyubimkov et al 2004)?(e.g. Lyubimkov et al 2004)?

• B and A supergiants have microturbulence B and A supergiants have microturbulence of several km/s, and macroturbulence of 15 of several km/s, and macroturbulence of 15 – 20 km/s (e.g. Przybilla et al 2006)! – 20 km/s (e.g. Przybilla et al 2006)!

• Is supergiant macroturbulence due to winds, Is supergiant macroturbulence due to winds, non-radial pulsations, convection, or…?non-radial pulsations, convection, or…?


Radial velocitiesRadial velocities

• In convecting stars (In convecting stars ( > 0), radial velocity of > 0), radial velocity of lines observed to vary with line strengthlines observed to vary with line strength

• Reflects typical velocity (average over Reflects typical velocity (average over flows) at depth where line is formedflows) at depth where line is formed

• Difficult to study: requires very accurate lab Difficult to study: requires very accurate lab wavelengths, sharp lineswavelengths, sharp lines

• Not yet studied over full HR diagramNot yet studied over full HR diagram

• Examples: Sun, Procyon (Allende Prieto et al Examples: Sun, Procyon (Allende Prieto et al 2002)2002)


Bisector curvature Bisector curvature (asymmetry)(asymmetry)• Line asymmetry (bisector curvature) Line asymmetry (bisector curvature)

reveals asymmetric flows reveals asymmetric flows • Should provide a direct means to observe Should provide a direct means to observe

convective velocity field in photosphereconvective velocity field in photosphere• Cool stars bisectors resemble solar Cool stars bisectors resemble solar

bisector, but with considerable variationsbisector, but with considerable variations• Gray & Nagel (1989) found bisectors Gray & Nagel (1989) found bisectors

reversed in hotter stars: a “granulation reversed in hotter stars: a “granulation boundary”boundary”

• Two “different” types of convection??Two “different” types of convection??


Bisector curvature Bisector curvature (asymmetry) - 2(asymmetry) - 2

• On MS, reversed bisectors also found On MS, reversed bisectors also found among A stars (Tamong A stars (Tee <10500 K) <10500 K)

• Late B stars show no bisector Late B stars show no bisector curvature, and have curvature, and have < 1 km/s < 1 km/s

• Bisector curvature not studied for Bisector curvature not studied for hotter stars, mainly because so few hotter stars, mainly because so few have v sin i < 5 km/shave v sin i < 5 km/s


Multi-parameter models of Multi-parameter models of flowflow• Modelling of cool stars by Dravins (1990) with Modelling of cool stars by Dravins (1990) with

four-component flow (2 hot upflows, 1 neutral, four-component flow (2 hot upflows, 1 neutral, 1 cool downflow) reproduces line profiles 1 cool downflow) reproduces line profiles reasonably and supports general picture of flow reasonably and supports general picture of flow behaviourbehaviour

• Frutiger et al (2000, 2005) have used multi-Frutiger et al (2000, 2005) have used multi-parameter models to derive temperature and parameter models to derive temperature and velocity structure of simple geometrical flow velocity structure of simple geometrical flow models for Sun, models for Sun, Cen A & B Cen A & B

• Useful for searches of parameter spaceUseful for searches of parameter space


3D hydrodynamic models3D hydrodynamic models

• Physically realistic modelling requires 3D Physically realistic modelling requires 3D hydrodynamic models (e.g. Nordlund & hydrodynamic models (e.g. Nordlund & Dravins) but such models are very costlyDravins) but such models are very costly

• 3D models of low-metal stars with 3D models of low-metal stars with convection reveal that temperature convection reveal that temperature stratification is changed significantly, stratification is changed significantly, perhaps also changing derived Li perhaps also changing derived Li abundance (Asplund & Garcia Perez abundance (Asplund & Garcia Perez 2001)2001)


3D models - 23D models - 2

• Detailed model of Procyon allows Detailed model of Procyon allows comparison of micro- macro-turbulence comparison of micro- macro-turbulence fits to fits of 3D line profiles (Allende Prieto fits to fits of 3D line profiles (Allende Prieto et al 2002)et al 2002)

• Without free model parameters (except Without free model parameters (except fundamental parameters of star), 3D fundamental parameters of star), 3D model lines provide excellent fit to model lines provide excellent fit to observationsobservations


3D models - 33D models - 3

• COCO55BOLD code used to compute coarse model BOLD code used to compute coarse model of entire M2 I star; find giant convection cells of entire M2 I star; find giant convection cells as suggested by images (Freytag et al 2002)as suggested by images (Freytag et al 2002)

• Same code computed convective models of A Same code computed convective models of A star, but found no reversed bisectors (Steffen star, but found no reversed bisectors (Steffen et al 2005)et al 2005)

• Limitation of 3D codes – if one disagrees with Limitation of 3D codes – if one disagrees with observation, testing changes is very costlyobservation, testing changes is very costly


MLT and other convection MLT and other convection modelsmodels

• MLT, FST and non-local convection models MLT, FST and non-local convection models provide alternative descriptionprovide alternative description

• Comparisons of predictions of such models Comparisons of predictions of such models with Balmer lines, uvby colours of star with Balmer lines, uvby colours of star (Smalley & Kupka 1997; Gardiner et al 1999) (Smalley & Kupka 1997; Gardiner et al 1999) show that observational tests of models are show that observational tests of models are possiblepossible

• Kupka & Montgomery (2002) seem to predict Kupka & Montgomery (2002) seem to predict correct sense of A star bisectors from non-correct sense of A star bisectors from non-local convection modellocal convection model


ConclusionsConclusions

• Stellar atmospheric velocity fields clearly Stellar atmospheric velocity fields clearly detectable in spectrum: microturbulence, detectable in spectrum: microturbulence, macroturbulence, bisector curvature, macroturbulence, bisector curvature, energy distribution,….energy distribution,….

• Behaviour over HR diagram quite varied; Behaviour over HR diagram quite varied; largest velocities in supergiantslargest velocities in supergiants

• Modelling making progress at connecting Modelling making progress at connecting convection theory with observationsconvection theory with observations

august 22, 2006iau symposium 239 observing convection in stellar atmospheres john landstreet london,...

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