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Arcturus Exposed: Arcturus Exposed: Non-LTE Analysis of Carbon and Oxygen Non-LTE Analysis of Carbon and Oxygen

Abundances in ArcturusAbundances in Arcturus

By: Jayme DerrahBy: Jayme Derrah

Supervisor: Dr. Ian ShortSupervisor: Dr. Ian Short

AUPAC 2007AUPAC 2007

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Chemical Abundances of Chemical Abundances of StarsStars

Why do we care?Why do we care?Important for understanding stellar Important for understanding stellar

evolutionevolution

When stars die, elements are released When stars die, elements are released into space that form other stars and into space that form other stars and planetsplanets

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Carbon, Oxygen and the Carbon, Oxygen and the SunSun

Why Carbon and Oxygen?Why Carbon and Oxygen?Oxygen is the 3Oxygen is the 3rdrd most common element most common element

in the universein the universeCNO cycle in stars produces C and OCNO cycle in stars produces C and ORecent studies by Asplund et al. Recent studies by Asplund et al.

revealed that revisions need to be made revealed that revisions need to be made in the C and O abundances in the sun in the C and O abundances in the sun Used a realistic 3-D code that had never Used a realistic 3-D code that had never

been done before been done before

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Red GiantsRed Giants

Stage in stellar evolution after Stage in stellar evolution after leaving the main sequenceleaving the main sequence

Stars’ outer layers expand and coolStars’ outer layers expand and coolAre big, bright objects that can be Are big, bright objects that can be

seen at large distancesseen at large distancesThis makes it easier to take the This makes it easier to take the

spectrumspectrum

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Arcturus: Standard Red Arcturus: Standard Red GiantGiant

General General Information: Information:

Spectral Type:Spectral Type: K1.5III K1.5III

Radius:Radius: ~15.9 R ~15.9 Rsolsol

TTeffeff:: 4300 K 4300 K

Surface GravitySurface Gravity: ~1.5: ~1.5

Metallicity: Metallicity: -0.3 to - -0.3 to -0.7 (20-50% of the 0.7 (20-50% of the sun)sun)

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Arcturus: Observed Arcturus: Observed SpectrumSpectrum

Observations done at Kitt Peak Observations done at Kitt Peak Observatory, using the Coude Feed Observatory, using the Coude Feed telescope.telescope.

Hinkle, Wallace, Valenti, Harmer: Hinkle, Wallace, Valenti, Harmer: Visible and Near Infrared Atlas of Visible and Near Infrared Atlas of the Arcturus Spectrum 3727-9300 Athe Arcturus Spectrum 3727-9300 A; ; 2003, ASP2003, ASP

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Arcturus: Observed Arcturus: Observed Spectrum-2Spectrum-2

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Stellar Spectroscopy Stellar Spectroscopy

Two methods used:Two methods used:Line Profile FittingLine Profile Fitting

Qualitative measure of line strengthQualitative measure of line strengthEquivalent Width MeasurementsEquivalent Width Measurements

Quantitative measure of line strengthQuantitative measure of line strengthWidth of a rectangle centred on a spectral Width of a rectangle centred on a spectral

line that has the same area as the line line that has the same area as the line

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PHOENIXPHOENIX

Developed By: Hauschildt, P.H.; Developed By: Hauschildt, P.H.; Baron, E. ; Allard, F.Baron, E. ; Allard, F.

PHOENIX is a state of the art stellar PHOENIX is a state of the art stellar atmosphere codeatmosphere codeCan calculate the spectra for all types Can calculate the spectra for all types

of starsof starsSimultaneously treats most elements in Simultaneously treats most elements in

non-LTEnon-LTEMany more in LTEMany more in LTE

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LTE vs. Non-LTE: LTELTE vs. Non-LTE: LTE Local Thermodynamic Local Thermodynamic

Equilibrium (LTE):Equilibrium (LTE): Only valid in a perfect Only valid in a perfect

blackbody blackbody environmentenvironment

Boltzmann and Saha Boltzmann and Saha relations can be usedrelations can be used

In reality, radiation In reality, radiation leaks into space, so leaks into space, so LTE is invalid. LTE is invalid.

Crude approximation Crude approximation to realityto reality

So why use LTE?So why use LTE?

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LTE vs. Non-LTE: LTELTE vs. Non-LTE: LTE Local Local

Thermodynamic Thermodynamic Equilibrium (LTE):Equilibrium (LTE):

Only valid in a perfect Only valid in a perfect blackbody blackbody environmentenvironment

Boltzmann and Saha Boltzmann and Saha relations can be usedrelations can be used

In reality, radiation In reality, radiation leaks into space, so leaks into space, so LTE is invalid. LTE is invalid.

Crude approximation Crude approximation to realityto reality

So why use LTE?So why use LTE? It’s easy! It’s easy!

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LTE vs. Non-LTE: Non-LTELTE vs. Non-LTE: Non-LTE

Non- Local Non- Local Thermodynamic Thermodynamic Equilibrium (Non-Equilibrium (Non-LTE):LTE): Must solve coupled 1Must solve coupled 1stst

order rate equations order rate equations for the atomic for the atomic transitions to get transitions to get level populationslevel populations

Much more realistic Much more realistic approximationapproximation

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LTE vs. NLTE: SpectraLTE vs. NLTE: Spectra

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- Enhanced Model- Enhanced Model

Scaled Solar models scaled down for Scaled Solar models scaled down for elements where Z>2. Why? – Easyelements where Z>2. Why? – Easy

However, all elements are made at However, all elements are made at different rates throughout galactic different rates throughout galactic chemical evolutionchemical evolution

-capture makes even numbered elements-capture makes even numbered elementsOld metal poor stars have more even Old metal poor stars have more even

numbered elements than odd, relative to numbered elements than odd, relative to the Sunthe Sun

More realistic model for ArcturusMore realistic model for Arcturus

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Results: Equivalent Widths Results: Equivalent Widths (mA)(mA)

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Results: Equivalent Widths 2Results: Equivalent Widths 2

LegendLegend x = Observed x = Observed

valuesvalues y = LTE valuesy = LTE values z = NLTE valuesz = NLTE values v = NLTE- alpha v = NLTE- alpha

enhanced valuesenhanced values

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ProblemsProblems

Only one Carbon lineOnly one Carbon lineTelluric absorption in IRTelluric absorption in IRLine blending in IR Line blending in IR

Two Oxygen lines could not be usedTwo Oxygen lines could not be usedToo weakToo weakEquivalent width could not be Equivalent width could not be

determined (blending in observed determined (blending in observed spectrum)spectrum)

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ConclusionsConclusions

Are these models accurate for Are these models accurate for Arcturus?Arcturus?

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ConclusionsConclusions

Are these models accurate for Are these models accurate for Arcturus?Arcturus?Nope Nope

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ConclusionsConclusions

Are these models accurate for Are these models accurate for Arcturus?Arcturus?Nope Nope

Further revisions need to be made to Further revisions need to be made to current models to accurately model current models to accurately model ArcturusArcturusC and O abundances need to be revisedC and O abundances need to be revisedOxygen abundances depend on which line- Oxygen abundances depend on which line-

shouldn’t be trueshouldn’t be trueModels unrealistic in other ways (i.e. 3-D Models unrealistic in other ways (i.e. 3-D

effects, starspots, etc.) effects, starspots, etc.)

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AcknowledgementsAcknowledgementsDr. Ian ShortDr. Ian Short

Joel TannerJoel Tanner

Questions ?Questions ?