Download - M Giants Gray and Corbally Chapter 8
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M Giants Gray and Corbally Chapter 8
Karen Garcia Georgia State University
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Outline• Basic characteristics• Spectral features• Mira variables• Carbon stars• S type stars• M MS S SC C• Symbiotic stars
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Basic Characteristics• Teff: 2200 – 3800K
• Mass: 0.3 – 8M☉
• Radii 20–100 R☉
Beta Pegasi
Radius : 95R☉
Mass: 2.1 M☉
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Molecular Bands
• Characteristic spectra in cool stars are caused by molecular bands
• In cooler stars more molecules form and survive
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Spectral Classification- Temperature
• Dominant feature Titanium Oxide (TiO)• Ca I λ4226 after M5• Vanadium Oxide (VO) at M7• Metallic lines decline – molecular
features remove the background continuum
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Gray and Corbally pg 297
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Spectral Classification-Luminosity
• Luminosity indicator Ca I λ4226 line • Negative luminosity effect –
strength varies inversely with stellar brightness
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Gray and Corbally pg 299
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NIR Spectra
• Temperature– TiO and VO bands – Increase with decreasing temperature
• Luminosity–CaH and Na D–negative luminosity effect
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Gray and Corbally pg 300
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Gray and Corbally pg 302
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Mira Variables
• Unstable interiors and atmospheres• Variation in temperature and luminosity
(irregular, semi-regular, or fairly-regular)• Long period variables: 80-1000 days• Amplitude in luminosity ranges of 2.5-10
magnitudes• Spectra change throughout their light cycle,
and from cycle to cycle
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Mira Variables - Spectra
• Similar to M giant spectra• Difference lies in the presence of H and
Fe II• H and Fe II are visible during pulsation
periods
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Gray and Corbally pg 305
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Carbon Stars
• Temperature and luminosities correspond to G, K, and M giants
• Difference between M giants lies in the large overabundances of carbon relative to oxygen
• Spectra is dominated by molecular bands due to molecules including CH, CN, C2
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CR Stars
Gray and Corbally pg 311
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CN Stars
Gray and Corbally pg 315
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CJ Stars
Gray and Corbally pg 318
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CH Stars
Gray and Corbally pg 321
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S type stars
• Long period variables• Zirconium Oxide (ZrO)• Cover the same range of temperatures of
M giants• Metallic oxides VO, YO, and LaO• Bridge between M giants and carbon stars
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Gray and CorballyPg 326
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M MS S SC C sequence
• M MS S strengthening of ZrO bands at expense of TiO bands
• S SC C fading of ZrO bands, strengthening of Na I D lines, and the appearance of C2 and other carbon molecules
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Physical Basis of Sequence• Increase in C/O ratio• Two physical effects– Change in mean opacity in the cool atmosphere• As C/O increases, decrease in H2O reduces mean
opacity
– Molecular dissociation effect• Metallic oxides with dissociation energies below 7eV
experience dissociation• As C/O approaches unity metallic oxides dissociate
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Stellar evolution M Giants
• Alpha capture of 13C nuclei can yield neutrons during helium shell burning which can lead to the production of many heavy elements via the s process (zirconium, technetium barium)
• Convective currents in envelope dredge nuclear-processed, carbon-rich material from the helium burning shell region to the surface
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• Thermal pulses cause the episodic formation of deep convective currents that are able to dredge the carbon-rich, s process-rich material up to the surface
• Successive dredge ups increase the C/O ratio in the atmosphere of the star as well as the abundance of s process elements moving the star through the spectral type sequence
Stellar evolution M Giants
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Symbotic StarsSymbiotic Stars
• Interacting Binaries• UV – white dwarf spectra• Optical - cool giant spectra• Distinguished from normal stars – strong
Hydrogen emission lines, He II, [O III],
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Gray and CorballyPg 332
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ReferencesReferences• Gray, Richard, and Christopher Corbally.
Stellar Spectral Classification. New Jersey: Princeton University Press, 2009
• Kaler, James. Stars and their Spectra: An Introduction to the Spectral Sequence. New York: Cambridge University Press, 2011