m giants gray and corbally chapter 8

M Giants Gray and Corbally Chapter 8

Karen Garcia Georgia State University

Outline• Basic characteristics• Spectral features• Mira variables• Carbon stars• S type stars• M MS S SC C• Symbiotic stars

Basic Characteristics• Teff: 2200 – 3800K

• Mass: 0.3 – 8M☉

• Radii 20–100 R☉

Beta Pegasi

Radius : 95R☉

Mass: 2.1 M☉

Molecular Bands

• Characteristic spectra in cool stars are caused by molecular bands

• In cooler stars more molecules form and survive

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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Spectral Classification-Luminosity

• Luminosity indicator Ca I λ4226 line • Negative luminosity effect –

strength varies inversely with stellar brightness

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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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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

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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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

CR Stars

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CN Stars

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CJ Stars

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CH Stars

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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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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

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

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

• 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

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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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