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

Gray and Corbally pg 297

Spectral Classification-Luminosity

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

strength varies inversely with stellar brightness

Gray and Corbally pg 299

NIR Spectra

• Temperature– TiO and VO bands – Increase with decreasing temperature

• Luminosity–CaH and Na D–negative luminosity effect

Gray and Corbally pg 300

Gray and Corbally pg 302

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

Gray and Corbally pg 305

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

Gray and Corbally pg 311

CN Stars

Gray and Corbally pg 315

CJ Stars

Gray and Corbally pg 318

CH Stars

Gray and Corbally pg 321

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

Gray and CorballyPg 326

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],

Gray and CorballyPg 332

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