Gray and Corbally Chapter 8 Karen Garcia Georgia State University Outline Basic characteristics Spectral f eatures Mira variables Carbon stars ID: 295166
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Slide1
M Giants
Gray and Corbally Chapter 8
Karen Garcia
Georgia State UniversitySlide2
Outline
Basic characteristics
Spectral
f
eatures
Mira variablesCarbon starsS type starsM MS S SC CSymbiotic stars Slide3
Basic Characteristics
T
eff
: 2200 – 3800K
Mass: 0.3 – 8M
☉Radii 20–100 R☉
Beta
Pegasi
Radius : 95R
☉
Mass: 2.1 M
☉Slide4
Molecular Bands
Characteristic spectra in cool stars are caused by molecular bandsIn cooler stars more molecules form and surviveSlide5
Spectral Classification- Temperature
Dominant feature Titanium Oxide (TiO)Ca I λ4226 after M5Vanadium Oxide (VO) at M7Metallic lines decline – molecular features remove the background continuumSlide6
Gray and
Corbally pg 297Slide7
Spectral Classification-Luminosity
Luminosity indicator Ca I λ4226 line Negative luminosity effect – strength varies inversely with stellar brightnessSlide8
Gray and
Corbally pg 299Slide9
NIR Spectra
Temperature
TiO
and VO bands
Increase with decreasing temperature
LuminosityCaH and Na Dnegative luminosity effectSlide10
Gray and
Corbally
pg
300Slide11
Gray and
Corbally
pg
302Slide12
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 cycleSlide13
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 periodsSlide14
Gray and
Corbally
pg
305Slide15
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, C
2Slide16
CR Stars
Gray and
Corbally
pg 311Slide17
CN Stars
Gray and
Corbally
pg
315Slide18
CJ Stars
Gray and
Corbally
pg
318Slide19
CH Stars
Gray and
Corbally
pg
321Slide20
S type stars
Long period variables
Zirconium Oxide (
ZrO
)
Cover the same range of temperatures of M giantsMetallic oxides VO, YO, and LaOBridge between M giants and carbon starsSlide21
Gray and
Corbally
Pg
326Slide22
M MS S SC C sequence
M MS S strengthening of
ZrO
bands at expense of
TiO
bandsS SC C fading of ZrO bands, strengthening of Na I D lines, and the appearance of C2 and other carbon moleculesSlide23
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 H
2O reduces mean opacityMolecular dissociation effectMetallic oxides with dissociation energies below 7eV experience dissociationAs C/O approaches unity metallic oxides dissociateSlide24
Stellar evolution M Giants
Alpha capture of
13
C 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 surfaceSlide25
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 GiantsSlide26
Symbotic Stars
Symbiotic Stars
Interacting Binaries
UV – white
d
warf spectraOptical - cool giant spectraDistinguished from normal stars – strong Hydrogen emission lines, He II, [O III], Slide27
Gray and
Corbally
Pg
332Slide28
References
References
Gray, Richard, and Christopher
Corbally
.
Stellar Spectral Classification. New Jersey: Princeton University Press, 2009Kaler, James. Stars and their Spectra: An Introduction to the Spectral Sequence. New York: Cambridge University Press, 2011