M Giants (and their friends) Gray & Corbally, Chapter 8 Eliot Halley Vrijmoet 2017 March 8 Image: ESO/ P. Kervella
Outline History and significance of M giants Assessing temperatures Assessing luminosities Mira variables Weird stars: - Carbon stars - S-type stars - Symbiotic and Algol systems
Significance Wide age range Type changes! Long-period variables Cool atmospheres Molecular bands Image: ESO
History 1802, 1814: Fraunhofer lines 1850s: Photography advances Angelo Secchi I II 1866: Angelo Secchi classifies spectra 1890: Draper Catalogue of Stellar Spectra - Harvard types developed III IV < M stars < Carbon stars
Temperature assessment GC Figure 8.1
Temperature assessment GC Figure 8.2
Temperature assessment M5 III M8 III M10 III GC Figure 8.6
Luminosity assessment Ca I triplet Ca I triplet GC Figure 8.4
Luminosity assessment M2 Ib M2 III M2 V Ca I triplet Ca I triplet GC Figure 8.5
Mira variables Periods: 80-100 days Luminosity amplitude: 2.5-10 mag Mechanism: thermally pulsing AGB stars + dredge-up Dusty winds
Mira variables Emission! Hβ? Mira Not Mira! Mira Mira GC Figure 8.8
Angelo Secchi Carbon stars 1890: Draper Catalogue of Stellar Spectra 1941: Keenan & Morgan C-classification Keenan 1993 classification I II III IV R, N C0, C1,, C7 C-R, C-N, C-J, C-H, C-Hd
C-R stars GC Figure 8.10
C-R stars GC Figure 8.11
C-R stars GC Figure 8.13
C-N stars GC Figure 8.14 C-R stars GC Figure 8.10
C-J stars GC Figure 8.17 C-N stars GC Figure 8.15
C-J stars Merrill-Sanford Bands GC Figure 8.19
Summary so far C-R C-J C-N weak s-process weak s-process strong s-process strong M-S bands M-S bands in early types strong violet flux weak violet flux weak violet flux
C-H stars GC Figure 8.20
C-Hd stars GC Figure 8.22
GC Figure 8.22 S-type stars
GC Table 8.3 S-type stars
Physical basis M > S > C: band strengths smoothly rise, fall TiO ZrO Na I D-lines C2, etc. Increasing C/O > Decreasing oxygen availability > Decreasing H 2 O > Decreasing mean opacity > Continuum formed deeper > Deeper spectral lines appear
Physical basis M > S > C: band strengths smoothly rise, fall TiO ZrO Na I D-lines C2, etc. Increasing C/O > Decreasing oxygen availability > Metallic oxides start dissociating > TiO, AlO, VO break up first > ZrO, LaO, YO break up later > C molecules left over when C/O > 1.0
Stellar Evolution for C and S-type stars S-type C-N C-R C-J Enhanced C, s-process > He burning products - Core He burning > surface? - Shell He burning > dredge-up! Enhanced C only Bonus: He burn also gives Sr, Y, Ba, Zr, Tc, La S types with no Tc? > Mass transfer
Stellar Evolution for C and S-type stars S-type C-N C-R C-J Close binary mergers (lots of mixing) Enhanced C only + = Hot Bottom Convective Envelope burning ( 13 C increases at expense of 12 C)
Symbiotic Systems Nothing should be taken for granted with respect to these SySs. GC Figure 8.26
Algol Systems Primary unevolved, more massive Secondary evolved, less massive Past Roche lobe overflow! Image: CHARA GC Figure 8.28 GC Figure 8.29
Summary M giants are cool, but they also have many cool friends. Because they are cool, these stars have lots of molecular bands. This is how we recognize them! Reliable, universal temperature and luminosity indicators are hard to come by. Accurate spectral typing/grouping is crucial for taking the next step: interiors and evolution.