1. C2H2, HCN and SiO bands of the AGB stars in the LMC/SMC/Sgr
Mikako Matsuura (UMIST, UK)
A.A. Zijlstra, J.Th. van Loon, I. Yamamura,
A.J. Markwick, P.M. Woods, L.B.F.M. Waters

2. Introduction
Lower metallicity should affect the abundance of molecules in the AGB stars
Metallicities: [Z/H]= -0.3 (LMC), -0.7 (SMC), (Sgr)
Molecular bands should be weaker in lower metallicity
L-band spectra
Oxygen-rich stars
SiO bands
Carbon-rich stars
HCN and C2H2 bands
SiO and C2H2
parents molecules for the dust grains (silicate; PAH)
Abundance of these molecules should be related to amount of dust formed in the AGB stars

3. Observations LMC 4 7 SMC 0 1(+1post-AGB) Sgr 0 3 VLT+ISAAC
Wavelength resolution and coverage
Oxygen-rich stars: R= , micron
Carbon-rich stars: R= , entire L-band
Targets
AGB stars (Long period variables) from Groenewegen & Blommaert (1998), van Loon et al. (1998), Whitelock et al. (1996)
Oxygen-rich stars Carbon-rich stars
LMC
SMC 0 1(+1post-AGB)
Sgr
SMC S2 (post-AGB)
PAH

4. Spectra of carbon-rich stars
HCN
C2H2
HCN + C2H2
V Cyg (galactic c-star)
IRAS
(LMC)
SMC S30

5. Equivalent Widths HCN is most abundant in Galactic AGB stars
Solar Neighbour
LMC
SMC
Sgr
3.1 micron HCN+C2H2
HCN is most abundant in Galactic AGB stars
Large C2H2 abundance in LMC and SMC than in Solar neighbour
3.8 micron C2H2
3.5 micron HCN
H-K

6. Interpretation with chemical model
At the same Teff and C/O ratio, both HCN and C2H2 abundances are less at lower metallicity
Higher C/O ratio will explain more abundant C2H2 in LMC/SMC
The third-dredge up
2000 K [Z/H]=0.0
(Solar Neighbour)
2000K [Z/H]=-0.3 (LMC)

7. Oxygen-rich stars Aringer et al (1999) for the galactic stars 28Si16O
Galactic stars: semi-regular variables
g Her (solar neighbour)
ISO/SWS, R=2000
Mira variables
29Si16O
IRAS (LMC) R=3300
S/N is determined by
the spectra of telluric standard
LMC (Mira)

8. Low Si abundance? Even lower SiO abundance than prediction in LMC
Aringer et al. (1997)
Even lower SiO abundance than prediction in LMC
Additional effects?
Filling with SiO in the extended atmosphere?
Higher C/O ratio after the third dredge-up?
Observed range
LMC
12.0+log(Si/H)~7.0
= EW~30A
(Dopita et al 1997)
Solar neighbour
12.0+log(Si/H)~7.6
= EW~55A

9. Summery Carbon-rich stars Oxygen-rich stars
HCN is the most abundant in the solar neighbour
More abundant C2H2 in LMC/SMC
Oxygen-rich stars
Non detection of SiO bands
EW(SiO) is below the predicted level
Lower abundance?
Filling the absorption by SiO in extended atmosphere?
High C/O ratio in LMC/SMC after the third dredge-up
In lower metallicity, maybe less dust grains in oxygen-rich stars, but more dust grains (PAHs) in carbon-rich stars (maybe not in AGB stars but in post-AGB stars)