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

Introduction Lower metallicity should affect the abundance of molecules in the AGB stars Metallicities: [Z/H]= -0.3 (LMC), -0.7 (SMC), -0.55 (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

Observations LMC 4 7 SMC 0 1(+1post-AGB) Sgr 0 3 VLT+ISAAC Wavelength resolution and coverage Oxygen-rich stars: R=2000-3300, 3.95-4.1 micron Carbon-rich stars: R=360-600, 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 4 7 SMC 0 1(+1post-AGB) Sgr 0 3 SMC S2 (post-AGB) PAH

Spectra of carbon-rich stars HCN C2H2 HCN + C2H2 V Cyg (galactic c-star) IRAS 04496-6958 (LMC) SMC S30

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

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)

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 05042-6720 (LMC) R=3300 S/N is determined by the spectra of telluric standard LMC (Mira)

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

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)