AGB stars as an origin of dust and gas in the interstellar medium of galaxies Mikako Matsuura National Astronomical Observatory of Japan A.A. Zijlstra, P.R. Wood, G.C. Sloan, J. Bernard-Salas, P.A. Whitelock, J.W. Menzies, M. Feast, E. Lagadec, M.A.T. Groenewegen, M.R. Cioni, J.Th. van Loon, G. Harris

Asymptotic Giant Branch (AGB) stars Late evolutionary phase of low and intermediate mass stars Mass losing stars

Infrared spectra of carbon-rich AGB stars SiC C2H2 HCN + C2H2 SMC S30 IRAS 04496-6958 LMC star Wavelength [micron] 3 MgS? C3 CO dust Flux [Jy] (scaled) V Cyg (galactic star) PAHs? MSX LMC 749 MSX LMC 219 ISAAC/VLT (ground) Spitzer Space Telescope

Asymptotic Giant Branch (AGB) stars PAHs Institut d’Astrophysique AGB stars Late evolutionary phase of low and intermediate mass stars Mass losing stars One of the important sources of dust grains and gas in the interstellar medium (ISM) of galaxies (c.f. super novae) Dust and gas Carbonaceous dust PAHs (Dwek 1998)

Problems related with AGB stars in our Galaxy and nearby galaxies Large Magellanic Cloud JAXA The amount of dust and gas ejected from AGB stars to the ISM Environmental factors: metallicities of galaxies

Influence of metallicities Many extra-galaxies have lower metallicities than our Galaxy Si, O, C etc…. Initial predictions At low metallicity, abundances of elements consisting of dust grains are lower Fewer dust grains: SiC, amorphous carbon, (PAHs) Lower mass-loss rates Our observations Self produced carbon influences the amount of dust grains and mass-loss rates

Observations Instruments Galaxy names [Fe/H] Distance (kpc) Very Large Telescope (VLT) + ISAAC spectroscopic mode : 3.0-4.2 m, R=360~600 Spitzer Space Telescope (SST) + IRSpectrometer (IRS) : 5-35 m (for LMC/SMC) or 5-14 m (for Fornax dSph) R=64~128 Galaxy names [Fe/H] Distance (kpc) Number of carbon stars known Observed with Spitzer Observed VLT LMC (Large Magellanic Cloud) -0.3 50 1000> 30 (+ ~30) 35 SMC (Small Magellanic Cloud) -0.7 59 ~800 13 2 Sagittarius dSph -1.1 24 30? (7) 4 Fornax dSph -1.3 138 104 5 (+6) (3) Sculptor dSph -2.2 87 8 *dSph (dwarf Spheroidal) galaxy

Dust compositions+amount SiC C2H2 HCN + C2H2 SMC S30 IRAS 04496-6958 LMC star Wavelength [micron] 3 MgS? C3 CO V Cyg (galactic star) MSX LMC 749 MSX LMC 219 ISAAC/VLT (ground) Spitzer Space Telescope

Strength of SiC excess (SiC / continuum) SiC strength 1/2 solar metallicity 1/4 solar metallicity ~ solar metallicity 1/10 solar metallicity (SiC / continuum) SiC strength

SED fitting : (dust) mass-loss rates from stars SiC 1 100 Wavelength [m] LMC + SMC stars Groenewegen et al. (2007) 1 Wavelength [m] 100 Stars in Fornax dSph Matsuura et al. (in press)

Dust compositions At low metallicity [Fe/H] SiC Amorphous carbon SiC reduction Amorphous carbon grain [Fe/H] SiC Amorphous carbon Galaxy 0.0 10% 90% LMC -0.3 2-8% 98-92% SMC -0.7 Fornax -1.3 5% 95%

Mass-loss rates 1/2 solar metallicity 1/4 solar metallicity Radiation pressure on dust grains -> luminosity & mass-loss rate Metallicity dependence of mass-loss rates: expansion velocity & dust-to-gas ratio Gas-to dust ratio: 200 (assumed) No reduction of mass-loss rate (An increase at lower metallicity and higher C/O ratio?) Matsuura et al. in press

Molecular bands C3 CO ISAAC/VLT (ground) Spitzer Space Telescope SiC C2H2 HCN + C2H2 SMC S30 IRAS 04496-6958 LMC star Wavelength [micron] 3 MgS? C3 CO Flux [Jy] (scaled) V Cyg (galactic star) MSX LMC 749 MSX LMC 219 ISAAC/VLT (ground) Spitzer Space Telescope

Equivalent widths of molecular bands LMC MW SMC Equivalent widths of molecular bands 7.5 m C2H2 Large equivalent width of C2H2 at lower metallicity Influence of high C/O ratio at low metallicity 13.7 m C2H2 Infrared colour [6.4]–[9.3]

The influence of C/O ratio – from chemical model Matsuura et al. (2005)

Conclusions At low metallicity Low Si abundance Fraction of SiC dust grains reduces Carbon: more sensitive to carbon produced in AGB stars Higher C/O ratio Fraction of amorphous carbon dust grains increases Increase C2H2 More PAHs? No reduction of mass-loss rates Total mass-loss rates from all AGB populations to the Interstellar Medium No reduction More carbon-rich AGB stars than oxygen-rich stars More carbon-rich dust grains