1. 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

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

3. Infrared spectra of carbon-rich AGB stars
SiC
C2H2
HCN + C2H2
SMC S30
IRAS
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

4. 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)

5. 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

6. 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

7. Observations Instruments Galaxy names [Fe/H] Distance (kpc)
Very Large Telescope (VLT) + ISAAC spectroscopic mode
: 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

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

9. 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

10. 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)

11. 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%

12. 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

13. Molecular bands C3 CO ISAAC/VLT (ground) Spitzer Space Telescope
SiC
C2H2
HCN + C2H2
SMC S30
IRAS
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

14. 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]

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

16. 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