We investigate the molecular bands in carbon-rich AGB stars in the Large Magellanic Cloud ( LMC ) , using the InfraRed Spectrograph ( IRS ) on board the Spitzer Space Telescope ( SST ) over the 5–38Â \mu m range .
All 26 low-resolution spectra show acetylene ( C _ { 2 } H _ { 2 } ) bands at 7 and 14Â \mu m. The hydrogen cyanide ( HCN ) bands at these wavelengths are very weak or absent .
This is consistent with low nitrogen abundances in the LMC .
The observed 14Â \mu m C _ { 2 } H _ { 2 } band is reasonably reproduced by an excitation temperature of 500Â K. There is no clear dilution of the 14Â \mu m C _ { 2 } H _ { 2 } band by circumstellar dust emission .
This 14Â \mu m band originates from molecular gas in the circumstellar envelope in these high mass-loss rate stars , in agreement with previous findings for Galactic stars .
The C _ { 2 } H _ { 2 } column density , derived from the 13.7Â \mu m band , shows a gas mass-loss rate in the range 3 \times 10 ^ { -6 } to 5 \times 10 ^ { -5 } M _ { \sun } yr ^ { -1 } .
This is comparable with the total mass-loss rate of these stars estimated from the spectral energy distribution .
Additionally , we compare the line strengths of the 13.7Â \mu m C _ { 2 } H _ { 2 } band of our LMC sample with those of a Galactic sample .
Despite the low metallicity of the LMC , there is no clear difference in the C _ { 2 } H _ { 2 } abundance among LMC and Galactic stars .
This reflects the effect of the 3rd dredge-up bringing self-produced carbon to the surface , leading to high C/O ratios at low metallicity .