We report the detection of absorption from interstellar C _ { 2 } and C _ { 3 } toward the moderately reddened star Sk 143 , located in the near ’ wing ’ region of the SMC , in optical spectra obtained with the ESO VLT/UVES .
These detections of C _ { 2 } ( rotational levels J =0–8 ) and C _ { 3 } ( J =0–12 ) absorption in the SMC are the first beyond our Galaxy .
The total abundances of C _ { 2 } and C _ { 3 } ( relative to H _ { 2 } ) are similar to those found in diffuse Galactic molecular clouds – as previously found for CH and CN – despite the significantly lower average metallicity of the SMC .
Analysis of the rotational excitation of C _ { 2 } yields an estimated kinetic temperature T _ { k } \sim 25 K and a moderately high total hydrogen density n _ { H } \sim 870 cm ^ { -3 } – compared to the T _ { 01 } \sim 45 K and n _ { H } \sim 85–300 cm ^ { -3 } obtained from H _ { 2 } .
The populations of the lower rotational levels of C _ { 3 } are consistent with an excitation temperature of about 34 K .