Detection of interstellar C2 and C3 in the Small Magellanic Cloud

Journal article, 2013

We report the detection of absorption from interstellar C2 and C3 towards the moderately reddened star Sk 143, located in the near `wing' region of the SMC, in optical spectra obtained with the European Southern Observatory Very Large Telescope (VLT)/Ultraviolet and Visual Echelle Spectrograph. These detections of C2 (rotational levels J = 0-8) and C3 (J = 0-12) absorption in the SMC are the first beyond our Galaxy. The total abundances of C2 and C3 (relative to H2) 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 Small Magellanic Cloud. Analysis of the rotational excitation of C2 yields an estimated kinetic temperature Tk ˜ 25 K and a moderately high total hydrogen density nH ˜ 870 cm-3 - compared to the T01 ˜ 45 K and nH ˜ 85-300 cm-3 obtained from H2. The populations of the lower rotational levels of C3 are consistent with an excitation temperature of about 34 K.