Monday 10/01/12

Detection of Interstellar C_2 and C_3 in the Small Magellanic Cloud

Daniel E. Welty (1), J. Christopher Howk (2), Nicolas Lehner (2), John H. Black (3) ((1) Univ. of Chicago, (2) Univ. of Notre Dame, (3) Chalmers Inst.)
(Submitted on 28 Sep 2012)

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 ~ 25 K and a moderately high total hydrogen density n_H ~ 870 cm^-3 — compared to the T_01 ~ 45 K and n_H ~ 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.

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