Published online by Cambridge University Press: 26 May 2016
We have searched 44 optical carbon stars for circumstellar C2 Swan (0,0) band absorption lines using the high dispersion echelle spectrograph for the 188 cm reflector at Okayama Astrophysical Observatory. We have detected the circumsteller C2 lines definitely in 18 stars, possibly in 6 stars, and not detected in 20 stars. We discuss the observed properties of the circumstellar C2 lines. We infer the C2 line-forming region is located at the innermost part of the circumstellar envelope.