Column density of C₄H represented as a function of the column density of CH₃OH (left panel), HCOOCH₃ (middle panel), and CH₃OCH₃ (right panel) in various starless/prestellar cores, Class 0 sources, and hot corinos. All column densities are beam-averaged values obtained with single-dish telescopes. See references in the caption of Fig. 5 for the column densities of C₄H and CH₃OH in TMC-1, L1544, B1-b, and L1527. Column densities of HCOOCH₃ and CH₃OCH₃ are taken from Marcelino et al. (in prep.) for TMC-1 (upper limits), Jiménez-Serra et al. (2016) for L1544, Cernicharo et al. (2012) for B1-b, and Yoshida et al. (2019) for L1527 (upper limits). For the three hot corino sources, column densities for C₄H are from Sakai et al. (2009a), for CH₃OH from Maret et al. (2005), and for HCOOCH₃ and CH₃OCH₃ from Cazaux et al. (2003) (values have been averaged for a 10″ source size) and from Bottinelli et al. (2007).