2012ApJ...744..163A

We report a sensitive search for the rotational transitions of the carbon-chain alcohol HC₄ OH in the frequency range 21.2-46.7 GHz in the star-forming region L1527 and the dark cloud TMC-1. The motivation was laboratory detection of HC₄ OH by microwave spectroscopy. Despite achieving rms noise levels of several millikelvin in the antenna temperature using the 45 m telescope at Nobeyama Radio Observatory, the detection was not successful, leading to 3σ upper limits corresponding to the column densities of 2.0x10¹² and 5.6x10¹²/cm2 in L1527 and TMC-1, respectively. These upper limits indicate that [HC₄OH]/[HC₅N] ratios are less than 0.3 and 0.1 in L1527 and TMC-1, respectively, where HC₅ N is an HC₄-chain cyanide and HC₄OH is a hydroxide. These ratios suggest that the cyano carbon-chain molecule dominates the hydroxyl carbon-chain molecule in L1527 and TMC-1. This is contrary to the case of saturated compounds in hot cores, e.g., CH₃ OH and CH₃CN, and can be a chemical feature of carbon-chain molecules in L1527 and TMC-1. In addition, the column densities of the "unsubstituted" carbon-chain molecule C₄ H and the sulfur-bearing molecules SO and HCS⁺ were determined from detected lines in L1527.