A Search for Interstellar Carbon Chain Alcohol HC4OH in Star-Forming Region L1527 and Dark Cloud TMC-1

TL;DR

This study conducted a sensitive search for the interstellar molecule HC4OH in star-forming region L1527 and dark cloud TMC-1, setting upper limits on its abundance and comparing it to related molecules to understand chemical compositions.

Contribution

First detection attempt of HC4OH in interstellar space, providing upper limits and chemical ratio insights in star-forming regions and dark clouds.

Findings

HC4OH was not detected, with upper limits on its column density.

The ratio [HC4OH]/[HC5N] is less than 0.3 in L1527 and 0.1 in TMC-1.

Cyano carbon chain molecules dominate hydroxyl ones in these regions.

Abstract

We report a sensitive search for the rotational transitions of the carbon chain alcohol HC4OH in the frequency range of 21.2-46.7 GHz in the star-forming region L1527 and the dark cloud TMC-1. The motivation was laboratory detection of HC4OH 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 3 sigma upper limits corresponding to the column densities of 2.0 \times 1012 and 5.6 \times 1012 cm-2 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 HC5N 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., CH3OH 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 C4H and the sulfur-bearing molecules SO and HCS+ were determined from detected lines in L1527.