A Search for O_2 in CO-depleted Molecular Cloud Cores with Herschel

TL;DR

This study used Herschel HIFI to search for molecular oxygen in four CO-depleted dense cores, finding no detections and setting upper limits that challenge existing chemical models of oxygen abundance in such environments.

Contribution

The paper provides the first observational upper limits on O_2 in CO-depleted cores using Herschel, testing theoretical predictions about oxygen chemistry in cold dense regions.

Findings

No O_2 detected in the four cores.

Upper limits on O_2 abundance are significantly lower than model predictions.

Results suggest revisions are needed in astrochemical models of oxygen in dense cores.

Abstract

The general lack of molecular oxygen in molecular clouds is an outstanding problem in astrochemistry. Extensive searches with SWAS, Odin and Herschel have only produced two detections; upper limits to the O_2 abundance in the remaining sources observed are about 1000 times lower than predicted by chemical models. Previous atomic oxygen observations and inferences from observations of other molecules indicated that high abundances of O atoms might be present in dense cores exhibiting large amounts of CO depletion. Theoretical arguments concerning the oxygen gas-grain interaction in cold dense cores suggested that, if O atoms could survive in the gas after most of the rest of the heavy molecular material has frozen out on to dust, then O_2 could be formed efficiently in the gas. Using Herschel HIFI we searched a small sample of four depletion cores - L1544, L694-2, L429, Oph D - for emission in the low excitation O_2 N_J=3_3-1_2 line at 487.249 GHz. Molecular oxygen was not detected and we derive upper limits to its abundance in the range N(O_2)/N(H_2) = (0.6 - 1.6)x10^{-7}. We discuss the absence of O_2 in the light of recent laboratory and observational studies.