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Search for molecular oxygen in dense interstellar clouds.

P F Goldsmith1, R L Snell, N R Erickson

  • 1Department of Physics and Astronomy, University of Massachusetts, Amherst, USA.

The Astrophysical Journal
|February 15, 1985
PubMed
Summary
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Researchers searched for oxygen-2 (O2) in giant molecular clouds but found none. This suggests the clouds are either young or have a high carbon-to-oxygen ratio, limiting O2 as a tracer.

Area of Science:

  • Astrochemistry
  • Interstellar Medium Physics

Background:

  • Molecular oxygen (O2) is a key molecule for understanding interstellar cloud chemistry and evolution.
  • The ratio of O2 to carbon monoxide (CO) is sensitive to cloud age and elemental abundance ratios (C/O).

Purpose of the Study:

  • To search for the 234 GHz N=2→0, J=1→1 transition of 16O18O in six giant molecular clouds.
  • To constrain the abundance of O2 and the [O2]/[CO] ratio in these clouds.
  • To assess the chemical maturity and elemental composition of the observed clouds based on O2 limits.

Main Methods:

  • Utilized the 13.7 m Five College Radio Astronomy Observatory (FCRAO) radio telescope.
  • Conducted observations targeting the specific rotational transition of 16O18O.
  • Measured C18O column densities using its 220 GHz J=2→1 transition to establish CO abundance.

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Main Results:

  • No emission from the targeted 16O18O transition was detected in any of the six giant molecular clouds.
  • Derived upper limits for the [O2]/[CO] ratio ranging from <0.5 to <4.
  • The observed O2 upper limits are inconsistent with chemical models predicting [O2]/[CO] approaching unity for mature clouds with C/O < 1.

Conclusions:

  • The studied giant molecular clouds are likely not chemically mature, or they possess a carbon-to-oxygen ratio greater than 1.
  • Current observational sensitivity limits the use of O2 as a reliable tracer for cloud structure and evolution.
  • More sensitive observations, potentially from space-based platforms, are needed for significant advancements in O2 astrochemistry studies.