[Paper Review] New upper limits on the interstellar O2 abundance

F. Combes, Tommy Wiklind+4 more|arXiv (Cornell University)|Sep 3, 1997

Astrophysics and Star Formation Studies3 citations

TL;DR

This study presents new upper limits on interstellar molecular oxygen (O₂) abundance using radio telescope observations of absorption lines at 33.4 and 70.5 GHz toward the quasar B0218+357. By analyzing the filling factor of the absorbing cloud and comparing with CO lines, the authors constrain the O₂/CO abundance ratio to ≤ 2 × 10⁻³ at 1σ, seven times lower than previous limits, significantly reducing the allowed O₂ abundance in the interstellar medium.

ABSTRACT

We report new observations of molecular oxygen in absorption at z=0.685 in front of the radio source B0218+357. The lines at 56.3 and 118.7 GHz have been observed, redshifted to 33.4 and 70.5 GHz respectively, with the 12m at Kitt Peak, 43m at Green Bank telescopes, and the 45m Nobeyama radio telescope. Deriving the surface filling factor of the absorbing dark cloud with other lines detected at nearby frequencies, we deduce from the upper limits on the O2 lines a relative abundance of molecular oxygen with respect to carbon monoxyde of O2/CO $\la$ 2 10$^{-3}$ at 1$σ$, seven times lower than the previous limit. The consequences of this result are discussed.

Motivation & Objective

• To improve constraints on the interstellar abundance of molecular oxygen (O₂) in the diffuse interstellar medium.
• To address the long-standing discrepancy between theoretical predictions and observational non-detections of O₂ in space.
• To reduce the upper limit on O₂ abundance by leveraging high-sensitivity radio observations and cloud filling factor analysis.
• To assess the implications of low O₂ abundance for interstellar chemistry and oxygen depletion mechanisms.

Proposed method

• Observations of O₂ rotational lines at 56.3 and 118.7 GHz, redshifted to 33.4 and 70.5 GHz due to z = 0.685, were conducted using the 12 m telescope at Kitt Peak, the 43 m telescope at Green Bank, and the 45 m telescope at Nobeyama.
• The surface filling factor of the absorbing dark cloud was derived from other molecular lines observed at nearby frequencies.
• The O₂ line intensities were measured with high sensitivity, and upper limits were established based on non-detection.
• The O₂/CO abundance ratio was calculated by comparing the derived O₂ column density with that of CO, using the filling factor and excitation temperature estimates.
• Statistical analysis at 1σ confidence level was applied to determine the upper bound on the O₂/CO ratio.
• The results were interpreted in the context of interstellar chemistry and oxygen reservoir models.

Experimental results

Research questions

• RQ1 What is the upper limit on the interstellar abundance of molecular oxygen (O₂) in the diffuse cloud toward B0218+357?
• RQ2 How does the observed O₂ abundance compare to previous observational limits and theoretical expectations?
• RQ3 What is the contribution of the absorbing cloud's filling factor to the O₂ column density determination?
• RQ4 How does the O₂/CO abundance ratio constrain models of interstellar oxygen depletion?
• RQ5 What are the implications of a low O₂ abundance for the chemical evolution of the interstellar medium?

Key findings

• The O₂/CO abundance ratio is constrained to be ≤ 2 × 10⁻³ at 1σ confidence level, representing a significant improvement over previous limits.
• This new upper limit is seven times lower than the previous best constraint, indicating a much tighter bound on interstellar O₂ abundance.
• No significant O₂ absorption lines were detected, leading to the upper limit based on sensitivity and noise levels of the instruments.
• The derived filling factor of the absorbing cloud was essential in converting line strength to column density and improving the abundance limit.
• The result implies that molecular oxygen is not a major reservoir of interstellar oxygen, challenging current chemical models.
• The low O₂ abundance suggests that oxygen may be locked in other forms, such as ice mantles on dust grains or in other molecules not yet detected.