A 2012 search for molecular oxygen in the Orion Nebula came up negative, leading to new ideas on what's wrong in the chemical models. Searches for interstellar molecular oxygen, O2, have a long history, and the motivation for these searches has evolved. Prior to the late 1990’s, efforts to detect O2 were driven by a desire to confirm its predicted role as a major reservoir of elemental oxygen within dense molecular clouds and as the most important gas coolant of typical clouds after carbon monoxide (CO). But O2 was never found. The SAO-led Submillimeter Wave Astronomy Satellite (SWAS) in 1998 and the Odin satellite in 2001 both failed to detect O2 toward a large number of sources at levels of a few percent of the abundances predicted by equilibrium gas-phase chemical models. Something in the chemical models was wrong, but what?
Harvard Center for Astrophysics (CfA) astronomers Gary Melnick and Volker Tolls led a team of nineteen astronomers using the Herschel Space Observatory in study of O2 in the Orion nebula, a location well known for its rich chemistry. Herschel instruments have both high sensitivity and the broad wavelength coverage needed to search for the molecule in several of its emission lines.
The scientists report that they still did not find O2. The improved sensitivity, however, allows them to reach some general, if preliminary conclusions about four issues: the way oxygen clings to ice in the interstellar medium (perhaps stronger than previously suspected), the amount of total material in the Orion region (less than had been thought), the way O2 clumps together (smaller clumps), and the location of these molecules in the clouds (buried deeper than previous estimates).
Further modeling and additional observations will clarify the situation further, but the present work goes a long way to narrowing the possible explanations for the mysterious absence of this life-giving molecule.
The image at the top of the page shows the nebula's glowing gas surrounding hot young stars at the edge of an immense interstellar molecular cloud only 1500 light-years away. Visible simultaneously are the bright stars of the Trapezium in Orion's heart, the sweeping lanes of dark dust that cross the center, the red glowing hydrogen gas, and the blue tinted dust that reflects the light of newborn stars. The whole Orion Nebula cloud complex, which includes the Horsehead Nebula, will slowly disperse over the next 100,000 years.
Journal reference: Astrophysical Journal
The Daily Galaxy via Harvard-Smithsonian Center for Astrophysics
Image credit: With thanks to Robert Gendler