Orion Bar. Credit: Weilbacher et al.
In all galaxies, stars are dying and forming. And while life on Earth is based on a rich jumble of elements and molecules, the cold, dense gas out of which stars form is pretty monotonous, composed of 99 percent molecular hydrogen (H2). So mapping where stars are born requires a detecting H2. Unfortunately, this material is hard to observe due to a lack of a strong characteristic signal at low temperatures—unlike its atomic cousin (H), which emits radio waves at an easily distinguishable wavelength of 21 cm. Astronomers from SRON Netherlands Institute for Space Research and the University of Groningen have now discovered a new tool to measure H2 indirectly by mapping hydrogen fluoride (HF) and linking its abundance to that of H2.
The new tool comes in handy when other tools fail, for example, in the Orion Bar, in between regions around the Orion Trapezium stars and the Orion Molecular Cloud. In these areas, carbon is ionized, meaning that carbon monoxide (CO)—usually a reliable trace molecule to find H2—can't work as a tracer. Floris van der Tak (SRON/RuG) and his team were surprised to find a characteristic HF signal in data from the Herschel telescope coming from the Orion Bar, as astronomers have previously only detected hydrogen fluoride as a silhouette: HF absorbing other radiation. HF and H2 abundance can be linked because HF is produced in a chemical reaction where H2 reacts with atomic fluorine (F) to form HF and atomic hydrogen (H). Without H2, there is no HF.
The team, led by SRON Ph.D. student Ümit Kavak, used their map of HF to examine a few mechanisms through which it could emit its signal. Collisions of HF molecules with electrons and molecular hydrogen turns out to be the main mechanism. The collisions excite the HF molecules to a higher energy state, after which they drop to their ground state while emitting infrared light at a characteristic wavelength of 1.2 THz.
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