News — Most stars form in clusters, where hundreds to thousands of stars coevolve in proximal environmental conditions that dictate how planetary systems might develop. Nearby massive stars can also contribute intense ionizing radiation, adding external dynamics that form a shell of ionized gas around a protoplanetary disk and emit unique hydrogen recombination spectral lines. Now, a team of astronomers has used archived Atacama Large Millimeter/submillimeter Array (ALMA) observations to identify for the first time the characteristic radio recombination lines associated with these ionized shells surrounding Solar System-sized proplyd disks in the Orion Nebula Cluster, at a distance of 1000 light-years from Earth.
ALMA, of which the U.S. National Science Foundation National Radio Astronomy Observatory (NSF NRAO) is a partner, has long been used to identify recombination lines from large astrophysical objects, and to study various features of protoplanetary disks. Now, a team of astronomers led by Ryan Boyden of the University of Virginia has discovered that ALMA’s state-of-the-art interferometers are powerful enough to detect the specific radio-wavelength hydrogen recombination lines of individual proplyd disks even in dense stellar clusters. Boyden says happily, “This discovery was serendipitous, and sometimes those are the most exciting science projects to work on—coming across a new finding somewhat by accident.”
Whereas a protoplanetary disk is defined as the disk of dust and gas around a young star, a proplyd carries the added distinction that the disk is being ionized by strong radiation from nearby, external massive stars. Thus, beyond the disk itself, a cocoon-like shell of ionized gas surrounds the system. The energetic boundary of this shell appears almost comet-like as the neighboring star drives photoevaporation across the disk, signaling to astronomers that these proplyd disks evolve quite differently.
Boyden and his team used data from prior ALMA observations to probe the ionized gas surrounding 200 protoplanetary disks around stars in the Orion Nebula Cluster. Of these, 17 proplyd disks were uniquely identified by a very particular hydrogen recombination line.
Although ionized hydrogen is common in energetic environments such as these star-forming regions, Boyden and his team searched for the specific energy signature released when a free electron combines with a hydrogen ion and “falls” from hydrogen’s 42nd energy level to its 41st. That fall, called the H41
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