Cha 1107-7626: The object with the lowest mass known still hosting a protoplanetary disc with active accretion and a rich organic chemistry
Too small to be a star, too large to be a planet, Cha 1107-7626 is the smallest known isolated object (6–10 Jupiter masses) surrounded by a protoplanetary disk rich in gas and dust, which is still actively accreting gas and hosts organic molecules such as methane and ethylene.
Brown dwarfs are fascinating objects that represent a bridge between stars and planets. Difficult to detect and study due to their low luminosity, they still harbor many unresolved mysteries—such as their primary formation mechanism: are they the lowest-mass outcome of the stellar formation process, or do they form like planets? Their protoplanetary disks also remain poorly understood. These disks are of particular interest: around higher-mass stars, they can evolve into planetary systems as a result of the rapid (on the order of a few million years) formation of rocky bodies that can later accumulate gas. One open question remains: can the disks around brown dwarfs also form planets?
Brown dwarfs are not only faint, but also cool objects, with effective temperatures between 1500 and 2500 degrees. At these temperatures, their emission is primarily in the infrared band. Their low brightness and infrared emission make brown dwarfs ideal targets for the James Webb Space Telescope (JWST), developed by NASA, ESA, and CSA. JWST is not only capable of detecting brown dwarfs in distant systems (recent studies have identified brown dwarf candidates even in the Magellanic Clouds), but also of producing high-quality spectroscopic observations that enable the study of the chemistry of their atmospheres and protoplanetary disks, particularly in nearby objects.
Recent JWST spectroscopic observations of Cha 1107-7626, located in the Chamaeleon I star-forming region, are presented in the study “Detection of Hydrocarbons in the Disk around an Actively-Accreting Planetary-Mass Object”, led by astrophysicist Laura Flagg of Johns Hopkins University. Using JWST NIRSpec and MIRI instruments, the authors built and analyzed the mid-infrared spectrum of Cha 1107-7626, where the disk emission is dominant over that of the central object. This allowed them to identify clear signs of gas accretion from the disk onto the object, with an estimated accretion rate, measured from ground-based observations, between 10-10 and 10-11 solar masses per year. They also detected spectroscopic signals indicating the presence of organic molecules such as ethylene and methane. These characteristics, commonly found in disks around more massive young stars—but observed here for the first time in such a low-mass object—suggest that the chemistry and evolution of disks around brown dwarfs may be similar to those around stars.
The study, recently published in The Astrophysical Journal, also includes the contribution of the astronomer V. Almendros Abad from INAF – Osservatorio Astronomico di Palermo.
The cover image (click here to view it in full) shows the spectrum of Cha 1107-7626 as observed by the NIRSpec-PRISM (green) and MIRI-LRS (blue) spectrographs on JWST. Previously obtained observations from ISPI and Spitzer are also shown. The lower panels highlight the spectroscopic features identified as emission from methane and ethylene molecules.
Mario Giuseppe Guarcello
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