For the first time, researchers have identified a turnover in the initial mass function of a star cluster. The new finding suggests that newborn stars and sub-stellar objects become more prevalent with decreasing mass down to the turnover at 12 Jupiter masses. The study also suggests that the fundamental lower limit of the process through which stars and brown dwarfs form may lie around 3 Jupiter masses.
How Low Does the Mass Function Go?
Take a cloud of turbulent hydrogen gas and give it time to coalesce into a cluster of stars. How many stars do you get, and how massive are they? These questions are answered by the initial mass function, which describes the mass distribution of newborn objects in a cluster. The initial mass function decreases from just a few stars at high masses to myriad stars, brown dwarfs, and planetary-mass objects at low masses.
However, the increase in number with decreasing mass doesn’t go on forever; star-formation theories predict that the initial mass function turns over, with objects becoming less prevalent below the turnover mass. The turnover of the initial mass function has never been observed — until now.
Searching for the Turnover
NGC 2024, the Flame Nebula, in visible light
This Hubble Space Telescope view of NGC 2024 shows the thick clouds of dust throughout the region. [NASA, ESA, and N. Da Rio (University of Virginia); Processing: Gladys Kober (NASA/Catholic University of America)]
In a research article published today, a team led by Matthew De Furio (University of Texas at Austin) presented their investigation of the low-mass end of the initial mass function in a nearby star cluster. The target of this search was NGC 2024 (the Flame Nebula), a star-forming region that is less than 1 million years old, just 1,300 light-years away, and suffused with thick clouds of dust.
NGC 2024’s extreme dustiness was actually a plus for this investigation. Though dust is often a hindrance for astronomers, in this case it helped to screen out background sources that might contaminate the exploration of the cluster. Using JWST’s Near-Infrared Camera (NIRCam), De Furio’s team picked out the faint point sources indicating low-mass objects in the star cluster. Existing star-finding algorithms were stymied by the dusty nebula, so the team developed a new automated routine, which detected 100 point sources in their images.
De Furio’s team trimmed this initial sample by removing background objects and sources that saturated the detector, were too heavily obscured by dust, or were likely more massive than the tail end of the initial mass function being studied here. This left 28 objects, for which the team used evolutionary models to calculate the masses. The least-massive object in the sample weighed in around 3 Jupiter masses.
histogram of object masses studied in NGC 2024
Histogram showing the masses of the 28 cluster members used in this study. The vertical red dashed line shows the breakpoint mass, or where the initial mass function turns over. [Adapted from De Furio et al. 2025]
Sampling Sub-Stellar Sources
Using Bayesian statistics, the team constructed the initial mass function for the cluster given the masses of the detected objects and the varying sensitivity across the field of view; NIRCam can detect objects down to 0.5–2.0 Jupiter masses, depending on the amount of obscuring dust. The resulting initial mass function shows a clear turnover around 12 Jupiter masses — the first time this feature has been identified for any cluster.
Although NIRCam is sensitive to objects down to 0.5–2.0 Jupiter masses, the team found no objects below 3 Jupiter masses. This suggests that the end of the initial mass function — the minimum-mass of an object that a turbulent cloud can create through fragmentation — may lie around this mass. Previous work predicted the end of the initial mass function to be between 1 and 10 Jupiter masses.
De Furio and coauthors plan to continue their investigation of the initial mass function in NGC 2024, using JWST spectra to confirm the masses of the objects in the sample, as well as to investigate whether the mass function varies from the center of the cluster, where this study was performed, to its outskirts.
Citation
“Identification of a Turnover in the Initial Mass Function of a Young Stellar Cluster Down to 0.5 MJ,” Matthew De Furio et al 2025 ApJL 981 L34. doi:10.3847/2041-8213/adb96a