Webb Focuses on Flame Nebula’s Brown Dwarfs

Astronomers using the NASA/ESA/CSA James Webb Space Telescope have explored the lowest mass limit of brown dwarfs within the Flame Nebula, a hotbed of star formation in the constellation of Orion.

The Flame Nebula is located approximately 1,400 light-years away in the constellation of Orion.

Otherwise known as NGC 2024 and Sh2-277, this emission nebula is approximately 12 light-years wide and less than one million years old.

The Flame Nebula was discovered by the German-born British astronomer William Herschel on January 1, 1786.

It is part of the Orion Molecular Cloud Complex, which includes such famous nebulae as the Horsehead Nebula and the Orion Nebula.

In a new study, astronomers used Webb to explore the lowest mass limit of brown dwarfs within the Flame Nebula.

The result, they found, were free-floating objects roughly two to three times the mass of Jupiter, although they were sensitive down to 0.5 times the mass of Jupiter.

“The goal of this project was to explore the fundamental low-mass limit of the star and brown dwarf formation process,” said said Dr. Matthew De Furio, an astronomer at the University of Texas at Austin.

“With Webb, we’re able to probe the faintest and lowest mass objects.”

The low-mass limit the team sought is set by a process called fragmentation.

In this process large molecular clouds, from which both stars and brown dwarfs are born, break apart into smaller and smaller units, or fragments.

Fragmentation is highly dependent on several factors with the balance between temperature, thermal pressure, and gravity being among the most important.

More specifically, as fragments contract under the force of gravity, their cores heat up.

If a core is massive enough, it will begin to fuse hydrogen.

The outward pressure created by that fusion counteracts gravity, stopping collapse and stabilizing the object.

However, fragments whose cores are not compact and hot enough to burn hydrogen continue to contract as long as they radiate away their internal heat.

“The cooling of these clouds is important because if you have enough internal energy, it will fight that gravity,” said Dr. Michael Meyer, an astronomer at the University of Michigan.

“If the clouds cool efficiently, they collapse and break apart.”

Fragmentation stops when a fragment becomes opaque enough to reabsorb its own radiation, thereby stopping the cooling and preventing further collapse.

Theories placed the lower limit of these fragments anywhere between one and ten Jupiter masses.

This study significantly shrinks that range as Webb’s census counted up fragments of different masses within the nebula.

“As found in many previous studies, as you go to lower masses, you actually get more objects up to about ten times the mass of Jupiter,” Dr. De Furio said.

“In our study with Webb, we are sensitive down to 0.5 times the mass of Jupiter, and we are finding significantly fewer and fewer things as you go below ten times the mass of Jupiter.”

“We find fewer five-Jupiter-mass objects than ten-Jupiter-mass objects, and we find way fewer three-Jupiter-mass objects than five-Jupiter-mass objects.”

“We don’t really find any objects below two or three Jupiter masses, and we expect to see them if they are there, so we are hypothesizing that this could be the limit itself.”

“Webb, for the first time, has been able to probe up to and beyond that limit,” Dr. Meyer added.

“If that limit is real, there really shouldn’t be any one-Jupiter-mass objects free-floating out in our Milky Way Galaxy, unless they were formed as planets and then ejected out of a planetary system.”

A paper on the findings was published in the Astrophysical Journal Letters.

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Matthew De Furio et al. 2025. Identification of a Turnover in the Initial Mass Function of a Young Stellar Cluster Down to 0.5 MJ. ApJL 981, L34; doi: 10.3847/2041-8213/adb96a