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NASA, ESA, CSA, M. Meyer (University of Michigan), A. Pagan (STScI)
The Flame Nebula, located about 1,400 light-years away from Earth, is a hotbed of star formation less than 1 million years old. Within the Flame Nebula, there are objects so small that their cores will never be able to fuse hydrogen like full-fledged stars-brown dwarfs.
Brown dwarfs, often called "failed stars," over time become very dim and much cooler than stars. These factors make observing brown dwarfs with most telescopes difficult, if not impossible, even at cosmically short distances from the Sun. When they are very young, however, they are still relatively warmer and brighter and therefore easier to observe despite the obscuring, dense dust and gas that comprises the Flame Nebula in this case.
NASA's James Webb Space Telescope can pierce this dense, dusty region and see the faint infrared glow from young brown dwarfs. A team of astronomers used this capability 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. With Webb, we're able to probe the faintest and lowest mass objects," said lead study author Matthew De Furio of the University of Texas at Austin.
Image A: Flame Nebula: Hubble and Webb Observations
The Hubble image on the left represents light at wavelengths of 1.05 microns (filter F105W) as blue, 1.3 microns (F130N) as green, and 1.39 microns (F129M) as red. The two Webb images on the right represent light at wavelengths of 1.15 microns and 1.4 microns (filters F115W and F140M) as blue, 1.82 microns (F182M) as green, 3.6 microns (F360M) as orange, and 4.3 microns (F430M) as red.
Smaller Fragments
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 (then known as a star). 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," says Michael Meyer of 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. In our study with the James Webb Space Telescope, 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," De Furio explained. "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."
Meyer added, "Webb, for the first time, has been able to probe up to and beyond that limit. 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."
Image B: Low Mass Objects within the Flame Nebula in Infrared Light
The Webb images represent light at wavelengths of 1.15 microns and 1.4 microns (filters F115W and F140M) as blue, 1.82 microns (F182M) as green, 3.6 microns (F360M) as orange, and 4.3 microns (F430M) as red.
Building on Hubble's Legacy
Brown dwarfs, given the difficulty of finding them, have a wealth of information to provide, particularly in star formation and planetary research given their similarities to both stars and planets. NASA's Hubble Space Telescope has been on the hunt for these brown dwarfs for decades.
