The James Webb Space Telescope (JWST) has probed deep into the dusty shroud of a young nebula alight with star formation on the hunt for “failed star” brown dwarfs.
Brown dwarfs are stellar objects that are born like stars but fail to gather enough matter to reach the masses needed to trigger the fusion of hydrogen to helium in their cores. These bodies with masses between 13 and 75 times the mass of Jupiter (or 1.3% to 7.5% the mass of the sun) are, therefore, much fainter than regular main sequence stars, despite the fact that some nuclear fusion does happen within them.
Brown dwarfs are hotter and brighter in their youth, and that makes them easier to spot in a young nebula like the Flame Nebula, which is around 1 million years old (if that seems ancient, consider our own middle-aged solar system is 4.6 billion years old).
The JWST was able to cut through the thick gas and dust obscuring the Flame Nebula to hunt its lowest mass limit of brown dwarfs.
The search turned up free-floating objects roughly two to three times the mass of Jupiter. By “free-floating,” astronomers mean objects that aren’t orbiting a parent star.
These could be stellar fragments that are on their way to becoming brown dwarfs.
“The goal of this project was to explore the fundamental low-mass limit of the star and brown dwarf formation process,” team leader Matthew De Furio of the University of Texas at Austin said in a statement.“With the JWST, we’re able to probe the faintest and lowest mass objects.”
I go to pieces…
The JWST hunted for free-floating planetary mass bodies that have masses of at least around half that of Jupiter. This was set by a process called “fragmentation” that sees large dense clouds of gas and dust, so-called “molecular clouds,” break down and condense to form stars and brown dwarfs.
Fragmentation is highly dependent on the balance between temperature, thermal pressure, and gravity, in addition to other slightly less critical factors.
As molecular cloud fragments contract under their own gravity, their cores rise in temperature. A core with enough mass becomes a protostar that will begin the fusion of hydrogen. This results in outward energy balancing the inward push of gravity and halting the collapse. The stabilized object is now a main-sequence star fusing hydrogen to helium in its core.
However, if a core isn’t dense and hot enough to kickstart hydrogen fusion, there is nothing to balance gravity, and the collapse continues unabated. These failed fragments continue to radiate away heat, a “proto-brown dwarf” in essence.
“The cooling of these clouds is important because if you have enough internal energy, it will fight that gravity,” team member Michael Meyer of the University of Michigan said. “If the clouds cool efficiently, they collapse and break apart.”
Fragmentation ceases when the gas of a fragment is dense enough to become opaque. This means it can reabsorb its own radiation, which stops it from cooling and halts its collapse.
The lower mass limit of these fragments has been theorized to be between 1 and 10 times the mass of Jupiter. These findings could reduce that mass range.
“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 JWST, 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 said. “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 that with the JWST, astronomers have for the first time been able to probe up to and beyond the brown dwarf mass limit.
“If that limit is real,” Meyer continued, “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.”
Rethinking failed stars
The faintness of brown dwarfs makes them tough to spot, but this effort is worthwhile as these failed stars can deliver a wealth of information about star formation and the differences and similarities between stars and planets.
This study by the JWST builds upon prior research by the Hubble Space Telescope, which wasn’t sensitive enough to study brown dwarfs of such low-masses in the Flame Nebula but was able to identify prime targets for further investigation.
“It’s really difficult to do this work, looking at brown dwarfs down to even ten Jupiter masses, from the ground, especially in regions like this,” said De Furio. “And having existing Hubble data over the last 30 years or so allowed us to know that this is a really useful star-forming region to target. We needed to have the JWST to be able to study this particular science topic.”
Astronomer Massimo Robberto of the Space Telescope Science Institute described the baton passing of Hubble to the JWST as a “quantum leap” in astronomers capability to understand the nature of brown dwarfs.
The team will now continue to study the Flame Nebula using the JWST, searching for objects lurking within its dense, dusty veil.
“There’s a big overlap between the things that could be planets and the things that are very, very low-mass brown dwarfs,” Meyer concluded. “And that’s our job in the next five years: to figure out which is which and why.”
The team’s research has been accepted for publication in The Astrophysical Journal Letters.
