A new census of stars answers the question: How small can stars and brown dwarfs get when they form? Part of the Orion Molecular Cloud Complex, the Flame Nebula is a much-studied region and the birthplace of new stars. pictureNASA'sTelescopes like the Hubble Space Telescope have observed it for years, but the smallest stars hidden deep within its dense, dusty core have remained out of reach—until now.
This collage of images of the Flame Nebula shows a near-infrared image taken by NASA's Hubble Space Telescope on the left, and the two insets on the right show a near-infrared image taken by NASA's James Webb Space Telescope. Much of the dark dense gas and dust in the Hubble image, as well as the surrounding white clouds, are clearly visible in the Webb image, allowing us to see a more transparent cloud layer shuttled with infrared-producing objects, which are young stars and brown dwarfs. Astronomers used the Webb telescope to conduct a census of the lowest-mass objects in this star-forming region. Image credits: NASA, ESA, Canadian Space Agency, Space Telescope Institute, Michael Meyer (University of Michigan), Matthew De Frio (University of Texas at Austin), Massimo Roberto (Space Telescope Institute), Alyssa Pagen (Space Telescope Institute)
NASA's James Webb Space Telescope used its powerful infrared capabilities to detect and count the faintest and smallest objects in the region for the first time, helping astronomers determine the minimum mass required to form a brown dwarf.
This animation alternates between Hubble Space Telescope and James Webb Space Telescope observations of the Flame Nebula. The Flame Nebula is a nearby star-forming nebula that is less than 1 million years old. In the comparison, three low-mass objects are highlighted. In Hubble's observations, these low-mass objects are obscured by the region's dense dust and gas. However, these objects showed up in Webb's observations due to Webb's sensitivity to faint infrared light. Image credits: NASA, ESA, CSA, Alyssa Pagan (Space Telescope Science Institute)
The Flame Nebula, located about 1,400 light-years from Earth, is a young and active star-forming region—less than a million years old. Within this stellar nursery, astronomers have discovered extremely tiny objects that are not massive enough to ignite hydrogen fusion in their cores. These objects are called brown dwarfs.
Brown dwarfs, often called "failed stars," cool and dim over time, becoming much dimmer than ordinary stars and harder to detect. Because of this, they are difficult to detect with most telescopes, even though they are relatively close to the sun. However, when brown dwarfs are very young, they are still warm and bright enough to be observed - even though they are hidden in thick clouds of dust and gas like the Flame Nebula.
Artistic conception of the James Webb Space Telescope in space. Image credit: NASA-GSFC, Adriana M. Gutierrez (CI Laboratory)
NASA's James Webb Space Telescope can penetrate the dense dust and capture the faint infrared light of these newborn brown dwarfs. Using the power of the Webb telescope, a team of astronomers set out to study just how small these free-floating objects are. They detected some objects with about two to three times the mass of Jupiter, and the telescope is sensitive enough to detect objects with only half the mass of Jupiter.
"The goal of this project is to explore the fundamental low-mass limit in the formation of stars and brown dwarfs. With the Webb telescope, we are able to detect the faintest, least massive objects," said study lead author Matthew Deverio of the University of Texas at Austin.
This near-infrared image of part of the Flame Nebula, taken with NASA's James Webb Space Telescope, highlights three low-mass objects in the inset on the right. These objects are much cooler than their protostars, requiring Webb's highly sensitive instruments to detect them. The study of these objects aims to explore the lowest mass limit for brown dwarfs in the Flame Nebula. The Weber image shows light at wavelengths 1.15 microns and 1.4 microns (filters F115W and F140M) as blue, light at 1.82 microns (F182M) as green, light at 3.6 microns (F360M) as orange, and light at 4.3 microns (F430M) as red. Image credits: NASA, ESA, Canadian Space Agency, Space Telescope Science Institute, Michael Meyer (University of Michigan)
The low-mass limit the research team sought is determined by a process called fragmentation. In the process, the large molecular clouds from which stars and brown dwarfs are born break into smaller and smaller units, or fragments.
Fragmentation is highly dependent on a number of factors, with the balance between temperature, thermal pressure and gravity being the most important. More specifically, as the fragment shrinks under gravity, its core heats up. If the core is large enough, it will start fusing hydrogen. The outward pressure generated by fusion counteracted gravity, preventing collapse and stabilizing the object (then known as a star). However, if the cores of the fragments are not dense enough and hot enough to burn hydrogen, they will continue to shrink as long as they continue to radiate internal heat.
At right are two images of the Flame Nebula (NGC 2024) taken by the Webb Space Telescope's Near Infrared Camera (NIRCam), with compass arrows, scale bars, and color scales for reference. These images are magnified regions of the interior of the Flame Nebula taken by the Hubble Space Telescope. Compass arrows pointing north and east indicate the direction of the image in the sky. Note that the relationship between north and east in the sky (viewed from below) is flipped relative to the directional arrows on the ground map (viewed from above). Image credits: NASA, ESA, CSA, STScI, Michael Meyer (University of Michigan), Matthew De Furio (University of Texas at Austin), Massimo Robberto (STScI), Alyssa Pagan (STScI)
"The cooling of these clouds is critical because if there is enough energy inside, it can resist gravity. If the clouds cool effectively, they will collapse and break apart," said Michael Meyer of the University of Michigan.
Fragmentation stops when the fragments become opaque enough to reabsorb their own radiation, halting cooling and preventing further collapse. Theoretically, the lower limit of these fragments is between 1 and 10 Jupiter masses. This study significantly narrows that range because Webb's census counted fragments of varying masses within the nebula.
"As many previous studies have found, the lower the mass, we actually find more objects in the range below ten times the mass of Jupiter. In our studies with the James Webb Space Telescope, we are sensitive to objects up to 0.5 times the mass of Jupiter, and below ten times the mass of Jupiter we find fewer and fewer objects," Deverio explained. "We found that there are fewer objects at five Jupiter masses than at 10 Jupiter masses, and there are much fewer objects at three Jupiter masses than at five Jupiter masses. We didn't actually find anything below two or three Jupiter masses, and if they existed, we would expect to see them, so we hypothesized that this might be the limit itself."
Meyer added: "For the first time, the Webb Space Telescope was able to detect and even exceed this limit. If this limit is real, then there should not be any objects with the mass of Jupiter floating freely in our galaxy, unless they started out as planets and were ejected by the planetary system."
Although brown dwarfs are difficult to discover, their similarities to stars and planets provide a wealth of information for star formation and planetary research. NASA's Hubble Space Telescope has been searching for these brown dwarfs for decades.
Although Hubble cannot observe the lower-mass brown dwarfs in the Flame Nebula like Webb, it will be crucial in identifying candidates for further study. This study is an example of how the Webb telescope is taking decades of Hubble data from the Orion Molecular Cloud Complex and conducting in-depth studies.
"This is a very difficult job, observing brown dwarfs as small as ten times the mass of Jupiter from the ground, especially in a region like this. Hubble observations over the past 30 years or so have shown us that this is a very valuable star-forming region. We need the Webb telescope to study this special science topic," Deverio said.
"This is a huge leap forward in our ability to understand Hubble's observations. Webb really opens up a whole new realm of possibilities to better understand these objects," explained Massimo Roberto, an astronomer at the Space Telescope Science Institute.
The team is continuing to study the Flame Nebula, using Webb's spectroscopic tools to further characterize the different objects within its dusty cocoon. "There is a large overlap between material that could become planets and very low-mass brown dwarfs," Meyer said. "And our mission over the next five years is to figure out what they are and why."
These results have been accepted for publication in The Astrophysical Journal Letters.
Compiled from /ScitechDaily