Astronomers using the NASA/ESA/CSA James Webb Space Telescope have spotted a faint, distant, and cold brown dwarf in the images of the massive galaxy cluster Abell 2744.
GLASS-JWST-BD1. Image credit: Nonino et al., arXiv: 2207.14802.
Brown dwarfs are relatively cool, dim objects that have a size between that of a gas giant planet, such as Jupiter or Saturn, and that of a Sun-like star.
Sometimes called failed stars, these objects are too small to sustain hydrogen fusion reactions at their cores, yet they have star-like attributes.
Brown dwarfs are often defined with a lower limit of 13 Jupiter masses, the approximate mass at which an object can begin to ignite deuterium fusion in its core, and with an upper limit of 80 Jupiter masses, the approximate mass at which an object becomes sufficiently massive to fuse hydrogen nuclei into helium nuclei within its core (principal characteristic of a main-sequence star).
However, these boundaries are not clear-cut as the exact masses where deuterium and hydrogen fusion occur depend on the chemical composition of the object.
Brown dwarfs are classified spectrally into M-, L-, T- and Y-type dwarfs. Despite their name, they are of different colors. Many brown dwarfs would likely appear magenta or orange-red to the human eye.
Their low mass, low temperature, and lack of internal nuclear reactions make them extremely faint and difficult to detect.
“A significant fraction of Milky Way stars near the Sun are brown dwarfs, non-fusing stars which have masses <= 0.07 solar masses, effective temperatures of T = 2,000 K, and are classified as late-L, T, and Y dwarfs,” said Dr. Mario Nonino from the INAF-Trieste Astronomical Observatory and colleagues.
“These objects are intrinsically faint and emit primarily at infrared wavelengths. Hence, wide-field imaging surveys of brown dwarfs are largely limited to the immediate solar neighborhood (d<=100 parsecs), and few metal-poor thick disk and halo brown dwarfs have been identified to date.”
“Webb represents a major step forward in the detection of cool and distant brown dwarfs, with imaging and spectroscopy extending to 5 μm,” they added.
The newly-discovered brown dwarf lies roughly 2,000 light-years away from Earth in the constellation of Sculptor.
Designated as GLASS-JWST-BD1, this object is classified as type T8 or T9.
It has a temperature of 600 K, a mass of 0.03 solar masses, and an age of 5 billion years.
“We estimate the distance of GLASS-JWST-BD1 to be 570-720 parsecs in a direction perpendicular to the Galactic plane, making it a likely thick disk or halo brown dwarf,” the astronomers said.
GLASS-JWST-BD1 was detected in the field of the galaxy supercluster Abell 2744 (Pandora’s Cluster).
The detection was made using data from Webb’s Near Infrared Camera (NIRCam) instrument as part of the GLASS-JWST-ERS program.
“The large estimated distance of GLASS-JWST-BD1 confirms the power of Webb to probe the very low-mass end of the stellar and substellar mass function in the Galactic thick disk and halo, enabling exploration of metallicity dependence on low-mass star formation and the evolution of brown dwarf atmospheres,” the researchers said.
Their work will be published in the Astrophysical Journal Letters.
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M. Nonino et al. 2022. Early results from GLASS-JWST. XIII. A faint, distant, and cold brown dwarf. ApJL, in press; arXiv: 2207.14802
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