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| In serpens, there's a nebula. The LMU team identified deuterated methane in a proto-brown dwarf in this region of the sky. Credit: ESO |
Brown dwarfs are weird celestial bodies that exist between stars and planets, sort of in the middle. Because they lack the mass to burn hydrogen in their cores and glow like stars, astronomers refer to them as "failed stars." The question of whether the genesis of brown dwarfs is merely a scaled-down version of the formation of Sun-like stars is constantly discussed. Astrophysicists are concentrating their efforts on the juvenile brown dwarfs, also known as proto-brown dwarfs. They are only a few thousand years old and in the early phases of creation. They're curious if the gas and dust in these proto-brown dwarfs are similar to that of the youngest Sun-like proto-stars.
Methane is
the centre of attention because it is a simple and very stable gas molecule
that can only be destroyed by high-energy physical processes once generated. It
has been discovered on a number of extrasolar planets. Methane has previously
been used to identify and investigate the features of the galaxy's oldest brown
dwarfs, which are hundreds of millions to billions of years old.
A team
lead by LMU astronomer Basmah Riaz has now reliably found deuterated methane
(CH3D) in three proto-brown dwarfs for the first time. It's the first time CH3D
has been found outside of our solar system. This is a surprising outcome.
The
proto-brown dwarfs are extremely cold and dense. This makes studying methane
fingerprints in the near-infrared difficult. In the millimetre wave region, on
the other hand, they are easily visible. Deuterated methane (CH3D) can be
detected at millimetre wavelengths, unlike methane, which has no spectral
signature in the radio domain due to its symmetry.
The
discovery of CH3D was all the more remarkable because proto-brown dwarfs are
cooler (approximately 10 Kelvin or less) and denser than proto-stars, according
to theories on brown dwarf formation. CH3D is generated preferentially when the
gas is heated, around 20 to 30 Kelvin, according to chemical theory. According
to Basmah Riaz, "the measurements show that at least a major fraction of
the gas in a proto-brown dwarf is warmer than 10 Kelvin, otherwise CH3D should
not be there at all." The scientists can determine the amount of methane
in the atmosphere using the CH3D abundance measurement.
While
there has only been one Sun-like proto-star where CH3D has been tentatively
found, the LMU Team has confirmed the presence of CH3D in three proto-brown
dwarfs. This implies that proto-brown dwarfs have a complex heated organic
chemistry, implying that these cool compact astrophysical objects aren't just
scaled-down proto-stars.
"Methane in proto-brown dwarfs may or may not survive or keep such high abundance in the oldest brown dwarfs," explains Max Planck Institute for Extraterrestrial Physics co-author Wing-Fai Thi. Because a warm atmosphere is conducive to the formation of more complex compounds, proto-brown dwarfs are attractive targets for future searches for these molecules.
References:
- B Riaz et al, First CH3D detection in Class 0/I proto-brown dwarfs: constraints on CH4 abundances, Monthly Notices of the Royal Astronomical Society: Letters (2022). DOI: 10.1093/mnrasl/slac007
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