Researchers Discover Intact Lithium Deposits in Ancient Brown Dwarf

Scientists at the Instituto de Astrofísica de Canarias (IAC) and the Instituto Nacional de Astrofísica, Óptica y Electrónica (INAOE), Mexico, have discovered lithium in the oldest and coldest brown dwarf where the existence of this prized element has been proven so far.

Comparison between different objects, showing the different levels of preservation and destruction of lithium. The Spanish-Mexican team has found that the boundary between those objects which destroy lithium and those which preserve it lies at 51.5 times the mass of Jupiter. The brown dwarf Reid 1B is a major deposit of lithium which will never be destroyed. Planets such as Jupiter and the Earth are even less massive and do not destroy their lithium. The Sun has destroyed all the lithium that was in its nucleus and preserves some in its upper layers, which are slowly mixing with its interior. (Image Credit: Gabriel Pérez Díaz, SMM (IAC)).

This substellar object, known as Reid 1B, has kept intact the earliest identified lithium deposit in the nearby cosmic community, dating back to a time before the realization of the binary system to which it is a part of.

The OSIRIS spectrograph on the Gran Telescopio Canarias (GTC), at the Roque de los Muchachos Observatory (Garafía, La Palma), in the Canary Islands aided in this discovery. The research was recently published in the journal Monthly Notices of the Royal Astronomical Society.

Brown dwarfs, also called “coffee-coloured dwarfs” or “failed stars,” are the natural connection between planets and stars. They are more enormous than Jupiter but currently adequate to burn hydrogen, which is the energy the stars utilize to shine.

For that reason, these substellar components were not detected until observers spotted them in the mid-1990s. They are especially interesting because it was anticipated that a few of them could preserve intact their content of lithium, occasionally referred to as “white petroleum” because of its uncommonness and its significance.

In the past two decades, astronomers have observed and tracked the orbital motions of binaries created by brown dwarfs in the solar vicinity. They have established their masses dynamically using Kepler’s laws, the mathematical formulae generated in the XVII century by Johannes Kepler to define the motions of astronomical bodies moving under the impacts of their mutual gravitation, such as the system created by the Sun and the Earth.

In a few of these systems, the main component has a mass adequate to burn lithium while the secondary component may not possess this mass. However, so far, the theoretical models had not been explored.

Using the OSIRIS spectrograph on the Gran Telescopio Canarias (GTC, or Grantecan) presently the world’s largest optical and infrared telescope, at the Roque de los Muchachos Observatory (ORM), a group of scientists at the Instituto de Astrofísica de Canarias (IAC) and the Instituto Nacional de Astrofísica, Óptica y Electrónica (INAOE) carried out high-sensitivity spectroscopic observations of two binaries whose components are brown dwarfs between February and August 2021.

They did not discover lithium in three of them, but they did locate it in Reid 1B, the faintest and coolest of the four. Doing this they made an extraordinary discovery, a deposit of cosmic lithium which is not ruined, whose origin goes back in time before the development of the system to which Reid 1B is a part of.

It is, in fact, the coolest, faintest extrasolar entity where lithium has been discovered, in a quantity 13 thousand times greater than the quantity found on Earth. This entity, which is about 1.100 million years in age, and a dynamical mass 41 times larger than that of Jupiter (the largest planet in the Solar System), is 16.9 light-years away from us.

A Chest of Hidden Treasure

Lithium observations in brown dwarfs allow researchers to predict their masses with a certain level of accuracy, based on nuclear reactions. The thermonuclear masses discovered in this way must be consistent with the dynamical masses discovered, with less ambiguity, from the orbital analysis.

However, scientists have learned that the lithium is preserved up to a dynamical mass which is 10% lesser than that estimated by the latest theoretical models. This difference seems to be substantial and indicates that there is something in the actions of brown dwarfs that is still not known.

We have been following the trail of lithium in brown dwarfs for three decades and finally we have been able to make a precise determination of the boundary in mass between its preservation and its destruction, and compare this with the theoretical predictions. There are thousands of millions of brown dwarfs in the Milky Way. The lithium contained in brown dwarfs is the largest known deposit of this valuable element in our cosmic neighbourhood.

Eduardo Lorenzo Martín Guerrero de Escalante, Study First Author, Researcher, and Professor, Higher Council for Scientific Research (CSIC), IAC

Carlos del Burgo Díaz co-author of the study, a scientist at the INAOE, a public research Center of the Mexican CONACYT, elucidates that “although primordial lithium was created 13.800 million years ago, together with hydrogen and helium, as a result of the nuclear reactions in the primordial fireball of the Big Bang, now there is as much as four times more lithium in the Universe.”

Although this element can be destroyed, it is also created in explosive events such as novae and supernovae, so that brown dwarfs such as Reid 1B can wrap it up and protect it as if it was a chest of hidden treasure.

Carlos del Burgo Díaz, Study Co-Author and Researcher, INAOE

This study has been funded by the Spanish Ministry of Economic Affairs and Digital Transformation (MINECO) and by the European Fund for Regional Developomente (FEDER) via project PID2019-109522GB-C53.

The Gran Telescopio Canarias and the Observatories of the Instituto de Astrofísica de Canarias (IAC) belong to the network of Singular Scientific and Technical Infrastructures (ICTS) of Spain.

Journal Reference:

Martín, E.L., et al. (2021) New constraints on the minimum mass for thermonuclear lithium burning in brown dwarfs. Monthly Notices of the Royal Astronomical Society. 10.1093/mnras/stab2969.


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