Gliese 229 B’s Newfound Companion Solves Brown Dwarf Mystery

 

     From - Sky & Telescope

   By- AAS NOVA

   Edited -  Amal Udawatta

  

A 1995 Hubble Space Telescope image of the brown dwarf Gliese 229 B next to its far brighter host star, the M dwarf Gliese 229 A.
S. Kulkarni (Caltech), D.Golimowski (JHU) and NASA

Astronomers recently discovered a companion to Gliese 229 B, the first confidently identified brown dwarf. This discovery resolves the conflict between Gliese 229 B’s observed mass and the predictions of evolutionary models, potentially illuminating the nature of other poorly understood brown dwarf systems as well.

An illustration of a brown dwarf. Brown dwarfs aren’t actually brown, likely spanning a range of colors from reddish-orange to nearly black.
NASA / JPL-Caltech

In 1995, Gliese 229 B became the first object to be unambiguously classified as a brown dwarf: an object that bridges the gap between planets and stars. At roughly 13–80 times the mass of Jupiter, brown dwarfs aren’t massive enough to sustain fusion of hydrogen in their cores, as stars do, but they are able to burn a heavier form of hydrogen called deuterium, setting them apart from planets. (The most massive brown dwarfs can burn lithium as well.) After exhausting their supply of deuterium, brown dwarfs steadily cool, sliding down the spectral-type ladder. The youngest and most massive brown dwarfs occupy late M spectral types, while older or less massive brown dwarfs are classified as L, T, or Y dwarfs.

While improved telescopes have advanced our understanding of brown dwarfs, there’s still much we don’t know about these objects, and attempts to study and classify brown dwarfs have been confounded by their complex properties. This is the case for the first confirmed T-class brown dwarf, Gliese 229 B, which recently became the subject of an astronomical mystery.

A Mass Mystery

Soon after Gliese 229 B was discovered, researchers used substellar evolution models to interpret the object’s spectrum and luminosity and estimate its mass at 30–50 Jupiter masses. More than two decades later, refined observations of the brown dwarf’s orbit around its red dwarf host star allowed researchers to calculate its mass dynamically. The newly calculated mass — 71 Jupiter masses — was troubling. According to models of how substellar objects cool as they age, it simply wasn’t possible for a 71-Jupiter-mass object of Gliese 229 B’s age to have cooled to its present temperature.

The large relative radial velocity between Gliese 229 A and 229 B and the large difference in Gliese 229 B’s radial velocity between the two time periods provides firm evidence for the existence of an unseen companion.
S. Whitebook et al. / Astrophysical Journal Letters 2024

This conflict between dynamical mass measurements and evolutionary model predictions led researchers to suspect that Gliese 229 B is actually a binary system — a brown dwarf harboring an unseen companion. In March and November of 2022, Samuel Whitebook (University of California, Santa Barbara; California Institute of Technology) and coauthors turned one of the giant telescopes of Keck Observatory toward the Gliese 229 system, using the sensitive High Resolution Echelle Spectrometer to search for evidence of a companion tugging on Gliese 229 B. The team found a clear difference in Gliese 229 B’s radial velocity compared to expectations for an orderly orbit around its host star. Its radial velocity changed by 11σ between the observations, completely ruling out the possibility that Gliese 229 B is a single object.

Single No More

Likelihood distribution of the orbital period and mass for the companion object.
S. Whitebook et al. / Astrophysical Journal Letters 2024

What do these observations tell us about the newfound companion? While it’s not possible to fully pin down the properties of the companion object from current observations, Whitebook’s team estimated the companion’s mass to be somewhere between 15 and 35 Jupiter masses with an orbital period between a few days and 60 days. Future observations will refine the companion’s orbit and provide an accurate estimate of the masses of the two components.

In addition to solving the mystery of Gliese 229 B, this discovery may help to explain other seemingly over-massive T dwarfs orbiting main-sequence stars, several of which have been discovered in the past decade. If future work reveals that these too-massive T dwarfs are actually pairs of brown dwarfs, that may suggest that T dwarfs orbiting main-sequence stars are more likely to host companions than T dwarfs in the field, which are usually solo.