Youngest Brown Dwarf Yet in a Multiple Stellar System
… and the Sharpest Optical Image (0.18 arcsec) from the VLT so far … !
Astronomers are eager to better understand the formation of stars and planets — with an eye on the complex processes that lead to the emergence of our own solar system some 4600 million years ago.
Brown Dwarfs (BDs) play a special role in this context. Within the cosmic zoo, they represent a class of "intermediate" objects. While they are smaller than normal stars, they shine by their own energy for a limited time, in contrast to planets.
Recent observations with the ESO Very Large Telescope (VLT) of a "young" Brown Dwarf in a multiple stellar system are taking on a particular importance in this connection. An evaluation of the new data by an international team of astronomers [1] shows that it is by far the youngest of only four such objects found in a stellar system so far. The results are now providing new insights into the stellar formation process.
This small object is known as TWA-5 B and with a mass of only 15 – 40 times that of Jupiter, it is near the borderline between planets and Brown Dwarfs, cf. the explanatory Appendix to this Press Release. However, visible and infrared VLT spectra unambiguously classify it in the latter category. Accurate positional measurements with the Hubble Space Telescope (HST) and the VLT hint that it is orbiting the central, much heavier and brighter star in this system, TWA-5 A (itself a close double star of which each component presumably has a mass of 0.75 solar masses), with a period that may be as long as 900 years.
And, by the way, an (I-band) image of the TWA-5 system is the sharpest delivered by the VLT so far, with an image size of only 0.18 arcsec [2]!
Brown Dwarfs: a cool subject
In current astronomical terminology, Brown Dwarfs (BDs) are objects whose masses are below those of normal stars — the borderline is believed to be about 8% of the mass of our Sun — but larger than those of planets, cf. [3]. Unlike normal stars, Brown Dwarfs are unable to sustain stable nuclear fusion of hydrogen. Once they have been formed, they enter into a very long phase of slow contraction. This process releases (potential) energy that is emitted in the form of electromagnetic radiation. Their brightness decreases with time, as they become smaller and smaller and their energy reservoir dwindles.
A few dozen "free-floating", isolated Brown Dwarfs have been discovered so far in space. They include members of the well-known, comparatively young Pleiades cluster (120 million years old) and some much older ones (some thousands of million years) only a few light-years away. A typical example is Kelu-1 that was found at ESO in 1997, see PR 07/97.
However, despite extensive searches and much invested effort, astronomers have so far only found three Brown Dwarfs that have been confirmed as companions to normal stars: Gl 229 B, G196-3 B, and Gl 570 D.
The younger a Brown Dwarf is, the more luminous it is, and the nearer it is to us, the brighter it appears in the sky. Old Brown Dwarfs are intrinsically so faint that, with the currently available instruments, they can only be found if they are nearby. It is therefore no surprise that the known, nearby Brown Dwarfs are generally older than the more distant ones, e.g. those found in the Pleiades.
A programme to find young Brown Dwarfs
It is on this background, that the international astronomer team [1] is now searching for young Brown Dwarfs that are companions to young, nearby stars.
