ESO Press Release 16/00 21 July 2000 For immediate release

Youngest Brown Dwarf Yet in a Multiple Stellar System

... and the Sharpest Optical Image (0.18 arcsec) from the VLT so far...!

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

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

However, young stars are quite rare in the solar neighbourhood. Only a few were known before the very successful ROSAT X-ray survey that discovered about 100 young and nearby stars, less than 100 million years old and within ~ 300 light-years distance. The new research programme attempts to find brown dwarf companions to these and other young and nearby stars.

For this, state-of-the-art infrared imaging cameras are used at the 3.6-m New Technology Telescope (NTT) with the SOFI (and SHARP) instrument on La Silla, as well as the 8.2-m VLT/ANTU telescope with the ISAAC multi-mode instrument at Paranal.

The first step is to take high-resolution images of the stars from the ROSAT list to look for possible faint companions. However, any faint object found near one of the programme stars may of course be a completely unrelated fore- or background object and it is therefore imperative to check this by means of supplementary observations.

Two methods are available. The first implies taking spectra of the companion candidates that demonstrate whether they are bona-fide Brown Dwarfs that display spectral lines typical for the cool atmospheres of this class, e.g., of Titanium Oxide (TiO) and Vanadium Oxide (VO). Infrared spectra are particularly useful for a measurement of the atmospheric temperature.

The other involves obtaining a second image some years later. If the companion candidate and the brighter star belong to the same stellar system, they must move together on the sky or, as astronomers say, their measured "proper motions" must be (nearly) the same.

If both checks are positive, the fainter object is most likely to be a bona-fide Brown Dwarf companion to the young and nearby star. To be absolutely certain, its orbital motion should also be detected, but it will be very slow and can only be perceived after several years of continued observations.

VLT observations of TWA-5 B

This faint companion ("TWA-5 B") was first detected in 1998 with the Hubble Space Telescope (HST), but until now, no spectrum had been published, nor had the proper motion been measured. It is indeed a difficult object to observe: it is 100 times fainter than the bright star and is located only two arcsec away in the sky.
 

ESO PR Photo 17a/00 [Preview - JPEG: 400 x 463 pix - 128k] [Normal - JPEG: 800 x 925 pix - 272k]

Caption: An image of TWA-5 A (lower, bright object) and TWA-5 B (upper), taken with the FORS-2 multi-mode instrument at the 8.2-m VLT/KUEYEN telescope on 21 February 2000. The integration time was 1 second through an I-band filter (wavelength 900 nm) with the high-resolution collimator (0.1 arcsec per pixel). The image quality is 0.18 arcsec FWHM (full-width-half maximum). The lines emerging from the bright image are caused by optical reflection in the telescope. The angular distance is 2 arcsec, cf. the indicated scale.

In order to investigate the nature of this object, the team obtained images and spectra with the Very Large Telescope (VLT) at Paranal.

An optical image was taken by ESO staff on 21 February 2000 during a technical test period ([4]) with the FORS-2 (FOcal Reducer/low dispersion Spectrograph) at the 8.2-m VLT/KUEYEN telescope, cf. PR Photo 17a/00. This is actually the sharpest optical image so far taken with the VLT, with a FWHM (full-width-at-half-maximum) of only 0.18 arcsec [2] and it shows the images of the primary star ("TWA-5 A") and the 100 times fainter companion ("TWA-5 B") very well separated.

An infrared image was taken on 16 April 2000 with the ISAAC (Infrared Spectrograph and Array Camera) multi-mode instrument at the 8.2-m VLT/ANTU telescope. This image was obtained by ESO staff in service mode and again, TWA-5 A and B are both clearly seen.

More recently, spectra of TWA-5 B were taken with FORS-2 (optical wavelength region) and ISAAC (infrared). These observations were particularly difficult, because of the need to avoid contamination from the strong light of the much brighter object, only 2 arcsec away.