Discovery Of The First X-Ray Emitting Brown Dwarf

Observations carried out with the ROSAT X-ray satellite, and with several telescopes of the European Southern Observatory in La Silla, Chile, have revealed the first brown dwarf known to emit X-rays. The brown dwarf, called Cha Ha 1, is a very young member of the Chamaeleon dark cloud number I (Cha I), a star forming region located 550 light-years away from us.

Brown dwarfs are objects intermediate between stars and planets. The temperatures and pressures at their centers are insufficient to sustain the nuclear reactions that provide stars with their longlasting source of energy. Since central temperature and pressure are essentially a function of the mass of the object, the borderline between stars and brown dwarfs can be drawn at a mass that theoretical models accurateley place at 7,5 % of the mass of our Sun, or about eight times the mass of Jupiter.

Lacking the source of energy that provides stars with a force preventing collapse under their own weight, brown dwarfs continue to shrink, cool, and fade out almost indefinitely after they are formed. It is precisely their very low luminosity what makes brown dwarfs so difficult to detect: it was not until late 1995 that the first bona-fide brown dwarf was identified. Since then, the number of known brown dwarfs has kept increasing, reaching about one dozen nowadays. When they are very young, brown dwarfs shrink rather rapidly, releasing large amounts of gravitational potential energy that makes them relatively bright and hot. Thus, apromising way of detecting brown dwarfs is to look at nearby regions where star fomation has taken place recently or is still going on, such as Chamaeleon I.

The history of the discovery

The history of the recognition of the special character of Cha Ha 1 began in March 1998, when Dr. Ralph Neuhäuser of the Max Planck Institute for Extraterrestrial Physics in Garching bei München, Germany, was analyzing some long exposure X-ray images taken by the ROSAT satellite. The Roentgen-Satellite ROSAT, a collaboration between Germany, the UK, and the US, was launched in June 1990 and has since performed not only the first all-sky survey in the soft X-ray regime, but also about nine thousand pointed observations. One of these pointings with an exposure time of 10 hours was centered on the Chamaeleon I star forming region. Most of the X-ray sources detected in this image are young stars whose X-ray emission was already well known. However, given the higher sensitivity of these images with respect to those used by previous investigators of that region, Neuhäuser expected to identify new members of Chamaeleon I whose X-ray emission was too faint to have been noticed previously. Since X-ray luminosity is obsereved to be correlated with the total luminosity of the star producing it, and the luminosity is in turn correlated with the mass, the new, faint X-ray emitters could be among the lowest mass stars of Chamaeloeon I, and it might be possible to find some brown dwarfs among them.

Meanwhile, an investigation also aimed at the detection of the faintest members of Chamaeleon I had been carried out by Dr. Fernando Comerón, of the European Southern Observatory in Garching bei München. Comerón tried to identify these objects by means of two features often found in young stars: the excess luminosity displayed at infrared wavelengths, due to the circumstellar material left over from the process of star formation, and the emission in Ha , a spectral line resulting when free protons and electrons become bound to form hydrogen atoms. For this purpose, Comerón had surveyed the entral region of Chamaeleon I using the infrared camera at the 2.2 m telescope in La Silla, and had obtained objective prism spectra of the same area in the wavelength region around the Ha line using the 1,5 m Danish telescope, also in La Silla. These surveys hat revealed two new faint objects with infrared excess luminosity, and six new Ha emission objects. Their luminosity placed them near, and possibly below, the theoretical borderline separating stars from brown dwarfs in a region of the age of Chamaeleon I.

The comparison between the ROSAT observations and the surveys carried out from La Silla yielded the surprising result that the faintest object detected with Ha emission was coincident, with the strongest X-ray source in that area of the sky without a previously identified visible counter part.

This was an exciting discovery, but it was made uncertain by the fact that Cha Ha 1 was too faint to allow a good quality spectrum to be obtained with the 1.5 Danish telescope. Such a spectrum was necessary to determine the temperature of the object, which would in turn allow the comparison with theoretical models and therefore the estimate of its mass and its age. Fortunately, in May 1998 it was possible to obtain new spectra, in the visible and infrared spectral ranges, using the ESO 3.5 m New Technology Telescope in La Silla. These spectra allow the temperature of Cha Ha 1 to be determined with an accuracy of about 150 degrees Kelvin. The derived temperature and luminosity of Cha Ha 1, when compared to the predictions of theoretical models, tell us that Cha Ha 1 is a brown dwarf with a mass of only 4 to 5 % of the mass of the Sun, and an age of one million years.

The significance of the discovery

Despite their expected brightness, and the fact that many candidates exist, very few brown dwarfs have been reliably identified so far in star forming regions. The reason is that it is difficult to discriminate observationally a very low mass star from a brown dwarf when they are only a few million years old. However, the very low luminosity and temperature derived for Cha Ha 1 leaves little uncertainty as to its true brown dwarf character: it can be considered to be only the second bona-fide brown dwarf identified so far in a star forming region, and the youngest one known, the second youngest being three to ten times older. As such, the study of its properties is of a primary importance for the understanding of this new class of objects, their evolution, and their structure.

One of these properties is the X-ray emission, which is known to be very common among young stars with masses of a few tenths of that of the Sun. Such stars are fully convective, meaning that the gas in their interior forms ascending and descending currents extending from deep inside the star to the surface. Such large scale motions of ionized gas, coupled with the general rotation of the star, can generate a strong magnetic field by dynamo effect. The magnetic field transports energy from the interior of the star to the chromosphere, a layer of hot gas lying above the atmosphere of the star. It is precisely the energy conveyed by the magnetic field, when deposited in the chromosphere, what produces the very high temperatures which give rise to X-ray emission. A similar mechanism of energy transport also operates in our Sun, although its details are not well understood. The detection of X-ray emission, and its correlation with various properties of the star such as its luminosity or its rotational period, can be of a great help to understand the details of the process of magnetic field generation and the internal structure of the star.

According to this general picture, one would expect that brown dwarfs, which are fully convective as well, should also possess magnetic fields and the hot chromospheres responsible for the X-ray emission. So far, this had been only theoretical conjecture without an observational confirmation. It had been suggested, on the other hand, that the fact that brown dwarfs cannot sustain nuclear reactions in their cores may have a negative impact on the process that generates the magnetic field. The role of rotation in generating the magneticfield has been also controversial: theoreticians disagree on whether fast rotation should enhance the magnetic field, or rather inhibit it, due to the difficulty of sustaining a well ordered convective pattern inside a rapidly rotating object. Answering to these questions requires observations of actual brown dwarfs and of their X-ray emission produced by magnetic activity.The results of Neuhäuser and Comerón, obtained with ROSAT and several ESO telescopes on La Silla, revealing brown dwarfs as a new class of X-ray emitting sources, are thus an important step towards a detailed understanding of these faint but important objects.

Ralph Neuhäuser, Fernando Comerón: ROSAT X-ray Detection of a Young Brown Dwarf in the Chamaeleon I Dark Cloud, Science, Vol. 282, 2 October 1998, 83 - 85