Max Planck Institute for Extraterrestrial Physics
Garching bei Munchen/Germany
Contact: Ralp Neuhauser
Phone: +49 89 - 32 99 - 33 98
Fax: +49 89 - 32 99 - 36 69
European Southern Observatory
Garching bei M=FCnchen/Germany
Phone: +49 89 - 3 20 06 - 531
Fax: +49 89 3 20 06 - 480
Discovery of the first X-ray emitting brown dwarf
Scientists at the Max Planck Institute for Extraterrestrial Physics and at
the European Southern Observatory in Garching/Germany reported not only the first X-ray detection of a brown dwarf, but also the discovery of the
youngest brown dwarf known so far (Science, Vol. 282, 2 October 1998).
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 H-alpha 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 H-alpha 1
Began in March 1998, when Dr. Ralph Neuhauser of the Max Planck Institute
for Extraterrestrial Physics in Garching bei Munchen, 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, Neuhauser 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 Comeron, of
the European Southern Observatory in Garching bei M=FCnchen. Comeron 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 H-alpha , a spectral line resulting when free protons and
electrons become bound to form hydrogen atoms. For this purpose, Comer=F3n 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 H-alpha line using the 1.5m Danish telescope, also in La Silla. These surveys hat revealed two new faint objects with infrared excess luminosity, and six new H-alpha 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
The comparison between the ROSAT observations and the surveys carried out
from La Silla yielded the surprising result that the faintest object detected
with H-alpha 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 H-alpha 1 was too faint to allow a good quality spectrum to be obtained
with the 1.5 m 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 H-alpha 1 to be determined with an accuracy of about 150 degrees
Kelvin. The derived temperature and luminosity of Cha H-alpha 1, when
compared to the predictions of theoretical models, tell us that Cha
H-alpha 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 H-alpha
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 asits
luminosity or its rotational period, can be of a great help to