Information from the European Southern Observatory ESO Press Release 08/98 24 June 1998 For immediate release | | First Circumstellar Disk around a Massive Star Observations
with an infrared-sensitive instrument at the ESO 3.6-m telescope at La
Silla have for the first time shown the presence of a disk around a hot
and massive star, known as G339.88-1.26 . Until
now, disks have only been found around less massive stars. Planets are
formed in such disks. The new discovery may thus have important
implications for our understanding of the formation of planetary
systems around stars. TIMMI observations Observations at mid-infrared wavelengths were carried out in July 1997 by Bringfried Stecklum (Landessternwarte Thüringen, Tautenburg, Germany) and Hans-Ulrich Käufl (ESO), using the TIMMI instrument at the ESO 3.6-m telescope. Additional measurements were carried out in March 1998. TIMMI ( T hermal I nfrared M ulti M ode I
nstrument) is a general-purpose camera spectrometer operating at a
wavelength of 10 µm. To reach sufficient sensitivity, the camera must
be cooled to approx. -260 o C, i.e. a few degrees above the absolute
minimum, by use of liquid Helium. Astronomical objects whose
temperatures are between -120 o C and 300 o C radiate most of their
energy at this wavelength. In addition, dust and haze that are
absolutely impenetrable for light visible to the human eye, are often
found to be nearly transparent at this wavelength. This is why
fire-fighters now use similar equipment to look through smoke. G339.88-1.26: A very special object ESO PR Photo 22a/98 [JPEG, 800k] This image is a true-color composite of near-infrared observations of the sky region around the radio source G339.88-1.26
with the ESO/MPI 2.2-m telescope at La Silla. In this image, the
visible colors red, green and blue have been used to represent the
infrared filters J, H and K (at 1.25, 1.63 and 2.2 µm wavelength,
respectively). No object is visible at the position of the radio
source, even at these near-infrared wavelengths. A dark band of
absorbing dust is clearly visible, exactly at the position of the
object (indicated by an arrow). Earlier observations with radio telescopes of the object G339.88-1.26
, deeply embedded in an interstellar nebula, had been interpreted in
terms of the possible existence of a circumstellar disk around a
high-mass star. It was concluded that the star responsible for heating
the surrounding gas must be very hot and also that it must be
intrinsically very bright. The star, most likely of spectral type O9,
would have a luminosity 10,000 times higher than that of the Sun and a
mass of about 20 times that of the Sun. From the measured velocity, the
likely distance of this object is about 10,000 light-years. The
object is associated with several "spots" of very strong radio emission
from methanol molecules (methanol masers). Interestingly, they form a
chain in the sky and the measured velocities of the individual spots
are indicative for orbital motion in a rotating disk around the central
star. The circumstellar disk ESO PR Photo 22b/98 [JPEG, 640k] The TIMMI 10 µm image of the inclined dust disk around a hot O9 star at the G339.88-1.26
radio source. The diameter of the disk is of the order of 5 arcsec,
i.e. at the most probable distance to the object (10,000 lightyears) it
is 20,000 times larger than the diameter of the Earth's orbit around
the Sun. The new TIMMI observations of G339.88-1.26
showed an elliptical object with strong infrared radiation. The peak of
this radiation (as seen in the sky) coincides with the peak of the
radio emission. Furthermore, the apparent orientation of the disk is
well aligned with that of the methanol maser "spots". There is little
doubt that this object is indeed the infrared image of a circumstellar
disk, viewed at an angle. As far as known, this is the first
direct image of a disk around a very massive star. At a wavelength of
10 µm, however, the central star that is responsible for heating the
dust disc, cannot be seen in spite of its rather high luminosity. This
is because it radiates mostly in the ultra-violet part of the spectrum.
Moreover, the dust disk in which the hot star is embedded, absorbs the
stellar ultraviolet light extremely efficiently, thereby re-emitting
this energy in the infrared. And any stellar light that escapes the
dust shroud is in any case completely blocked by intervening
interstellar material in the nebula. Implications of the discovery The
formation of disks of dust and gas around young stars is now considered
to be a normal feature of star formation. This is well established for
stars of about the size and mass of our Sun. However, until now there
has been no direct evidence of such disks being also formed around
young massive stars. In view of the extremely high luminosity
of such massive stars, any surrounding disks are subject to a fierce
attack by the enormous flux of light to which they are exposed. Hence
the existence of such disks around massive and luminous stars has been
questioned by astronomers interested in the physical process of star
formation. The discovery of the disk around G339.88-1.26
now settles the question whether such disks can indeed be formed and
are stable over periods long enough that they can be observed. Supplementary measurements to investigate the molecules and dust around G339.88-1.26
have been performed with the SEST submillimeter telescope at La Silla
in March 1998. Near-infrared images have also been taken at the NTT and
at the 2.2-m telescope. The team responsible for this project also includes Thomas Henning and Markus Feldt (Astrophysikalisches Institut & Universitäts-Sternwarte, Jena, Germany), Andreas Eckart (Max-Planck-Institut für extraterrestrische Physik, Garching, Germany) and Lars-Åke Nyman (ESO). Further information is available at URL: http://www.tls-tautenburg.de/research/g339.html . This Press Release is accompanied by ESO PR Photo 22a/98 and ESO PR Photo 22b/98 . They may be reproduced, if credit is given to the European Southern Observatory. How to obtain ESO Press Information ESO Press Information is made available on the World-Wide Web (URL: http://www.eso.org/outreach/press-rel/ ). ESO Press Photos may be reproduced, if credit is given to the European Southern Observatory. |