Key mechanism for star formation found?
2 June 1999What causes new stars to form inside clouds of gas and dust in space? A team of astronomers using the European Space Agency's infrared space observatory, ISO, believes they have taken a big step towards answering this question. They announced today in Chicago (US), at the American Astronomical Society meeting, the first ever space-based detection of weak magnetic fields in a distant region in which stars are being formed. The differences between these magnetic fields and those from regions with no star formation have revealed what could be the key factor in triggering the birth of new stars.
The team, led by Dan Clemens, from Boston University Institute for Astrophysical Research, examined a distant cloud of gas and dust called GF9, located about 1300 light years away. It shows a filamentary or wispy appearance, with dark "globules" distributed along its length. The new observation with ISO focused on two of these dark globules, one in which a young star is forming and one in which no new stars are currently forming.
The results show that the magnetic fields in the star-forming region are more ordered and aligned than in the star-free region. It is this alignment of the weak magnetic fields that astronomers suspect is the key ingredient needed for gas clouds to become star-forming clouds.
"The finding is very important", says ESA astronomer Reni Laureijs at the ISO Data Centre in Villafranca (Spain), who last week chaired a workshop on this topic attended by Clemens. "It shows that our theories describing star-forming clouds are incomplete. People expected that the magnetic fields would follow the same pattern of the gas and dust filaments observable in the cloud, but we now know that the magnetic fields have a different, unexpected direction. Because of the weakness of the magnetic fields it has been a very difficult detection, made possible thanks to the sensitivity of ESA's ISO telescope"
The observations were made with the photopolarimeter ISOPHOT, one of the four instruments on board ISO. The polarimeter module of ISOPHOT worked rather similarly to the way in which polaroid sunglasses block light which has been polarized (caused to vibrate in the same direction) by reflection off metal or water. In the dust and gas cloud GF9 the light is polarized by grains of dust, which act as tiny magnets that alter the way the light propagates. By measuring the direction of this polarized light the ISO polarimeter could also sense the direction of the faint magnetic fields in GF9.
As Clemens explains, "the dust grains respond to the magnetic field by spinning around the lines of magnetic force, and we sense those dust grains through the polarized infrared light they emit".
The data show that the magnetic fields detected toward the star-forming globule in GF9 are surprisingly uniform and parallel, while in the star-less globule the magnetic field lines are tangled and disordered. Astronomers think that tangled magnetic field lines may be an important source of support that keeps some clouds from collapsing to form new stars.
This new discovery was made possible because of the great improvement in sensitivity to infrared light brought about by the cooled ISO telescope and its location above the Earth's atmosphere. Clemens believes more space missions with infrared polarimetric capabilities are needed. "Similar studies of other nearby star-forming clouds will have to await development of a new generation of small, highly capable cooled telescope missions", he says.
Footnote on ISOESA's infrared space observatory, ISO, was placed in orbit in November 1995, by an Ariane 44P launcher from the European Spaceport in Kourou, French Guiana. Its operational phase lasted till 16 May 1998, almost a year longer than expected. As an unprecedented observatory for infrared astronomy, able to examine cool and hidden places in the Universe, ISO made nearly 30 000 scientific observations. These are now available to the scientific community via the ISO Archive (http://www.iso.vilspa.esa.es) at the ISO Data Centre, in Villafranca, near Madrid, Spain.
For more information:
ESA Public Relations Division: Tel: +33(0)220.127.116.11.55 Fax:+33(0)18.104.22.168.90
Martin F. Kessler, ISO Project Scientist: Tel.: + 34 91 813 1253 mkessleriso.vilspa.esa.es
Reni Laureijs, ISO Data Centre in Villafranca (Madrid, Spain) Tel: +34 91 813 1367 rlaureijiso.vilspa.esa.es
Other science contact points:
Dan Clemens, Director of the Boston University Institute for Astrophysical Research Tel: +1 617 353 6140 clemensbu.edu
The ISO photopolarimeter, ISOPHOT, was built by a large international team led by Dietrich Lemke, MPI Fuer Astronomie, Heidelberg Tel: +49 6221.528.259 lemkempia-hd.mpg.de