Astronomers find clue to riddle of spinning star formation

Armagh Observatory, 20 January 2000:

Astronomers may have found another piece of the puzzle of why stars are born. The material, from which stars form, is supported by its own spin. If a star is to appear, the spin must be deposited elsewhere. It could go into orbiting planets, or be blown out in a rotating wind. Instead, new evidence obtained using the UK's Infrared Telescope (UKIRT) in Hawaii, spins a different story. Streams of spinning bullets have been discovered.

``These infrared bullets, moving at prodigious speeds, deposit the spin far away from the birth region'', stated Armagh Observatory astronomer Dr. Michael Smith. ``The cloud of gas and dust may then continue its transformation into a young star.''

The formation of a star resembles a pregnancy. Stars are born within stellar wombs: giant clouds of molecules and dust. The young embryos can be detected through observations with radio waves, analogous to the ultrasound scans which can follow early pregnancies. Astronomers have found that the womb suffers a series of strong kicks as the growing star seeks to throw away waste products. These kicks generate strings of molecular bullets, which can be directly observed as they approach the skin of the stellar womb or even while still close to the stellar embryo. For this, telescopes with infrared cameras are used. Now, using the UK Infrared telescope in Hawaii fitted with sensitive instruments, a team of astronomers have found evidence that the jets of bullets are spinning.

The team, led by Dr. Antonio Chrysostomou (University of Hertfordshire) and Dr. Chris Davis (Joint Astronomy Centre, Hawaii), studied the bullets of the most beautiful, symmetric jets called HH212 which protrude from a very deeply embedded star-to-be, or prototstar. They also examined a second series of bullets called HH7-11. The speed at which the bullets are moving was investigated in two ways. ``We imaged the trail of bullets in HH7-11 in 1994 and 1998, and found that the bullets must have moved at a remarkable speed across space,'' said Dr. Chrysostomou. ``Then we looked at the speed of the bullets in our direction,'' added Chris Davis, ``and found that they were organised. So we sent the results to Dr. Smith''.

``I discovered that there were systematic motions not only along the jets but also across the jets'', explained Dr. Smith, ``that are consistent with rotation. The data could be interpreted in other ways, of course, so we needed some further support.'' The support came in the form of simultaneous work going on by another team of astronomers led by Jennifer Wiseman (Johns Hopkins University, USA) which discovered a disk of rotating gas surrounding the star. The disk lies transverse to the line of bullets, and rotates in the same sense as the jet (clockwise looking down from the north). ``This is exactly what we expect to find if the bullets are to extract the spin from the disk. Then, the rest of the gas can be accumulated into a protostar''.

Dr. Smith went on to speculate: ``We would all like to know how to travel fast without being shaken to pieces. This gas is moving at Mach 500, and the molecules survive. If we could travel at Mach 500 from London to New York, we would be there within a minute.''

Further science information:

Spin is a form of angular momentum. It is usually conserved, which means it is hard to shake off. Thus, the moon spins around the earth and the earth spins around the Sun. Without these spins, gravity would not be balanced and the whole solar system would collapse. Therefore, the outward centrifugal force created by the spin is essential throughout the Universe. So, if spin is ubiquitous, then how do the stellar embryos collapse? Where does the spin go? As the new observations suggest, the spin is ejected out of the system at high speed within the bullets. It remains to determine the mechanism behind this. Smith explained ``I believe we know the principles. It is analogous to that of a thin wire which gets coiled up into a spring by the rotating disk. The spring is then triggered, releasing the bullet. Here magnetic lines of force act like the wire. The molecules we observe are like beads which thread the wire, forced to move in unison.''

The observations may lead to the solution of one problem but they intensify a second problem. The astronomers are stunned by the high speeds with which fragile molecules are ejected. ``How can molecules of hydrogen, which are easily destroyed in collisions of only 10 kilometers per second, attain speed of almost 500 kilometers per second?'', asks Dr. Smith. ``Each molecule should at least dissociate, back into two distinct atoms, and each atom ionise into a proton and an electron.''

Background information:

The images were obtained with two generations of infrared cameras at UKIRT, first IRCAM3 and later the Ukirt Fast-Track Imager, UFTI. UFTI has a smaller pixel scale than IRCAM3 and so allows us to make the most of the continually-improving image quality at UKIRT. UFTI was built in Oxford and Edinburgh and was delivered to UKIRT in 1998. UFTI continues to live up to our expectations, delivering super-sharp images that are comparable to those of the infrared camera on the Hubble Space Telescope.

The radial (along the line of sight) velocity measurements were obtained with UKIRT's work-horse spectrometer, CGS4. CGS4 is equiped with a very high resolution echelle grating and is still very much the premier infrared spectrometer in the world.

Manuscripts:

Two papers are in press of the Monthly Notices of the Royal Astronomical Society, and are available from http://www.jach.hawaii.edu/~cdavis or upon request to the authors.

Astronomers involved in this project:

Chris Davis; Joint Astronomy Centre, Hilo, Hawaii, USA
Antonio Chrysostomou; University of Hertfordshire, Hatfield, England
Michael Smith; Armagh Observatory, N. Ireland
Jason Hobson and Aaron Berndsen; University of Victoria, Canada.

For further information of UKIRT and its instrumentation, visit the UKIRT web pages (http://www.jach.hawaii.edu/UKIRT/)
or contact Chris Davis (phone: US+808 969 6520, Fax: US+808 961 6516)

For further information about Armagh Observatory, visit the Observatory web pages
or contact Michael Smith (phone: UK+28-37522928, Fax: UK+28-37527174)

For further information about jets, bullets and star formation try contacting Michael Smith (phone: UK+28-37522928, Fax: UK+28-37527174)

For further information on the rotating disk detected in the ammonia NH3 (J,K)=(1,1) emission, contact Dr. Wiseman at Johns Hopkins University, (phone: USA-410-516-6539).

HH212 was discovered by Hans Zinnecker, Mark McCaughrean and John Raynor. For further information on the 'most beautiful' jets of HH212 contact Dr Zinnecker or Dr McCaughrean at the Astrophysikalisches Institut, Potsdam, (phone: 49-331-7499-347), or read the Nature paper from 1998: Nature 394, 862.

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