Contact: Dr. Göran Sandell
Email: gsandell @ mail. arc. nasa. gov
Contact: Dr. Melvyn Wright
Email: wright @ creek2. berkeley. edu
A team of Berkeley astronomers led by Dr. Göran Sandell (USRA-SOFIA) has found the youngest high-mass star ever observed. The star is surrounded by a huge rotating disk of gas more than 100 times the mass of our Sun and embedded inside a compact dense cloud core of more than a 1000 solar masses. Further studies of this protostar, named NGC 7538 S, will yield important clues about the formation process of very high-mass stars. Dr. Sandell and his two collaborators, Dr. Melvyn Wright and Dr. James R. Forster, both at the Radio Astronomy Laboratory, University of California, Berkeley, studied the object using the Berkeley-Illinois-Maryland Array at Hat Creek Radio Observatory in California. Their findings are being presented at the American Astronomical Society meeting in Nashville, Tennessee and in the June 10 issue of the Astrophysical Journal Letters.
The young massive protostar is embedded in a huge gas cloud in the constellation Cepheus near a bright emission nebula known as NGC 7538. Located approximately 10,000 light years from Earth, this protostellar system consists of a central protostar of 20 - 40 solar masses surrounded by a rotating disk seen nearly edge on with a diameter more than 1,000 times that of our solar system. The protostellar disk drives a jet-like outflow which may be as young as only 2,000 years.
Figure 1 shows thermal dust emission for the cloud core and the protostellar disk in pseudo color. The cloud core is shown in green and the protostellar disk is yellow. The intense central area, hiding the protostar is coded as red. The color contours show the rotation of the disk measured in deuterated hydrogen cyanide. The blue color contours show that the northeastern part of the disk is coming towards us and the red contours show the receding part of the disk.
"Until very recently we did not have a clue as to how high mass stars form," says Sandell, "because once a massive core about 10 times the mass of the Sun is formed, theoretical model calculations showed that the radiation pressure from the young hot star should halt the infalling gas, preventing it from growing.
"The key to forming a high mass star is to have a large mass reservoir in a small volume" says Sandell. "This creates a high pressure environment that overpowers the pressure from the newly formed protostar allowing it to continue growing. Our observations show that once we have these conditions, a high mass protostar looks rather similar to a protostar of the size of our Sun."
Stars form in the gravitational collapse of dense regions of interstellar gas clouds. In the collapse, angular momentum is conserved, and part of the collapsing material forms a rotating disk around the protostar. As the protostar accretes mass, excess angular momentum is carried off in material which is ejected in a jet of high velocity gas perpendicular to the rotating disk. "Our observations show that NGC 7538 S has both of these essential characteristics of a protostar: (1) a rotating disk, and (2) a jet of high velocity gas, both centered on NGC 7538 S," says Wright.
Figure 2 shows the molecular outflow (blue and red contours) superimposed on a grey scale image of the protostellar disk and the dense cloud surrounding it, as measured in the 13C isotope of hydrogen cyanide. The protostar is marked by a red dot while the white line shows the symmetry axis of the outflow.
"Another thing which is unique to our high-mass protostar is a cold, dense cloud core," says Sandell. "Most young high mass stars or protostars are found in the immediate vicinity of compact ionized regions and near or in so-called hot cores. Here we find what appears to be a massive protostar, but which apparently has not yet had time to heat up the surrounding cloud, i.e., it is the first high mass star forming in this massive cloud core.
"McKee and Tan (Nature 2002) show that the typical individual star formation time scale for a massive star is 100,000 years," says Sandell, "What we are presumably witnessing is a much younger system, still surrounded by a rotating disk-like structure. However, we also know that this 'disk' cannot be stable (it is far too massive compared to the central protostar). It is almost certainly accreting material and at the same time it is blowing off mass in the outflow which we observe."
"The phase of a star's life where it is forming from gravitational collapse, is relatively short, perhaps only 1000 - 10,000 years for massive stars. The jet in NGC 7538 S is less than 10,000 years old, further evidence that this is a very young object. Because of the short formation time scale of a high mass star, it is very difficult to discover such a young object," says Wright.
Most of the objects detected with rotating accretion disks and jets are powered by nuclear reactions which will make them shine for millions of years. Even though one cannot "see" a new star in optical light one can detect the material which is ionized by powerful ultraviolet radiation from the star using radio wavelength observations. There is no evidence for ionizing radiation from NGC 7538 S, thus the authors believe that NGC 7538 S is a true protostar, still in the gravitational collapse phase.
Figure 3 is a cartoon representation of the rotating disk and the outflow. It shows that the blue outflow is still confined by the dense cloud core in which it was formed, but that the red outflow appears to have broken out of the cloud.
Göran Sandell is a senior scientist at the Universities Space Research Association, a 34-year old private, U. S. non-profit group of 90 institutions formed so that universities can cooperate effectively with one another, with the government, and with other organizations to further space science and technology and to promote education in those areas.
Melvyn Wright and James, R., Forster are astronomers with the Radio Astronomy Laboratory at UC Berkeley. Wright is one of the pioneers of millimeter wave aperture synthesis, the technique used to image the protostar and its massive rotating disk. Forster is the resident astronomer in charge of running the Hat Creek Telescope.
The Berkeley-Maryland-Illinois-Array located at Hat Creek, California, is operated by the Universities of California (Berkeley), Illinois, and Maryland with support from the National Science Foundation.