S106FIR: The Youngest Jet in the Universe???

Ray Furuya, Masao Saito, Yoshimi Kitamura,Ryohei Kawabe, Mark Claussen, Al Wootten

Many young stars are known to possess gas jets or outflows, but firm observational evidence for the mechanism of their generation has proven to be surprisingly elusive. The best evidence comes from observing newly ejected gas very close to its driving protostar. That gas, rapidly excaping a central forming star, drives a shock into the enveloping dense molecular gas cloud. Here we present observations of outwardly flowing gas from a protostar, called S106FIR, lying at the center of a dense cloud of gas and dust. We have measured the expansion of an extremely compact protostellar jet by monitoring the motion of water vapor masers as they punch their way out of the dense circumstellar gas. This flow must be very young because it is very compact; it occurs near a very young protostar with no evidence of a larger scale flow, and it displays an unusual 'U' shape in the distribution of the masers. On the left above is a 5 GHz radio image of S106, showing the large outflow from the more luminous, more evolved source S106. Within the dark lane to the right lies the S106FIR source, one tenth as luminous overall, but not visible at the wavelength of either of these images.

Please obtain a copy of a paper on the masers of S106FIR by Furuya et al. 1999


	At submillimeter wavelengths, the dust which obscures objects at visible or near infrared wavelengths glows proportionally to its mass. In this submillimeter image from the JCMT, the invisible knot containing S106FIR is prominent to the right of S106IR.

Many astronomers have come to believe in recent years that planetary systems such as our solar system are common in the Universe. Many disks of dust and gas have been found spinning around young stars, each containing enough raw material to form planetary systems. Accopanying even the youngest stars, astronomers have found protostellar jets emerging from the central protostar at its poles, perpendicular to the disk. These jets are thought to ease accumulation of mass by the protostar, and may be essential to star formation. However, how and when these jets form is uncertain; most young stars already possess well-developed jets. S106FIR has showed no evidence for a flow, despite detailed searches. The large amount of gas and dust at its center suggested the star might be exceptionally young. For these regions we searched more thoroughly for evidence of a jet. S106FIR lies about 2000 light years away, with several times the mass of the Sun and hundreds of times its luminosity. It lies at the center of a dense molecular cloud stretching over 60 times the size of the Solar System. Material from this core is expected to fall, through gravitation, onto the central protostar which in turn powers a jet which drives core material outward from the star's poles. As the jet impacts the cloud, water masers, the microwave equivalent of lasers, are formed, as occurs near many similar though older protostars. Being very bright, they reveal the detailed structure of the jet to the continent-wide telescope, the Very Long Baseline Array (VLBA), which we have used to image them. This telescope measured details in the water maser structures smaller than the Sun's distance from Mercury in our Solar System. The masers lie in a 'U' shaped pattern of width and length 4 astronomical units (AU; 1 AU is the mean Earth-Sun distance), a little less than Jupiter's distance from the Sun. The pattern is located about 25 AU from the central protostar. This is many times smaller than any other known protostellar outflow; it is the smallest known jet in the Universe, which we call a microjet. Theoretical predictions show that when very young, these shocks will ahve a 'U' shape, widening to a rounded V shape as they age. The latter shape is found in another young object, with a much older flow, IRAS05413-0104, for instance. We measured the structures on four days about a month apart from 1997 November 5 to 1998 February 1. The 'U' shaped constellation of masers was measured to be moving away from the protostar at nearly 100,000 miles per hour. At this speed, it may have been as little as a few years since the jet began to penetrate the cloud; there is no evidence for larger scale flow despite sensitive searches. Thus, this smallest known protostellar jet is also the youngest known. This discovery is a very important step towards understanding the formation mechnaism of protostellar jets--when they appear, how they evolve, and what role they play in a forming star's ability to gain mass. The ability of the VLBA to study these jets traced by water masers in such detail provides researchers with a powerful new tool for determining how jets form and evolve. New telescopes, currently being designed, operating at much higher frequency, will image even fainter chemical components of the jets. The Millimeter Array (MMA) or Large Millimeter and Submillimeter Array (LMSA) planned by the United States and Japan for a high site in the Chilean Desert, will provide detailed images of the high speed jets as they pound their way through interstellar clouds, showing not only the spatial progress of the shocks generated by the jet, but also the chemical changes which occur in the accelerated and heated material