@misc{oai:ir.soken.ac.jp:00000389,
 author = {新永, 玲 and シンナガ, レイ and SHINNAGA, Rei},
 month = {2016-02-17},
 note = {The formation of stars begins with collapse of a dense interstellar cloud core to a protostar surrounded by an accretion disk.  Materials in the core falls inwards to feed further growth of the protostar through the disk.  At certain point during the accretion phase, a jet of gas is generated by the forming protostar along the disk's rotation axis.  Subsequently, protostellar jet drives molecular outflow, and which will destroy the core and will terminate the mass accretion.  Such evolution of young stellar objects (YSOs) had been classified into three categories, Class I, II, and III, based on the slope of their infrared spectral energy distribution (SED).  Recently, the new fourth class "Class 0" has been suggested for the youngest protostellar objects which have not been detected in the near-infrared and which display an SED with a peak in the submillimeter wavelength.  The jet and outflow are an ubiquitous phenomenon in YSOs ranging from "Class O" to "Class I" sources.  Despite the substantial observational progress, it is still uncertain when the jets are ejected during the accretion phase because most of Class 0 sources already posses jets and outflows.
  The Class 0 source, S106 FIR (d = 600 pc) is a good candidate to investigate star formation process at an extremely early stage before formation of outflow.  This is because this source is known to be associated with no extensive molecular outflows, making it unique among all 40 known Class 0 sources.  First, we conducted aperture synthesis observations of H20 maser emission at 22 GHz toward S106 FIR using the Very Large Array (VLA) of the National Radio Astronomy Observatory (NRAO) at 60 mas (milli-arcsecond) resolution on two epochs separated by ～3 months.  Two compact clusters of the maser spots separated by ～50 AU were found in the center of the submillimeter core of S106 FIR.  The western cluster, which has three maser components, was blueshifted and the eastern cluster, which has a single component, was redshifted with respect to the ambient cloud velocity.  Each component was composed of a few spatially localized maser spots and was aligned on a line connecting the two clusters.  We found relative proper motions of the components with 〓30 mas year-1 along the line.  In addition, a series of single-dish observations show that the maser components drifted with an accelerations of ～1 km s-1 year-1.  We conclude that the masers are excited by a 10 AU-scale jetlike accelerating flow ejected from an assumed protostar located between the two clusters.
  Second, we made the higher resolution observations to reveal the detailed structure of the jetlike flow and to track its motion using the Very Long Baseline Array (VLBA) of the NRAO on four epochs separated by ～1 month.  The attained angular resolution of ～0.5 mas corresponds to 0.3 AU.  The observations have revealed the presence of a U-shaped distribution of the masers in the western blueshifted cluster, which has a length of 4 AU and a width of 3 AU.  Three-dimensional velocity of the U-shape structure was analyzed using the transverse velocities of the masers and line-of-sight velocities. Since the masers are thought to be excited in a "micro bowshock" behind the head of the jetlike flow drilling the ambient cloud medium, we call the jetlike flow as a "micro jet'.  The U-shaped structure suggests that the micro bowshock does not comprise of interior knots but lies at the head of the Micro jet.  The presence of the Micro jet and the absence of extensive molecular outflows together with the cold SED argue that S106 FIR is an object in the earliest stages of star formation just after the onset of the protostellar jet.
  In order to investigate the youth of S106 FIR, we have made deep outflow search by 12CO (1-0) emission using the Nobeyama Millimeter Array (NMA), and by the (3-2) and (2-1) transitions of the molecule using the Caltech Submillimeter Observatory 10.4 m telescope.  We could not identify any evidence for the presence of the molecular outflow driven by the Micro jet.  Our attained 10σ sensitivities are of orders of 10-7 M〓 per km s-1 for the (1-0), (2-l), and (3-2) transitions:  these sensitivities would have detected any of the CO outflows associated with Class 0 sources.  This negative detection suggest that S106 FIR is too young to sweep up the ambient cloud medium to form the molecular outflow.  We discussed its age to be 〓 0.5-1.0x10 3 years from an upper limit of dynamical time scale of outflow, and from the number ratio of sources without and with outflow in Class 0 sources and the lifetime of "Class 0".  We conclude from these observations that S106 FIR has not swept up enough ambient gas to generate a large scale molecular outflow.
  With the original definition, S106 FIR is certainly categorized into "Class 0".  However, S106 FIR is obviously different from the other Class 0 sources in the sense that it has not yet generated a large scale molecular outflows.  This fact might mean that the SED classification scheme is inadequate to describe in detail the evolutionary stages, which might be regulated by jets and outflows.  In this sense, S106 FIR is the prototype of the earliest stage, which is characterized by the lack of an extensive molecular outflow, the presence of a Micro jet and a cold SED.  Identifying similar protostars and studying their physical properties will be of great importance towards understanding the evolution from cloud cores to protostars.  In addition, the physical properties of the Micro jet itself will help to clarify the acceleration and collimation mechanisms of astrophysical jets.
  In addition, we conducted a multi-epoch survey of H20 masers towards 〓 200 known cloud cores and YSOs using the Nobeyama 45m-telescope and the VLA.  The goal of this survey is to establish a general view of the maser emissions associated with low-mass YSOs.  We have found that the masers have been detected for 46 ± 8% of Class 0 sources, while a detection rate of only 4 ± 3% for Class I sources.  We interpreted this result as follows.  Since Class 0 sources possess a large amount of circumstellar gas, the energetic jets and outflows of Class 0 sources easily enhance the maser emission., application/pdf, 総研大甲第455号},
 title = {An Observational Study of the Extremely Young Stageof Star Formation},
 year = {}
}