Tomography of Galactic star-forming regions and spiral arms with the Square Kilometre Array

Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike LicenceVery Long Baseline Interferometry (VLBI) at radio wavelengths can provide astrometry accurate to 10 micro-arcseconds or better (i.e. better than the target GAIA accuracy) without being limited by dust obscuration. This means that unlike GAIA, VLBI can be applied to star-forming regions independently of their internal and line-of-sight extinction. Low-mass young stellar objects (particularly T Tauri stars) are often non-thermal compact radio emitters, ideal for astrometric VLBI radio continuum experiments. Existing observations for nearby regions (e.g. Taurus, Ophiuchus, or Orion) demonstrate that VLBI astrometry of such active T Tauri stars enables the reconstruction of both the regions' 3D structure (through parallax measurements) and their internal kinematics (through proper motions, combined with radial velocities). The extraordinary sensitivity of the SKA telescope will enable similar "tomographic mappings" to be extended to regions located several kpc from Earth, in particular to nearby spiral arm segments. This will have important implications for Galactic science, galactic dynamics and spiral structure theories.Final Published versio

The Gould's Belt Distances Survey (GOBELINS). III. The Distance to the Serpens/Aquila Molecular Complex

Interstellar matter and star formatio

Discharge cleaning and plasma purity in ISX-B

Two simply measured parameters are shown to be useful in characterizing the vacuum vessel and plasma cleanliness and in predicting plasma performance in the ISX-B tokamak. It is demonstrated that the parameter P/sub rad//anti n/sub e/, measured at the start of each tokamak discharge, is related to both the available operating space (I/sub p/, anti n/sub e/) with Ohmic heating and the energy confinement times achieved with neutral beam injection. An assessment of P/sub rad//anti n/sub e/ on both a shot-to-shot and a day-to-day basis then determines the changing cleanliness of the plasma. It is further shown that P/sub rad//anti n/sub e/ can be predicted by the results of a residual gas analysis performed after discharge cleaning; specifically, it is directly proportional to the fractional concentration of water vapor

Gettering in ISX-B

Gettering is used in the ISX-B tokamak to reduce the impurity concentration. This paper documents the gettering process used, and compares the expected changes in recycling and radiation with those observed experimentally. The enlargement of the operating regime (1/q, anti n/sub e/ R/B/sub phi/ space) is discussed. Finally, the effect on one of the objectives of the experimental program, that of obtaining high values of beta, is described

V4641 Sgr returns to quiescence; deep observations planned

Observations of the X-ray binary V4641 Sgr (e.g., ATEL #309, ATEL #303) with the Very Large Array (VLA) show that the object has returned to quiescence. Our last clear detection was on 20 July 2004, when the 4.86 GHz flux density remained steady at 2.0 +/- 0.2 mJy in two five-minute observations centered on 05:06 and 08:53 UT. Data taken at the same frequency on 23 July 2004 gave a nominal flux density at the position of the source of 0.27 +/- 0.09 mJy/beam, while 8.46 GHz observations on 23, 27, 28, 31 July, 1 August gave nominal flux densities of 0.09 +/- 0.05, 0.06 +/- 0.08, 0.05 +/- 0.04, 0.105 +/- 0.034, and 0.00 +/- 0.10 mJy/beam, respectively