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The kinematics of gas and stars in the solar neighbourhood Goulet, Thomas


The kinematic properties of gas and stars in the solar neighbourhood are described in terms of the line-of-sight component of a three-dimensional first order Taylor series expansion of the local velocity field. The types of object analysed are: 1) 21-cm absorbing clouds, 2) HI intercloud medium, 3) main sequence B stars, 4) B stars of luminosity class ranging from I to IV, 5) main sequence A stars, 6) K-giant stars. The least squares fitting procedure used to derive the 10 coefficients describing the fields for various types of objects was essentially the same so that a valid comparison could be made. Marked departures from circular motion are found in most cases but the only systematic trend is a correlation between δu/δx (u being the velocity component along the x-axis directed towards the galactic center) and stellar spectral type, where the gas behaves like a medium "younger" than the early type stars. The analysis of the gas indicates that the standard plane-parallel model provides a good description of the intercloud medium but is inadequate for the absorbing clouds. A velocity ellipsoid description of the residuals is presented for each group of objects. The influence of the Gould belt and of nearby spiral arms on local kinematics is discussed.

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