Astronomisches Rechen-Institut Heidelberg

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ARI-Heidelberg Mitteilungen Serie B, No. 49

Author(s): Wielen, R.
Title: Spiralarm-Indikatoren, Dichtewellentheorie und optische Beobachtungen
Source: In: Optical Observing Programs on Galactic Structure and Dynamics. Proc. Conf., Bochum, Germany; T. Schmidt-Kaler (ed.), Astron. Inst., Ruhr-Univ., Bochum, p. 59-74
Year: 1975
Abstract: Spiral-arm tracers, density-wave theory and optical observations: We discuss the implications of the density-wave theory on future programs of optical observations for the spiral structure and kinematics of the Galaxy. We conclude that (1) the 'Grand Design' of the spiral structure should be obtained as far as possible from optical studies instead of using 21-cm-profiles of HI, (2) the age-dependence of spiral structure (i.e. drift and broadening of arms as a function of the age of the stars used for defining the structure) should be studied more carefully , and (3) the spiral structure should be obtained not only from the density distribution (i.e. locations) of young objects but also from their kinematical behaviour. We emphasize that the usual formalism of the density-wave theory is valid only for a (quasi)-stationary wave, which may exist in the gas and among the older stars, but certainly not among the young stars. Although young stars reflect some properties of the density wave in the gas, they are merely in the process of migrating away from their places of formation instead of forming a stationary wave. We show that the commonly adopted model of the density wave in our Galaxy does not predict any measurable drift of spiral arms in the region around R=10 kpc, since the newly born stars move, on the average, along the spiral shock front for about 7 107 years.
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