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Abstract:

We received a sample of 6840 Hipparcos stars south of declination -26 degrees that (i) have MK spectral types in the Michigan catalogues and (ii) had spectroscopic parallaxes that placed them within 80 pc of the Sun. Of these, 3727 are well determined as luminosity class V and actually lie within 100 pc. From this subsample we can determine the distribution in M-V of main-sequence stars of given spectral type for spectral types that range from early F to early K. These distributions are significantly non-Gaussian, but when fitted to Gaussians they yield central values of M-V in good agreement with earlier estimates of the absolute magnitudes of main-sequence stars. We also determine anew the distribution of B-V at each spectral type. We find that the dispersion in B-V at given spectral type is very small.

Abstract:

A parameterized model of the mass distribution within the Milky Way is fitted to the available observational constraints. The most important single parameter is the ratio of the scale length R_d* of the stellar disk to R0. The disk and bulge dominate v_c(R) at R> R0. For example, changing the disk slightly from an exponential surface-density profile significantly changes the form of v_c(R) at R >> R0, where the disk makes a negligible contribution to v_c. Moreover, minor changes in the constraints can cause the halo to develop a deep hole at its centre that is not physically plausible. These problems call into question the proposition that flat rotation curves arise because galaxies have physically distinct halos rather than outwards-increasing mass-to-light ratios. The mass distribution of the Galaxy and the relative importance of its various components will remain very uncertain until more observational data can be used to constrain mass models. Data that constrain the Galactic force field at z > R and at R > R0 are especially important.

Abstract:

A straightforward determination of the circular-speed curve vc(R) of the Milky Way suggests that near the Sun, vc starts to rise approximately linearly with R. If this result were correct, the Galactic mass density would have to be independent of radius at R ~> R0. We show that the apparent linear rise in v_c arises naturally if the true circular-speed curve is about constant or gently falling at R0 < R ~< 2 R0, but most tracers that appear to be at R ~> 1.25 R0 are actually concentrated into a ring of radius ~1.6 R0.