## 2. Halo modelsIn Paper I constraints on the halo fraction in clustered and unclustered low-mass stars are derived assuming the stars have zero metallicity and that the halo density varies with Galactocentric cylindrical coordinates () as where, in Paper I, the local density
pc The assumption of zero metallicity is maintained in the present analysis since one expects the halo to be perhaps the oldest of the Galactic components, and hence its constituents to have more or less primordial metallicity. The expected absolute magnitude in various photometric bands for such stars between the hydrogen-burning limit mass () and has been calculated by Saumon et al. (1994) and their results are employed here as in Paper I. The model dependency of the conclusions in Paper I is assessed by re-calculating the constraints for a number of different, but plausible, halo models. For ease of comparison the models selected are 5 of the reference halo models used by the MACHO collaboration (Alcock et al. 1996) in its analysis. (MACHO considers a total of 8 Galactic models, though only 5 of the halo models have distinct functional forms.) All halo models assume kpc and kpc. The 5 models are denoted by MACHO as models A-D and S (for `standard'), and this labelling is maintained here. The standard model S has the same functional form as the halo
investigated in Paper I (i.e. it is described by Eq. 1) but
uses the slightly larger IAU value for above
and assumes a lower local density
pc where is the velocity normalisation,
The particular parameters for models A-D, along with those of model
S are listed in Table 1. Model A is the closest analogy to model
S within the power-law family of models, whilst model B has a rising
rotation curve at large radii, model C a falling rotation curve, and
model D a flattening equivalent to an E6 halo. When combined with the
MACHO canonical Galactic disc (Alcock et al. 1996), the models give
values for the local Galactic rotation speed
within 15% of the IAU standard value of
220 km s
© European Southern Observatory (ESO) 1997 Online publication: March 24, 1998 |