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Many evidences demonstrate that the Milky Way is a barred galaxy with the near end of the bar pointing in the first Galactic quadrant (e.g. Gerhard 1999 for a review), thus offering an exceptional chance to study the detailed three-dimensional structure and dynamics of a real bar. One of the best tracer of the Galactic bulge/bar are the red clump stars: they are very numerous, sufficiently bright to be detected throughout the bulge, and their absolute I-magnitude has an intrinsic dispersion of only 0.2 magnitude, with a mean value almost independent of colour and metallicity (Paczyński & Stanek 1998, Udalski 2000). Here we outline an ongoing project aiming to recover the 3D bulge stellar distribution as primarily traced by these stars, as in Stanek et al. (1997), but in a non-parametric way and from the VR database of the MACHO experiment within an almost fully observed ∼ 8° × 8° bulge area centred on (l, b) ≈ (4°, − 6°) (e.g. Alcock et al. 1997), soon complemented with I-band photometry.

We have analyzed a sample of 1150 type ab, and 550 type c RR Lyrae stars found in 24 bulge fields of the MACHO database. These fields cover a range in Galactocentric distances from 0.3 to 1.6 kpc. In combination with the data on the outer bulge fields of Alard (1997) and Wesselink (1987), here we present the surface density distribution of bulge RR Lyrae between 0.3 and 3 kpc. The distribution along the line of sight of the bulge RR Lyrae population was examined on the basis of the mean magnitudes, and it was shown that the bulk of the RR Lyrae population is not barred (Alcock et al. 1998). There is a hint of a bar only in the RR Lyrae of the inner fields closer to the Galactic center. The red giant clump stars in the MACHO fields, however, clearly show a barred distribution, confirming the results of previous studies (e.g. Dwek et al. 1995, Stanek et al. 1996). In the MACHO fields studied there are about 550 clump giants per RR Lyrae star. The RR Lyrae trace metal-poor stars, which are a minor component of the bulge population. The clump giants, however, should trace the bulkmore » of the metal-rich population, foUowing underlying mass of the bulge more closely. Given the different spatial distribution, we concluded that the RR Lyrae and the clump giants trace two dif+erent populations (Alcock et al. 1998). The RR Lyrae would represent the inner extension of the Galactic halo in these fields (Minniti 1996). The observed surface distribution of RR Lyrae in the bulge fields was computed after discarding background RR Lyrae that belong to the Sgr dwarf galaxy (Alard 1996, Alcock et al. 1997). This distribution yields a power law density distribution. There is no turnover or flattening of this distribution even in the innermost fields, indicating that the RR Lyrae population is very concentrated, with core radius RC < 0.5 kpc. We also determine that the RR Lyrae surface distribution in the bulge fields is flattened, with b/a = 0.7.« less

Most of the mass near the Sun takes the form of stars less luminous than the Sun. A central problem of Galactic astronomy is the determination of how much mass these stars contain.The only thoroughly reliable way of detecting mass is through its gravitational field. Classically this has been done by studying the Galaxy’s rotation curve and the Oort limit, both of which remain active areas of research. In the last few years an exciting new way of probing the Galaxy’s gravitational field has been opened up by large surveys for microlensing events. These surveys are yielding important information about the numbers of low-mass objects both in the disk and above it. Another truly dynamical probe for low-luminosity objects is provided by studies of clusters. JD 10 reviewed what we have learned about the density of low-luminosity stars from each of these approaches.An important orthogonal approach to determining the density of low-luminosity stars involves seperately determining the luminosity function and the mass-luminosity relation for stars of a given spectral class (e.g. main-sequence stars) and then combining them to obtain the mass function. The development of infrared detectors and the refurbishment of the The Hubble Space Telescope have recently yielded important advances in each of these areas. A major difficulty with this line of research is the fact that, in the neighbourhood of M = 0.08 Mʘ, the main-sequence mass-luminosity relation is (i) steep and non-linear, and (ii) dependent not only on the masses of the faintest stars, but also on their ages, metallicities, rotation rates, binarity etc. Moreover, cool low-luminosity stars have extremely complex spectra, which are difficult to simulate with radiative transfer calculations. Similar difficulties are encountered in the determination of the mass-luminosity relation for white dwarfs. JD 10 reviewed the current status of these very difficult problems.