Star counts and the warped and flaring Milky Way disk

This paper reports the star count predictions of warped and flaring models of the outer Milky Way disk. These have been suggested as possible location s of the lenses responsible for the microlensing events toward the Large Ma gellanic Cloud (LMC). Three such models are investigated in detail, and the theoretical predictions are confronted with Hubble Space Telescope star co unt data in seven low-latitude fields (30 degrees < \b\ < 40 degrees). If t he warped and flaring disk population has the same characteristics as disk stars in the solar neighborhood, then the models can be unambiguously ruled out. Metallicity gradients are well known in disk galaxies and may cause t he outer disk population to differ in colors and luminosity from the local population. This effect is studied using a simple Ansatz for the variation in the color-magnitude relation with position, and while it is shown to lea d to better agreement with the star counts, upper limits on the contributio n of the warped and flaring disk to the optical depth are still below the v alue measured toward the LMC. Only if the warp is very asymmetric or if the luminosity function changes strongly with Galactocentric radius can the mo dels be made consistent with the star counts.