Astrometric masses of 21 asteroids, and an integrated asteroid ephemeris

We apply the technique of astrometric mass determination to measure the masses of 21 main-belt asteroids; the masses of 9 Metis (1.03 +/- 0.24 x 10(-11) M-circle dot), 17 Thetis (6.17 +/- 0.64 x 10(-13) M-circle dot), 19 Fortuna (5.41 +/- 0.76 x 10(-12) M-circle dot), and 189 Phthia (1.87 +/- 0.64 x 10(-14) M-circle dot) appear to be new. The resulting bulk porosities of 11 Parthenope (12 +/- 4%) and 16 Psyche (46 +/- 16%) are smaller than previously-reported values. Empirical expressions modeling bulk density as a function of mean radius are presented for the C and S taxonomic classes. To accurately model the forces on these asteroids during the mass determination process, we created an integrated ephemeris of the 300 large asteroids used in preparing the DE-405 planetary ephemeris; this new BC-405 integrated asteroid ephemeris also appears useful in other high-accuracy applications.