LONG-PERIOD OXYGEN-RICH OPTICAL MIRAS IN THE SOLAR NEIGHBORHOOD

We use available surveys and the K-band period-luminosity relationship to determine the spatial distribution of oxygen-rich Miras with periods longer than 400 days in the neighborhood of the Sun. We restrict the analysis to '' optical '' Miras, stars with F(nu)(12 mum)/F(nu)(2.2 mum) < 1.0. We identify a total of 48 such stars within 1 kpc of the Sun and argue that this sample is probably better than 80% complete. We derive an exponential scale height of these long-period Miras of approximately 240 pc and a surface density projected onto the Galactic plane of 18 kpc-2. The derived scale height indicates a main-sequence progenitor population with masses approximately 1.1 M . and appears to be the same as that of the intermediate-period oxygen-rich Miras (400 > P greater-than-or-equal-to 300 days). The data are consistent with the hypothesis that the intermediate-period Miras in the solar neighborhood evolve into long period Miras in approximately 5 x 10(5) yr. From the observed spatial distribution and estimates of the death rates of the progenitor main-sequence stars, we estimate a lifetime of the local long-period Miras (P greater-than-or-equal-to 400 days) of 30,000 yr. The typical mass-loss rate is approximately 4 x 10(-7) M. yr-1, with a total mass loss during this phase of approximately 0.01 MD. The suggestion that the relatively short lifetime of the long-period Miras is a consequence of the development of very rapid mass loss in these stars is qualitatively consistent with the relative numbers of intermediate- and long-period Miras, but does not seem to agree in detail with our estimates for the mass-loss rates. There is a marked contrast between the Mira population at approximately 1 kpc from the Galactic center where there are nearly as many long-period oxygen-rich Miras as intermediate-period oxygen-rich Miras, and the local, solar neighborhood population of oxygen-rich Miras where the fraction of Miras of intermediate period is much greater. We suggest that at approximately 1 kpc from the Galactic center, the main-sequence stars with masses larger than 1 MD have higher metallicities than main-sequence stars with the same masses in the solar neighborhood. In the solar neighborhood such main-sequence stars become carbon rich on the AGB; in the region near the Galactic center, we propose that they become long-period oxygen-rich Miras.