OXYGEN-RICH SEMIREGULAR AND IRREGULAR VARIABLES

We have compiled all the known oxygen-rich semiregulars and irregulars of spectral classes K and M with \b\ greater-than-or-equal-to 30-degrees, DELTAm greater-than-or-equal-to 0.5 mag and, for the semiregulars, periods in the range 50 less-than-or-equal-to P < 400 days. As with the Miras, the properties of the semiregulars are correlated with their periods. In contrast to Miras, which typically have longer Periods, the bulk of semiregulars have periods of less than 200 days. Our sample of optically identified semiregulars and irregulars is probably complete to within a factor of 2 to a distance of 600 pc. In an Appendix, we list stars in this region of the sky that display infrared colors characteristic of mass-losing red giants but which are not listed in the General Catalog of Variable Stars. Many of these stars are as-yet unclassified variables; some may be losing mass without exhibiting much variation in their optical light. By comparing their distributions on the sky and their brightnesses in the K band at 2.2 mum, we argue that the semiregulars with 300 less-than-or-equal-to P < 400 days appear to belong to the same population as the "thin disk" Miras in the same period range and have an exponential scale height from the Galactic plane of approximately 250 pc. In contrast, the semiregulars with 200 less-than-or-equal-to P < 300 days belong to the population of "thick disk" Miras with an exponential scale height from the Galactic plane of approximately 500 pc. On the basis of the similarities of their space distributions and the presence of Tc in their atmospheres, and using the pulsational mode calculations by Ostlie and Cox, we suggest that the semiregulars with 100 less-than-or-equal-to P < 150 days are probably first and second overtone pulsations of the same population of AGB stars as the Miras in their fundamental pulsational mode with periods longer than 300 days. There are at least 2/3 as many semiregulars in overtone pulsations as there are Miras in the fundamental mode. The overtone pulsators with 100 less-than-or-equal-to P < 150 days appear to lose mass at approximately 2/3 the rate of the Miras with symmetric light curves in the period interval 300 less-than-or-equal-to P < 400 days. The semiregulars with 50 less-than-or-equal-to P < 100 days appear to have an exponential scale height from the Galactic plane of at least 190 pc, indicating that they evolve from main-sequence stars with masses of less than 1.5 M.. The irregulars appear to have an exponential scale height of at least 250 pc. This implies that they evolve from the same population as do the "thin disk" Miras with 300 less-than-or-equal-to P < 400 days or the semiregulars with 100 less-than-or-equal-to P < 150 days. The derived space density of irregulars is comparable to that of the Miras of the same population. Most of the semiregulars and irregulars are losing mass at rates between 10(-8) and 10(-7) M. yr-1. Even though there are large numbers of these stars, the total mass injected into the interstellar medium by mass-losing red giants appears to be dominated by the relatively few "very dusty" stars with high mass-loss rates near 10(-5) M. yr-1 rather than the more numerous stars with lower mass-loss rates.