Infrared photometry and mass loss rates for OBA supergiants and Of stars

We present infrared observations of 34 OBA supergiants and 10 Of and Oe stars. Excess free- free emission is detectable at 10 m but not at shorter wavelengths for the supergiants. From our 2.2 and 3.6 t m photometry of the supergiants, in conjunction with existing UBV data, we derive an invariant ratio of total to selective extinction, R = 3.10 + 0.03. Using published radio and infrared data, we derive the velocity law for the mass loss outflow from P Cygni, and we apply this velocity law to the measured 10 m excess fluxes from our sample of supergiants in order to derive mass loss rates. Our preliminary survey of Of and Oe stars shows excess free-free emission to be common at 3.6 m and sometimes detectable at 2.2 t m. Using our 2.2-10 m data, we derive mass loss rates for several 0 stars. Our results are consistent with radiation pressure being the driving mechanism for mass loss from OBA supergiants and Of stars. Subject headings: interstellar: matter - stars: circumstellar shells - stars: early-type - stars: mass loss - stars: Of-type - stars: supergiants