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The NASA High Resolution Microwave Survey (HRMS) consists of two complementary elements: a Sky Survey of the entire sky to a moderate level of sensitivity, and a Targeted Search of nearby stars, one at a time, to a much deeper level of sensitivity. For the main survey of a large sample of nearby solar-type stars, we argue that the selection criteria should be heavily biased by what little we know about the origin and evolution of life. We propose that observations of stars with stellar companions orbiting near the habitable zone should be de-emphasized, because such companions would prevent the formation of habitable planets or would disrupt their orbits if they did manage to form. We also propose that observations of stars younger than about 3 billion years should be de-emphasized in favor of older stars, because we know that our technical civilization took longer than 3 billion years to evolve here on earth.
To provide the information needed for the preparation of specific target lists, we have undertaken an inventory of all the solar-type stars out to a distance of 60 pc, with the goal of characterizing the relevant astrophysical properties of these stars, especially their ages and companionship. The initial sample has been assembled from two different sources: a) an all-sky list of 3347 G stars drawn from the Henry Draper Catalog, and classified by Erik Olsen as dwarfs, based on extensive Stromgren photometry; b) the spectral classification work by Nancy Houk in the southern hemisphere. In order to identify the half of the sample that is older than roughly 3 billion years, new spectra are being obtained of southern stars at the Ca~II H and K lines, to complement the continuing work on northern stars by Sallie Baliunas. Radial velocities will be used to identify those stars with companions having orbital periods of ten years or less. The northern half of the sample is being monitored with the CfA Digital Speedometers, while the southern half is being observed with the CORAVELs by our colleagues from Geneva and Copenhagen. After selection we expect to end up with a list of roughly 1000 solar-type dwarfs for the Targeted Search.
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