The “Living with a Red Dwarf” Program

Red Dwarfs (main-sequence / dwarf M or dM) stars are the most common stars in the Galaxy. These cool, faint, low mass stars comprise over 75% of all stars. Because of their low luminosities (∼0.0008–0.06 of the Sun’s luminosity), the circumstellar habitable zones (HZs) of dM stars are located within ∼0.05–0.4 AU of the host star. Nevertheless, the prospect of life on a planet located within the HZ of a red dwarf is moderately high, based on the longevity of these stars (>50 Gyr), their constant luminosities and high space densities. Here we describe the aims and early results of the “Living with a Red Dwarf” Program – a study of dM stars that we have been carrying out over the last few years. The primary focus of our research on dM stars is the study of their magnetic dynamos and resulting star spots & coronal X-ray and chromospheric UV emissions as a function of age, rotation and spectral type. This program will provide datasets that can be used as inputs for the study of all aspects of dM stars, along with the planets already discovered hosted by them and the probable hundreds (thousands?) of planets expected to be uncovered in the near future by missions such as Kepler & Darwin/TPF. These datasets will be invaluable to those who model exo-planetary atmospheres, as well as exobiologists & astrobiologists who are studying the possibilities of life elsewhere in the universe. Our expected results will provide fundamental information on the most numerous stars in the Galaxy – including their ages, as well as radiative (irradiances) and magnetic dynamo properties. A significant element of our program is the determination of accurate stellar magnetic-driven X-ray– UV (X-UV) irradiances that are generated by the dM stars’ vigorous magnetic dynamos. These X-UV irradiances (and flare frequencies) are strongly dependent on rotation, and thus age, and diminish as the stars lose angular momentum and spin-down over time via magnetic braking. 1. Background & Introduction Red dwarf stars (also known as main-sequence M or dwarf M – dM stars) are by far the most numerous stars in our Galaxy, comprising more than 75% of all stars (see Fig. 1a). dM stars are cool, low luminosity stars with deep convective zones and luminosities that range from L ≈ 0.0008–0.06L ̄ (for dM8–dM0 stars, respectively). These diminutive low mass stars (∼0.1–0.6 R ̄; ∼0.1–0.6 M ̄) have very slow nuclear fusion rates and thus very long lifetimes that range from ∼40 Gyr for dM0 stars to >200 Gyr for the lower mass, very low luminosity dM5–8 stars. Because of their long stable lifetimes, it is possible that planets hosted by older dM stars could harbor life – possibly even advanced intelligent life. For example, the Sloan Digital Sky Survey has found millions of dM stars as old as, and older than, the Sun (Bochanski et al. 2007). Given their large numbers and long lifetimes, determining the number of dM stars with planets