Nomads of the Galaxy

الأصل · EN

We estimate that there may be up to 10⁵ compact objects in the mass range 10⁻⁸ -10⁻² solar mass per main sequence star that are unbound to a host star in the Galaxy. We refer to these objects as nomads; in the literature a subset of these are sometimes called free-floating or rogue planets. Our estimate for the number of Galactic nomads is consistent with a smooth extrapolation of the mass function of unbound objects above the Jupiter-mass scale, the stellar mass density limit, and the metallicity of the interstellar medium. We analyze the prospects for detecting nomads via Galactic microlensing. The Wide-Field Infrared Survey Telescope (WFIRST) will measure the number of nomads per main sequence star greater than the mass of Jupiter to 13%, and the corresponding number greater than the mass of Mars to 25%. All-sky surveys such as GAIA and LSST can identify nomads greater than about the mass of Jupiter. We suggest a dedicated drift scanning telescope that covers approximately 100 square degrees in the Southern hemisphere could identify nomads as small as 10⁻⁸ solar mass via microlensing of bright stars with characteristic lightcurve timescales of a few seconds.