Astrophysical Parameters and Habitable Zone of the Exoplanet Hosting Star GJ 581

GJ 581 is an M dwarf host of a multiplanet system. We use long-baseline interferometric measurements from the CHARA Array, coupled with trigonometric parallax information, to directly determine its physical radius to be 0.299 ± 0.010 R _sun. Literature photometry data are used to perform spectral energy distribution fitting in order to determine GJ 581's effective surface temperature T _EFF = 3498 ± 56 K and its luminosity L = 0.01205 ± 0.00024 L _sun. From these measurements, we recompute the location and extent of the system's habitable zone and conclude that two of the planets orbiting GJ 581, planets d and g, spend all or part of their orbit within or just on the edge of the habitable zone.