Hubble Space Telescope/Faint Object Spectrograph Imaging of Wolf 424 AB: Is This Binary a Double Brown Dwarf System?

Abstract

The low-mass binary system Wolf 424 AB (GJ 473 AB) was spatially resolved in an image obtained on 1996 April 16 with the imaging mode of the Hubble Space Telescope Faint Object Spectrograph. The separation was measured to be 0.42"+/-0.03" at a position angle of 353.2d+/-2.5d. The new measurement was combined with previously published orbital positions to update the orbital elements and obtain an estimate of the total mass. Spectroscopy and photometry of these objects suggest they are nearly equal in mass. The derived orbital solution, using all of the observations since 1938 appropriately weighted, yields a total mass of m_1+m_2 of 0.143 M_solar, and component masses of ~0.07 M_solar, just under the theoretical limit for a brown dwarf. An alternate orbital solution derived solely from speckle observations since 1986 and the Faint Object Spectrograph image yields a total mass of 0.232 M_solar, placing both components well above the brown-dwarf limit. The mass depends on the value for the semimajor axis, which is controlled in the latter solution by an optical speckle observation made near the last period of maximum separation. Further high-resolution observations during the upcoming maximum separation, and through the next secondary maximum, are needed to constrain the orbit. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.