On the age and binarity of Fomalhaut

Abstract

The nearby (d = 7.7pc) A3V star Fomalhaut is orbited by a resolved dusty debris disk and a controversial candidate extrasolar planet. The commonly cited age for the system (200 ± 100Myr) from Barrado y Navascues et al. relied on a combination of isochronal age plus youth indicators for the K4V common proper-motion system TW PsA. TW PsA is 196 away from Fomalhaut and was first proposed as a companion by Luyten, but the physicality of the binarity is worth testing with modern data. I demonstrate that TW PsA is unequivocally a physical stellar companion to Fomalhaut, with true separation 0.280+0.019- 0.012pc (57.4+3.9- 2.5kAU) and sharing velocities within 0.1±0.5kms-1 - consistent with being a bound system. Hence, TW PsA should be considered "FomalhautB." Combining modern H-R diagram constraints with four sets of evolutionary tracks, and assuming the star was born with protosolar composition, I estimate a new isochronal age for Fomalhaut of 450 ± 40Myr and mass of 1.92±0.02 M .Various stellar youth diagnostics are re-examined for TW PsA. The star's rotation, X-ray emission, and Li abundances are consistent with approximate ages of 410, 380, and 360Myr, respectively, yielding a weighted mean age of 400 ± 70Myr. Combining the independent ages, I estimate a mean age for the Fomalhaut-TW PsA binary of 440 ± 40Myr. The older age implies that substellar companions of a given mass are approximately 1mag fainter at IR wavelengths than previously assumed. © 2012. The American Astronomical Society. All rights reserved..