ON THE AGE AND BINARITY OF FOMALHAUT

Abstract

The nearby ( d = 7.7 pc) 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 ± 100 Myr) 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 1 96 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 − 0.012 pc (57.4 +3.9 − 2.5 kAU) and sharing velocities within 0.1 ± 0.5 km s −1 —consistent with being a bound system. Hence, TW PsA should be considered "Fomalhaut B." 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 ± 40 Myr 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 360 Myr, respectively, yielding a weighted mean age of 400 ± 70 Myr. Combining the independent ages, I estimate a mean age for the Fomalhaut–TW PsA binary of 440 ± 40 Myr. The older age implies that substellar companions of a given mass are approximately 1 mag fainter at IR wavelengths than previously assumed.