emamajek on 16-Oct-2011 18:05:49 GMT about * alf PsA
Fomalhaut
has stellar common proper motion companion
TW PsA
situated 1.96 degrees away. The case for their physical association is quite strong.
The stars are co-distant: Distances can be calculated using the revised Hipparcos parallaxes from van Leeuwen (2007). For Fomalhaut d = 7.70+-0.03 pc, for TW PsA d = 7.61+-0.04 pc. Their 3D physical separation is 57500+-2500 AU.
The stars are co-moving: Gontcharov (2006) lists a literature mean radial velocity for Fomalhaut of 6.5+-0.5 km/s, which is almost identical to the Holmberg et al. (2007) mean radial velocity for TW PsA (6.6+-0.1 km/s). If one combines these radial velocities with the astrometry for both stars from van Leeuwen (2007), one can calculate 3D velocity vectors:
Fomalhaut: (U,V,W) = -5.70+-0.15, -8.26+-0.25, -11.07+-0.34 km/s.
TW PsA: (U,V,W) = -5.43+-0.06, -8.08+-0.03, -11.53+-0.02 km/s.
Hence the stars are not only in close proximity, but comoving within ~0.56+-0.45 km/s. Hence, their velocities are statistically consistent with co-motion and they could constitute a bound binary system.
Age indicators for both stars are suggestive of ages of ~400 Myr, but a detailed discussion is beyond the scope of these notes.
To the author's knowledge the first mention in the literature of Fomalhaut and TW PsA constituting a binary comes from Eggen (1955;
1955AJ.....60...65E
).
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