Close stellar conjunctions of α Centauri A and B until 2050 An mK=7.8 star may enter the Einstein ring of α Cen A in 2028

The rapid proper motion of the alpha Cen pair (approximate to 3.7 arcsec yr(-1)) and its location close to the galactic plane on a rich stellar background combine constructively to make them excellent candidates for close stellar conjunctions with more distant stars. Adding new differential astrometry to archival data, we have refined the orbital parameters, barycentric proper motion and parallax of alpha Cen and compute its apparent trajectory on sky over the coming decades. Based on NTT/SUSI2, NTT/SOFI, and VLT/NACO maps of the field stars around the trajectories of alpha Cen A and B, we present a catalog of the expected close conjunctions until 2050. An exceptional event will take place in early May 2028, when alpha Cen A will come within rho(min) = 0.015 +/- 0.135 arcsec of the m(K) = 7.8 star 2MASS 14392160-6049528 (hereafter S5). In terms of impact parameter and contrast, this is the most favorable stellar conjunction of alpha Cen within at least the next three decades. With an angular diameter of theta(LD) = 0.47 +/- 0.05 mas, it is likely that S5 is a red giant or super-giant located at several kiloparsecs. The approached stars will act as moving light probes in transmission through the environment of alpha Cen. The observation of these close conjunctions holds great promises to search for planets and other low mass objects in the alpha Cen system using photometry and astrometry. The relativistic displacement of the approached star images will be observable, with significant deflection angles in the milliarcsecond range. The small impact parameter of the conjunction with S5 means that this star has a probability of 45% of entering the Einstein ring of alpha Cen A. The gravitational amplification of the flux of S5 could reach a factor five for the combination of the two lensed images. The proper motion, orbital parameters and parallax of alpha Cen will be measurable with an extreme accuracy from differential astrometry with the S stars. This will be valuable, for example to prepare the recently announced Breakthrough Starshot initiative to send interstellar nanocrafts to alpha Centauri.