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With archival and new Hubble Space Telescope observations we have refined thespace-velocity measurements of the stars in the central region of the remnantof Tycho's supernova (SN) 1572, one of the historical Galactic Type Iasupernova remnants (SNRs). We derived a proper motion for Tycho-G of(mu_RA_cos_dec;mu_dec)=(-2.63;-3.98)+/-(0.06;0.04)[formalerrors]+/-(0.18;0.10)[expected errors] mas/yr. We also reconstruct the binaryorbit that Tycho-G should have followed if it were the surviving companion ofSN 1572. We redetermine the Ni abundance of this star and compare it with newabundance data from stars of the Galactic disk, finding that [Ni/Fe] is about1.7 sigma above the Galactic trend. From the high velocity (v_b = -50+/-14km/s) of Tycho-G perpendicular to the Galactic plane, its metallicity, and itsNi excess, we find the probability of its being a chance interloper to be P <0.00037 at most. The projected rotational velocity of the star should be belowcurrent observational limits. The projected position of Tycho-G is, within theuncertainties, consistent with the centroid of the X-ray emission of Tycho'sSNR; moreover, its brightness is generally consistent with the post-explosionevolution of the luminosity of a SN companion. Among the other 23 stars havingV<22 mag and located within 42 arcsec from the X-ray centroid, only 4 are atdistances compatible with that of the SNR, and none of them shows anypeculiarity. Therefore, if even Tycho-G is not the surviving companion of SN1572, the absence of other viable candidates does favor the merging of twowhite dwarfs as the producer of the SN.

Improved Hubble Space Telescope Proper Motions for Tycho-G and Other Stars in the Remnant of Tycho's Supernova 1572