We have redetermined the parallax and proper motion of the nearby isolated neutron star RX~J185635-3754. We used eight observations with the high resolution camera of the HST/ACS taken from 2002 through 2004. We performed the astrometric fitting usin
g five independent methods, all of which yielded consistent results. The mean estimate of the distance is 123 (+11, -15) pc (1 sigma), in good agreement with our earlier published determination.
We show that microscopic calculations based on chiral effective field theory interactions constrain the properties of neutron-rich matter below nuclear densities to a much higher degree than is reflected in commonly used equations of state. Combined
with observed neutron star masses, our results lead to a radius R = 9.7 - 13.9 km for a 1.4 M_{solar} star, where the theoretical range is due, in about equal amounts, to uncertainties in many-body forces and to the extrapolation to high densities.
