CONSTRAINING the AGE and DISTANCE of the GALACTIC SUPERNOVA REMNANT G156.2+5.7 by Hα EXPANSION MEASUREMENTS

We present deep Hα images of portions of the X-ray bright, but optically faint, Galactic supernova remnant G156.2+5.7, revealing numerous and delicately thin non-radiative filaments, which mark the location of the remnant's forward shock. These new images show that these filaments have a complex structure not visible on previous lower resolution optical images. By comparing Hα images taken in 2004 at the McDonald Observatory and in 2015-2016 at the Kiso Observatory, we set a stringent 1σ upper limit of expansion to be yr^-1. This proper motion, combined with a shock speed of 500 km s^-1, inferred from X-ray spectral analyses, gives a distance of 1.7 kpc. In addition, a simple comparison of expansion indices of several supernova remnants allows us to infer the age of the remnant to be a few tens of thousands years old. These estimates are more straightforward and reliable than any other previous studies, and clearly rule out the possibility that G156.2+5.7 is physically associated with part of the Taurus-Auriga cloud and dust complex at a distance of 200-300 pc.