The Preshock Gas of SN 1006 from Hubble Space Telescope Advanced Camera for Surveys Observations

Abstract

We derive the preshock density and scale length along the line of sight for the collisionless shock from a deep HST image that resolves the Hα filament in SN 1006 and updated model calculations. The very deep ACS high-resolution image of the Balmer line filament in the northwest quadrant shows that 0.25 cm^-3<=n₀<=0.4 cm^-3 and that the scale along the line of sight is about 2×10¹⁸ cm, while bright features within the filament correspond to ripples with radii of curvature less than 1/10 that size. The derived densities are within the broad range of earlier density estimates, and they agree well with the ionization timescale derived from the Chandra X-ray spectrum of a region just behind the optical filament. This provides a test for widely used models of the X-ray emission from SNR shocks. The scale and amplitude of the ripples are consistent with expectations for a shock propagating through interstellar gas with ~20% density fluctuations on parsec scales as expected from studies of interstellar turbulence. One bulge in the filament corresponds to a knot of ejecta overtaking the blast wave, however. The interaction results from the rapid deceleration of the blast wave as it encounters an interstellar cloud.