Dust-to-gas ratios in the Kepler supernova remnant

A new method to evaluate the dust-to-gas ratios in the Kepler SNR is presented. Dust emission in the infrared and bremsstrahlung are calculated consistently, considering that dust grains are collisionally heated by the gas throughout the front and downstream of both the expanding and the reverse shocks. The calculated continuum SED is constrained by the observational data. The dust-to-gas ratios are determined by the ratio of the dust emission bump and bremsstrahlung in the infrared. The shell-like morphological similarity of X-ray and radio emission, and of images in Halpha and in infrared wavelengths confirms that both radio and X-ray emissions are created at the front of the expanding shock and that dust and gas are coupled crossing the expanding and reverse shock fronts. The results show that large grains with radius of similar to1 mum with dust-to-gas ratios <4 x 10(-3) survive sputtering and are heated to a maximum temperature of 125 K downstream of the shock expanding outwards with a velocity of about 1000 km s(-1). The high velocity shocks become radiative for dust-to-gas ratios > 10(-3). Such shocks do not appear in the NE region, indicating that dust grains are not homogeneously distributed throughout the remnant. Smaller grains with radius of about 0.2 mum and dust-to-gas ratios of similar to4 x 10(-4) are heated to a maximum temperature of similar to50 K downstream of the reverse shock corresponding to velocities of about 50 km s(-1). A maximum dust mass <0.16 M. is calculated.