Proper motions of Hα filaments in the supernova remnant RCW 86

We present a proper motion study of the eastern shock-region of the supernova remnant RCW 86 (MSH 14-63, G315.4−2.3), based on optical observations carried out with Very Large Telescope/FOcal Reducer and low dispersion Spectrograph 2 in 2007 and 2010. For both the north-eastern and south-eastern regions, we measure an average proper motion of Hα filaments of 0.10 ± 0.02 arcsec yr−1, corresponding to 1200 ± 200 km s−1 at 2.5 kpc. There is substantial variation in the derived proper motions, indicating shock velocities ranging from just below 700 km s−1 to above 2200 km s−1.

The optical proper motion is lower than the previously measured X-ray proper motion of north-eastern region. The new measurements are consistent with the previously measured proton temperature of 2.3 ± 0.3 keV, assuming no cosmic ray acceleration. However, within the uncertainties, moderately efficient (<27 per cent) shock acceleration is still possible. The combination of optical proper motion and proton temperature rule out the possibility that RCW 86 has a distance less than 1.5 kpc.

The similarity of the proper motions in the north-east and south-east is peculiar, given the different densities and X-ray emission properties of the regions. The north-eastern region has lower densities and the X-ray emission is synchrotron dominated, suggesting that the shock velocities should be higher than in the south-eastern, thermal X-ray dominated, region. A possible solution is that the Hα emitting filaments are biased towards denser regions, with lower shock velocities. Alternatively, in the north-east the shock velocity may have decreased rapidly during the past 200 yr, and the X-ray synchrotron emission is an afterglow from a period when the shock velocity was higher.