Spitzer Mapping of RCW 86: Examining Small Spatial Scales in the Post Shock Environment

Astronomy and Astrophysics – Astronomy

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We present a mid-IR mapping observation of the supernova remnant RCW 86, the proposed remnant of SN 185 A.D. and a source of TeV gamma-rays, obtained with the MIPS and IRAC instruments on Spitzer. The entire shell is detected at 24 μm, and the remnant's large size combined with Spitzer's resolution allow study on small spatial scales ( 0.1 pc). Emission from both radiative and non-radiative regions is observed, attributable primarily to warm dust in the ISM heated by the shock. Short-wavelength images show emission in the bright southwest corner, where radiative shocks dominate. Strong emission from the southwest is also observed at 70 μm, as is a thin filament in the northwest portion of the
remnant. This filament has a 70/24 flux ratio of 3.3, and is spatially coincident with a filament seen in archival X-ray images from XMM-Newton. A plane-shock model with variable abundances, a temperature of 0.33 keV and ionization timescale of 1.67 × 1011 cm-3 s and an additional non-thermal component provide a good fit to the X-ray data. We use these parameters and assume a shock speed of 600 km s-1 to fit a dust model to IR data, finding a post-shock gas density of np = 8.0 cm-3 and a dust mass of 7.5 × 10-4 M&sun;. From the X-ray emission measure of the thermal component, we find a swept gas mass of 0.75 M&sun; in this region, giving a dust-to-gas mass ratio of 2.5 × 10-3, a factor of several lower than what is expected in the Galactic ISM, even accounting for sputtering of dust grains by energetic ions. Accounting for this discrepancy would require a significant mass in dust to be too cold to emit efficiently at 70 μm. We find some evidence for cosmic ray modification of the shock front.

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