ChandraObservations and Models of the Mixed‐Morphology Supernova Remnant W44: Global Trends

Abstract

We report on the Chandra observations of the archetypical mixed-morphology (or thermal composite) supernova remnant W44. As with other mixed-morphology remnants, W44's projected center is bright in thermal X-rays. It has an obvious radio shell but no discernible X-ray shell. In addition, X-ray-bright knots dot W44's image. The spectral analysis of the Chandra data shows that the remnant's hot bright projected center is metal-rich and that the bright knots are regions of comparatively elevated elemental abundances. Neon is among the affected elements, suggesting that ejecta contributes to the abundance trends. Furthermore, some of the emitting iron atoms appear to be underionized with respect to the other ions, providing the first potential X-ray evidence for dust destruction in a supernova remnant. We use the Chandra data to test the following explanations for W44's X-ray-bright center:(1) entropy mixing due to bulk mixing or thermal conduction, (2) evaporation of swept-up clouds, and (3) a metallicity gradient, possibly due to dust destruction and ejecta enrichment. In these tests, we assume that the remnant has evolved beyond the adiabatic evolutionary stage, which explains the X-ray dimness of the shell. The entropy-mixed model spectrum was tested against the Chandra spectrum for the remnant's projected center and found to be a good match. The evaporating-cloud model was constrained by the finding that the ionization parameters of the bright knots are similar to those of the surrounding regions. While both the entropy-mixed and the evaporating-cloud models are known to predict centrally bright-X-ray morphologies, their predictions fall short of the observed brightness gradient. The resulting brightness gap can be largely filled in by emission from the extra metals in and near the remnant's projected center. The preponderance of evidence (including that drawn from other studies) suggests that W44's remarkable morphology can be attributed to dust destruction and ejecta enrichment within an entropy-mixed, adiabatic-phase supernova remnant. The Chandra data prompt a new question-by what astrophysical mechanisms are the metals distributed so inhomogeneously in the supernova remnant. Subject headingg s: ISM: abundances -ISM: individual (W44) -supernova remnants -X-rays: ISM

On the metal abundances inside mixed-morphology supernova remnants: the case of IC 443 and G166.0+4.3

“…The temperature profiles, although not entirely consistent with the model, appear to indicate very limited variations, and therefore points toward efficient thermal conduction inside both of the remnants. The failure of the cloud evaporation model and the high efficiency of thermal conduction suggest an explanation in terms of the entropy-mixed thermally conductive model developed by Shelton et al (2004) to explain the observational data of MMSNR W44, possibly amended to take into account the peculiar environment in which these remnants expand. A detailed numerical model of the metal-rich MMSNRs that takes into account mixing between the ejecta and shocked ISM material, thermal conduction, and a realistic model of the environment, is required to verify this scenario.…”

“…Several authors proposed that some of the X-ray emission of MMSNRs may be due to thermal plasma of high metal abundances, as in the case of stellar ejecta inside young historical SNRs (e.g., Shelton et al 2004;Chen et al 2008), or in other Galactic (e.g., Vela SNR, Miceli et al 2008; Cygnus Loop, Katsuda et al 2008; Puppis A, Hwang et al 2008) and Magellanic Cloud SNRs (e.g., 0103-72.6, Park et al 2003a;N49B, Park et al 2003b). This important result was addressed and summarized by Lazendic & Slane (2006), who compiled a sample of 26 MMSNRs, 10 of which appear to show enhanced metal abundances in their X-ray spectrum.…”

XMM–Newtonstudies of the supernova remnant G350.0−2.0

“…Alternatively, the metal enrichment can be provided by the dust destruction (see e.g. Shelton et al 2004). …”

Supernova Remnants Interacting with Molecular Clouds: X-Ray and Gamma-Ray Signatures

“…At present, there are ∼ 40 known MM SNRs (see summary by Vink 2012), but the nature of the central X-ray emission is poorly understood. While abundance determinations from X-ray spectra indicate evidence for the presence of ejecta in some such remnants (e.g., Shelton et al 2004;Lazendic and Slane 2006;Bocchino et al 2009;Pannuti et al 2010;, the estimated mass of X-ray emitting material in the central regions is generally much too high to be composed primarily of ejecta.…”

“…However, in many systems the X-ray emitting mass, M x , is also large. Modeling of the emission from W44 indicates roughly 100M of hot gas in the remnant interior (Shelton et al 2004), for example. The roughly solar abundances for this swept-up material will thus act to severely dilute the enhanced abundances of any (much smaller) ejecta component.…”