Modeling W44 as a Supernova Remnant in a Density Gradient with a Partially Formed Dense Shell and Thermal Conduction in the Hot Interior. II. The Hydrodynamic Models

Abstract

In a previous paper, we presented the analytical background for a new model for W44; in this paper, we report hydrocode experiments verifying many of the details. Our model remnant is evolving in a moderately dense (~6 cm-3), ambient medium having a substantial density gradient. At the observed age (~20,000 yr), the shock is radiative over much of the surface, with expansion speeds of only ~130-200 km s-1 for the dense and rare ends, respectively. With these speeds, the remnant has a cool periphery and does not produce a limb-brightened X-ray image. It has thermal conduction within its hot interior, resulting in a nonnegligible density there, and its center is X-ray luminous. The combined effect creates a "center-filled" X-ray image. A thin, very dense cool shell has formed over the denser half of the remnant's surface, and its radio synchrotron emission derives from the highly compressed cosmic rays and swept up magnetic field, producing the usual "shell-type" image associated with radiative remnants. This combination of emission patterns results in the remnant being characterized as having a "thermal composite morphology." Our previous paper demonstrated that the intensities and qualitative distributions of the anticipated optical, IR line, X-ray, radio synchrotron, and gamma-ray emissions from the model are comparable to those actually observed in W44. In this paper we first use a two-dimensional hydrocode to follow the remnant evolution in a density gradient, verifying that the spatial and velocity structure of the H I shell are a good match to the observations, without the complications suggested by Koo & Heiles, and demonstrating that the remnant's asymmetry does not substantially affect the distribution of X-ray emitting material. We also calculate the distribution of radio-continuum emission expected from the compression of the ambient magnetic field and cosmic rays into the dense shell (the van der Laan mechanism) and examine the role of surface crinkling in generating filamentation. The results are in good agreement with observations, though the density of ambient cosmic-ray electrons required to match the observed flux is about 4 times greater than that estimated for the solar neighborhood. A one-dimensional hydrocode model was then used to explore the effects of nonequilibrium ionization on the X-ray spectrum and intensity. Our model is similar to but more comprehensive than the recent one by Harrus et al., and, because their model lacked thermal conduction, ours is more successful in providing the thermal X-rays from the hot interior, including a better match to the spectrum. Neither provides the sharpness of the central peaking of the X-ray distribution without further complications. (For that matter, however, the earlier models with evaporating clouds did no better.) The paper closes with a discussion of the OH masers associated with W44, proposing a specific site for their origin, in the dense trailing edge of the cooling region following the radiative shock.