The Nature of the Young Supernova Remnant S8 in the Dwarf Galaxy IC 1613
Abstract
We present sub-arcsecond optical images and low- to moderate-resolution spectra of the compact, X-ray and optically bright supernova remnant known as S8 in the nearby dwarf galaxy IC 1613. Deep Hα images of the remnant show a sharply defined crescent shaped nebula, while narrow passband images reveal a coincident and unexpectedly bright continuum nebulosity exhibiting a size and morphology like that seen for the remnant's line emissions. Low-dispersion spectra covering 3600 - 9000 A show numerous low-ionization line emissions such as [O I] and [Fe II], along with higher-ionization emission lines including He II and optical coronal lines [Fe VII], [Fe X], [Fe XI], and [Fe XIV]. This suggests the presence of a wide range of shock velocities from 50 to over 350 km s-1, corresponding to preshock densities of 1 - 30 cm-3. Higher resolution spectra indicate an expansion velocity around 180 km s-1 with a 45 km s-1 wide central cavity. Hα emission spans rest frame velocities of +120 to -240 km s-1 and we estimate a total nebula mass of 119 34 M. We conclude S8 is a relatively young supernova remnant (2700 - 4400 yr) exhibiting properties remarkably like those seen in the young LMC remnant N49, including age, physical size, shock velocities, filament densities, optical line strengths, X-ray and optical luminosities, and coronal line and continuum emissions.