Physical Structures of the Type Ia Supernova Remnant N103B
  N103B is a Type Ia supernova remnant (SNR) projected in the outskirt of the
superbubble around the rich cluster NGC 1850 in the Large Magellanic Cloud
(LMC). We have obtained H$\alpha$ and continuum images of N103B with the
$\textit{Hubble Space Telescope}$ ($\textit{HST}$) and high-dispersion spectra
with 4m and 1.5m telescopes at Cerro Tololo Inter-American Observatory. The
$\textit{HST}$ H$\alpha$ image exhibits a complex system of nebular knots
inside an incomplete filamentary elliptical shell that opens to the east where
X-ray and radio emission extends further out. Electron densities of the nebular
knots, determined from the [S II] doublet, reach 5300 cm$^{-3}$, indicating an
origin of circumstellar medium, rather than interstellar medium. The
high-dispersion spectra reveal three kinematic components in N103B: (1) a
narrow component with [N II]6583/H$\alpha$ $\sim$ 0.14 from the ionized
interstellar gas associated with the superbubble of NGC 1850 in the background,
(2) a broader H$\alpha$ component with no [N II] counterpart from the SNR's
collisionless shocks into a mostly neutral ambient medium, and (3) a broad
component, $\Delta V$ $\sim$ 500 km s$^{-1}$, in both H$\alpha$ and [N II]
lines from shocked material in the nebular knots. The Balmer-dominated
filaments can be fitted by an ellipse, and we adopt its center as the site of
SN explosion. We find that the star closest to this explosion center has colors
and luminosity consistent with a 1 $M_\odot$ surviving subgiant companion as
modelled by Podsiadlowski. Follow-up spectroscopic observations are needed to
confirm this star as the SN's surviving companion.
