The Shellless SNR B0532-67.5 in the Large Magellanic Cloud
  The supernova remnant (SNR) B0532$-$67.5 in the Large Magellanic Cloud (LMC)
was first diagnosed by its nonthermal radio emission and its SNR nature was
confirmed by diffuse X-ray emission; however, no optical SNR shell is detected.
The OB association LH75, or NGC 2011, is projected within the boundary of this
SNR. We have analyzed the massive star population in and around SNR
B0532$-$67.5: using optical photometric data to construct color-magnitude
diagrams (CMDs), using stellar evolutionary tracks to estimate stellar masses,
and using isochrones to assess the stellar ages. From these analyses, we find a
20-25 Myr population in LH75 and a younger population less than 10 Myr old to
the southwest of LH75. The center of SNR B0532$-$67.5 is located closer to the
core of LH75 than the massive stars to its southwest. We conclude that the SN
progenitor was probably a member of LH75 with an initial mass $\sim$15
$M_\odot$. The SN exploded in an H I cavity excavated by the energy feedback of
LH75. The low density of the ambient medium prohibits the formation of a
visible nebular shell. Despite the low density in the ambient medium, physical
properties of the hot gas within the SNR interior do not differ from SNRs with
a visible shell by more than a factor of 2-3. The large-scale H I map shows
that SNR B0532$-$67.5 is projected in a cavity that appears to be connected
with the much larger cavity of the supergiant shell LMC-4.
