An Investigation of the Interstellar Environment of Supernova Remnant
  CTB 87
  We present a new millimeter CO-line observation towards supernova remnant
(SNR) CTB 87, which was regarded purely as a pulsar wind nebula (PWN), and an
optical investigation of a coincident surrounding superbubble. The CO
observation shows that the SNR delineated by the radio emission is projectively
covered by a molecular cloud (MC) complex at V$_{\rm {LSR}}$ = $-60$ to $-54$
km s$^{-1}$. Both the symmetric axis of the radio emission and the trailing
X-ray PWN appear projectively to be along a gap between two molecular gas
patches at $-58$ to $-57$ km s$^{-1}$. Asymmetric broad profiles of $^{12}$CO
lines peaked at $-58$ km s$^{-1}$ are found at the eastern and southwestern
edges of the radio emission. This represents a kinematic signature consistent
with an SNR-MC interaction. We also find that a superbubble, $\sim 37'$ in
radius, appears to surround the SNR from HI 21cm (V$_{\rm {LSR}} \sim -61$ to
$-68$ km s$^{-1}$), WISE mid-IR, and optical extinction data. We build a
multi-band photometric stellar sample of stars within the superbubble region
and find 82 OB star candidates. The likely peak distance in the stars'
distribution seems consistent with the distance previously suggested for CTB
87. We suggest the arc-like radio emission is mainly the relic of the part of
blastwave that propagates into the MC complex and is now in a radiative stage
while the other part of blastwave has been expanding into the low-density
region in the superbubble. This scenario naturally explains the lack of the
X-ray emission related to the ejecta and blastwave. The SNR-MC interaction also
favours a hadronic contribution to the {\gamma}-ray emission from the CTB 87
region.
