2018ApJ...859..173L

We present a new millimeter CO-line observation toward 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_LSR=-60 to -54kms^–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 -57kms^–1. Asymmetric broad profiles of ¹²CO lines peaked at -58kms^–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, ∼37' in radius, appears to surround the SNR from H I 21 cm (V_LSR∼-61 to -68kms^–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 a relic of the part of the blast wave that propagates into the MC complex and is now in a radiative stage while the other part of the blast wave has been expanding into the low-density region in the superbubble. This scenario naturally explains the lack of X-ray emission related to the ejecta and blast wave. The SNR-MC interaction also favors a hadronic contribution to the γ-ray emission from the CTB 87 region.