Interaction between Noncommutative Open String and Closed-String Tachyon

We construct a vertex operator which describes an emission of the ground-state tachyon of the closed string out of the noncommutative open string. Such a vertex operator is shown to exist only when the momentum of the closed-string tachyon is subject to some constraints coming from the background $B$ field. The vertex operator has a multiplicative coupling constant $g(\sigma )$ which depends on $\sigma$ as $g(\sigma)=\sin^2 \sigma$ in $0 \le \sigma \le \pi$. This behavior is the same as in the ordinary B=0 case.