Operations under which the class of undecidable languages isn't closed Do there exist undecidable languages such that their union/intersection/concatenated language is decidable? What is the physical interpretation of such example because in general, undecidable languages are not closed under these operations?

What can we say about the kleene closure? Do we have examples for it too?  I.e. can the closure of an undecidable language be decidable?

Also, can we generalize such undecidable classes?
Yes, let $H$ be binary encoding of the halting problem and $A=0H\cup 1\{0,1\}^{\ast}\cup\{\epsilon\}$, $B=1H\cup0\{0,1\}^{\ast}\cup \{\epsilon\}$, then $AB=\{0,1\}^{\ast}$ (why?)