You will most likely need a root canal although your dentist will make the call. They may try to do a "pulp cap" on it and re-bond the broken tooth on there again possibly. If its too far exposed, then it may require a root canal and a crown.
Adhesive only although the bond strength of enamel to enamel with a bond should be excellent.
If they do re-bond it, will it eventually fuse together or will it only be held there be the adhesive?