Explosive common-envelope ejection: implications for gamma-ray bursts and low-mass black-hole binaries

We present a new mechanism for the ejection of a common envelope in a massive binary, where the energy source is nuclear energy rather than orbital energy. This can occur during the slow merger of a massive primary with a secondary of 1-3 M(circle dot) when the primary has already completed helium core burning. We show that in the final merging phase, hydrogen-rich material from the secondary can be injected into the helium-burning shell of the primary. This leads to a nuclear runaway and the explosive ejection of both the hydrogen and the helium layers, producing a close binary containing a CO star and a low-mass companion. We argue that this presents a viable scenario to produce short-period black-hole binaries and long-duration gamma-ray bursts (LGRBs). We estimate an LGRB rate of similar to 10(-6) yr(-1) at solar metallicity, which implies that this may account for a significant fraction of all LGRBs and that this rate should be higher at lower metallicity.