COMMON ENVELOPE: ENTHALPY CONSIDERATION

Abstract

In this Letter we discuss a modification to the criterion for the common\nenvelope (CE) event to result in envelope dispersion. We emphasize that the\ncurrent energy criterion for the CE phase is not sufficient for an instability\nof the CE, nor for an ejection. However, in some cases, stellar envelopes\nundergo stationary mass outflows, which are likely to occur during the slow\nspiral-in stage of the CE event. We propose the condition for such outflows, in\na manner similar to the currently standard $\\alpha_{\\rm\nCE}\\lambda$-prescription but with an addition of $P/\\rho$ term in the energy\nbalance equation, accounting therefore for the enthalpy of the envelope rather\nthan merely the gas internal energy. This produces a significant correction,\nwhich might help to dispense with an unphysically high value of energy\nefficiency parameter during CE phase, currently required in the binary\npopulation synthesis studies to make the production of low-mass X-ray binaries\n(LMXBs) with a black hole companion to match the observations.\n