On the maximum efficiency of the propeller mass-ejection mechanism

¹ CEA Saclay, DSM/DAPNIA/Service d'Astrophysique (CNRS FRE 2591), 91191 Gif-sur-Yvette, France  e-mail: mfalanga@cea.fr
² INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00044 Rome, Italy
³ Dipartimento di Fisica – Università di Roma “Tor Vergata”, via della Ricerca Scientifica 1, 00133 Rome, Italy
⁴ Università degli studi di Cagliari, Dipartimento di Fisica, SP Monserrato-Sestu, KM 0.7, 09042 Monserrato, Italy
⁵ Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO 80309, USA

Received: 11 January 2005
Accepted: 11 January 2007

Abstract

Aims.We derive simple estimates of the maximum efficiency with which matter can be ejected by the propeller mechanism in disk-fed, rotating magnetic neutron stars. Some binary evolution scenarios envisage that this mechanism is responsible for expelling to infinity the mass inflowing at a low rate from the companion star, therefore limiting the total amount of mass that can be accreted by the neutron star.

Methods.We demonstrate that, for typical neutron star parameters, a maximum of times more matter than accreted can be expelled through the propeller mechanism at the expenses of the neutron star rotational energy (P_-3 is the NS spin period in unit of 10^-3 s). Approaching this value, however, would require a great deal of fine tuning in the system parameters and the properties of the interaction of matter and magnetic field at the magnetospheric boundary.

Results.We conclude that some other mechanism must be invoked in order to prevent that too much mass accretes onto the neutron stars of some low mass X-ray binaries.