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Astron. Astrophys. 361, 795-802 (2000)

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8. Discussion and conclusions

In the lower crust region that seems most likely to be relevant for the explanation of the large glitches observed in the Vela pulsar one would expect the corotating constituent to be characterised by a density [FORMULA] (attributable mainly to protons and bound neutrons in the atomic type ions forming a solid lattice) having a range of values that is roughly comparable with that of the corresponding neutron superfluid density [FORMULA] (quantitatively round about [FORMULA] g/cm3). Thus although they are of opposite sign (tending to push the crust material outward in the case (43) of vortex pinning, but to push it inwards in the case (45) for which pinning is absent) the alternative formulae (48) and (49) both predict the same rough order of magnitude for the stress induced on the crust by the existence of a difference between the angular velocity [FORMULA] of the neutron superfluid constituent and the (externally observable) angular momentum [FORMULA] characterising the crust.

The implication is that, as a candidate for explaining the large magnitude of the discontinuous changes [FORMULA] that are commonly observed in a pulsar such as Vela, the previously overlooked buoyancy deficit mechanism characterised by the formula (48), i.e.


(pushing outward along the cylindrical radial direction) seems at first sight to be just as promising as the more thoroughly investigated vortex pinning mechanism, which, if the chemical contribution [FORMULA] were unimportant, would be given according to (49) by


(pushing inward along the cylindrical radial direction). In order to obtain definitive conclusions it is clear however that much more work on both kinds of mechanism will be needed. In particular it will be necessary to pay more attention than hitherto to the role of the chemical excess force (31) and other potentially relevant mechanisms such as dynamical drag on vortices.

The present situation can be summarised by the statement that the large magnitude of the observed glitches in Vela provides strong evidence for the existence of angular velocity differences - and hence for the existence of superfluidity - in the pulsar interior, but that it is premature to claim it also provides strong evidence for vortex pinning because stresses of comparable magnitude could be produced in the absence of pinning by the centrifugal buoyancy deficit mechanism.

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© European Southern Observatory (ESO) 2000

Online publication: October 2, 2000