Jet Formation in BL Lacertae Objects with Different Accretion Modes

We estimate the masses of massive black holes in BL Lac objects from theirhost galaxy luminosity. The power of jets and central optical ionizingluminosity for a sample of BL Lac objects are derived from their extended radioemission and the narrow-line emission, respectively. The maximal jet power canbe extracted from a standard thin accretion disk/spinning black hole iscalculated as a function of dimensionless accretion rate $\dot{m}$($\dot{m}=\dot{M}/\dot{M}_{\rm Edd}$). Comparing with the derived jet power, wefind that the accretion disks in most BL Lac objects should not be standardaccretion disks. For a pure advection dominated accretion flow (ADAF), there isan upper limit on its optical continuum luminosity due to the existence of anupper limit $\dot{m}_{\rm crit}$ on the accretion rate. It is found that a pureADAF is too faint to produce the optical ionizing luminosity of BL Lac objectsderived from their narrow-line luminosity. We propose that an ADAF is presentin the inner region of the disk and it becomes a standard thin disk in theouter region in most BL Lac objects, i.e., ADAF+SD(standard disk) scenario.This ADAF+SD scenario can explain both the jet power and optical ionizingcontinuum emission of these BL Lac objects. The inferred transition radiibetween the inner ADAF and outer SD are in the range of $40-150 GM_{bh}/c^2$,if the disks are accreting at the rate $\dot{m}=0.01$.Comment: accepted by Ap