Star formation in self-gravitating disks in active galactic nuclei. II.
  Episodic formation of broad line regions
  (abridged) We study the consequence of star formation (SF) in an self-gravity
dominated accretion disk in quasars. The warm skins of the SF disk are governed
by the radiation from the inner part of the accretion disk to form Compton
atmosphere (CAS). The CAS are undergoing four phases to form broad line
regions. Phase I is the duration of pure accumulation supplied by the SF disk.
During phase II clouds begin to form due to line cooling and sink to the SF
disk. Phase III is a period of preventing clouds from sinking to the SF disk
through dynamic interaction between clouds and the CAS. Finally, phase IV is an
inevitable collapse of the entire CAS through line cooling. This CAS evolution
drives the episodic appearance of BLRs. Geometry and dynamics of BLRs can be
self-consistently derived from the thermal instability of the CAS during phases
II and III by linear analysis. The metallicity gradient of SF disk gives rise
to different properties of clouds from outer to inner part of BLRs. We find
that clouds have column density N_H < 10^22cm^{-2} in the metal-rich regions
whereas they have N_H > 10^22 cm^{-2} in the metal-poor regions. The metal-rich
clouds compose the high ionization line (HIL) regions whereas the metal-poor
clouds are in low ionization line (LIL) regions. Metal-rich clouds in HIL
regions will be blown away by radiation pressure, forming the observed
outflows. The LIL regions are episodic due to the mass cycle of clouds with the
CAS in response to continuous injection by the SF disk, giving rise to
different types of AGNs. Based on SDSS quasar spectra, we identify a spectral
sequence in light of emission line equivalent width from Phase I to IV. A key
phase in the episodic appearance of the BLRs is bright type II AGNs with no or
only weak BLRs. We discuss observational implications and tests of the
theoretical predictions of this model.
