  In this paper we present evidence suggesting that the absence or presence of
Hidden Broad Line Regions (HBLRs) in Seyfert 2 galaxies is regulated by the
rate at which matter accretes onto a central supermassive black hole, in units
of Eddington rate. We use the intrinsic (i.e. unabsorbed) X-ray luminosities of
these sources and their black hole masses (estimated by using the well known
relationship between nuclear mass and bulge luminosity in galaxies) to derive
the nuclear accretion rate in units of Eddington. We find that virtually all
HBLR sources have accretion rate larger than a threshold value of
$\dot{m}_{thres} \simeq 10^{-3}$ (in Eddington units), while non-HBLR sources
lie at $\dot{m} \ls \dot{m}_{thres}$.
  These data nicely fit predictions from a model proposed by Nicastro (2000),
in which the Broad Line Regions (BLRs) are formed by accretion disk
instabilities occurring in proximity of the critical radius at which the disk
changes from gas pressure dominated to radiation pressure dominated. This
radius diminishes with decreasing accretion rates; for low enough accretion
rates (and therefore luminosities), the critical radius becomes smaller than
the innermost stable orbit, and BLRs cannot form.
