  Spectral properties of Jacobi operators $J$ are intimately related to an
asymptotic behavior of the corresponding orthogonal polynomials $P_{n}(z)$ as
$n\to\infty$. We study the case where the off-diagonal coefficients $a_{n}$
and, eventually, diagonal coefficients $ b_{n}$ of $J$ tend to infinity in such
a way that the ratio $\gamma_{n}:=2^{-1}b_{n} (a_{n}a_{n-1})^{-1/2} $ has a
finite limit $ \gamma $. %We study an asymptotic behavior as $n\to\infty$ of
the orthogonal polynomials $P_{n}(z)$ defined by Jacobi recurrence coefficients
$a_{n}$ (off-diagonal terms) and $ b_{n}$ (diagonal terms). %We consider the
case $a_{n}\to\infty$ and suppose that the sequence $\gamma_{n}:=2^{-1}b_{n}
(a_{n}a_{n-1})^{-1/2} $ has a limit $ \gamma $ as $n\to\infty$. In the case
$|\gamma | < 1$ asymptotic formulas for $P_{n}(z)$ generalize those for the
Hermite polynomials and the corresponding Jacobi operators $J$ have absolutely
continuous spectra covering the whole real line. If $|\gamma | > 1$, then
spectra of the operators $J$ are discrete. Our goal is to investigate the
critical case $| \gamma |=1$ that occurs, for example, for the Laguerre
polynomials. The formulas obtained depend crucially on the rate of growth of
the coefficients $a_{n}$ (or $b_{n}$) and are qualitatively different in the
cases where $a_{n}\to \infty$ faster or slower then $n$. For the fast growth of
$a_{n}$, we also have to distinguish the cases
  $|\gamma_{n}|
  \to 1-0$ and $|\gamma_{n}|
  \to 1+0$. Spectral properties of the corresponding Jacobi operators are quite
different in all these cases. Our approach works for an arbitrary power growth
of the Jacobi coefficients.
