  The newly observed $P_c(4312)$, $P_c(4440)$ and $P_c(4457)$ at the LHCb
experiment are very close to the $\Sigma_c\bar{D}$ and $\Sigma_c\bar{D}^\ast$
thresholds. In this work, we perform a systematic study and give a complete
picture on the interactions between $\Sigma_c^{(\ast)}$ and $\bar{D}^{(\ast)}$
systems in the framework of heavy hadron chiral perturbation theory, where the
short-range contact interaction, long-range one-pion-exchange contribution, and
intermediate-range two-pion-exchange loop diagrams are all considered. We first
investigate the three $P_c$ states without and with considering the $\Lambda_c$
contribution in the loop diagrams. It is difficult to simultaneously reproduce
the three $P_c$s unless the effect of $\Lambda_c$ is included. The coupling
between $\Sigma_c^{(\ast)}\bar{D}^{(\ast)}$ and $\Lambda_c\bar{D}^{(\ast)}$
channels is crucial for the formation of these $P_c$s. Our calculation supports
the $P_c(4312)$, $P_c(4440)$ and $P_c(4457)$ to be the $S$-wave hidden-charm
$[\Sigma_c\bar{D}]_{J=1/2}^{I=1/2}$, $[\Sigma_c\bar{D}^\ast]_{J=1/2}^{I=1/2}$
and $[\Sigma_c\bar{D}^\ast]_{J=3/2}^{I=1/2}$ molecular pentaquarks,
respectively. Our calculation disfavors the spin assignment $J^P=\frac{1}{2}^-$
for $P_c(4457)$ and $J^P=\frac{3}{2}^-$ for $P_c(4440)$, because the
excessively enhanced spin-spin interaction is unreasonable in the present case.
We obtain the complete mass spectra of the
$[\Sigma_c^{(\ast)}\bar{D}^{(\ast)}]_J$ systems with the fixed low energy
constants. Our result indicates the existence of
$[\Sigma_c^{\ast}\bar{D}^{\ast}]_J~(J=\frac{1}{2},\frac{3}{2},\frac{5}{2})$
hadronic molecules. The previously reported $P_c(4380)$ might be a deeper bound
one. Additionally, we also study the hidden-bottom
$\Sigma_b^{(\ast)}B^{(\ast)}$ systems, and predict seven bound molecular
states, which could serve as a guidance for future experiments....
