  We investigate the charm quark system using the relativistic heavy quark
action on 2+1 flavor PACS-CS configurations previously generated on $32^3
\times 64$ lattice. The dynamical up-down and strange quark masses are set to
the physical values by using the technique of reweighting to shift the quark
hopping parameters from the values employed in the configuration generation. At
the physical point, the lattice spacing equals $a^{-1}=2.194(10)$ GeV and the
spatial extent $L=2.88(1)$ fm. The charm quark mass is determined by the
spin-averaged mass of the 1S charmonium state, from which we obtain $m_{\rm
charm}^{\msbar}(\mu = m_{\rm charm}^{\msbar}) = 1.260(1)(6)(35)$ GeV, where the
errors are due to our statistics, scale determination and renormalization
factor. An additional systematic error from the heavy quark is of order
$\alpha_s^2 f(m_Q a)(a \Lambda_{QCD})$, which is estimated to be a percent
level if the factor $f(m_Q a)$ analytic in $m_Q a$ is of order unity. Our
results for the charmed and charmed-strange meson decay constants are
$f_D=226(6)(1)(5)$ MeV, $f_{D_s}=257(2)(1)(5)$ MeV, again up to the heavy quark
errors of order $\alpha_s^2 f(m_Q a)(a \Lambda_{QCD})$. Combined with the CLEO
values for the leptonic decay widths, these values yield $|V_{cd}| =
0.205(6)(1)(5)(9)$, $|V_{cs}| = 1.00(1)(1)(3)(3)$, where the last error is on
account of the experimental uncertainty of the decay widths.
