  We constrain a unified dark matter (UDM) model from the latest observational
data. This model assumes that the dark sector is degenerate. Dark energy and
dark matter are the same component. It can be described by an affine equation
of state $P_X= p_0 +\alpha \rho_X$. Our data set contains the newly revised
$H(z)$ data, type Ia supernovae (SNe Ia) from Union2 set, baryonic acoustic
oscillation (BAO) observation from the spectroscopic Sloan Digital Sky Survey
(SDSS) data release 7 (DR7) galaxy sample, as well as the cosmic microwave
background (CMB) observation from the 7-year Wilkinson Microwave Anisotropy
Probe (WMAP7) results. By using the Markov Chain Monte Carlo (MCMC) method, we
obtain the results in a flat universe:
$\Omega_\Lambda$=$0.719_{-0.0305}^{+0.0264}(1\sigma)_{-0.0458}^{+0.0380}(2\sigma)$,
$\alpha$=$1.72_{-4.79}^{+3.92}(1\sigma)_{-7.30}^{+5.47}(2\sigma)(\times10^{-3})$,
$\Omega_bh^2$=$0.0226_{-0.0011}^{+0.0011}(1\sigma)_{-0.0015}^{+0.0016}(2\sigma)$.
Moreover, when considering a non-flat universe,
$\Omega_\Lambda$=$0.722_{-0.0447}^{+0.0362}(1\sigma)_{-0.0634}^{+0.0479}(2\sigma)$,
$\alpha$=$0.242_{-0.775}^{+0.787}(1\sigma)_{-1.03}^{+1.10}(2\sigma)(\times10^{-2})$,
$\Omega_bh^2$=$0.0227_{-0.0014}^{+0.0015}(1\sigma)_{-0.0018}^{+0.0021}(2\sigma)$,
$\Omega_k$=$-0.194_{-1.85}^{+2.02}(1\sigma)_{-2.57}^{+2.75}(2\sigma)(\times10^{-2})$.
These give a more stringent results than before. We also give the results from
other combinations of these data for comparison. The observational Hubble
parameter data can give a more stringent constraint than SNe Ia. From the
constraint results, we can see the parameters $\alpha$ and $\Omega_k$ are very
close to zero, which means a flat universe is strongly supported and the speed
of sound of the dark sector seems to be zero.
