A unification of RDE model and XCDM model
  In this Letter, we propose a new generalized Ricci dark energy (NGR) model to
unify Ricci dark energy (RDE) and XCDM. Our model can distinguish between RDE
and XCDM by introducing a parameter $\beta$ called weight factor. When
$\beta=1$, NGR model becomes the usual RDE model. The XCDM model is
corresponding to $\beta=0$. Moreover, NGR model permits the situation where
neither $\beta=1$ nor $\beta=0$. We then perform a statefinder analysis on NGR
model to see how $\beta$ effects the trajectory on the $r-s$ plane.
  In order to know the value of $\beta$, we constrain NGR model with latest
observations including type Ia supernovae (SNe Ia) from Union2 set (557 data),
baryonic acoustic oscillation (BAO) observation from the spectroscopic Sloan
Digital Sky Survey (SDSS) data release 7 (DR7) galaxy sample and cosmic
microwave background (CMB) observation from the 7-year Wilkinson Microwave
Anisotropy Probe (WMAP7) results. With Markov Chain Monte Carlo (MCMC) method,
the constraint result is
$\beta$=$0.08_{-0.21}^{+0.30}(1\sigma)_{-0.28}^{+0.43}(2\sigma)$, which
manifests the observations prefer a XCDM universe rather than RDE model. It
seems RDE model is ruled out in NGR scenario within $2\sigma$ regions.
Furthermore, we compare it with some of successful cosmological models using
AIC information criterion. NGR model seems to be a good choice for describing
the universe.
