We present a mass determination for the transiting super-Earth \rho ^ { 1 } Cancri e based on nearly 700 precise radial velocity ( RV ) measurements .
This extensive RV data set consists of data collected by the McDonald Observatory planet search and published data from Lick and Keck observatories ( Fischer et al .
2008 ) .
We obtained 212 RV measurements with the Tull Coudé Spectrograph at the Harlan J. Smith 2.7 m Telescope and combined them with a new Doppler reduction of the 131 spectra that we have taken in 2003-2004 with the High-Resolution-Spectrograph ( HRS ) at the Hobby-Eberly Telescope ( HET ) for the original discovery of \rho ^ { 1 } Cancri e. Using this large data set we obtain a 5-planet Keplerian orbital solution for the system and measure an RV semi-amplitude of K = 6.29 \pm 0.21 m s ^ { -1 } for \rho ^ { 1 } Cnc e and determine a mass of 8.37 \pm 0.38 M _ { \oplus } .
The uncertainty in mass is thus less than 5 \% .
This planet was previously found to transit its parent star ( Winn et al .
2011 , Demory et al .
2011 ) , which allowed them to estimate its radius .
Combined with the latest radius estimate from Gillon et al .
( 2012 ) , we obtain a mean density of \rho = 4.50 \pm 0.20 g cm ^ { -3 } .
The location of \rho ^ { 1 } Cnc e in the mass-radius diagram suggests that the planet contains a significant amount of volitales , possibly a water-rich envelope surrounding a rocky core .