New 13 CO observations towards the Keplerian gaseous disk around

We present new high angular resolution observations of the 13 CO 1→0 rotational line towards the HBe star R Mon, obtained with the IRAM Bure In-terferometer, along with the previous results in the observed transitions 12 CO 1→0 and 2→1. We have used a flat disk model to fit the 12 CO 1→0 and 2→1, and 13 CO 1→0 emission, in a strip perpendicular to the outflow axis. The model assumes standard abundances (X(12 CO) = 8·10 −5 , X(13 CO) = 9·10 −7), radial potential temperature and density laws (T (K) = T o r −q , ρ (cm −3) = ρ o r −p), and Local Thermodynamic Equilibrium. The 13 CO and 12 CO emission is consistent with a flat disk at an inclination angle of 20 • in Keplerian rotation around the star. The gaseous disk is fitted with a mass of 0.014 M ⊙ , an outer radius of 1500 AU, a temperature of 4500 K at the inner radius (1 AU), and values of q=0.62 and p=1.3 for the indexes of the temperature and density laws. This new 13 CO observations allow us to conclude that the disk around R Mon is flat. Our result confirms previous works suggesting a predominant flat geometry in disks around early Be stars.