Session 40 - The Interstellar Medium.
Display session, Tuesday, June 09
Atlas Ballroom,

## [40.14] EGRET Observations of Diffuse Gamma-Ray Emission in Monoceros

S. W. Digel (Raytheon STX, NASA/GSFC), I. A. Grenier (Université Paris 7 and Service d'Astrophysique, CE Saclay), S. D. Hunter (NASA/GSFC), T. M. Dame, P. Thaddeus (Harvard-Smithsonian CfA)

We present a study of the diffuse, high-energy gamma-ray emission observed toward Monoceros by the EGRET instrument on the Compton Gamma-Ray Observatory. The EGRET data for this region (l = 210-250\arcdeg, b = -15 - +20\arcdeg) include two observations, viewing periods 510.0 and 510.5, taken specifically for this work. The diffuse emission, largely produced by cosmic-ray interactions with interstellar gas, is used to trace the variation of the density of high-energy cosmic rays across the outer Galaxy in the third quadrant. This study complements an earlier work for the second quadrant (Digel et al. 1996, ApJ, 463, 609). Both regions contain segments of the Perseus arm, but the gap between the local arm and the Perseus arm in Monoceros is great enough that the density of cosmic rays in the interarm region can be measured separately. The density inferred from the gamma-ray emissivity of the gas is found to be significantly less in the interarm than in the local or Perseus arms. This finding supports models of Galactic cosmic rays which assume that cosmic rays are coupled to interstellar gas. For the local molecular clouds in Monoceros, which include Mon R2 and CMa OB1, we are also able to measure the mass-calibrating ratio N(H_2)/W_CO=(1.56 \pm 0.29)\times10^20 cm^-2 (K km s^-1)^-1, and set limits on the variation of cosmic-ray density within the clouds.