Fermi LAT Study of the Cosmic-rays and the Interstellar Medium in Nearby Molecular Clouds
Abstract
We report an analysis of the interstellar gamma-ray emission from nearby molecular clouds Chamaeleon, R Coronae Australis (R CrA), and Cepheus and Polaris flare regions with the Fermi Large Area Telescope (LAT). They are among the nearest molecular cloud complexes, within 300 pc from the solar system. The gamma-ray emission produced by interactions of cosmic-rays (CRs) and interstellar gas in those molecular clouds is useful to study the CR densities and distributions of molecular gas close to the solar system. The obtained gamma-ray emissivities from 250 MeV to 10 GeV for the three regions are about (6--10) × 10-27 photons s-1 sr-1 H-atom-1, indicating a variation of the CR density by 20% even if we consider the systematic uncertainties. The molecular mass calibration ratio, X CO = N (H2)/W CO, is found to be about (0.6--1.0) × 1020 H2-molecule cm-2 (K km s-1)-1 among the three regions, suggesting a variation of X CO in the vicinity of the solar system. From the obtained values of X CO, we calculated masses of molecular gas traced by \ in these molecular clouds. In addition, similar amounts of dark gas at the interface between the atomic and molecular gas are inferred.
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