We report an analysis of the interstellar gamma-ray emission from the Chamaeleon, R CrA, and Cepheus and Polaris flare regions with the Fermi Large Area Telescope (LAT). They are among the nearest molecular cloud complexes, within about 300 pc from the solar system. The gamma-ray emission produced by interactions of cosmic rays and interstellar gas in those molecular clouds is useful to study the CR densities and distributions of interstellar gas close to the solar system. At the interface between the atomic and molecular gas, a considerable amount of “dark gas” is inferred to be associated with cold dust but not traced by HI and CO lines.
Thanks to the excellent performance of the LAT, we have obtained unprecedentedly high-quality emissivity spectra of the atomic and molecular gas in these regions in the 250 MeV - 10 GeV range. We carefully examined systematic uncertainties due to the optical depth correction of the atomic gas and backgrounds such as the inverse Compton scattering and isotropic components.
The obtained emissivities indicate a variation of the CR density of about 20 % among three regions, even if we take into account the combined statistical and systematic uncertainties. The molecular mass calibration ratio, Xco, is found to be 0.6 –1.0 (x 1020 H2-molecule cm-2 (K km s-1)-1 in the three regions, suggesting a variation of Xco in the vicinity of the solar system. From the obtained values of Xco, we calculated masses of molecular gas traced by Wco in these molecular clouds. In addition, similar amounts of dark gas at the interface between the atomic and molecular gas are inferred.