Fermi LAT Study of the Cosmic-rays and the Interstellar Medium in Nearby Molecular Clouds

TL;DR

This study uses Fermi LAT gamma-ray data to analyze cosmic-ray densities and molecular gas properties in nearby molecular clouds, revealing variations in cosmic-ray density and molecular mass calibration ratios close to the solar system.

Contribution

First detailed gamma-ray analysis of multiple nearby molecular clouds to measure cosmic-ray densities and molecular gas properties near the solar system.

Findings

Cosmic-ray density varies by about 20% among the regions.

The molecular mass calibration ratio, X_CO, ranges from 0.6 to 1.0 x 10^{20} H2 molecules cm^{-2} (K km s^{-1})^{-1}.

Similar amounts of dark gas are inferred at atomic-molecular interfaces.

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 {\it Fermi} Large Area Telescope (LAT). They are among the nearest molecular cloud complexes, within $\sim$ 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) $\times$ 10$^{-27}$ photons s$^{-1}$ sr$^{-1}$ H-atom$^{-1}$, indicating a variation of the CR density by $\sim$ 20% even if we consider the systematic uncertainties. The molecular mass calibration ratio, $X_{\rm CO} = N{\rm (H_2)}/W_{\rm CO}$, is found to be about (0.6--1.0) $\times$ 10$^{20}$ H$_2$-molecule cm$^{-2}$ (K km s$^{-1}$)$^{-1}$ among the three regions, suggesting a variation of $X_{\rm CO}$ in the vicinity of the solar system. From the obtained values of $X_{\rm CO}$, 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.