Volume 530, June 2011
|Number of page(s)
|Interstellar and circumstellar matter
|20 May 2011
Effects of magnetic fields on the cosmic-ray ionization of molecular cloud cores
1 Institut de Ciències de l’Espai (CSIC–IEEC), Campus UAB, Facultat de Ciències, Torre C5-parell 2 a, 08193 Bellaterra, Spain
2 INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
Received: 8 March 2011
Accepted: 26 April 2011
Context. Low-energy cosmic rays are the dominant source of ionization for molecular cloud cores. The ionization fraction, in turn, controls the coupling of the magnetic field to the gas and hence the dynamical evolution of the cores.
Aims. The purpose of this work is to compute the attenuation of the cosmic-ray flux rate in a cloud core taking into account magnetic focusing, magnetic mirroring, and all relevant energy loss processes.
Methods. We adopt a standard cloud model characterized by a mass-to-flux ratio supercritical by a factor of ~2 to describe the density and magnetic field distribution of a low-mass starless core, and we follow the propagation of cosmic rays through the core along flux tubes enclosing different amount of mass. We then extend our analysis to cores with different mass-to-flux ratios.
Results. We find that mirroring always dominates over focusing, implying a reduction of the cosmic-ray ionization rate by a factor of ~2−3 over most of a solar-mass core with respect to the value in the intercloud medium outside the core. For flux tubes enclosing larger masses the reduction factor is smaller, since the field becomes increasingly uniform at larger radii and lower densities. We also find that the cosmic-ray ionization rate is further reduced in clouds with stronger magnetic field, e.g. by a factor ~4 for a marginally critical cloud.
Conclusions. The magnetic field threading molecular cloud cores affects the penetration of low-energy cosmic rays and reduces the ionization rate by a factor 3 − 4 depending on the position inside the core and the magnetization of the core.
Key words: cosmic rays / ISM: clouds / ISM: magnetic fields
© ESO, 2011
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.