The Orion Region: Evidence of enhanced cosmic-ray density in a stellar wind forward shock interaction with a high density shell
arXiv:1901.01764 · doi:10.1051/0004-6361/201833651
Abstract
Context. In recent years, an in-depth gamma-ray analysis of the Orion region has been carried out by the AGILE and Fermi-LAT (Large Area Telescope) teams with the aim of estimating the H2-CO conversion factor, XCO. The comparison of the data from both satellites with models of diffuse gamma-ray Galactic emission unveiled an excess at (l,b)=[213.9, -19.5], in a region at a short angular distance from the OB star k-Ori. Possible explanations of this excess are scattering of the so-called "dark gas", non-linearity in the H2-CO relation, or Cosmic-Ray (CR) energization at the k-Ori wind shock. Aims. Concerning this last hypothesis, we want to verify whether cosmic-ray acceleration or re-acceleration could be triggered at the k-Ori forward shock, which we suppose to be interacting with a star-forming shell detected in several wavebands and probably triggered by high energy particles. Methods. Starting from the AGILE spectrum of the detected gamma-ray excess, showed here for the first time, we developed a valid physical model for cosmic-ray energization, taking into account re-acceleration, acceleration, energy losses, and secondary electron contribution. Results. Despite the characteristic low velocity of an OB star forward shock during its "snowplow" expansion phase, we find that the Orion gamma-ray excess could be explained by re-acceleration of pre-existing cosmic rays in the interaction between the forward shock of k-Ori and the CO-detected, star-forming shell swept-up by the star expansion. According to our calculations, a possible contribution from freshly accelerated particles is sub-dominant with respect the re-acceleration contribution. However, a simple adiabatic compression of the shell could also explain the detected gamma-ray emission. Futher GeV and TeV observations of this region are highly recommended in order to correctly identify the real physical scenario.
arXiv comment8 pages, 5 figures, accepted by A&A
References in corpus (23)
- Maps of Dust IR Emission for Use in Estimation of Reddening and CMBR Foregrounds
- The Leiden/Argentine/Bonn (LAB) Survey of Galactic HI: Final data release of the combined LDS and IAR surveys with improved stray-radiation corrections
- The Milky Way in Molecular Clouds: A New Complete CO Survey
- PAMELA Measurements of Cosmic-ray Proton and Helium Spectra
- Detection of the Characteristic Pion-Decay Signature in Supernova Remnants
- Simulations of Ion Acceleration at Non-relativistic Shocks. I. Acceleration Efficiency
- Physical Parameters and Wind Properties of Galactic Early B Supergiants
- Supernova Remnants in Molecular Clouds
- Dispersion of Magnetic Fields in Molecular Clouds. II
- Neutral pion emission from accelerated protons in the supernova remnant W44
- Limits on diffusive shock acceleration in supernova remnants in the presence of cosmic-ray streaming instability and wave dissipation
- Nonthermal Emission from a Supernova Remnant in a Molecular Cloud
- A Uniform CO Survey of the Molecular Clouds in Orion and Monoceros
- The origin of galactic cosmic rays
- A Map of Dust Reddening to 4.5 kpc from Pan-STARRS1
- Non-linear shock acceleration in the presence of seed particles
- Revisiting the Westerlund 2 Field with the H.E.S.S. Telescope Array
- The Supernova Remnant W44: confirmations and challenges for cosmic-ray acceleration
- The supernova remnant W44: a case of cosmic-Ray reacceleration
- Gamma-ray observations of the Orion Molecular Clouds with the Fermi Large Area Telescope
- EGRET Observations of the Diffuse Gamma-Ray Emission in Orion: Analysis Through Cycle 6
- A star forming ring around Kappa Ori 250 pc from the Sun
- A new gamma-ray source unveiled by AGILE in the region of Orion