Cosmic Rays in the Orion Bar

  • Yago Ascasibar
  • Aura C. Obreja
  • Ángeles I. Díaz
Conference paper
Part of the Astrophysics and Space Science Proceedings book series (ASSSP, volume 34)


The detailed structure of HII regions and the surrounding photodissociation regions (more precisely, the extent and chemical composition of the ionized, neutral, and molecular layers) poses strong constraints on the intensity of the magnetic field and the presence of cosmic rays. Conversely, an accurate knowledge of these physical parameters is necessary in order to understand the observable properties of nearby regions, as well as those of unresolved, distant star-forming galaxies. Due to its close distance to the Earth and its nearly edge-on geometry, the Orion Bar provides an excellent testbed for detailed photoionization models. We will show that, although the observed structure of the ionized HII region can be reproduced with remarkable accuracy, the agreement in the atomic layer is only qualitative, and the predicted abundances of several molecular species are off by several orders of magnitude. Consistent with previous work, our results hint that the density of cosmic rays is high (well above the Galactic background, though not as large as implied by equipartition arguments), but their role on the molecular chemistry is still unclear. In particular, we consider that cosmic rays alone cannot probably solve all the present discrepancies between models and observations, and a more detailed treatment of the three-dimensional structure of the region is clearly required.


Column Density Molecular Region Hydrostatic Equilibrium Orion Nebula Dust Extinction 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.



We would like to thank the organizers for such a stimulating conference, bringing together people from so many different communities and points of view. Y. Ascasibar receives financial support from project AYA 2010-21887-C04-03 as well as from the Ramón y Cajal programme, under contract RYC-2011-09461.


  1. 1.
    Ascasibar Y (2008) FiEstAS sampling – a Monte Carlo algorithm for multidimensional numerical integration. Computer Physics Communications 179:881–887, DOI 10.1016/j.cpc.2008.07.011, 0807.4479Google Scholar
  2. 2.
    Ascasibar Y (2010) Estimating multidimensional probability fields using the Field Estimator for Arbitrary Spaces (FiEstAS) with applications to astrophysics. Computer Physics Communications 181:1438–1443, DOI 10.1016/j.cpc.2010.04.011, 1006.1296Google Scholar
  3. 3.
    Ascasibar Y, Binney J (2005) Numerical estimation of densities. MNRAS356:872–882, DOI 10.1111/j.1365-2966.2004.08480.x, arXiv:astro-ph/0409233Google Scholar
  4. 4.
    Ascasibar Y, Diaz AI (2010) Photoionized gas in hydrostatic equilibrium: the role of gravity. MNRAS404:275–282, DOI 10.1111/j.1365-2966.2010.16270.x, 1001.3035Google Scholar
  5. 5.
    Ascasibar Y, Obreja AC, Diaz AI (2011) Hydrostatic photoionization models of the Orion Bar. MNRAS416:1546–1555, DOI 10.1111/j.1365-2966.2011.19151.x, 1106.3990Google Scholar
  6. 6.
    Baldwin JA, Ferland GJ, Martin PG, Corbin MR, Cota SA, Peterson BM, Slettebak A (1991) Phys- ical conditions in the Orion Nebula and an assessment of its helium abundance. ApJ374:580– 609, DOI 10.1086/170146Google Scholar
  7. 7.
    Elitzur M, Ferland GJ (1986) Radiation pressure and emission clouds around active galactic nuclei. ApJ305:35–44, DOI 10.1086/164226Google Scholar
  8. 8.
    Feigelson ED, Getman K, Townsley L, Garmire G, Preibisch T, Grosso N, Montmerle T, Muench A, McCaughrean M (2005) Global X-Ray Properties of the Orion Nebula Region. ApJS160:379–389, DOI 10.1086/432512, arXiv:astro-ph/0506503Google Scholar
  9. 9.
    Ferland GJ, Elitzur M (1984) Radiation pressure and the stability of broad-line region clouds. ApJ285:L11–L14, DOI 10.1086/184354Google Scholar
  10. 10.
    Ferland GJ, Korista KT, Verner DA, Ferguson JW, Kingdon JB, Verner EM (1998) CLOUDY 90: Numerical Simulation of Plasmas and Their Spectra. PASP110:761–778, DOI 10.1086/316190Google Scholar
  11. 11.
    Fuente A, Rodrguez-Franco A, Garca-Burillo S, Martn-Pintado J, Black JH (2003) Observa- tional study of reactive ions and radicals in PDRs. A&A406:899–913, DOI 10.1051/0004- 6361:20030712Google Scholar
  12. 12.
    Hogerheijde MR, Jansen DJ, van Dishoeck EF (1995) Millimeter and submillimeter observations of the Orion Bar. 1: Physical structure. A&A294:792–810Google Scholar
  13. 13.
    Jansen DJ, Spaans M, Hogerheijde MR, van Dishoeck EF (1995) Millimeter and submillimeter observations of the Orion Bar. II. Chemical models. A&A303:541Google Scholar
  14. 14.
    Kraus S, Balega YY, Berger J, Hofmann K, Millan-Gabet R, Monnier JD, Ohnaka K, Pedretti E, Preibisch T, Schertl D, Schloerb FP, Traub WA, Weigelt G (2007) Visual/infrared interferom- etry of Orion Trapezium stars: preliminary dynamical orbit and aperture synthesis imaging of the ?1 Orionis C system. A&A466:649–659, DOI 10.1051/0004-6361:20066965, arXiv:astro- ph/0702462Google Scholar
  15. 15.
    Kurucz RL (1979) Model atmospheres for G, F, A, B, and O stars. ApJS40:1–340, DOI 10.1086/190589Google Scholar
  16. 16.
    ODell CR, Harris JA (2010) Spectrophotometry of the Huygens Region of the Orion Nebula, the Extended Orion Nebula, and M 43: Scattered Light Systematically Distorts Conditions Derived from Emission Lines. AJ140:985–1006, DOI 10.1088/0004-6256/140/4/985, 1008.1002Google Scholar
  17. 17.
    Pellegrini EW, Baldwin JA, Ferland GJ, Shaw G, Heathcote S (2009) Orions Bar: Physical Conditions Across the Definitive H + /H0/H2 Interface. ApJ693:285–302, DOI 10.1088/0004- 637X/693/1/285, 0811.1176Google Scholar
  18. 18.
    Schilke P, Pineau des Forets G, Walmsley CM, Martin-Pintado J (2001) Observations of SiO to- wards photon dominated regions. A&A372:291–301, DOI 10.1051/0004-6361:20010470Google Scholar
  19. 19.
    Simon R, Stutzki J, Sternberg A, Winnewisser G (1997) Chemical stratification in the Orion Bar region: CN and CS submillimeter observations. A&A327:L9–L12Google Scholar
  20. 20.
    Stoerzer H, Stutzki J, Sternberg A (1995) CO + in the Orion Bar, M17 and S140 star-forming regions. A&A296:L9 + Google Scholar
  21. 21.
    van der Werf PP, Stutzki J, Sternberg A, Krabbe A (1996) Structure and chemistry of the Orion bar photon-dominated region. A&A313:633–648Google Scholar
  22. 22.
    Wen Z, Odell CR (1995) A three-dimensional model of the Orion Nebula. ApJ438:784–793, DOI 10.1086/175123Google Scholar

Copyright information

© Springer-Verlag Berlin Heidelberg 2013

Authors and Affiliations

  • Yago Ascasibar
    • 1
  • Aura C. Obreja
    • 1
  • Ángeles I. Díaz
    • 1
  1. 1.Universidad Autónoma de MadridMadridSpain

Personalised recommendations