Compression and ablation of the photo-irradiated molecular cloud the Orion Bar


The Orion Bar is the archetypal edge-on molecular cloud surface illuminated by strong ultraviolet radiation from nearby massive stars. Our relative closeness to the Orion nebula (about 1,350 light years away from Earth) means that we can study the effects of stellar feedback on the parental cloud in detail. Visible-light observations of the Orion Bar1 show that the transition between the hot ionized gas and the warm neutral atomic gas (the ionization front) is spatially well separated from the transition between atomic and molecular gas (the dissociation front), by about 15 arcseconds or 6,200 astronomical units (one astronomical unit is the Earth–Sun distance). Static equilibrium models2,3 used to interpret previous far-infrared and radio observations of the neutral gas in the Orion Bar4,5,6 (typically at 10–20 arcsecond resolution) predict an inhomogeneous cloud structure comprised of dense clumps embedded in a lower-density extended gas component. Here we report one-arcsecond-resolution millimetre-wave images that allow us to resolve the molecular cloud surface. In contrast to stationary model predictions7,8,9, there is no appreciable offset between the peak of the H2 vibrational emission (delineating the H/H2 transition) and the edge of the observed CO and HCO+ emission. This implies that the H/H2 and C+/C/CO transition zones are very close. We find a fragmented ridge of high-density substructures, photoablative gas flows and instabilities at the molecular cloud surface. The results suggest that the cloud edge has been compressed by a high-pressure wave that is moving into the molecular cloud, demonstrating that dynamical and non-equilibrium effects are important for the cloud evolution.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Figure 1: Multiphase view of the Orion nebula and molecular cloud.
Figure 2: ALMA images of the Orion Bar.


  1. 1

    Walmsley, C. M., Natta, A., Oliva, E. & Testi, L. The structure of the Orion Bar. Astron. Astrophys. 364, 301–317 (2000)

    ADS  CAS  Google Scholar 

  2. 2

    Tielens, A. G. G. M. & Hollenbach, D. J. Photodissociation regions. I: basic model. Astrophys. J. 291, 722–754 (1985)

    ADS  CAS  Google Scholar 

  3. 3

    Andree-Labsch, S., Ossenkopf, V. & Röllig, M. 3D modelling of clumpy PDRs: understanding the Orion Bar stratification. Preprint at (2014)

  4. 4

    Tielens, A. G. G. M. et al. Anatomy of the photodissociation region in the Orion Bar. Science 262, 86–89 (1993)

    ADS  CAS  PubMed  Google Scholar 

  5. 5

    Hogerheijde, M. R., Jansen, D. J. & van Dishoeck, E. F. Millimeter and submillimeter observations of the Orion Bar. 1: physical structure. Astron. Astrophys. 294, 792–810 (1995)

    ADS  CAS  Google Scholar 

  6. 6

    Young Owl, R. C., Meixner, M. M., Wolfire, M., Tielens, A. G. G. M. & Tauber, J. HCN and HCO+ images of the Orion Bar photodissociation region. Astrophys. J. 540, 886–906 (2000)

    ADS  Google Scholar 

  7. 7

    Sternberg, A. & Dalgarno, A. Chemistry in dense photon-dominated regions. Astrophys. J. Suppl. Ser. 99, 565–607 (1995)

    ADS  CAS  Google Scholar 

  8. 8

    Le Petit, F., Nehmé, C., Le Bourlot, J. & Roueff, E. A model for atomic and molecular interstellar gas: The Meudon PDR code. Astrophys. J. Suppl. Ser. 164, 506–529 (2006)

    ADS  CAS  Google Scholar 

  9. 9

    Röllig, M. et al. A photon dominated region code comparison study. Astron. Astrophys. 467, 187–206 (2007)

    ADS  Google Scholar 

  10. 10

    Genzel, R. & Stutzki, J. The Orion molecular cloud and star-forming region. Annu. Rev. Astron. Astrophys. 27, 41–85 (1989)

    ADS  CAS  Google Scholar 

  11. 11

    Goicoechea, J. R. et al. Velocity-resolved [Cii] emission and [Cii]/FIR mapping along Orion with Herschel. Astrophys. J. 812, 75 (2015)

    ADS  PubMed  PubMed Central  Google Scholar 

  12. 12

    O’Dell, C. R. The Orion Nebula and its associated population. Annu. Rev. Astron. Astrophys. 39, 99–136 (2001)

    ADS  Google Scholar 

  13. 13

    van der Werf, P. P. Goss, W. M. & O’Dell, C. R. Tearing the veil: interaction of the Orion Nebula with its neutral environment. Astrophys. J. 762, 101 (2013)

    ADS  Google Scholar 

  14. 14

    Hollenbach, D. J. & Tielens, A. G. G. M. Photodissociation regions in the interstellar medium of galaxies. Rev. Mod. Phys. 71, 173–230 (1999)

    ADS  CAS  Google Scholar 

  15. 15

    Weilbacher, P. M. et al. A MUSE map of the central Orion Nebula (M 42). Astron. Astrophys. 582, A114 (2015)

    Google Scholar 

  16. 16

    Draine, B. T. Physics of the Interstellar and Intergalactic Medium (Princeton Univ. Press, 2011)

  17. 17

    Bertoldi, F. & Draine, B. T. Nonequilibrium photodissociation regions: ionization-dissociation fronts. Astrophys. J. 458, 222–232 (1996)

    ADS  Google Scholar 

  18. 18

    Störzer, H. & Hollenbach, D. J. Nonequilibrium photodissociation regions with advancing ionization fronts. Astrophys. J. 495, 853–870 (1998)

    ADS  Google Scholar 

  19. 19

    Lis, D. C. & Schilke, P. Dense molecular clumps in the Orion Bar photon-dominated region. Astrophys. J. 597, L145–L148 (2003)

    ADS  CAS  Google Scholar 

  20. 20

    Spitzer, L. Physical Processes in the Interstellar Medium (Wiley, 1978)

  21. 21

    Hill, J. K. & Hollenbach, D. J. Effects of expanding compact H ii regions upon molecular clouds: molecular dissociation waves, shock waves, and carbon ionization. Astrophys. J. 225, 390–404 (1978)

    ADS  CAS  Google Scholar 

  22. 22

    Hosokawa, T. & Inutsuka, S.-i. Dynamical expansion of ionization and dissociation front around a massive star. II: on the generality of triggered star formation. Astrophys. J. 646, 240–257 (2006)

    ADS  CAS  Google Scholar 

  23. 23

    Hennebelle, P. & Falgarone, E. Turbulent molecular clouds. Astron. Astrophys. Rev. 20, 55 (2012)

    ADS  Google Scholar 

  24. 24

    Federrath, C. & Klessen, R. S. On the star formation efficiency of turbulent magnetized clouds. Astrophys. J. 763, 51 (2013)

    ADS  Google Scholar 

  25. 25

    Tremblin, P., Audit, E., Minier, V., Schmidt, W. & Schneider, N. Three-dimensional simulations of globule and pillar formation around H ii regions: turbulence and shock curvature. Astron. Astrophys. 546, A33 (2012)

    ADS  Google Scholar 

  26. 26

    Gorti, U. & Hollenbach, D. J. Photoevaporation of clumps in photodissociation regions. Astrophys. J. 573, 215–237 (2002)

    ADS  CAS  Google Scholar 

  27. 27

    Elmegreen, B. G. & Lada, C. J. Sequential formation of subgroups in OB associations. Astrophys. J. 214, 725–741 (1977)

    ADS  CAS  Google Scholar 

  28. 28

    Berné, O., Marcelino, N. & Cernicharo, J. Waves on the surface of the Orion molecular cloud. Nature 466, 947–949 (2010)

    ADS  PubMed  Google Scholar 

  29. 29

    García-Segura, G. & Franco, J. From ultracompact to extended H ii regions. Astrophys. J. 469, 171–188 (1996)

    ADS  Google Scholar 

  30. 30

    Lefloch, B. & Lazareff, B. Cometary globules. 1: formation, evolution and morphology. Astron. Astrophys. 289, 559–578 (1994)

    ADS  Google Scholar 

  31. 31

    Pety, J. & Rodríguez-Fernández, N. J. Revisiting the theory of interferometric wide-field synthesis. Astron. Astrophys. 517, A12 (2010)

    ADS  Google Scholar 

  32. 32

    van der Werf, P. P., Stutzki, J., Sternberg, A. & Krabbe, A. Structure and chemistry of the Orion bar photon-dominated region. Astron. Astrophys. 313, 633–648 (1996)

    ADS  CAS  Google Scholar 

  33. 33

    Allers, K. N., Jaffe, D. T., Lacy, J. H., Draine, B. T. & Richter, M. J. H2 pure rotational lines in the Orion Bar. Astrophys. J. 630, 368–380 (2005)

    ADS  CAS  Google Scholar 

  34. 34

    O’Dell, C. R. & Yusef-Zadeh, F. High angular resolution determination of extinction in the Orion Nebula. Astron. J. 120, 382–392 (2000)

    ADS  Google Scholar 

  35. 35

    Cardelli, J. A., Clayton, G. C. & Mathis, J. S. The relationship between infrared, optical, and ultraviolet extinction. Astrophys. J. 345, 245–256 (1989)

    ADS  CAS  Google Scholar 

  36. 36

    Goicoechea, J. R. et al. Low sulfur depletion in the Horsehead PDR. Astron. Astrophys. 456, 565–580 (2006)

    ADS  CAS  Google Scholar 

  37. 37

    Arab, H. et al. Evolution of dust in the Orion Bar with Herschel. I: radiative transfer modelling. Astron. Astrophys. 541, A19 (2012)

    Google Scholar 

  38. 38

    Cuadrado, S. et al. The chemistry and spatial distribution of small hydrocarbons in UV-irradiated molecular clouds: the Orion Bar PDR. Astron. Astrophys. 575, A82 (2015)

    Google Scholar 

  39. 39

    Langer, W. D. & Penzias, A. A. 12C/13C isotope ratio across the Galaxy from observations of 13C18O in molecular clouds. Astrophys. J. 357, 477–492 (1990)

    ADS  CAS  Google Scholar 

  40. 40

    Inutsuka, S.-i. & Miyama, S. M. A production mechanism for clusters of dense cores. Astrophys. J. 480, 681–693 (1997)

    ADS  Google Scholar 

  41. 41

    Noel, B., Joblin, C., Maillard, J. P. & Paumard, T. New results on the massive star-forming region S106 by BEAR spectro-imagery. Astron. Astrophys. 436, 569–584 (2005)

    ADS  CAS  Google Scholar 

  42. 42

    Burton, M. G., Hollenbach, D. J. & Tielens, A. G. G. M. Line emission from clumpy photodissociation regions. Astrophys. J. 365, 620–639 (1990)

    ADS  CAS  Google Scholar 

  43. 43

    Wyrowski, F., Schilke, P., Hofner, P. & Walmsley, C. M. Carbon radio recombination lines in the Orion Bar. Astrophys. J. 487, L171–L174 (1997)

    ADS  CAS  Google Scholar 

  44. 44

    Tremblin, P. et al. Ionization compression impact on dense gas distribution and star formation. Probability density functions around HII regions as seen by Herschel. Astron. Astrophys. 564, A106 (2014)

    Google Scholar 

  45. 45

    Brogan, C. L., Troland, T. H., Abel, N. P., Goss, W.M. & Crutcher, R. M. in Astronomical Polarimetry: Current Status and Future Directions (eds Adamson, A . et al.) 183–184 (ASP Conference Series 343, Astronomical Society of the Pacific, 2005)

  46. 46

    Planck Collaboration. Planck intermediate results. XXXIV: The magnetic field structure in the Rosette Nebula. Astron. Astrophys. 586, A137 (2016)

  47. 47

    Roberge, W. G. & Draine, B. T. A new class of solutions for interstellar magnetohydrodynamic shock waves. Astrophys. J. 350, 700–721 (1990)

    ADS  Google Scholar 

  48. 48

    Raga, A. C., Cantó, J. & Rodríguez, L. F. Analytic and numerical models for the expansion of a compact HII region. Mon. Not. R. Astron. Soc. 419, L39–L43 (2012)

    ADS  CAS  Google Scholar 

  49. 49

    Hollenbach, D. J. & Natta, A. Time-dependent photodissociation regions. Astrophys. J. 455, 133–144 (1995)

    ADS  CAS  Google Scholar 

  50. 50

    Bertoldi, F. ISO: A Novel Look at the Photodissociated Surfaces of Molecular Clouds 67–72 (Special Publication 419, European Space Agency, 1997)

  51. 51

    Neufeld, D. A. & Wolfire, M. G. The chemistry of interstellar molecules containing the halogen elements. Astrophys. J. 706, 1594–1604 (2009)

    ADS  CAS  Google Scholar 

  52. 52

    Neufeld, D. A. et al. Discovery of interstellar CF+. Astron. Astrophys. 454, L37–L40 (2006)

    ADS  CAS  Google Scholar 

  53. 53

    Guzmán, V. et al. The IRAM-30m line survey of the Horsehead PDR. I: CF+ as a tracer of C+ and as a measure of the fluorine abundance. Astron. Astrophys. 543, L1 (2012)

    ADS  Google Scholar 

  54. 54

    Agúndez, M., Goicoechea, J. R., Cernicharo, J., Faure, A. & Roueff, E. The chemistry of vibrationally excited H2 in the interstellar medium. Astrophys. J. 713, 662–670 (2010)

    ADS  Google Scholar 

  55. 55

    Nagy, Z. et al. The chemistry of ions in the Orion Bar. I. CH+, SH+, and CF+: the effect of high electron density and vibrationally excited H2 in a warm PDR surface. Astron. Astrophys. 550, A96 (2013)

    Google Scholar 

  56. 56

    Goicoechea, J. R. et al. OH emission from warm and dense gas in the Orion Bar PDR. Astron. Astrophys. 530, L16 (2011)

    ADS  Google Scholar 

  57. 57

    Stoerzer, H., Stutzki, J. & Sternberg, A. CO+ in the Orion Bar, M17 and S140 star-forming regions. Astron. Astrophys. 296, L9–L12 (1995)

    ADS  CAS  Google Scholar 

Download references


We thank the ERC for support under grant ERC-2013-Syg-610256-NANOCOSMOS. We also thank MINECO, Spain, for funding support under grants CSD2009-00038 and AYA2012-32032. This work was in part supported by the French CNRS programme ‘Physique et Chimie du Milieu Interstellaire’. We thank P. Schilke and D. Lis for sharing their IRAM-PdBI observations of the H13CN J = 1–0 condensations inside the Orion Bar, and M. Walmsley for sharing his H2 v = 1–0 S(1) and O i 1.3 μm infrared images. ALMA is a partnership of the ESO (representing its member states), the NSF (USA) and NINS (Japan), together with the NRC (Canada), the NSC and ASIAA (Taiwan) and KASI (Republic of Korea) in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by the ESO, the AUI/NRAO and the NAOJ. This Letter makes use of observations obtained with the IRAM 30 m telescope. IRAM is supported by the INSU/CNRS (France), the MPG (Germany), and the IGN (Spain).

Author information




J.R.G. was the principal investigator of the ALMA project. He led the scientific analysis, modelling and wrote the manuscript. J.P. and E.C. carried out the ALMA data calibration and data reduction. S.C. and N.M. carried out the single-dish maps observations with the IRAM 30 m telescope. All authors participated in the discussion of results, determination of the conclusions and revision of the manuscript.

Corresponding author

Correspondence to Javier R. Goicoechea.

Additional information

We used the ALMA data ADS/JAO.ALMA#2012.1.00352.S available at

Reviewer Information Nature thanks R. Plume and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Figure 1 Structure of a strongly ultraviolet-irradiated molecular cloud edge.

The incident stellar ultraviolet radiation comes from the left. The velocity of the advancing ionization and dissociation fronts are represented by vIF and vDF respectively. In the Orion Bar, the dissociation front is at about 15″ (about 0.03 pc) from the ionization front. UV, ultraviolet; PAH, polycyclic aromatic hydrocarbons. The snow line refers to the inner cloud layers where molecular gases start to freeze and dust grains become coated by ices.

Extended Data Figure 2 Comparison with other tracers.

a, ALMA HCO+ J = 4–3 line integrated intensity. b, ALMA CO J = 3–2 line peak (Orion Bar velocity component). The red contours represent the H13CN J = 1–0 emission (from 0.08 to 0.026 in steps of 0.02 Jy beam−1 km s−1) of dense condensations inside the Orion Bar19. The black contours show the brightest regions of H2 v = 1–0 S(1) emission1 (from 1.5 to 4.5 in steps of 0.5·10−4 erg s−1 cm−2 sr−1). The H2 image is saturated between δx = 19″ and 23″ (that is, no data are shown). Figures have been rotated 127.5° anticlockwise to bring the incident ultraviolet radiation from the left.

Extended Data Figure 3 Excitation models for different gas temperatures and densities.

a, CO J = 3–2 line peak (for N(CO) = 1018 cm−2). b–d, HCO+ J = 4–3 integrated line intensity at 100 K (b), 200 K (c) and 300 K (d). Each curve represents a different electron abundance model: xe = 0 (blue) and xe = 10−4 (red). Continuous curves are for N(HCO+) = 5 × 1013 cm−2 and dotted lines for N(HCO+) = 2 × 1014 cm−2 (appropriate for deeper inside the Orion Bar, δx > 30″). The horizontal green dashed line represents the average (a) and (b–d) with their standard deviation (grey shaded) towards the dissociation front (at δx ≈ 15″).

Extended Data Figure 4 Line velocity centroid, dispersion and profiles.

a, Vertically averaged cuts perpendicular to the Orion Bar in the HCO+ J = 4–3 line velocity centroid (magenta curve) and FWHM velocity dispersion (grey curve). b, CO and HCO+ spectra at representative positions. The top and middle plots show positions between the ionization and dissociation fronts, the bottom plot is inside the molecular Orion Bar. Offsets are given with respect to the rotated images in Extended Data Fig. 2. The velocity of the background cloud is vLSR ≈ 8.5 km s−1 (black dashed line), whereas the velocity of the Orion Bar is vLSR ≈ 11 km s−1 (green line).

Extended Data Table 1 Gas pressures and estimated magnetic field strengths

PowerPoint slides

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Goicoechea, J., Pety, J., Cuadrado, S. et al. Compression and ablation of the photo-irradiated molecular cloud the Orion Bar. Nature 537, 207–209 (2016).

Download citation

Further reading


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing