1. Physical Conditions in Orion’s Veil Nicholas Abel – University of Cincinnati, Clermont Campus Collaborators: Crystal Brogan, Gary Ferland, Bob O’Dell, Gargi Shaw, Phillip Stancil, Tom Troland, and Matt Lykins No Veil Veil

2. Outline I.Background II.Observational Properties III.Why is Veil important? IV.What we want to know? V.Data Analysis – Results

3. Sketch of Orion Nebula made by Christiaan Huygens (1659). Taken from The Orion Nebula (O’Dell – 2004) HST image of Orion Nebula (O’Dell & Wong 1996)

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5. τ 4861 ~ C Hβ /0.434 O’Dell and Yusef-Zadeh (2000) Extinction Map, From of 20 cm and Hβ Emission High extinction here, hence “Dark Bay” Towards Trapezium, A V ~ 1.6 mag Veil is at least as large as the H II region.

6. Lyα Absorption with STIS (Cartledge et al. 2001) Towards Trapezium N(HI) = (4.8 ± 1.1)x10 21 cm -2 About 60% of that seen in the ISM R ISM ~3.1. Grains in Veil are larger than typical, not as efficient in extinguishing UV and Optical

7. Magnetic Field Map of Veil (Troland et al. 1989) Magnetic Field map, from Zeeman 21cm splitting. Magnetic field strength B los ~50μG towards Trapezium Also have 21 cm optical depth profiles throughout the Veil.

8. Proportional to N(H)/T spin 21 cm Optical Depth Profile Towards Trapezium Component BComponent A N(H)/T spin [A] = 1.78x10 19 cm -2 K -1 N(H)/T spin [B] = 2.35x10 19 cm -2 K -1 Abel et al. (2004, 2006)

9. S III Abel et al. (2004, 2006)

10. BA H2H2 Component B Component A The Veil is also seen in UV and optical absorption studies, using the Trapezium as a continuum source. Veil has two main neutral components towards the Trapezium, both redshifted. Has an ionized component, which is redshifted. Both components seen in radio, UV, and optical ( I will focus on radio, UV). Linewidth of each component in UV puts constraints on kinetic temperature and turbulent motions. STIS Spectra VLA

11. BA H2H2 Component B Component A b-values for Hydrogen puts constraints on kinetic temperature and turbulent motions. Component A = 1.2 km/s (+/- 0.02). Component B = 2.3 km/s (+/- 0.02). STIS data also, for the first time, observed H 2 in absorption. STIS Spectra VLA

12. Savage et al. (1977)

13. Why is Veil important? Distance of neutral components (is/was?) one of the major uncertainties in the geometry of the Orion environment. Wealth of observational data on gas and dust provides excellent test for calculations of physical conditions in the ISM. H 2 formation and destruction processes fundamentally important in study of PDRs, molecular clouds, star-formation. Star formation controlled by balance of thermal, magnetic, gravitational, and turbulent energies. Veil offers the chance to compare each in a region associated with a nearby region of active star-formation.

14. What do we want to know about the Veil? Wealth of data allows us to investigate physical characteristics. This includes. 1.Energy and pressure balance in the Veil Thermal, Turbulent, and Magnetic Relationship of kinetic and spin temperatures 2.Geometry Distance of Veil from Trapezium Thickness of each component Dynamical evolution 3.Molecular hydrogen in the Veil Abundance. Why so low? In order to do this, we need to determine the density, temperature, and column density in each component. Combination of theoretical calculations and observational analysis

15. H I Column Density in Each Component Observations of Kr I Absorption in UV (this works for O I, Cu II, and C I too). Kr should all be Kr 0 for neutral layers Kr not depleted, assume Kr 0 /H 0 ratio is constant We know all but N (H 0 ) in components A and B, so we can solve for it.

16. Can determine N(H) in each component

17. Column Density in Each Component Final result is: From 21 cm optical depth, we immediately get the spin temperature (ranges include error, ( ) is value from central H I column density)

18. Calculations A series of theoretical calculations were performed using the spectral synthesis code Cloudy (Ferland et al. 1998). CI, CI*, and CI** constrain density and temperature. Observations of ions in multiple ionization stages, such as S, Mg, and C constrain distance to Trapezium. Unfortunately, we do not have such data for each component, only average over both components from IUE data. Can only get average distance of both components. Upper limit on H 2 constrains ratio of UV field to density. Linewidths constrain temperature for each component. Cloudy and Friends, 1999

19. Calculations Well known set of elemental abundances for Orion Assuming Veil abundances mimic Orion. Well known grain properties Large R model from Weingartner & Draine (2001), whose application in Cloudy is described in van Hoof et al. (2004). Well known stellar continuum O6V star θ 1 Ori C, with some contributions from the other Trapezium stars. Know when to stop the model, at N (H 0 ) column density of each component.

20. Calculations Perform a series of calculations which varies density of Veil and distance of Veil from Trapezium. Problem, we do not see more than one ionization state of an element in multiple components We do see averages (over both components) of several elements ionization stages seen in IUE data (Shuping & Snow 1997). Mg 0 /Mg + ~ 10 -3.4 C 0 /C + ~10 -3.7 S 0 /S + ~ 10 -3.0 We also know that distance cannot be so far away that no H 2 forms. Distance cannot be so close that we see the Veil glow in emission, so models have to predict less Hβ than coming from M42.

21. Taken from Abel et al. (2004) Log[N(H 2 )] Must be < 17.35

22. Log[I Hβ ] Must be significantly less than -11.3, the value observed in M42.

23. Taken from Abel et al. (2004) H2 and Hβ Constraint: Combination of density/distance dramatically reduced.

24. CI**/C itot very sensitive to density over this region of parameter space.

25. T kinetic very sensitive to density over this region of parameter space. T kinetic

26. 3 P 0,1,2

27. BA

28. T kinetic BA

29. Distance and Density Given all constraints from observations, best fit model to all data gives the average distance of neutral components (not each component) as: 10 18.8 (+/- 0.1) (about two parsecs) CI*, CI** data, combined with observational constraints on the model, give the density of each component as: n H = 10 2.5 (component A) n H = 10 3.4 (component B)

30. Results From density, we get temperature in each component. T kin = 50 K (component A) T kin = 80 K (component B) As a result, for both components in the Veil, T kin < T spin (H I level populations are not in thermal equilibrium). Since we know the density and column density for each component, we can calculate average thickness, which is: L A = 1.3pc L B = 0.5pc

31. Energetics Now that we have density, temperature, and magnetic field in each component, can compare energetics. Using the formalism in Heiles & Troland (2005), we computed the ratio of thermal to magnetic energy density β therm, and turbulent to magnetic energy density β turb. In the ISM, β therm < 1 and β turb ~ 1 β turb ~ 1 means equipartition between magnetic and turbulent energies, while β therm < 1 means magnetic energy dominates of thermal in the ISM

32. Energetics For component A Very narrow linewidth, combined with T kin ~50, means less velocity for turbulent motions. Smaller density in component A than component B. β turb =0.01 in component A, making Veil unique as a region dominated by magnetic energy. Component B, β turb =0.5, so in rough equipartition.

33. Molecular Hydrogen Absorption Observations

34. Molecular Hydrogen Absorption Calculations Our calculations predicted that most H 2 in Veil would be in states with J >1. H 2 level populations in Veil pumped by UV radiation, not controlled by collisions. Veil is on verge of forming large amounts of H 2, just barely not thick enough.

35. Dependence of N(H 2 ) on Grain Size

36. Summary 1.Neutral Veil layers are about 2 parsecs from Trapezium. UV field about 2000 greater than ISRF ( G 0 = 2000). In addition, ionized and neutral components are moving towards each other, and will collide in about 10 5 years. 2.Computed density, kinetic temperature, spin temperature, and thickness of each component. 3.Compared magnetic, thermal, and turbulent energetics in the Veil, for each component. Component A is magnetically dominated and NOT in equilibrium with turbulent energies, as is often found in many regions of the ISM. 4.H 2 and H I (21 cm) level populations not in thermal equilibrium Lack of H 2 due to close proximity to Trapezium (high G 0 ), but also due to large grains.

37. Thank you for your time, and for the invitation to come to the NCAC. And thanks to Bob O’Dell for his friendship over the years.

38. Trapezium stars ORIONNEBuLAORIONNEBuLA OMC1OMC1 1-2 Scale (parsecs) 0.10.25~1 EARTHEARTH IONIZEDGASIONIZEDGAS NEUTRALGASNEUTRALGAS NEUTRALGASNEUTRALGAS 450 10 km/s1 - 5 km/s ~1