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Magnetic realignment - Observer's perspective SALT fluorescence pilot observations: NGC2023

Atomic fluorescence and prospects for observing magnetic geometry using magnetic realignment of atomic ground states. Magnetic realignment - Observer's perspective SALT fluorescence pilot observations: NGC2023 A “Spreadsheet” Model Which Ions Comparison to observations

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Magnetic realignment - Observer's perspective SALT fluorescence pilot observations: NGC2023

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  1. Atomic fluorescence and prospects for observing magnetic geometry using magnetic realignment of atomic ground states • Magnetic realignment - Observer's perspective • SALT fluorescence pilot observations: NGC2023 • A “Spreadsheet” Model • Which Ions • Comparison to observations • Planning polarization observations • Future/ Observations Polarized Fluorescence in RNe - AstroPol08

  2. Magnetic Realignment • Linear polarization of atomic resonance/ fluorescence lines • anisotropic UV pumping produces anisotropic angular momentum distribution ("alignment") of ground state if photon rate > collisions (i.e., certainly within 1-10 pc of OB*) • presence of magnetic field alters alignment if Larmor freq > photon rate • ISM: B > ~0.1 μGauss: effect is “saturated”, field geometry only • CSM: B > 10-104μGauss: could depend on field strength also • Unique polarization signatures: • non-zero net polarization of IS/ CS absorption lines • distortion of position angles of scattered emission lines • Depends on: 3D geometry of magnetic field, ion ground state configuration and pumping (Yan & Lazarian 2006 - 2008) • Potentially more powerful than • 21 cm Zeeman: sensitive to weaker fields; works in hot gas • dust alignment: sensitive to 3D geometry, gas props and velocity Polarized Fluorescence in RNe - AstroPol08

  3. θ1 θ2 How to Observe it?I. Absorption • Ions: need at least 3 fine states in ground level (J ≥ 1) • Neutral: NI, OI, SII, FeII • Resolution. For sensitivity, resolve IS lines (R > 20,000) • Wavelengths: almost entirely in the FUV (except TiII, FeI - really complicated) • Tough, but will be trying it with Far Ultraviolet SpectroPolarimeter on θ1 and θ2 Ori through "Orion Veil" Polarized Fluorescence in RNe - AstroPol08

  4. How to Observe it?II. Emission • Ions: need at least 3 fine/ hyperfine states in ground level (F ≥ 1) • InterPlan, PDR: NaI, KI • Neutral: NI, OI, SII, FeII, AlII • Resolution. For sensitivity, against dust continuum (R > 5,000) • Wavelengths: • Resonance: UV, except NaI, KI • Fluorescence: UV/Vis/NIR • Signal: position angle rotation from reflection polarization (centrosymmetric) • Can observe this with ground-based high-resolution spectropolarimeter! Polarized Fluorescence in RNe - AstroPol08

  5. Pilot Observation: Fluorescence in Reflection Nebulae • OI, NI fluorescence previously seen only in HII, PNe: weak lines, with many excitation processes • Better: in RNe, is only excitation process; but need to verify/ model, prepare for spectropolarimetry • Robert Stobie Spectrograph on SALT 11m: • NGC2023 RN, HD37903 B1.5V central star • 0.6”x8’ slit, 1st order, R = 7500 - 9500 • ~1000 Ang coverage blue, yellow, red • Many fluorescence lines found! Inner 2’ of nebula, within PDR: Polarized Fluorescence in RNe - AstroPol08

  6. Predicted Ions • Which Ions? • Neutral medium: principle ion with IP < 13.6 eV • Abundance/H > 10-10 • 1st Resonance < 13.6 eV • 8 with LS Coupling (primary, secondary, alignable): • 6 with Non-LS coupling: ArI, TiII, CrII, MnII, FeII, NiII • We do indeed see these! Polarized Fluorescence in RNe - AstroPol08

  7. Predicted Equivalent Widths • Model scattered line / visible continuum = Equivalent Width • Observed lines: get generally correct EW • one OI may be confused with FeII • MgII not seen. depletion? • Predict more lines for future NIR instrument • Predict more Vis lines: AlII, strong MgII Polarized Fluorescence in RNe - AstroPol08

  8. Expected Polarization Signals • Alignment polarization • for alignable ions, ground state aligned by pumping via all UV resonance transitions • changes pol and PA depending on scattering angle, 3Dmagnetic field orientation • recognizable signal is Uperp = p sin 2ΔPA. Use map of this to deduce mag field orientation • Reflection polarization • from unaligned ground state (“thermalized”) • like electron scattering (100% at 90°) times “polarizability” E1: depends only on J of levels • position angle perp or parallel to radius vector • pol depends only on scattering angle: deduce geometry Polarized Fluorescence in RNe - AstroPol08

  9. Diagnostic Diagram • Plot Realignment sensitivity vs polarizability to select lines to map • Geometry, calibration • Mg II 9246: E1 = 50%! Use to deduce scattering angle map • MgII 9221, Si 5981: E1 = 0. Use to measure foreground interstellar polarization • Magnetic field determination • OI 7997, Al 8643: Uperp => magnetic field map Polarized Fluorescence in RNe - AstroPol08

  10. NGC2023 Possible Hitch: Optical Depth • Effect of optical depth of "trapped" UV transitions (τ >> 1): • Fluorescence intensity keeps growing with τ • depolarization due to more isotropic diffuse radiation • Good news: many fluorescent excitors never trapped: τeff < 1/(escape prob): retain reflection polarization (small symbols) • Not as good: many alignable ions have trapped pumping lines: alignment polarization signal depolarized (large symbols) • Remedies: • look at thinner nebulae (signal still good) • look at FeII: not trapped Polarized Fluorescence in RNe - AstroPol08

  11. Orion Neb Hα IC2118 “Witch’s Head” Summary/ Future • Atomic Fluorescence lines seen for the first time in a Reflection nebula: NI, OI, SiII, FeII • Intensities consistent with simple model • Important to understand pumping in optically thick nebula • Expect linear polarization signal • Reflection polarization (polarizability) should be easily observable • Magnetic realignment polarization distortion observable with OI, AlII, but may be reduced by trapped pumping lines • Best bet: FeII • Future NGC2023 • Spectroscopy: look for MgII, AlII lines, • Slit spectropolarimetry: verify best polarized lines • Fabry-Perot spectropolarimetry: field map • Model with CLOUDY, do realignment calculations for FeII • Move on to thinner nebulae, HII regions... Polarized Fluorescence in RNe - AstroPol08

  12. Extra Slides Polarized Fluorescence in RNe - AstroPol08

  13. What Gas is This? • Line signal goes to zero 1’ from illuminating star HD37903 • peaks sharply in clump 30” N • Lies almost entirely inside H2 emission in well studied PhotoDissociation Region • New probe of this warm neutral material: previous absorption studies dominated by cold foreground material • velocity probe • temperature probe • magnetic field probe, too warm & thick for HI Zeeman Polarized Fluorescence in RNe - AstroPol08

  14. Model: Illumination • Fluorescence excited by FUV: 912 – 1200 Ang • Unextincted HD37903: use FUSE HD121300 • Assume physically thin shell, standard dust/ gas, standard extinction, variable AV • H2 absorption important: allow variable H2/H Polarized Fluorescence in RNe - AstroPol08

  15. NI, AlII Polarized Fluorescence in RNe - AstroPol08

  16. MgII, Si II Polarized Fluorescence in RNe - AstroPol08

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