July 8, 2008Polarized Fluorescence in RNe - AstroPol081 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

July 8, 2008Polarized Fluorescence in RNe - AstroPol082 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 > μGauss: 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 ) 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

July 8, 2008Polarized Fluorescence in RNe - AstroPol083 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 θ 1 and θ 2 Ori through "Orion Veil" θ2θ2 θ1θ1

July 8, 2008Polarized Fluorescence in RNe - AstroPol084 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!

July 8, 2008Polarized Fluorescence in RNe - AstroPol085 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 = –~1000 Ang coverage blue, yellow, red Many fluorescence lines found! Inner 2’ of nebula, within PDR: NIOISiIITiIICrIIFeII? 10531? 176

July 8, 2008Polarized Fluorescence in RNe - AstroPol086 Predicted Ions Which Ions? – Neutral medium: principle ion with IP < 13.6 eV –Abundance/H > –1 st Resonance < 13.6 eV 8 with LS Coupling (primary, secondary, alignable): AtomIonResVis1Vis2 CII200 NI4018 OI>1211>2 MgII7104 AlII220 SiII1645 PII1500 SII500 6 with Non-LS coupling: ArI, TiII, CrII, MnII, FeII, NiII We do indeed see these!

July 8, 2008Polarized Fluorescence in RNe - AstroPol087 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

July 8, 2008Polarized Fluorescence in RNe - AstroPol088 Expected Polarization Signals Reflection polarization –from unaligned ground state (“thermalized”) –like electron scattering (100% at 90°) times “polarizability” E 1 : depends only on J of levels –position angle perp or parallel to radius vector –pol depends only on scattering angle: deduce geometry Alignment polarization –for alignable ions, ground state aligned by pumping via all UV resonance transitions –changes pol and PA depending on scattering angle, 3D magnetic field orientation –recognizable signal is U perp = p sin 2ΔPA. Use map of this to deduce mag field orientation

July 8, 2008Polarized Fluorescence in RNe - AstroPol089 Diagnostic Diagram Plot Realignment sensitivity vs polarizability to select lines to map Geometry, calibration –Mg II 9246: E 1 = 50%! Use to deduce scattering angle map –MgII 9221, Si 5981: E 1 = 0. Use to measure foreground interstellar polarization Magnetic field determination –OI 7997, Al 8643: U perp => magnetic field map

July 8, 2008Polarized Fluorescence in RNe - AstroPol0810 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: τ eff < 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 NGC2023

July 8, 2008Polarized Fluorescence in RNe - AstroPol0811 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... Orion Neb Hα IC2118 “Witch’s Head”

July 8, 2008Polarized Fluorescence in RNe - AstroPol0812 Extra Slides

July 8, 2008Polarized Fluorescence in RNe - AstroPol0813 What Gas is This? Line signal goes to zero 1’ from illuminating star HD37903 peaks sharply in clump 30” N Lies almost entirely inside H 2 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

July 8, 2008Polarized Fluorescence in RNe - AstroPol0814 Model: Illumination Fluorescence excited by FUV: 912 – 1200 Ang Unextincted HD37903: use FUSE HD Assume physically thin shell, standard dust/ gas, standard extinction, variable A V H 2 absorption important: allow variable H 2 /H

July 8, 2008Polarized Fluorescence in RNe - AstroPol0815 NI, AlII

July 8, 2008Polarized Fluorescence in RNe - AstroPol0816 MgII, Si II