july 8, 2008polarized fluorescence in rne - astropol081 atomic fluorescence and prospects for...
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July 8, 2008 Polarized Fluorescence in RNe - AstroPol08 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
July 8, 2008 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 > 10-104 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 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
July 8, 2008 Polarized Fluorescence in RNe - AstroPol08 3
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" θ2
θ1
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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!
Pmax = 19.1%O I
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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:
NI OI SiII TiII CrII FeII ?
10 5 3 1? 1? 17 6
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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):
Atom Ion Res Vis1 Vis2
C II 2 0 0
N I 4 0 18
O I >12 11 >2
Mg II 7 10 4
Al II 2 2 0
Si II 16 4 5
P II 15 0 0
S II 5 0 0
• 6 with Non-LS coupling: ArI, TiII, CrII, MnII, FeII, NiII
• We do indeed see these!
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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
Primary Fluorescence
0.0
0.1
1
10
100
3000 5000 7000 9000 11000 13000 15000 17000
N I
O I
S i II
A l II
M g II
`
Pre
dic
ted
EW
(A
ng
)
R SS N IRR SS Vis
Secondary FluorescenceN I
O I
S i II
A l II
M g II
N G C 2023 F luorescence Predictions
0.01
0.1
1
10
0.001 0.01 0.1 1 10Predicted EW
Ob
serv
ed
EW
(A
ng
)
N I
O I
S i II
M g II
F illed - P rim aryO pen - Secondary
A (m ag)V 0.20H /H2
0.000b(km /s) 5.0log Ly -11.2
July 8, 2008 Polarized Fluorescence in RNe - AstroPol08 8
Expected Polarization Signals
• 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
PA magnified 30x Pmax = 18%Al II 8643
• 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 Uperp = p sin 2ΔPA. Use map of this to deduce mag field orientation
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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
O I (D ->)
M g II (3 /2->3/2)
A l II (->S)
S i II (3 /2->1/2)S i II (3 /2->3/2)
M g II (3 /2->1/2)M g II (3 /2->5/2)
A l II (->D )O I (S ->)
M g II (1 /2->)S i II (S ->)
-5
0
5
10
-60 -30 0 30 60Polarizability E (%)1
Alig
nm
ent
U (
%)
per
p
July 8, 2008 Polarized Fluorescence in RNe - AstroPol08 10
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
NGC2023
July 8, 2008 Polarized Fluorescence in RNe - AstroPol08 11
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”
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Extra Slides
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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
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H /H = 0.1A = 0.13
2
V
0
1
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
S i II
A l II
M g II
O I
N I
900 1000 1100 1200W avelength (Ang)
Flu
x
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NI, AlII
July 8, 2008 Polarized Fluorescence in RNe - AstroPol08 16
MgII, Si II