what can emission lines tell us? lecture 1 grażyna stasińska
TRANSCRIPT
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What can emission lines tell us?
lecture 1
Grażyna Stasińska
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What can emission lines tell us?
lecture 1
Grażyna Stasińska
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some basic literature dealing with the ionized ISM
books•Physical Processes in the Interstellar Medium, Spitzer, 1978 •Astrophysics of Gaseous Nebulae and Active Galactic Nuclei, Second Edition,
Osterbrock & Ferland, 2005
•Astrophysics of the Diffuse Universe, Dopita & Sutherland, 2003
lectures
•Stasinska 2002 astro-ph/0207500
« Abundance determinations in HII regions and planetary nebulae »
•Stasinska 2007 astro-ph « What can emission lines tell us? »
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What can emission lines tell us?
The mere presence of emission lines indicates
• the existence of gas
• eg emission line galaxies contain gas in large amount while galaxies emitting only a continuum with absorption features (such as elliptical galaxies) do not
• the existence of an ionizing agent (most emission lines come from ionized species)
• hot star(s)
• active nucleus
• (shocks) …
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a gallery of nebular spectra
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a g the galaxy
Te diagnostic
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IFU data for the most metal-poor HII galaxy I Zw 18
Kehrig et al 2006
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[OIII] image H image
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deep UV-FIR spectrum of the high excitation planetary nebula NGC 7027
Zhang et al 2005
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observed
dereddened
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high resolution spectrum of the PN NGC 6153 showing many recombination lines
Liu et al 2000
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SPITZER IRS spectrum of the PN SMP83 in the LMCBernard-Salas et al 2004
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XMM spectrum of the corona of α Cen Liefke & Schmitt 2006
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line displacements tell about radial velocitiesand allow one to measure
• redshifts of galaxies
• dark matter mass in galaxies using PNe as test particles (eg Romanowsky et al 2003)
• internal motions in zones of line emission (eg line broadening in AGNs)
• expansion velocities
high resolution multislit spectroscopy of the PN NGC 7009 in the [NeIII] line showing expansion of the envelope
Wilson 1958
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Basic mechanisms in ionized nebulae and emission line production
• ionisation and recombination processes• heating and cooling processes• line production mechanisms• about radiation transfer in nebulae• equilibrium versus out of equilibrium• the nebular physicist’s compendium
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ionisation and recombination processes
ionization• Photoionization
• Collisions
• Charge exchange
Recombination• Radiative recombination
• Dielectronic recombination
• Charge exchange
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heating and cooling processes
Heating
• Photoionization
• Collisional ionization
cooling• Free-free radiation
• Free-bound radiation
• Bound-bound radiation
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what determines the ionic fractions and the temperature?
Ionization
Ionic fractions
Recombination
Heating
Electron temperature
Cooling
• Photoionization• Collisions• Charge exchange
• Radiative recombination• Dielectronic recombination• Charge exchange
• Photoionization (mainly H and He)• Collisional ionization
• Free-free radiation• Free-bound radiation• Bound-bound radiation (mainly O)
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line production mechanisms
Recombination followed by cascade
(these lines are named with the recombined ion)
• H lines: Balmer …, Paschen etc …• He I lines (He I 5876…)• He II lines (He II 4686…)
Collisional excitation followed by radiative deexcitation
• Forbidden lines: [OIII]5007, [NII]6584• Semi-forbidden lines : CIII]1907 …• Resonance lines: CIV 1550, NV 1240, OVI 1035, SiIV 1400
Photoexcitation and fluorescence• Bowen lines: OIII 3133, 3444 (Bowen 1934)• Fe K line (probe of astrophysical black holes Fabian et al 2000)
notes
• Each line can be produced by several processes, but usually only one dominates• H Ly is produced both by recombination and by collisional excitation
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about radiation transfer in ionized nebulae
• Lyman continuum photons from the ionizing source
• ionizing photons produced by the nebula
• non-ionizing photons
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Lyman continuum photons from the ionizing source
• They suffer geometrical dilution away from the source
• They suffer line-of-sight absorption (main absorbers H and He)
• The first ones to be absorbed are the ones with energies close to the ionization threshold (-3)
• => hardening of the ionizing radiation field in external zones
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ionizing photons produced by the nebula
• those photons are emitted in all directions
The “on the spot approximation”
• assumes that all ground-state recombination photons are reabsorbed OTS• is justified for analytical order of magnitude estimations• but computed Te is incorrect by ~1000K-2000K (Gruenwald et al’s 3D code)
The “outward only approximation”• Radial outward only (Ferland’s Cloudy)• Full outward only (Stasinska’s PHOTO)
“complete” treatment• Traditional iterative way (Harrington, Rubin)• Using Monte-Carlo transfer (Ercolano’s Mocassin)
• resonance iine radiation is locally scattered many times• Can be treated with a “quasi on the spot” approximation (Ferland’s Cloudy)• Can be treated with an “local escape probability” approximation • Can be treated “exactly” (Dumont’s Titan)
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non-ionizing photons (including lines emitted by the nebula)
In general • they escape freely (the optical thickness of the nebulae is small enough)• They can be attenuated by dust absorption
Exceptions• resonance lines
• which suffer scattering (eg H Ly ) and may be selectively destroyed by dust• FIR lines
• can suffer self-absorption as abs is larger than for optical lines (Rubin 1968)• but turbulent/expansion velocities favour their escape (Abel et al 2003)
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equilibrium versus out of equilibrium
typical timescales for nebulae with n=103cm-3
Recombination time• trec= 1/(ne) 100 yr
Cooling time• tcool= (nkTe)/50 yr
Dynamical time• tdyn=R/vexp 2x104 yr for a single star HII region
Stellar evolution time • t* 5x106 yr for HII regions (=tMS)
1x104 yr for PNe (=tPAGB)
Most ionized nebulae are in ionization and thermal equilibrium
low density plasmas can be out of equilibrium
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The nebular physicist’s compendium
• The Stromgren sphere
• The H luminosity and surface brightness
• What drives the electron temperature?
• What determines the ionization structure of a nebula?
• Why is the gas temperature roughly uniform in photoionized nebulae?
• Comments on energy losses
• Other comments on the gas temperature
• Comments on line intensities
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The nebular physicist’s compendium
• The Stromgren sphere
• The H luminosity and surface brightness
• What drives the electron temperature?
• What determines the ionization structure of a nebula?
• Why is the gas temperature roughly uniform in photoionized nebulae?
• Comments on energy losses
• Other comments on the gas temperature
• Comments on line intensities
![Page 25: What can emission lines tell us? lecture 1 Grażyna Stasińska](https://reader035.vdocuments.net/reader035/viewer/2022070306/5518bce5550346b31f8b53cb/html5/thumbnails/25.jpg)
The nebular physicist’s compendium
• The Stromgren sphere
• The H luminosity and surface brightness
• What drives the electron temperature?
• What determines the ionization structure of a nebula?
• Why is the gas temperature roughly uniform in photoionized nebulae?
• Comments on energy losses
• Other comments on the gas temperature
• Comments on line intensities
![Page 26: What can emission lines tell us? lecture 1 Grażyna Stasińska](https://reader035.vdocuments.net/reader035/viewer/2022070306/5518bce5550346b31f8b53cb/html5/thumbnails/26.jpg)
QH [ph/sec] M*[M] Mion [M] Mion [M]
(n=102) (n=104)
PN 3x1047 .6 15 .15
O7 star 3x1048 30 150 1.5
starburst 3x1050 104 15000 150
the Strömgren radius
• In a homogeneous medium of density n and filling factor , the radius RS up to which gas is fully ionized is obtained from
QH = 4/3 RS3 n2 B(H) => RS [cm] = 9720 (QHn-2-1)1/3
• nb: The transition region between ionized and neutral gas is usually small:
maximum nebular mass that can be ionized• Mion= 4/3 RS
3 n mH => Mion [M] = 5 x 10-45 QH n-1
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The nebular physicist’s compendium
• The Stromgren sphere
• The H luminosity and surface brightness
• What drives the electron temperature?
• What determines the ionization structure of a nebula?
• Why is the gas temperature roughly uniform in photoionized nebulae?
• Comments on energy losses
• Other comments on the gas temperature
• Comments on line intensities
![Page 28: What can emission lines tell us? lecture 1 Grażyna Stasińska](https://reader035.vdocuments.net/reader035/viewer/2022070306/5518bce5550346b31f8b53cb/html5/thumbnails/28.jpg)
density bounded case:
LH = Mneb n (H)/ mH
LH is independent of QH
ionization bounded case:
LH = QH (H)/ B(H)
LH is a measure of QH
Hluminosity: LH = 4/3 R3 n2 (H)
H surface brightness: SH = LH / (4 R2)
ionization bounded case:
SH = A (QH n4 2)1/3
narrow slit spectra are of better quality for denser nebulae
density bounded case:
SH = B (Mneb n5 2)1/3
narrow slit spectra are of better quality for denser nebulae
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The nebular physicist’s compendium
• The Stromgren sphere
• The H luminosity and surface brightness
• What drives the electron temperature?
• What determines the ionization structure of a nebula?
• Why is the gas temperature roughly uniform in photoionized nebulae?
• Comments on energy losses
• Other comments on the gas temperature
• Comments on line intensities
![Page 30: What can emission lines tell us? lecture 1 Grażyna Stasińska](https://reader035.vdocuments.net/reader035/viewer/2022070306/5518bce5550346b31f8b53cb/html5/thumbnails/30.jpg)
• energy gains :
G = i, jni j i
j
where the i j are the gains per ion (photoionization and collisional ionization)
• energy losses :
L = i, jni j i
j
where the i j the losses per ion
(recombination and collisional excitation followed by photon emission)
• net energy gain :dE / dt = G - L
• If thermal equilibrium is achieved, the temperature is determined by: G = L
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The nebular physicist’s compendium
• The Stromgren sphere
• The H luminosity and surface brightness
• What drives the electron temperature?
• What determines the ionization structure of a nebula?
• Why is the gas temperature roughly uniform in photoionized nebulae?
• Comments on energy losses
• Other comments on the gas temperature
• Comments on line intensities
![Page 32: What can emission lines tell us? lecture 1 Grażyna Stasińska](https://reader035.vdocuments.net/reader035/viewer/2022070306/5518bce5550346b31f8b53cb/html5/thumbnails/32.jpg)
ionization equilibrium equation
between ni and ni+1, at a fraction f of the Stromgren radius RS
ni 4J i d= ni+1 nei
where the mean intensity of the radiation field in photons s-1
is
4J = QH g (T*) / r2
=> expression of the ionization state as a function of QH, n, f,T*:
ni+1 / ni = (QHn2)1/3 f-2 g(T*)
ionization parameter • definition: U = QH / (4R2 n c)
• quivalent expression:e U = A (QHn2)1/3
the ionization state is fully defined by the product QHn2 once T* is specified
the average ionization is higher for larger QH and larger n
in a given nebula, regions of higher n are more recombined
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The nebular physicist’s compendium
• The Stromgren sphere
• The H luminosity and surface brightness
• What drives the electron temperature?
• What determines the ionization structure of a nebula?
• Why is the gas temperature roughly uniform in photoionized nebulae?
• Comments on energy losses
• Other comments on the gas temperature
• Comments on line intensities
![Page 34: What can emission lines tell us? lecture 1 Grażyna Stasińska](https://reader035.vdocuments.net/reader035/viewer/2022070306/5518bce5550346b31f8b53cb/html5/thumbnails/34.jpg)
• Energy gained by photoionization of H at a distance r from the source
G = n(H°) 4J (h-h°) derg cm-3 s-1]
• Ionization equilibrium equation of H at distance r
n(H°) J d= n(H+) ne B(H)
• Substituting:
G = n(H+) ne B(H) < E >
with < E > = 4J (h-h°) d4J d
<E> is the mean energy gain per absorbed photon
<E> ≈ A T*
Energy gains due to photoionization of H are • independent of distance to the star• proportional to the star temperature
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The nebular physicist’s compendium
• The Stromgren sphere
• The H luminosity and surface brightness
• What drives the electron temperature?
• What determines the ionization structure of a nebula?
• Why is the gas temperature roughly uniform in photoionized nebulae?
• Comments on energy losses
• Other comments on the gas temperature
• Comments on line intensities
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• The most important cooling process is collisionally excited line radiation
• For a given ion in a two-level approximation, the cooling rate is given by
Lcoll = n2 A21 h21 erg cm-3 s-1]
where n2 results from the equilibrium equation of levels 1 and 2 :
n1 ne q12 = n2 (A21 + ne q21)
• In the limit of small ne one has
Lcoll = n1 ne q12 h21
where q12 is the collisional excitation rate
q12 = 8.629 10-6 (1,2) /1 Te-0.5 exp (-E12/kTe)
note for « normal abundances » • the most important cooling ion is O++
• H and He have too high excitation potentials to be excited at “normal temperatures”
Cooling by collisional line excitation is more important for• abundant ions • lines corresponding to large• levels that can be easily attained at the temperature of the medium
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The nebular physicist’s compendium
• The Stromgren sphere
• The H luminosity and surface brightness
• What drives the electron temperature?
• What determines the ionization structure of a nebula?
• Why is the gas temperature roughly uniform in photoionized nebulae?
• Comments on energy losses
• Other comments on the gas temperature
• Comments on line intensities
![Page 38: What can emission lines tell us? lecture 1 Grażyna Stasińska](https://reader035.vdocuments.net/reader035/viewer/2022070306/5518bce5550346b31f8b53cb/html5/thumbnails/38.jpg)
Spatial variations of Te
• are mostly determined by • the mean energy of the absorbed photons• the populations of the main cooling ions
• are generally small • except at high metallicities
• in the O++ zone cooling is very efficient through emission of [OIII]52, 88 lines which have very low excitation potentials
• in the O+ zone the cooling efficiency is smaller (O+ has no low-lying levels)
photoionization models showing
the effect of metallicity
Stasinska 1978- - - Z < Z __ Z > Z
General dependence of Te with the defining properties of the nebulae
• for a given T*, Te as Z
• for a given Z, Te as T*
• for given T* , ionization state and Z, Te if n above ncrit
Te
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The nebular physicist’s compendium
• The Stromgren sphere
• The H luminosity and surface brightness
• What drives the electron temperature?
• What determines the ionization structure of a nebula?
• Why is the gas temperature roughly uniform in photoionized nebulae?
• Comments on energy losses
• Other comments on the gas temperature
• Comments on line intensities
![Page 40: What can emission lines tell us? lecture 1 Grażyna Stasińska](https://reader035.vdocuments.net/reader035/viewer/2022070306/5518bce5550346b31f8b53cb/html5/thumbnails/40.jpg)
Temperature dependence of emission lines
Collisionally excited lines (CEL)
I CEL= n1(X) ne 8.629 10-6 (1,2) /1 Te-0.5 exp (-E12/kTe) h21
Recombination lines (RL)
I RL= n(X) ne Te- ( with ≈ 1)
Temperature dependence of line ratios
• Ratios RL / RL are almost independent of Te
• Ratios CEL(IR) / RL almost independent of Te
• Ratios CEL(opt or UV) / CEL(opt or UV) usually depend on Te
• Ratios CEL(opt) / RL strongly depend on Te
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temperature dependence of emission lines
The example of lines emitted by O++
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vthe Orion Nebulao’Dell http://vis.sdsc.edu/research/orion.html
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volume visualization of the Orion Nebulao’Dell http://vis.sdsc.edu/research/orion.html
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a new view of the Orion Nebulaimages resolved in velocity and ionization
Garcia-Diaz & Henney 2006
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