photodissociation regions amiel sternberg school of physics & astronomy tel aviv university...
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Photodissociation Regions
Amiel SternbergSchool of Physics & AstronomyTel Aviv UniversityISRAEL
Dense ISM in GalaxiesZermatt Switzerland22-26 September 2003
Operational definition: Neutral interstellar hydrogen gas where (6-13.6 eV) FUV radiation dominates the physical and chemical structure. Hence also called “photon-dominated regions” (PDRs).
Molecular photodissociation a key process.
Interstellar PDRs include:
The diffuse WNM and CNM clouds.
Translucent clouds: AV < 5 nH < 1000 cm-3 weak interstellar FUV fields.
Dense molecular clouds: AV up to ~10 nH > 1000 cm-3 including intense FUV fields near OB stars
~90% of the Galactic molecular ISM may be “photon-dominated”.
Dense PDRs:
Dense PDRs exposed to intense FUV fields in star-forming molecular cloudsare particularly important as sources of luminous fine-structure and molecularline (cooling) radiation and far-IR thermal dust continuum emission.
vib
rotH2
Clumpy, time-dependent, PDRs:
Hubble Space Telescope NIRIM camera @ WIYN 3.5m
Speck et al. 2003 PASP 115, 170
Molecular Hydrogen emission in the Ring Nebula.
1-0 S(1) 2.12 m
Basic structure: 1-D steady-state model, escape probability method.
FUV6-13.6 eV
H2 self-shielding and dust attenuationvs. grain surface formation.
Controlling parameters:
1) cloud density/pressure2) FUV intensity3) grain scattering properties4) H2 formation rate coefficient5) geometry/clumpiness6) gas phase abundances7) magnetic field...
Local Far-Ultraviolet (FUV) Radiation Field (6 - 13.6 eV 912 - 2070 Å):
“Draine Field” (1978). Empirically basedfor solar neighborhood.
Mean energy densities:
Draine / Habing = 1.7
Paravanno, Hollenbach & McKee 2003 ApJ 584, 797
Abinitio computations of the local FUV field, based on the Galactic birth-rate of OB associations.
monte-carlo Key Results
1) At any point, dust opacity limits contributing associations to within 500 pc.
1) Large angle scattering produces a diffuse field at the 10% level.
1) Time-dependent due to statistical fluctuations in the OB-association formation rates.
1) Median predicted value within 6% of the Draine field!
Heating Mechanisms:
Photoelectric heatingbeginning with Spitzer 1948 ApJ, 107, 6
FUV-pumping of H2 in dense PDRs.
grain charging
net
photo
ele
ctro
n e
nerg
y
Draine & Weingartner 2001 ApJS, 234, 162
Grain photoelectric heating efficiency:
Heating dominated by smallestgrains/PAHs.
Half the heating from grainswith radii a < 15 Å.
The rest from grains with15 Å < a < 100 Å
for a dn = a -3.5 da(MRN) grain size distribution.
Bakes & Tielens 1994
Different assumedelectron-grain stickingcoefficients.
CII 158 m fine-structure line cooling:
Expected theoretically - e.g. Dalgarno & McCray 1972.
First far-IR (Lear jet) detections byRussell et al. 1980in NGC 2024 and Orion.
Widely detected since withKAO, ISO...
Tielens & Hollenbach 1985 ApJ 291 722
CNM
WNM
Wolfire et al. 2003
densePDRs
Accounting for CII 158 m OI 63 m and other “low-ionization” fine-structure emission lines observed in star-forming molecular clouds. Line strengths ~ 1% of IR continuum.
FUV (6-13.6 eV) heating via photoelectric emission from dust grains:
mean photon energy = 10 eVtypical grain work function = 6 eV (for neutral grains).photoelectric yield = 0.1
Thus,heating efficiency = (4/10) x 0.1 = 0.04 (smaller for positively charged grains.)
Low-ionization fine-structure emission lines in star-forming molecular clouds:
FUV intensity
line inte
nsi
ty (
erg
cm
-2 s
-1 s
r-1)
line inte
nsi
ty (
erg
cm
-2 s
-1 s
r-1)
[CII] 158m
[OI] 63m
Fine-Structure Line Emission:
Hollenbach & Tielens 1997 ARAA, 35, 179Hollenbach & Tielens 1999 Rev. Mod. Phys., 71, 173
FUV
inte
nsi
ty
log hydrogen density (cm-3)
[OI] 63 m / [CII] 158 m
Kaufman et al. 1999 ApJ, 527, 795
[OI] 63m can become optically thick.
Gas-Phase Chemistry:
Sternberg & Dalgarno 1995 ApJS 99, 565 Jansen et al. 1995
Gas-Phase Production of Hydroxyl and Water:
Ion-molecule chemistry in cold gas
OH H2O
ee
H3O+H2O+OH+
H3+
H2 H2
O
H2 H2
Neutral-neutral chemistry in warm gasregime ofion-moleculechemistry
regime ofneutral-neutralchemistry
Neufeld & Kaufmann 1995
OH and H2O in PDRs:
OH H2O
ee
H3O+H2O+OH+
H3+
H2 H2
O
H2 H2
Removal by photodissociation.
OH/H2O remains large in warm PDR gas.
OH
C+
CO+ HCO+
H2
Si+SiO+
S+
SO+
O
O2
N
NO
OH driven chemistry in warm H/H2 transition zone:
T = 300 – 1000 K
Prediction:
CO+/HCO+ large in PDR CO+/HCO+ small in dark core
For example, CO+ / HCO+
At interface:G0 = 2.4 x 103
n = 104 cm-3
Plateau de Bure & 30m
CO+/HCO+
MolecularPeak < 0.001
PDR Peak ~ 0.1
NGC 7023 (reflection nebula) Fuente et al. 2003, AA, 899, 913
Molecular Peak
PDR Peak
illuminating star(Herbig B3Ve)
HCO+
CO+
HCO+
CO+
Neutral Atomic Carbon:
C+/C/COconversion S + C+ S+ + C
Kamegai et al. 2003 ApJ 589, 378L1688 in Oph Mt Fuji submm-telescope 1.2m
[CI] 609 m fine-structure line emission in Oph:
Extended CI may be due:
1) Scattered FUV in a clumpy medium.
2) Possible time-dependent effects. Recombining C+ in shadowed PDR clumps where cooling-time is longer than C formation time.
Stoerzer, Stutzki & Sternberg 1997
Polycyclic Aromatic Hydrocarbons (PAHs):
NGC 7023 (reflection nebula)Optical “internal conversion” IR
PAHs in PDRs:
Bakes & Tielens 1998 ApJ 499, 258 PAH+ PAH PAH-
dashed – without PAHssolid – with PAHs (2 x 10-7)
mutual neutralization
X+ + PAH- X + PAH
competes with or dominatesradiative recombination
X+ + e X + h
Lepp & Dalgarno 1988, ApJ, 335, 769
Extragalactic PDRs: NGC 4038/4039 interacting galaxies CII emission from NGC 4038/4039 Nikola et al. 1998 ApJ 504, 749 (KAO)
Dense PDRs dominate:
G0 = 500nH = 105 cm-3
(CNM a small fraction.)
CII line deficit in Ultra-Luminous IR Galaxies (ULIRGs):
ISO-LWSULIRG sample.
LIR > 1012 L
Luhman et al. 2003 astro-ph 0305520Possible Explanations
1) G0 / n >> 1 cm3 in ULIRGs inefficient photoelectric gas heating.
1) Underabundance of OB stars (unlikely).
1) Absorption of UV photons in dusty HII regions. Requires high ionization- parameters (U dust).
Is this consistent with nebular emission-line excitation? TBD
(basic assumption: FIR is reradiated FUV energy.)
The interacting galaxiesNGC 4038/4039
Infrared Space Observatory
Keck
H2 lines
UV-pumped Molecular Hydrogen in the Antennae:
Mid-IR Cluster Gilbert et al. 2000, ApJ, 533, L57
FUV Pumping and Photodissociation of Molecular Hydrogen:
Near IR
FUV
Pumping
Dissociation
Effe
ctiv
e P
ote
ntia
l En
erg
y
v=0
12
3
Internuclear Separation
En
erg
y
Black & Dalgarno 1976Black & van Dishoeck 1987Sternberg 1988Sternberg & Dalgarno 1989Draine & Bertoldi 1996Sternberg & Neufeld 1999
H2 level populations in the mid-IR cluster: Indicating radiative excitation in PDRs.
v = 1 v = 2 v = 3
Tvib = 6000 K non-thermal
ortho
para
o/p ratio forexcited states 1.8
a signature of warmT > 300 K gas !
Curve of growth for absorption lines
log column density in ground state
log
eq
uiv
ale
nt w
idth
LINEAR
FLAT
SQUARE ROOT
thin ------------- thick -------------------- saturated
log
nu
mbe
r o
f pho
tons
ab
sorb
ed
Dopplercore
radiativedampingwings
The FUV H2 absorption lines are highly saturated.Therefore,
Sternberg & Neufeld 1999 ApJ, 516, 371
H2 ortho-to-para ratio:
Magnetically Regulated Star-Formation:
Ionization Structure:
Cosmic-rayionized core.
Rapid magneticflux loss.
Magnetic field “frozen in”.
ambipolar diffusion time tAD = 1014 xe yr
Cosmic ray
core
PDR
Photoionization Regulated Star-Formation:
McKee 1989 ApJ 345, 782
starformati
on
sterile envelope
Cosmic ray cores collapse via ambipolar diffusion
increased star-formation rate
Energy injection via protostellar outflows,cloud expansion, cosmic ray ionized massfraction decreases.
Photoionization regulated star-formation:
Reduced star-formation rate, cloudcontraction, enhanced shielding
Predicts:
1) Galactic star-formation rate of 4 M yr-1
2) Star-formation is restricted to regions of relatively high extinction, AV > 4.
(consistent with observations)
A golden future!
Space Infrared Telescope Facility (SIRTF)
Stratospheric Observatory for Infrared Astronomy
Atacama Large Millimeter Array (ALMA)
Herschel Space Observatory (submillimeter)