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)