Interstellar dust

Interstellar dust:
The hidden protagonist
Stéphanie Cazaux
Marco Spaans
Vincent Cobut
Paola Caselli
Rowin Meijerink
15 th February 2012

Overview
Star form in clouds made of gas + dust
Catalyst: Enrich gas
H 2
,H 2
O, O 2
, H 2
O 2
Reservoir:
Stealing the gas
Formation of ices
Impact of dust on the gas? Impact of dust on star formation?

Dust as catalyst/ reservoir
H 2
in MW NOT explained by gas phase reactions : Grain Gould & Salpeter 1963
In the MW, H 2
formation dust >> gas
Reaction exothermic → products gas
H 2
(60-70 %)
Extinction Av ~3
Gas phase species → dust and make ices
Hollenbach et al. 2009 ApJ, 690, 1497

Dust as catalyst:
H 2
observations
Diffuse clouds
Planetary Nebulae
Dying stars
Supernovae remnants
T grain
= 40-80K
T gas
< 6000K
T grain
~ 20K
T gas
~ 100K
T grain
~ 125K
T gas
~ 1000K
Photodissociation Regions
T grain
=10 - 100K
T gas
= 300 - 1000K
PAHs
Newly born stars
T grain
~ 70K
T gas
= 1500 - 2000K
Active galactic nuclei
CO H 2
T grain
= 15- 70K
T gazs
~ 2000K
H 2
forms for a wide range of physical conditions

Dust as a reservoir:
interstellar ices
CO depleted from the gas
H 2
O ices absorption in embedded
protostars (VLT-ISAAC)

Dust as reservoir:
interstellar ices

Interstellar dust grains
Grain size distribution
PAHs
Amorphous carbon
Weingartner & Draine 2001
Mathis, Rumpl & Nordsieck 1977
DUST= Silicates, Amorphous carbon, PAHs
PAHs = 50% of surface available for chemistry

Formation of molecules on dust
1) Interactions atom/surface
Experiments: TPD
Ab-initio calculations
2) Mobility on the surface
Rate equations and Monte carlo simulations
Comparison with observations

Interaction atom/surface:
experiments
Experiments on graphite, amorphous carbon, silicates
Pirronello et al. 1997, 1999, Zecho et al. 2002, Perets et al. 2007, Vidali et al. 2007
En
Amorphous Carbon
Low temp., Tsurf = 5 K
192s
96s
48s
24s
Physisorption
Van der Waals
Chemisorption
covalent
Graphite
High temp., Tsurf = 150 K

Model:
Interaction and mobility
Energy
600K
Distance from the
surface
Transmission coefficient of the barriers
mobility of H and D atoms
Physisorption
Chemisorption
3Å
Physisorption + chemisorption
tunnel + thermal hopping

Model:
Interaction and mobility
physisorbed H atoms
physisorbed D atoms
chemisorbed H atoms
chemisorbed D atoms
H2
HD
D2
En
Gr
Si
AC
Mechanisms:
LH
ER
Physisorption
Van der Waals
Chemisorption

Rate equations and
Monte Carlo simulations.
Rate equations Monte Carlo
D
H
Follow populations
Big grains → always 1 species
Follow each species
small grains
random accretion and
random walk
detail characteristic of
the surface → para sites

Formation of H 2
on dust

Model:
Interaction and mobility
Energy
600K
Distance from the
surface
Transmission coefficient of the barriers
mobility of H and D atoms
Physisorption
Chemisorption
3Å
Physisorption + chemisorption
tunnel + thermal hopping

Interaction atom/surface:
Density functional theory (DFT)
Sha et al, Surface Science 496, 318 (2002)
Eva Rauls

Interaction atom/surface:
experiment
Energy
Distance from the surface
Physisorption
Van der Waals ≈ meV
Graphite:
Chemisorption of H
C puckered out of the basal plane
associated with barrier ~ 0.2 eV.
Jeloaica & Sidis 1999
Sha & Jackson 2002
Chemisorption
Covalent ≈ eV
Recent studies:
Hoernekær et al. 2006
Rougeau et al. 2006
Bachellerie et al. 2007
STM @ 170K
Hornekaer et al. 2006
3 rd atom → no barrier to form H 2
1 st H barrier
2 nd H → no barrier
to enter para site if
spin opposite to 1 st H

Interaction atom/surface:
experiment
● 1 atom sticks → dust becomes catalyst → H 2
formation barrier-less
Atoms get grouped as
Dimers (2 atoms)
Trimers (3)
Hexamers (6)
Binding energy increases with number of atoms

Results
Formation of H2 and HD
physisorbed atoms @ low T dust
chemisorbed atoms @ high T dust
.
Inclusion para sites
Increase the efficiency >1 mag
PAHs
HD
H2

H 2
formation rate in the ISM
R(H 2
)=(1/2) n H
v H
• n H
number density of H atoms
• v H
speed of H atoms in the gas phase
• σ area of the grain
• n d
number density of dust grain
n d
S H
• S H
sticking coefficient of the H atoms on the grain
• ε H 2
recombination efficiency
Photo-dissociation regions

H 2
formation rate:
Photo-dissociation Regions
ISOCAM MAP (in the LW2 filter)
Rotational transitions of H 2
and PAHs emission
ISO SWS
Rotational transitions of H 2
ISO LWS
ISOCAM- CVF
Spectro- imaging
Rotational transitions of H 2
Abergel et al. 1996
Habart et al. 2003
Gas temperature
Photodissocation of H 2
Formation rate of H 2
Grain temperature

H 2
formation rate:
Photo-dissociation Regions
Observations of several PDRs
(Abergel et al. 1996; Habart et al. 2003)
T dust
= 15 - 90K
T gas
= 60 - 620K
R(H 2
)= 3 10 -17 - 1.5 10 -16 cm 3 s -1
H 2
formation @ high Tdust and Tgas → para sites
properties
Other factors: R(H 2
)=(1/2) n H
v H
n d
S H
abundances PAHs/very small grains (AC)
(Compiegne et al. 2008, Joblin et al )
Gry et al. 2002
Habart et al. 2004

H 2
: Summary and Conclusions
H 2
formation wide range of physical conditions (Shocks, high UV,
low metallicity).
Understand the formation of H 2
on cold and warm dust grains →
2 interactions atom/surface: physisorption and chemisorption.
Mobility: tunnelling effects and thermal hopping
Observations of PDRs → efficiency of H 2
formation on warm
grains is important
The inclusions of the barrier-less route to form H 2
on PAHs →
necessary to reproduce the observations of PDRs.

Experiment
H 2
formed on PAHs
L. Boschman, T Schlathoelter, Kernfysisch Versneller Instituut (KVI) Groningen
+ helium cryostat
Fix temperature
Increase temp.
H
H

H 2
forms on PAHs??
For different H beam energy → H does or does not stick?
Model the amount of H on the PAHs surface with various
conditions
Efficiency of the formation of H2 on PAHs

More complex species
Star form in clouds made of gas + dust
Molecules: CO, O 2
, H 2
O cool the gas (Neufeld et al. 1995)
Impact of grain surface chemistry on
interstellar gas?
Formation and evolution of ices during SF

Grain surface chemistry:
Ingredients
Flux density Binding energies:
Van der Waals
Cuppen & Herbst 2007
Bergeron et al. 2008
Mobility:
Tunnel & thermal
Formation
→no barrier
→barrier overcome
Prob(dust)
Prob(gas)

Grain surface chemistry:
Monte carlo simulations
Atoms arrive randomly from gas phase
Flux=n X
v X
σ (s -1 )
On the grid
random walk
UV + CR
Evaporation
O
H
Grain surface = grid
Each point of the grid:
site atom/molecule
Formation of molecules
D
H 2
, HD, D 2
, OH, OD, O 2
, H 2
O, HDO, D 2
O, O 3
, HO 2
, DO 2
, H 2
O 2
, HDO 2
, D 2
O 2

Grain surface chemistry:
Monte carlo simulations
2 species in 1 site: probability react VS probability escape.
P(escape)
P(reac)
reaction
product directly released in the gas
H + O OH 36 %
OH + H H 2
O 15 %
H 2
+ O OH + H 4 %
H 2
+ OH H 2
O + H 0.8 %
O + O O2 36 %
O + O2 O3 0.2 %
Based experiments (H 2
) Pironello et al. 1997, 1999
P(free)
P(stay)
binding energy
enthalpy of reaction.

Grain surface chemistry:
Monte carlo simulations
Based on experiments H 2
: Pironello et al. 1997, 1999
P(free)
60-70%
On bare grains: reaction product released in gas.
Depend on: binding energy and enthalpy of reaction.
H + O OH 36 %
OH + H H 2
O 15 %
H 2
+ O OH + H 4 %
H 2
+ OH H 2
O + H 0.8 %
O + O O2 36 %
O + O2 O3 0.2 %
Empirical!!!
10%
AC
Si
30%
50%
70%
90%
H 2

Grain surface
Results
Gas phase
Test case:
Grain 10K
nH=10 3
D/H =0.1
O/H=0.1
P0
P1
O+ H OH+ H H 2
O
O+ H OH+ H H 2
O
h h
Grain 10K
Hydrogenated species
UV boosts water desorption

Grain surface
Results
Gas phase
Test case:
Grain 30K
nH=10 3
D/H =0.1
O/H=0.1
P0
P1
O+ H OH+ H H 2
O
O+ H OH+ H H 2
O
h h
Grain 30K
Species rich in oxygen
UV boosts H 2
O desorption

Results
Grains 10 K favours hydrogenation
Warmer grains (30 K) favours oxygenation
UV photons dissociate species that recombine. “dissociationformation-dissociation”
boost gas phase.
P free
Etc.
Species released in gas photo-dissociated.
Boost VS photo-dissociation?

Diffuse clouds
Diffuse clouds: H atomic
T dust
=18K, T gas
=100K
nH=100 cm -3, O/H =3 10 -4 , D/H=2 10 -5
Grain surface
0.36 0.09
O+ H → OH+ H → H 2
O
h
O+ H ← OH+ H ← H 2
O
Gas phase
h
95 % confidence level

Photo-dissociation regions
PDR: H molecular
T dust
=30K, T gas
=30K, G 0
=10 3 , Av=5 H 2
O forms with O 2
and O 3
nH=1000 cm -3 , O/H =3 10 -4 , D/H=2 10 -5
Grain surface
Gas phase
95 % confidence level

UV and X-rays star forming regions
Makarian 231:
Ultraluminous infrared galaxy (ULIRGS)
Hosting accreting Black hole
Very important amount of warm water
N(H2O)=5*10 17 cm -2
Spire/Hershel
González-Alfonso et al. 2010, A&A, 518, L43
van der Werf et al. 2010, A&A, 518, L42

Impact of dust on gas:
Xray dominated regions
MRN dust
Mathis, Rumpl,& Nordsieck 1977
↑ amount of warm H 2
O
~1 order of magnitude
WD dust
Weingartner & Draine 2001
↑ H 2
= ↑ OH and H 2
O
Meijerink, R., Cazaux, S., & Spaans, M. 2012, A&A, 537, A102

Dust grains as catalyst
Formation species on dust → impact gas phase
Temperature dust → which chemistry (H or O)
Reactions with ≠ exothermicity: ≠ impact on gas
Chemistry gas + dust → enhancement of water (XDRs)
Uncertainties: fraction of new species formed on dust → gas

Impact of dust on gas:
uncertainties
Leaving the grain upon formation
Enthalpy of reaction
Binding energy
P(free)
What do we know:
P(stay)
Experimentally: H2 desorbs upon formation for 60-70 % on
Amorphous carbon/silicates grains
Dynamic calculations: H2 and OH 100% (but OH stays longer before
leaving the dust)

Impact of dust on gas:
experiments
Existing setup + minor modifications
O 2
+ H
+ helium cryostat
Fix temperature
Increase temp.
H 2
+ O 2
H 2
+ O
H 2
O + H
O 3
+ H
enthalpy
H
O 2
Determine a trend between P(free) and enthalpy/binding energy.

Summary and Conclusions
Very first stages of star formation: atomic / molecular
H2 forms onto dust.
Efficiency depends on sticking??
still unknown? Efficiency on PAHs??
Water forms on dust through different routes
Reactions involve different routes with exothermicity: impact on gas
UV photo-dissociate species that reform enhance fraction released in the gas.
Chemistry gas + dust during the collapse of a cloud
Interstellar gas
thermal balance
star formation efficiency

Dust in Flash
Flash + cooling at different Z ⊙
(from PDR)
Fragmentation of Molecular cloud (Hocuk & Spaans 2010).
Include fully dust impact → Z ⊙
, gas composition &
depletion (gas + dust)
Impact of dust on fragmentation/ star formation
efficiency and IMF.

Neufeld et al. 1995