Interstellar dust: The hidden protagonist. Stéphanie Cazaux Marco Spaans Vincent Cobut Paola Caselli Rowin Meijerink

Transcription

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

2 Overview Star form in clouds made of gas + dust Catalyst: Enrich gas H2,H2O, O2, H2O2 Reservoir: Stealing the gas Formation of ices Impact of dust on the gas? Impact of dust on star formation?

3 Dust as catalyst/ reservoir H2 in MW NOT explained by gas phase reactions : Grain Gould & Salpeter 1963 In the MW, H2 formation dust >> gas Reaction exothermic products gas H2 (60-70 %) Extinction Av ~3 Gas phase species dust and make ices Hollenbach et al ApJ, 690, 1497

4 Dust as catalyst: H2 observations Dying stars Diffuse clouds Planetary Nebulae Supernovae remnants Tgrain = 40-80K Tgas < 6000K Tgrain ~ 20K Tgas ~ 100K Photodissociation Regions Tgrain=10-100K Tgas = K Newly born stars Tgrain ~ 125K Tgas ~ 1000K Active galactic nuclei Tgrain ~ 70K Tgas = K PAHs CO H2 Tgrain = 15-70K Tgazs~ 2000K H2 forms for a wide range of physical conditions

5 Dust as a reservoir: interstellar ices CO depleted from the gas H2O ices absorption in embedded protostars (VLT-ISAAC)

6 Dust as reservoir: interstellar ices

7 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

8 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

9 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 En Amorphous Carbon Graphite Low temp., Tsurf = 5 K 192s 96s Physisorption Van der Waals 48s 24s Chemisorption covalent High temp., Tsurf = 150 K

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

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

12 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

13 Formation of H2 on dust

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

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

16 Interaction atom/surface: experiment Energy Graphite: Distance from the surfacechemisorption of H C puckered out of the basal plane associated with barrier ~ 0.2 ev. Physisorption Van der Waals mev Jeloaica & Sidis 1999 Sha & Jackson 2002 Chemisorption Covalent ev Recent studies: Hoernekær et al Rougeau et al Bachellerie et al st H barrier 2nd H no barrier to enter para site if spin opposite to 1st H 170K Hornekaer et al rd atom no barrier to form H2

17 Interaction atom/surface: experiment 1 atom sticks dust becomes catalyst H2 formation barrier-less Atoms get grouped as Dimers (2 atoms) Trimers (3) Hexamers (6) Binding energy increases with number of atoms

18 Results Formation of H2 and HD physisorbed low Tdust chemisorbed high Tdust. Inclusion para sites Increase the efficiency >1 mag H2 HD PAHs

19 H2 formation rate in the ISM R(H2)=(1/2) nh vh nd SH nh number density of H atoms vh speed of H atoms in the gas phase σ area of the grain nd number density of dust grain SH sticking coefficient of the H atoms on the grain ε H2 recombination efficiency Photo-dissociation regions

20 H2 formation rate: Photo-dissociation Regions ISOCAM MAP (in the LW2 filter) Rotational transitions of H2 and PAHs emission ISO SWS Rotational transitions of H2 ISO LWS ISOCAM- CVF Spectro- imaging Rotational transitions of H2 Gas temperature Photodissocation of H2 Formation rate of H2 Grain temperature Abergel et al Habart et al. 2003

21 H2 formation rate: Photo-dissociation Regions Observations of several PDRs (Abergel et al. 1996; Habart et al. 2003) Tdust= 15-90K Tgas= K R(H2)= cm3s-1 H2 high Tdust and Tgas para sites properties Other factors: R(H2)=(1/2) nh vh nd SH abundances PAHs/very small grains (AC) (Compiegne et al. 2008, Joblin et al ) Gry et al Habart et al. 2004

22 H2: Summary and Conclusions H2 formation wide range of physical conditions (Shocks, high UV, low metallicity). Understand the formation of H2 on cold and warm dust grains 2 interactions atom/surface: physisorption and chemisorption. Mobility: tunnelling effects and thermal hopping Observations of PDRs efficiency of H2 formation on warm grains is important The inclusions of the barrier-less route to form H2 on PAHs necessary to reproduce the observations of PDRs.

23 Experiment H2 formed on PAHs L. Boschman, T Schlathoelter, Kernfysisch Versneller Instituut (KVI) Groningen + helium cryostat Fix temperature H H Increase temp.

24 H2 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

25 More complex species Star form in clouds made of gas + dust Molecules: CO, O2, H2O cool the gas (Neufeld et al. 1995) Impact of grain surface chemistry on interstellar gas? Formation and evolution of ices during SF

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

27 Grain surface chemistry: Monte carlo simulations Atoms arrive randomly from gas phase Flux=nX vx σ (s-1) H O On the grid Grain surface = grid random walk Each point of the grid: UV + CR site atom/molecule Evaporation Formation of molecules D H2, HD, D2, OH, OD, O2, H2O, HDO, D2O, O3, HO2, DO2, H2O2, HDO2, D2O2

28 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 OH + H H2O H2 + O OH + H H2 + OH H2O + H O+O O2 O + O2 O3 36 % 15 % 4% 0.8 % 36 % 0.2 % Based experiments (H2) Pironello et al. 1997, 1999 P(free) P(stay) binding energy enthalpy of reaction.

29 Grain surface chemistry: Monte carlo simulations Based on experiments H2: 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 H2O 15 % H2 + O OH + H 4% H2 + OH H2O + H 0.8 % O+O O2 36 % O + O2 O3 0.2 % Empirical!!! 10% 30% 50% 70% 90% AC Si H2

30 Results Grain surface Gas phase Test case: Grain 10K nh=103 D/H =0.1 O/H=0.1 O+ H O+ H P0 h OH+ H P1 H2O OH+ H h H2O Grain 10K Hydrogenated species UV boosts water desorption

31 Results Grain surface Gas phase Test case: Grain 30K nh=103 D/H =0.1 O/H=0.1 O+ H O+ H P0 h OH+ H OH+ H P1 h H2O H2O Grain 30K Species rich in oxygen UV boosts H2O desorption

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

33 Diffuse clouds Diffuse clouds: H atomic Tdust=18K, Tgas=100K O+ H OH+ H H2O nh=100 cm-3, O/H =3 10-4, D/H= O+ H OH+ H H2O Grain surface h h Gas phase 95 % confidence level

34 Photo-dissociation regions PDR: H molecular Tdust=30K, Tgas=30K, G0=103, Av=5 nh=1000 cm-3, O/H =3 10-4, D/H= Grain surface H2O forms with O2 and O3 Gas phase 95 % confidence level

35 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*1017 cm-2 Spire/Hershel González-Alfonso et al. 2010, A&A, 518, L43 van der Werf et al. 2010, A&A, 518, L42

36 Impact of dust on gas: Xray dominated regions MRN dust Mathis, Rumpl,& Nordsieck 1977 amount of warm H2O ~1 order of magnitude WD dust Weingartner & Draine 2001 H2 = OH and H2O Meijerink, R., Cazaux, S., & Spaans, M. 2012, A&A, 537, A102

37 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

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

39 Impact of dust on gas: experiments Existing setup + minor modifications O2 + H H2 + O2 H2 + O H2O + H O3 + H + helium cryostat Fix temperature Increase temp. O2 enthalpy H Determine a trend between P(free) and enthalpy/binding energy.

40 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

41 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.

42 Neufeld et al. 1995