Chemical processes in highmass starforming regions

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Chemical processes in (high-mass) star-forming
regions

• Floris van der Tak
• MPIfR, Bonn

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Outline

• Basic molecular processes
• Models of star-forming regions
• Measuring molecular abundances
• Recent progress
• Future directions

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Molecular processes gas phase

• Ion-molecule reactions k 10-9 cm3 s-1
• e.g., CO (H3, H2) HCO
• Dissociative recombination k 10-6
• e.g., HCO (e-, H2) CO
• Neutral-neutral reactions k 10-11, EA 0.1 eV
• e.g., O (H2, H) OH (H2, H) H2O

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Molecular processes solid phase

• Accretion, not reaction limited
• outcome depends on atomic composition of
  gas phase
• create saturated species unlike
  ion-molecule reactions
• Low density n(H) gt n(O) hydrogenation
• e.g., CO ... H2CO ... CH3OH
• n gt 104 cm-3 n(O) gt n(H) oxygenation
• e.g., CO ... CO2
• Either case n(D) / n(H) gtgt D/H deuteration
• may also occur in gas phase

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Gas-grain interaction

• Freeze-out
• n gt 104 cm-3 or AV gt 3
• Thermal evaporation
• pure CO at T 20 K
• pure H2O at T 90 K
• Non-thermal evaporation
• mainly by cosmic-ray spot heating

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Shock chemistry

• v lt 40 km/s, xe lt 10-6 C-shock
• O into H2O, S into SO SO2
• v gt vA J-shock
• H2 dissociated reformed then as C-shock
• Grain-grain grain-H2 collisions
• erode mantles (CH3OH) and cores (SiO)

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Star-forming regions All of the above !

• Need multi-point radiative transfer (MC / ALI)
• Bonus can study chemical gradients

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For example OVRO BIMA maps of NGC 1333 IRAS 2A

• Jørgensen et al 2004 CS, SO, SiO CH3OH on N2H

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High-mass stars more heating more ionization
Detection of CO and SO toward AFGL 2591
Stäuber et al 2004
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Modern models chemistry dynamics
Collapsing envelope 31 shells, 207 species,
2337 reactions
Rodgers Charnley 2003
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Measuring molecular abundances

• Doty et al 2004 The modelers take revenge!

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High resolution infrared spectroscopyCH4
abundance variations towardNGC 7538 IRS9
P Cygni profiles shifted by 7 km/s Gas-phase CH4
/ H2 4 x 10-6 in inner envelope Outer envelope
CH4 frozen on grains Forms on grains, evaporates
near stars, destroyed in shocks Also possible
absorption by ethane, ethanol, and formaldehyde
ice (Boogert et al 2004a)
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Space-based infrared spectroscopySpitzer ice
inventory towardlow-mass protostars
High CO2 but low CH3OH abundance dense
pre-stellar core CO2 bending mode profile not
mixed with CH3OH CO and OCN- bands heating to
only 50 K (Boogert et al 2004b)
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New (sub)mm observations The 800 GHz survey of
Orion-KL
541 features from 929 transitions by 26
species Complex organic species in warm (250 K)
hot core component Possible detection of NH2
radical No sign of dust absorption non-spherical
geometry (?) (Comito et al 2005)
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Quantum chemical calculations flow tube
experiments no way to make methyl formate
Overall reaction downstream but significant
barriers half-way Reactions of normal
protonated H2CO CH3OH cornerstones of hot core
models ... (Horn et al 2004)
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Variations in the cosmic-ray ionization rate
Dense clouds 3 x 10-17 s-1 (van der Tak van
Dishoeck 2000) Diffuse clouds 10 x higher
(McCall et al 2003 Le Petit et al
2004) Pre-stellar cores 10 x lower (Caselli et
al 2002)
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Depletion Deuteration
Multi-deuterated ammonia methanol (Roueff Lis
vdTak Parise et al 2001-5) Highly abundant H2D
D2H (Caselli et al 2003 Vastel et al
2004) Strong link with CO depletion (Bacmann et
al 2002-3) Chemistry in completely depleted
limit (Walmsley, Flower PdF 2004-5) Much
stronger DD in pre-stellar than in protostellar
phase
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Future directions

• Submillimeter interferometry (ALMA)
• resolve abundance enhancements
• disk chemistry
• Terahertz spectroscopy (Herschel)
• inventory of gas phase
• hydrides other light species
• Mid-infrared spectra maps (JWST)
• complex ice species deuterated ices
• dominant N and S carriers
• Laboratory
• gas-phase reaction rates
• ice feature identification

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Relevant posters

• Benz et al (High-energy tracers)
• Codella et al (Shock chemistry in outflows)
• Pirronello et al (Grain surface chemistry)
• Rizzo et al (PDR chemistry)
• Wyrowski et al (Deuterium chemistry)

Further reading
http//astrochemistry.net http//rate99.co.uk http
//www.strw.leidenuniv.nl/moldata