Chemical models of star forming regions

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Chemical models of star forming regions
Pascal Stäuber, Arnold Benz, Institute of
Astronomy, ETH Zürich Ewine van Dishoeck,
Sterrewacht Leiden (NL) Steven Doty, Denison
University (USA) Jes Jørgensen, CfA Harvard (USA)
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Outline

• Why astrochemistry?
• Introduction Chemistry in star forming regions
• Influence of X-rays on the gas-phase chemistry
• Water destruction by X-rays in star-forming
  environments
• Tracing high-energy radiation with molecular lines

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Why astrochemistry?

• Only probe of earliest stages in star-formation
  through observations of molecular lines and dust
  continuum at infrared and millimeter wavelengths
• Probe of physical processes and conditions
  Temperature, density, mass (incl. inflow and
  outflow rates), ionization rate (cosmic rays,
  X-rays, FUV radiation), velocity
• Unique laboratory

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Introduction
van Dishoeck Hogerheijde 1999
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Introduction
What is the chemical budget acquired during the
protostellar phase and inherited by the forming
planets?
Similarities found between comets (e.g.,
Hale-Bopp) and interstellar ices (e.g.,
Ehrenfreund et al. 1997)
van Dishoeck Hogerheijde 1999
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The 129 reported interstellar and circumstellar
molecules

• Molecules with Two Atoms
• AlF AlCl C2 CH CH CN CO CO CP CS CSi HCl H2 KCl
  NH NO NS NaCl OH PN SO S0 SiN SiO SiS HF SH
  FeO(?) 
• Molecules with Three Atoms
• C3 C2H C20 C2S CH2 HCN HCO HCO HCS HOC H20 H2S
  HNC HNO MgCN MgNC N2H N20 NaCN OCS S02 c-SiC2
  CO2 NH2 H3 AlNC
• Molecules with Four Atoms
• c-C3H l-C3H C3N C30 C3S C2H2 CH2D? HCCN HCNH
  HNCO HNCS HOCO H2CO H2CN H2CS H30 NH3 SiC3
• Molecules with Five Atoms
• C5 C4H C4Si l-C3H2 c-C3H2 CH2CN CH4 HC3N HC2NC
  HCOOH H2CHN H2C20 H2NCN HNC3 SiH4 H2COH
• Molecules with Six Atoms
• C5H C50 C2H4 CH3CN CH3NC CH30H CH3SH HC3NH
  HC2CHO HCONH2 l-H2C4 C5N
• Molecules with Seven Atoms
• C6H CH2CHCN CH3C2H HC5N HCOCH3 NH2CH3
  c-C2H4O CH2CHOH 
• Molecules with Eight Atoms

National Radio Astronomy Observatory Nov. 2005
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Basic (gas-phase) molecular processes
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Solid phase processes

• Grain surface acts as catalyst for
  neutral-neutral reactions
• Hydrogenation H ? H2
• O ? H2O
• S ? H2S
• C ? CH4 , CH3OH
• N ? NH3
• Oxygenation CO2 , O2 , O3

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Basic (gas-phase) molecular processes
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Basic (gas-phase) molecular processes Cooling
Maloney et al. 1996
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Chemical models

• Start with a physical model, initial abundances
• (H2 1, CO 10-4, H2O 10-7-10-4, S
  10-8-10-6, N 10-5-10-4, metals 10-8)
• Time-dependent vs. steady state (chemical
  equilibrium 105-107 yrs)
• Solving rate equations for several 100 species
  and several 1000 reactions
• Comparison to observations (radiative transfer
  modeling)

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Constraining the physical and chemical structure
Doty et al. 2004
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X-ray chemistry Motivation

• Young stellar objects are strong X-ray emitters
  (up to 1000-10000 x higher in luminosity than
  the sun)
• (Typical luminosities LX1030-1031 ergs s-1)
• Onset of X-rays is not well known ? Do even the
  youngest sources (Class 0 objects) emit
  X-rays?
• Study influence on envelope and early disk
  chemistry
• Study fraction of ionization through chemistry

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X-ray chemistry

• Assume thermal X-ray spectrum
• Direct X-ray ionization and dissociation
• Secondary electron ionization and dissociation
  (1keV e leads to 27 ionizations)
• Photodissociation and ionization through
  electronically excited H2 (LyW bands)
• Charge transfer of doubly ionized species

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Photoionization cross section
Stäuber et al. 2005
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Photoionization cross section
X-rays can penetrate deep into the cloud
Stäuber et al. 2005
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Model for AFGL 2591
Input Distance 1kpc Lbol 2x104 Lsun Menv 44
Msun power-law index p (r-p) 1.0 van der Tak et
al. 1999
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X-ray chemistry Results for AFGL 2591
Stäuber et al. 2005
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X-ray chemistry Results for AFGL 2591
Water is destroyed
Stäuber et al. 2005
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X-ray chemistry Conclusion

• X-rays can influence the chemistry of the
  envelope even at large distances from the central
  source
• X-rays dominate the ionization rate for the inner
  part of the envelope
• X-ray ionization rate is dominated by secondary
  H2 ionization rate
• Many species are enhanced (e.g., HCO , SH) ?
  X-ray tracers

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Water destruction by X-rays Chemical models

• Study influence of X-rays on molecular species
  independent on physical (geometrical) structure
• Abundance study as a function of temperature,
• density and X-ray flux (erg s-1 cm-2)

Cover regions from envelopes, outflow hot-spots,
protoplanetary disk atmospheres (nH 104-109
cm-3 , T 10-1000 K)
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Water destruction by X-rays Chemical model
results
Stäuber et al. 2006
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Results Summary for H2O

• Water is destroyed within 5x104 yrs even for
  low X-ray fluxes for T lt 230 K
• Higher X-ray fluxes need less time
  (few 103 yrs)
• Water is mainly destroyed by internally produced
  FUV photons and in reactions with H3 and HCO
• For T gt 230 K OHH2?H2OH is very
  efficient, water can persist or even be enhanced

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Tracing X-rays in very young objects

• Young objects (Class 0) are deeply embedded in
  their natal molecular cloud
• ? X-rays absorbed, not directly observable
• Tracers for X-rays CN, HCO, CO (among others)
• Observations of molecular tracers with the James
  Clerk Maxwell Telescope on Mauna Kea
• (15m dish, 315-370 GHz)

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Tracing X-rays in very young objects
x(CO) 1 x 10-12
x(CO) 7 x 10-12
Stäuber et al. in prep.
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Tracing X-rays in very young objects
Abundances indicate X-ray emission with LX
1031 erg s-1
Stäuber et al. in prep.
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Tracing X-rays (?) in very young objects
Abundance ratios indicate X-ray emission with LX
1030 erg s-1
Stäuber et al. in prep.
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Conclusion Outlook

• X-ray models suggest a peculiar chemistry with
  enhanced abundances of ions and radicals
• Observations of ions and radicals suggest that
  very young objects emit X-rays
• Outlook
• ? Observations of molecular tracers with Herschel
  Space Observatory and ALMA (The Atacama Large
  Millimeter Array)
• ? Model influence of X-rays and chemical
  evolution of early disks

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THANK YOU
Literature Caselli 2005 (Chemical processes in
star forming regions) Feigelson Montmerle
1999 (High-energy processes in young stellar
objects) Hogerheijde 2004 (Chemical evolution of
protostars) van Dishoeck Hogerheijde
1999 (Models and observations of the
chemistry near young stellar objects)
A L M A