observing organic molecules in interstellar gases: non equilibrium excitation. laurent wiesenfeld,...
TRANSCRIPT
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Observing organic molecules in interstellar gases: Non equilibrium excitation.
LAURENT WIESENFELD, ALEXANDRE FAURE, Grenoble, FranceANTHONY REMIJAN, National Radio Astronomy Observatory, Charlottesville, VA KRZYSZTOF SZALEWICZ, University of Delaware, Newark, DE.
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Observing molecules:a question of collisions
Interactions
Molecule M, with dipole moment µ diluted in a gas of H2 (H, He, e-)• Strong interactions with
photons:Thermal bath of photon• Tphoton = T internal (M)
• Strong interaction with H2
• Tkinetic (H2) = T internal (M)
How ?
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Level populationsLevels and Temperature; Equilibrium with the photon gas or the buffer gas
Out of equilibrium : no T. Competition between photon interactions and
collisions
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Energy
J=0
J=1
J=2
J=3 T
Population
P = exp(-E/kT)
no T
Population
Energy
Super cooling
ALM
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Level populationsLevels and Temperature; Equilibrium with the photon gas or the buffer gas
Out of equilibrium : no T. Competition between photon interactions and
collisions
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Energy
J=0
J=1
J=2
J=3 T
Population
P = exp(-E/kT)
no T
Population
Energy
Masing
How to compute the collision coefficients:
ISMS Champaign June 2014
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Potential energy surface (Force field, classically thinking)
Dynamical calculation
1. Computing interaction of molecule and H2/He : ab initio quantum chemistry. Potential at N points
2. Fitting N points onto one functional F(R, angles)
3. Performing quantum or classical scattering
An exemple: H2CO – H2
• PES computed at CCSD(T) – R12 level aug cc- pVTZ level.
• Show one van der Waals minimum.
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OBSERVATIONS
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Heavy molecules: a question of spectroscopy (1)
• Both H2CO and HCOOCH3 are prolate asymmetric tops.
• But : HCOOCH3 does not come into ortho-para / A and E forms nearly equivalent
• Observations: HCOOCH3
weakly masing (superradiant)
Brown et al ApJ 1975,Menten 2004, Churchwell et al ApJ 1980.
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A. Faure, A.J. Remijan, K. Szalewicz and LW;ApJ, 2014.
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Heavy molecules: a question of spectroscopy (2)
• Both H2CO and HCOOCH3 are prolate asymmetric tops.
• But : H2CO comes into ortho and para forms• Observations: H2 CO supercooling
Absorption against the cosmic microwave background Tinternal ≈ 1.9 K
(Townes & Cheung, ApJ 1976; Garrison et al., JCP 1976)
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110 111absorption at 4.82966 GHz
N. Troscompt et al, A&A 2009.LW, A Faure, MNRAS 2013
GBT observation of pre-stellar core B68.
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METHYL FORMATE 10
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HCOOCH3 - He highly anisotropic potential
A Faure, K Szalewicz, LW JCP, 2011 ; A Faure P. Remijan,K. Szalewicz, LW ApJ 2013
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A - HCOOCH3
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A - HCOOCH3
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FORMALDEHYDE 14
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The opposite : H2CO
• Ortho and para forms are disconnected• Overpopulation of the lowest level, 1 11
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PREFERENTIAL COLLISION TOWARDS lower level 1 11
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The opposite : H2CO
• Ortho and para forms are disconnected• Overpopulation of the lowest level, 1 11
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PREFERENTIAL COLLISION TOWARDS the lower level 1 11
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Conclusion• Non equilibrium excitation of low-lying levels
(GBT/VLA, not ALMA frequencies !!)
• Need intimate knowledge of excitation/de-excitation schemes as well as opacity effects
• For high lying masers, same type of knowledge will be requested. Much tougher to compute• Ex: H2O v> 0 excitation tremendously long to compute (>>
week/energy point, 12 cores, up to date codes).• High lying levels of COM's also at the limit of possibilities, in an
exact fashion (same precision in any state-to-state collision)• QCT difficult, because of subsequent quantization.
• We keep on trying !
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Part of a large effort aiming at computing collision coefficients between light or heavy molecules with H2, both ortho and para.
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Who, where.
L. WIESENFELD, A. FAURE, C. RIST, C. CECCARELLI , B. LEFLOCH, N. Troscompt, M. Wernli, F. Daniel
Ongoing collaborations:Paris, Meudon, Le Havre,Dijon, Lille, Orsay,Rennes, Bordeaux (FR)Nijmegen (NL), JPL-Pasadena, Delaware, NRAO (US),Durham (UK), Madrid (SP),
€ , $ : EU FP6 ‘Molecular Universe’, CHESS KP HSO CNES, ANR FORCOMS - HYDRIDES COST programs ‘ The Chemical Cosmos’,'Our Astrochmical History'. PCMI-INSU-CNRS, LABEX OSUG@2020, NWO (NL), NASA-JPL