Observing organic molecules in interstellar gases: Non equilibrium excitation. LAURENT WIESENFELD, ALEXANDRE FAURE, Grenoble, France ANTHONY REMIJAN, National.

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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

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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