Laboratoire d’Études du Rayonnement et de la Matière en Astrophysique et Atmosphères

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Reactivity on cold Surfaces

par Jean-Hugues Fillion, Mathieu Bertin - publié le , mis à jour le


Francois Dulieu (Prof – Team Leader), Saoud Baouche (Engineer), Henda Chaabouni (Ass. Prof.), Vincent Cobut (Ass. Prof.), Emanule Congiu (Ass. Prof.), Stéphane Diana (Engineer) , Francois Lachèvre (Tech.), Henri Lemaître (PhD student), Audrey Moudens (Ass. Prof.), Thanh Nguyen (PhD student).

Context : How molecules are formed at the surface of cold grains ?

The molecules (H2O, CO2...) existed well before the birth of our Earth. Radioastronomy is able to decipher this chemical history, when the molecules are in the gas phase. Unfortunately, the molecular complexity remains almost invisible, as complex molecules are synthesized and frozen on the surface of cold dust particles. Therefore, only laboratory astrophysics can explore this micro world. Despite all the recent observational progress, the enigma of astrochemistry is still unresolved and this is why we built state-of-the-art surface science apparatus.

The team « Reactivity on cold surfaces » is mostly devoted to experimental physics. It is located at the Cergy-Pontoise University. We study the evolution of atoms and molecules on surfaces relevant to astrophysics. We are interested in reactivity but also in all related processes like sticking, diffusion and desorption. We use atomic and molecular beams targeted on surfaces (graphite, silicates, ices...) cooled down to 6K, in order to mimic the extreme conditions of star forming regions.

Experimental set-up

We mostly use two complementary set-up for our studies.

  • FORMOLISM – Developped since 2001.

2 atomic or molecular beams (H, N, O, CO, NO, H2CO…). Project : nanograins source (Coronene).
Surfaces : removable sample (graphite, gold, silicate) and an in-situ controlled water ice growing system (amorphous, porous, crystalline…).
Surface temperature control : 6-300K, project 10-800K.
Detection tools :
Mass spectrometry (its use is fourfold) : Beam compositions, During Exposure Detection, Thermally Programmed Desorption, Internal energy of atoms or molecules.
Reflection Absorption Infra Red Spectroscopy.
Laser system (REMPI 2+1) coupled with a time of flight detection.

  • VENUS – developed since 2011

Up to 5 atomic or molecular beams. Only 2 presently running.
Surfaces : Rotatable sample holder with 3 surfaces.
Temperature Range : 10-300K
Mass spectrometry (its use is fourfold) : Beam compositions, During Exposure Detection, Thermally Programmed Desorption, Internal energy of atoms or molecules.
Reflection Absorption Infra Red Spectroscopy.

Recent studies

  • Molecular synthesis : Molecular synthesis : H2O (Chaabouni et al 2012), NH2OH (Congiu et al 2012), Nitrogen oxides (Minissale et al 2013, 2014), CO2 (Noble et al 2011, Minissale et al 2012,2014)…
  • Diffusion and desorption of oxygen : Diffusion is faster than expected at low temperature (<10K) (Minissale et al 2013,2014, Congiu et al 2014), but desorption energy is larger than previously estimated (Minissale et al submitted).
  • Chemical desorption : Experimental evidence (Dulieu et al 2013, Minissale&Dulieu 2014) : It is an important step linking the solid-state chemistry and observations of the gas phase.
  • Thermal desorption  : Role and importance of surface coverage and surface type in sub-monolayer regime (Noble et al 2012a,b).
  • Water ice morphology  : After its synthesis or after H recombination, water ice is compact or compacted : (Accolla et al 2012, 2013)

Séminaires à venir

Vendredi 23 mars 2018, 14h00
Salle de l'atelier, Paris
New Statistical Evidence for AGN-Feedback Giving Rise to ‘Mass-Quenching’ in Central Galaxies
ETH, Zurich
résumé :
One of the most striking features of the population of local galaxies is that the distributions of several key galaxy properties are highly bimodal (e.g. color and star formation rate). In general, high mass galaxies in dense environments, with bulge-dominated morphologies and pressure supported kinematics are more frequently passive (non-star forming) than lower mass galaxies in low density environments, with disc-dominated morphologies and rotationally supported kinematics. Understanding which, if any, of these correlations is causally related to the ‘quenching’ of star formation in galaxies remains an active and hotly debated area of investigation in modern astrophysics. Theoretically, a wealth of physical processes have been evoked to account for central galaxy quenching, including halo mass quenching from virial shocks, feedback from active galactic nuclei (AGN; in either the quasar or radio mode), stabilizing torques from central mass concentrations, or even magnetic fields interacting with the hot gas halo. I will present strong new statistical evidence which suggests that the quenched fraction of local central galaxies is primarily related to their central kinematics (Bluck et al. 2016; 2018 in prep.). I will show that this is broadly consistent with quenching from AGN feedback, through a detailed comparison with a semi-analytic model and a cosmological hydrodynamical simulation.

Vendredi 6 avril 2018, 14h00
Salle de l'atelier, Paris
Realistic dust properties for dust growth in dense cores
Charlène Lefèvre
IRAM, Grenoble
résumé :
Grain growth is expected from far infrared observations but the emission process implies a degeneracy between dust properties and temperatures. On the contrary, the scattering phenomenon does not depend on temperature but relies on dust geometry (size and shape). Nowadays, no dust grain model is able to reproduce consistently the multi-wavelength observations of dense cores including both scattering and emission. Recent laboratory works show the impact of dust composition on emissivities. Dust grain models from the literature suffer from a lack of flexibility: either the dust composition is fixed and too constraining for dense cores (with a magnesium over iron ratio too low) or the dust size distribution is set for given density, porosity, and coagulation time, that likely differ from one core to the other.

Based on methods used to derive aggregate properties for protoplanetary disks, I calculate dust properties for dense cores that take into account dust composition, size distribution and shape. I will present a fast method used to derive realistic dust properties for dense cores in a short computation time. In particular, I will illustrate the impact of porosity on scattering efficiency and dust emissivity. I will also review the consequences on dust amount, and show that grain growth requests cold dust in starless cores. Finally, I will discuss the coagulation time needed to build large dust aggregates.
Vendredi 25 mai 2018, 14h00
Bâtiment B (Salle RdC proche cantine), Paris
Closing the gap between simulations and observations
résumé :
In this talk I will give an overview over my latest results to compare
current observations with latest 3D radiation MHD simulations of turbulent
protoplanetary disks. I will focus on the dust thermal emission in
protoplanetary disks, covering the outer regions which emit in the sub/mm.

In the talk I will present comparisons between the expected dust scale
height from simulations and observations
and how we could use this results to learn about the gas disk dynamics.
I will also show the possibility to explain mm polarization by dust grains
which are aligned by the magnetic field.

Finally I will draw conclusion on the results and discuss about the
possibility to observe MRI activity with current telescope facilities.
Vendredi 1 juin 2018, 14h00
Salle de l'atelier, Paris
Structures in protoplanetary disks: what do they tell us about planet formation ?
résumé :

Vendredi 8 juin 2018, 14h00
Salle de l'atelier, Paris
University of Michgan
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