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 21 décembre 2018, 14h00
Salle de l'atelier, Paris
Astrochemistry in star forming regions : new modeling approaches
Emeric BRON
résumé :
Star-forming regions present rich infrared and millimeter spectra emitted by the gas exposed to the feedback of young stars. This emission is increasingly used to study the star formation cycle in other galaxies, but results from a complex interplay of physical and chemical processes : chemistry in the gas and on grain surfaces, (de)excitation processes of the atoms and molecules, heating and cooling balance,... Its understanding thus requires detailed astrochemical models that include the couplings between these processes. In this talk, I will present several examples where new modeling approaches of specific processes and their couplings proved crucial to solve persistent observational riddles : from the driving role of UV irradiation in the dynamics of photodissociation regions (PDR) to the efficient reformation of molecular hydrogen in these regions.
Mardi 15 janvier 2019, 11h00
Salle de l'atelier, Paris
ATTENTION jour ET heure inhabituels
Thresholds for Globular Cluster Formation and their Dominance of Star Formation in the Early-Universe
IBM Research Division
résumé :
Young massive clusters (YMCs) are usually accompanied by lower-mass clusters and unbound stars with a total mass equal to several tens times the mass of the YMC. If this was also true when globular clusters (GCs) formed, then their cosmic density implies that most star formation before redshift ~2 made a GC that lasted until today. Star-forming regions had to change after this time for the modern universe to be making very few YMCs. Here we consider the conditions needed for the formation of a ~10^6 Msun cluster. These include a star formation rate inside each independent region that exceeds ~1 Msun/yr to sample the cluster mass function up to such a high mass, and a star formation rate per unit area of Sigma_SFR ~ 1 Msun/kpc^2/yr to get the required high gas surface density from the Kennicutt-Schmidt relation, and therefore the required high pressure from the weight of the gas. High pressures are implied by the virial theorem at cluster densities. The ratio of these two quantities gives the area of a GC-forming region, ~1 kpc^2, and the young stellar mass converted to a cloud mass gives the typical gas surface density of 500-1000 Msun/pc^2. Observations of star-forming clumps in young galaxies are consistent with these numbers, suggesting they formed today's GCs. Observations of the cluster cut-off mass in local galaxies agree with the maximum mass calculated from Sigma_SFR. Metal-poor stellar populations in local dwarf irregular galaxies confirm the dominant role of GC formation in building their young disks.
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