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 septembre 2018, 14h00
Salle de l'atelier, Paris
Understanding the structure of molecular clouds: Multi-line wide-field imaging of Orion B
résumé :
The new generation of wide-bandwidth high-resolution receivers turns
almost any radio observation into a spectral survey. In the case of
wide-field imaging of the interstellar medium, such a wealth of data
provides new diagnostic tools, but also poses new challenges in terms of
data processing and analysis.

The ORION-B project aims at observing 5 square degrees of the Orion B
molecular cloud, or about half of the cloud's surface, over the entire
3mm band. The emission of tens of molecular tracers have been mapped,
including CO isotopologues, HCO+, CN, HNC, N2H+, methanol, SO, CN...
Machine learning techniques have been applied to these maps, in order to
segment the molecular cloud into typical regions based on their
molecular emission, and to idenfify the most meaningful correlations of
different molecular tracers with each other and with physical quantities
such as density or dust temperature.

The spatial coverage, together with the spatial and spectral resolution,
also allow to characterize statistically the kinematics and dynamics of
the gas. The amount of momentum in the compressive and solenoidal
(rotational) modes of turbulence are retrieved, showing that the cloud
is dominated by solenoidal motions, with the compressive modes being
concentrated in two star-forming regions - which is in line with the
overall very low star formation efficiency of the cloud, and highlights
the role of compressive forcing in the star formation process. The
filamentary network of the molecular cloud also proves to have
particluarly low densities, and is very stable against gravitational
collapse and fragmentation, which also points at a young evolutionary
stage of the filaments.
Vendredi 28 septembre 2018, 14h00
Salle de l'atelier, Paris
The [CII] emission line as a molecular gas mass tracer in galaxies at low and high redshift
résumé :
So far the gas conditions in main-sequence galaxies at the peak of the cosmic star formation history have been mainly investigated through the CO emission lines. However, observing the CO transitions at higher redshift becomes challenging, since the lines luminosity weakens as metallicity decreases. A powerful alternative could be the [CII] emission at 158um instead: it is one of the brightest lines in the far IR regime observed in star-forming galaxies and it is the main coolant of the interstellar medium. Local studies show that the [CII] luminosity correlates with the galaxy star formation rate (SFR), although main-sequence sources and starbursts seem to have different behaviours. At higher redshift the picture is even less clear and only samples of starbursts have been analyzed so far. To remedy this situation we have observed with ALMA a sample of 10 main-sequence sources at z ~ 2 and we complemented our sample with literature data at lower and higher redshift. We found that the [CII] luminosity correlates with galaxies' molecular gas mass, independently of their depletion time, metallicity, and redshift. This lays foundations for future explorations of the interstellar medium of starbursts and galaxies at much higher redshift (z > 4).

Vendredi 5 octobre 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.
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