Research

My research focuses on the chemistry occurring in star-forming environments. I'm interested in using laboratory and observational techniques to understand molecule formation, destruction, phase changes and abundances in various regions of the interstellar medium. Below are more detailed descriptions of projects I have been involved in since I joined the astrochemistry field.

Non-thermal ice desorption

Ice covered dust grains are present in the cold parts of star forming regions. The chemical content of these ices can vary throughout the protostellar evolutionary stages but they also constitute a reservoir of molecules that can enrich the gas phase as a result of heating and irradiation from young stellar objects. The observation of cold gas in regions where molecules should be frozen out on the grains is due to non-thermal desorption processes. Both in Leiden and Paris, I have been co-leading measurement campaigns to quantify the impact of UV-photon induced desorption and understand the underlying molecular mechanisms using synchrotron light. With such experiments, accurate photodesorption rates for various molecules can be estimated therefore included in astrochemical models, taking into account the different UV fields encountered in various regions of the ISM. Moreover, we have been able to better constrain this (DIET) Desorption Induced by Electronic Transition process and to highlight the impact of the neighboring molecules on the desorption efficiency.


Diffusion of molecules and volatile entrapments

The main component of ice mantles are H2O, CO, and CO2. H2O and CO2 are thought to form together on the surface of the grains during the pre-stellar stage while CO, already in the gas phase, freezes out during the drop in temperature resulting from the contraction of the cloud to form the protostar. This bi-layered ice structure will evolve during the accretion of the envelope material onto the YSO and thermally induce diffusion and desorption of the icy molecules. Among these processes, the segregation of CO2 ice from a water rich layer has been observed in the infrared and quantified in the laboratory. Experiments supported by Monte-Carlo simulations show that segregation occurring through surface diffusion takes place at lower temperature (30 K) than segregation induced by swapping between molecules within the entire ice mantle.

The desorption of volatile species trapped in the water ice mantle is also greatly influenced by the ice thickness and volatile concentration. It will proceed in two steps, first at the pure desorption temperature of the volatile species and secondly close to 100 K when the water is changing phase from amorphous to crystalline as well as together with water when it desorbs. The amount of trapped volatiles has been quantified in the laboratory with respect to concentration and ice thickness. These results can now be used to parameterize desorption of volatiles around protostars.


Complex organics in MYSOs and link to the ice composition

Complex molecules are found around high-mass protostars but their presence around specific objects within the same star forming cloud as well as the various abundance ratios encountered are still a mystery. One possible origin lies in the initial ice content found in protostellar envelopes since these organic molecules are either formed on the grains and evaporated into the gas phase or result from a second generation gas phase chemistry involving the evaporated molecules. Such a link can be established by comparing ice and gas observation of the same high-mass protostars. Observations of high mass objects are, however, very difficult due to the limited amount of sources with both ice and gas features. Recent observations suggest that complex molecules are not only observed in objects with a massive hot-core chemistry but can also be present in fainter massive objects. The latest can be used to increase the ice-gas sample and explore the link between ice and complex organics chemistry.


Edith Fayolle

Harvard-Smithsonian Center for Astrophysics
60 Garden St, MS-15
Cambridge, MA 02138 (USA)
E-mail : efayolle@cfa.harvard.edu