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- Galactic Star Formation and the ISM
- Astrochemistry and the Birth of Massive Stars
- The Dust Grain Ice Formation Inverse Problem
UCLCHEM is a time and depth dependent gas-grain chemical model that can be used to estimate the fractional abundances (with respect to hydrogen) of gas and surface species in every environment where molecules are present. The model includes both gas and surface reactions. Regardless of the object that we model, the code will always start from the most diffuse state where all the gas is in atomic form and evolve the gas to its final density. Depending on the temperature, atoms and molecules from the gas freeze on to the grains and they hydrogenate where possible.
The advantage of this approach is that the ice
composition is not assumed but it is derived by a time-dependent
computation of the chemical evolution of the gas-dust interaction process.
The code includes some of the latest experimental data on desorption
processes and, in general, tries to 'keep up to date' with
novel experimental and theoretical work in astrochemistry. Our group is
actively collaborating with the chemistry Department at UCL (see
http://www.chem.ucl.ac.uk/cosmicdust/) as well as with others.
The code is very modular and has been used to model a variety of regions
(all published in several articles, since 1999).
The code is available on github.
Chemical Modelling of Star Formation
At the very early stages of star formation, molecules react either in the gas-phase or in their solid state. While it is possible to observe species in the gas-phase, there are not many observational clues on how species form on the grain surfaces; on the other hand, experimental chemists are working to reproduce, in laboratories, chemistry of interstellar medium relevance in order to look at the reaction products. Laboratory data (reaction rates as well as binding energies) can then be included in chemical models that actually simulate interstellar medium conditions. An example of chemical code is UCLCHEM, which has been employed to reproduce the physics and chemistry of diffuse, high- and low-mass star-forming regions. So far, the interstellar chemistry of complex organic molecules (such as methyl formate, ethylene oxide, acetaldehyde) has been reproduced with successful results. The study of most of these complex organic species has a great impact on the astronomical as well as the astrobiology communities because of the link of these molecules with prebiotic chemistry. Gas-phase reactions are also important for a better understanding of icy mantle composition and for reproducing the observed abundances of species. One of the key atoms in the interstellar medium is oxygen because it is known to be quite abundant in the diffuse medium. Thanks to a revised reaction scheme inserted into the UCLCHEM model, oxygen was found to show a different chemical behaviour when reacting with small, unsaturated hydrocarbons in the gas-phase compared to that on grains. At the moment, the focus is on sulphur atoms, because of their high reactivity, in both gas and solid phases, and the different molecular structures it might spawn. Finally, in order to improve the accuracy of the calculations, it is crucial to keep up to date with chemical data of all the molecules that are included in the UCLCHEM code, by frequent checking of the available astrochemical databases.
In Ngyen et al. (2002) we explored the chemistry of disks around massive young stellar objects, and disks around low-mass stars irradiated by nearby OB associations. Our aim was to examine the contribution of the PDR envelope makes to the molecular species that may be observed.
Low- and high-mass galactic star-forming regions
Figure 1: Chemical models for the Orion plateau and extended ridge: the plot shows the time evolution of the fractional abundances (with respect to the total number of hydrogen nuclei) of selected species.
UCLCHEM is often used to model hot cores and corinos (e.g. Viti et al. 2004a; Awad et al. 2010). These are small, compact, dense regions surrounding high and low mass stars, respectively. For these models, UCLCHEM is used in a two-phase calculation where the first phase starts from a fairly diffuse (~300 cm-3 ) medium in atomic form and undergoes a free-fall collapse until densities typical of hot cores or corinos are reached. Phase 2 follows the chemical evolution of the remnant core. We simulate the effect of the presence of an infrared source in the centre of the core or in its vicinity by subjecting the core to an increase in the gas and dust temperature. The temperature reaches its maximum (~300 K) at different times depending on the mass of the new-born star.
Diffuse and translucent clouds
We model diffuse and translucent clouds concentrates as part of a dynamically evolving interstellar medium (e.g. Price et al. 2003). In this picture, diffuse clouds represent a transient phase during this contraction from tenuous to dense molecular gas. Once star formation occurs, stellar winds, outflows, and explosions ensure that dense molecular gas is returned to a more tenuous form.
We are interested in understanding the origin and structure of low velocity, chemically rich clumps of gas observed along low- and intermediate-mass outflows (see Benedettini et al. 2006,7 and Viti et al. 2004b). UCLCHEM has been extensively used for this purpose and is one of the models used to interpret HIFI data on L1157 (Codella et al. 2010, Viti et al. 2011, Holdship et al. 2016). Our HIFI programs include:
This program aims at obtaining a comprehensive set of water
observations towards a large sample of protostars, covering a wide
range of masses and luminosities -from the lowest to the highest mass
protostars-, and a large range of evolutionary stages -from the first
stages represented by the pre-stellar cores to the last stages
represented by the pre-main sequence
stars surrounded only by their protostellar disks.
This program will provide the community with a coherent
study of the line spectra in the HIFI frequency range (500-2000GHz) of
star forming regions as a function of mass and evolution. The main goals
of the program are: i) to guide future observations with Herschel ii)
to provide a legacy database for the use of the general astronomy
For more information please contact Serena Viti.
Extragalactic star formation
UCLCHEM (together with UCL_PDR) is routinely used to investigate how stars form under various chemical and physical conditions in the Universe. See here for more details
For further information, please contact Serena Viti (sv AT star.ucl.ac.uk)
Page last modified on 17 nov 16 14:35 by Jonathan R Holdship