- Massive Star Binary Systems
- CoBRAS e-MERLIN Legacy Survey
- Dust Production in Supernovae
- Evolved Binaries
UCL_CHEM 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) and it can be coupled with
code (Martin et al. 2009) as well as with SMMOL (e.g. Lerate
et al. 2010).
The code is available on request from Serena Viti (sv AT star.ucl.ac.uk).
Below we list regions of interest that have been modelled with UCL_CHEM:
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.
UCL_CHEM 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, UCL_CHEM 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). UCL_CHEM 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). More information about HIFI is here.
Extragalactic star formation
UCL_CHEM (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 29 sep 10 11:33 by Roger Wesson