Practically all star formation in the Milky Way occurs in dense, massive reservoirs of gas called Giant Molecular Clouds (GMCs). Much of star formation theory thus relies on being able to constrain the total mass in molecular gas. In the Milky Way and similar galaxies, which contain gas that has a ‘solar’ metallicity (i.e. gas that has an elemental composition similar to the Sun), astronomers have to rely on the molecule CO to trace the mass of the clouds. This is because H2, which makes up the overwhelming bulk of the mass, has energy levels that are not excited at GMC temperatures (< 40K).
As we go to lower metallicities, such as those found in dwarf galaxies, the ability of CO to trace the molecular fraction in the gas breaks down, as it is easily dissociated by the light from nearby young stars. As a result, the CO emission only traces the very dense cores in low-metallicity clouds, and tells us very little about the cloud’s bulk properties.
However, CO is not the only carbon-bearing species that we can observe. Both neutral, and singly ionised carbon (C and C+) are easily excited in low-metallicity clouds, and can be observed with current telescopes (ALMA/APEX/SOFIA). Preliminary results from our numerical modelling has also shown that these species are present throughout these clouds, and so may be a good tracer of cloud conditions, properties and dynamics. If this is the case, then emission from these species can provide a new probe of star-forming environments in nearby galaxies that can help to constrain the theory of galactic evolution.
In this project the student would run numerical simulations of molecular cloud formation that self-consistently combine self-gravity, (magneto)hydrodynamics, time-dependent chemistry (including the main branch of carbon chemistry) and ISM thermodynamics. These simulations (similar to those in the figure) will capture the formation of the clouds from the warm neutral medium, and follow them until they have started to produce stars. The student will then use the output from these simulations as input to a radiative transfer calculation, to obtain maps (or more exactly, position-position-velocity cubes) of the C, C+ and CO emission from the clouds. They will then be able to explore how the emission from these species traces the star formation process and properties of the host molecular cloud.
The goal of the project will be to address questions such as, can we use these tracers to calculate the mass of the clouds? Do they depend on the history of cloud’s assembly? And, are they sensitive to the star formation rate in the clouds? All these questions are of interest to the wider galactic evolution community.
Summary: The student will use computer simulations to model 1) the star formation process in low metallicity molecular clouds, and 2) the line-emission that arises from these regions that can be seen by ALMA APEX and SOFIA. Over the course of the PhD, the student will have the opportunity to study a wide range of astrophysical processes, including fluid dynamics, physical chemistry, and radiative transfer. Major code development will not be required for the project, however there is some scope for adding new features to the models.
This project will be funded by the STFC.
Applicants should apply to the Doctor of Philosophy in Physics and Astronomy with a start date of 1st October 2020. https://www.cardiff.ac.uk/study/postgraduate/research/programmes/programme/physics-and-astronomy
In the research proposal section of your application, please specify the project title and supervisors of this project. If you are applying for more than one project, please list the individual titles of the projects in the text box provided. In the funding section, please select ’I will be applying for a scholarship/grant’ and specify that you are applying for advertised funding from the STFC.
Applicants will need to submit the following documents with their application:
- post high school certificates and transcripts to date
- academic CV
- personal statement
- two academic references. Your references can either be uploaded with your application, or emailed by the referee to [email protected]
or [email protected]
More details on the type of simulations and radiative transfer modelling that this project will require can be found in the following papers:
Clark P. C., Glover S. C. O., Ragan, S.E., Shetty, R. Klessen R. S., 2013, ApJ, 768L, 34
Glover S. C. O., Clark P. C., 2012, MNRAS, 426, 377
Clark P. C., Glover S. C. O., Klessen R. S., Bonnell I.A., 2012, MNRAS, 424, 2599
Glover S. C. O., Clark P. C., 2012, MNRAS, 421, 9