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  • Accretion and Outflow from Young Stars: Understanding the Formation of Jets, Accretion Disks, and Planetary Systems

    Dr Toby Moore

    Background information

    Young stars form via a spectacular and rather violent combination of accretion and outflow. Stars are produced deep inside the many dark clouds of gas and dust scattered throughout the Milky Way galaxy. Material from the surrounding cloud spirals down onto the central star through an extended envelope and central accretion disk while, at the same time, the star drives supersonic jets of gas from its poles to relieve the build-up of angular momentum.

    Figure 1: An infra-red image of the busy star-forming region NGC 1333, showing (in pink) a number of jets and outflows driven by young, nebulous stars in the region. The outflows are traced by MHOs, the pink features in this image. The image combines data taken at the United Kingdom Infra-red Telescope and the Spitzer Space Telescope. From Davis et al. 2007.

    The accretion disk will eventually evolve into a planetary system; in the meantime, the jets stir up the surrounding gas and dust, shaping the ambient medium and possibly influencing the efficiency with which the cloud can form additional stars.

    Jets and "outflows" from young stars are traced by Herbig-Haro (HH) objects, shock fronts driven into the surrounding cloud by the outflows. HH objects emit radiation from hot atoms and ions. Recent developments in infra-red astronomy have led to the discovery of molecular counterparts to HH objects. More than a thousand of these Molecular Hydrogen emission-line Objects, or MHOs, are now known to exist.

    Figure 2: An infra-red image showing two spectacular outflows from young stars in the L1448 dark cloud. The young stars driving the outflows are seen as bright red points midway along the axis of each outflow. The outflows themselves emit brightly in light from hot hydrogen molecules which, in this false-colour image, appear pink. The image combines data taken at the United Kingdom Infra-red Telescope and the Spitzer Space Telescope. From Davis et al. 2007.


    Research project

    The existence of collimated bipolar jets points to a disc-accretion model for the accumulation of mass onto the early-stage protostar which is common to the formation of stars of all masses. Jets and the related molecular outflows are therefore one of the few links between low-mass and high-mass star formation, which are otherwise thought to be fairly distinct processes. Two of the key outstanding questions related to this phenomenon are: (1) at what stage of the formation of a star does the jet/outflow begin and how long does it continue? (2) Are jets and outflows truly ubiquitous and associated with stars of all masses? Answering the former will tell us about the start and duration of active accretion via a disc in the life of a protostar and so the rate and process of mass accumulation. The latter will show whether disc accretion proceeds similarly in low- and high-mass young stars and reveal differences in the underlying formation process. for example, in some models of high-mass star formation, the accretion disc may be a transient phenomenon, forming and re-forming several times.

    This project will use three basic sets of survey data, one from the James Clerk Maxwell Telescope Galactic Plane Survey (JPS), which uses the SCUBA-2 bolometer array on the James Clerk Maxwell Telescope (JCMT) in Hawaii to map cool and cold dust in the inner Galaxy and traces the earliest stages of star formation; the second is the UWISH2 survey of H_2 emission obtained at the UK Infrared Telescope (also in Hawaii). H2 is an excellent tracer of shocked gas excited by the impact of bipolar outflows and the jets that drive them with the surrounding cloud. The third data set is from the CHIMPS and CHIMPS2 surveys of carbon monoxide in the inner Galaxy, detecting the high-velocity molecular gas by which outflows are often detected. CHIMPS2 has recently been awarded 400 hours of observing time at JCMT from 2017 to 2020 and students working on this project are likely to be invited to travel to Hawaii to assist in the observations.

    When combined with infrared photometric data from the Herschel Hi-GAL survey, these data can be used to create a sample of cold cores and young protostars with associated outflows. We can then estimate statistically the timescales associated with the outflow phenomenon as a function of the total luminosity of the newly-forming star, constraining the onset, duration and continuity of the process and, hence, the physics of mass accumulation to form stars.