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  • Understanding Star Formation in the Milky Way

    Dr Toby Moore (Publications)

    An infrared image of a busy star forming region in Orion.

    The following projects are closely related and aim to make key steps towards a predictive understanding of star formation by studying the large-scale mechanisms at work in the Milky Way. They make use of existing data but are also the subject of a new survey recently awarded 400 hours of observing time at the James Clerk Maxwell Telescope (JCMT) in Hawaii between 2017 and 2020. Students working on these projects will be invited to travel to JCMT to assist in the observations.

    Project 1: Identifying the mechanisms that regulate the efficiency of star formation

    One of the great unsolved problems in modern astrophysics is that we are still unable to predict the number and masses of the stars that form when a cloud of gas collapses under its own gravity. In particular, we do not yet know which effects regulate the star-formation efficiency (SFE), which is the fraction of the mass of a gas cloud that is converted into stars and is known to vary by large factors, or how that might depend on the physical properties of the cloud and its environment.

    However, in recent work at LJMU, we have made significant progress in this matter using large surveys of the Galactic Plane to quantify the SFE in a large sample of dense interstellar clouds located in a wide range of environments. We have demonstrated that the SFE on kilo-parsec scales is not strongly affected by the most obvious large-scale structures of the Galaxy, the spiral arms, but that there are very large variations from one cloud to another. We also know that there is no systematic dependence of SFE on the internal kinematics of clouds (i.e. on their internal velocity dispersion). Therefore the search is on to identify which environmental conditions or internal cloud properties can be related to variations in SFE.

    Recent results showing the variation of the molecular-gas column density (top), the Herschel 70-micron Flux density (middle) and the ratio of these two (bottom) as function of Galactic longitude, separated into four spiral arms. The latter ratio is a parameterised measure of the star-formation efficiency within individual clouds.

    The aims of the project will be to measure the star-formation efficiency and the luminosity function of the embedded, newly-forming stars within the sample of molecular gas clouds that we have identified. The variation in these key observables will be quantified and correlations sought with variables such as cloud density, position within the Galaxy, cloud-to-cloud velocity dispersion, local shearing stresses, local interstellar radiation field and pressure, and the proximity of external effects such as the winds and radiation from clusters of hot stars. A further aim will be to determine the spatial frequency power spectrum to find the dominant scale on which the amplitude of the SFE variations is largest. This will constrain the spatial scale on which the dominant controlling mechanisms operate and hence the nature of the mechanisms themselves.

    Project 2: Properties of dense filaments and their role in the star formation process

    The JCMT Plane Survey (JPS) is a large-scale, unbiased survey of the thermal 850-micron continuum emitted by the interstellar dust that is mixed with the gas of the interstellar medium. The survey covers the inner Galactic Plane and was made using the James Clerk Maxwell Telescope (JCMT) in Hawaii and led by LJMU. The JPS provides a complete census of star-forming and potential star-forming regions over a large fraction of the Milky Way with better sensitivity and spatial resolution than any other data set. One important result is that the data reveal a large population of dense filamentary structures. Such filaments are closely associated with star formation and may be a key stage in the sequence via which gas is funnelled from large clouds into protostars during the star-formation process. Filaments may therefore be instrumental in the determination of the final mass of the stars that are produced.

    A small section of the James Clerk Maxwell Telescope 850-micron Galactic Plane survey (JPS), showing multiple filamentary structures (in green, yellow and red) detected in the thermal emission from cold interstellar dust at temperatures of 10-20K

    The research project on offer is to identify a population of filaments detected by the JPS data and to find their relationship to the larger-scale molecular clouds in which they are embedded, as well as any variation in the population properties with environment and position within the Galaxy. This will be done by matching the structures identified in JPS to emission from CO and other molecules, giving velocities, distances and gas temperatures. Then, by quantifying the star formation associated with the filaments using infrared survey data, and the gas flow within them, the process of mass accumulation from molecular clouds, though filaments and onto new stars will be established.

    Project 3: The large-scale structures of the Milky Way and their relationship to star formation

    This project has two related parts, linked by an investigation into the effect of Galactic structure on star formation, using data from several surveys of the Galactic Plane; in particular, the CHIMPS survey of CO J=3-2 and the SEDIGISM CO 2-1 survey of the southern Galactic Plane. The first part of the project aims to analyse new large-scale structures found in the CHIMPS data, to identify whether they constitute a new spiral arm or are prominent inter-arm spurs and to compare the observed structures to current theoretical simulations of spiral structure in disc galaxies such as ours.

    The second part of the project will concentrate on the star-forming properties of the regions around the ends of the central Galactic bar, which are thought to be particularly favourable to star formation due to the intersection of circular and longitudinal orbits associated with such structures. Using SEDIGISM data, we will measure the star- and clump-formation efficiency at the far end of the central Galactic bar, from the ratios of infrared luminosity and dense-gas mass to the mass of gas in the molecular clouds that form the reservoir for star formation. The results will be compared to those of similar studies of the corresponding region around the near end of the bar and to those of gas at similar Galactic radii away from the bar ends.

    A position-velocity map of the inner Milky Way, showing spiral-arm structures detected in the CO J=3-2 rotational emission line by the CHIMPS survey (pale blue, green, yellow and red). The loci of spiral arms from current models of Galactic structure are shown and named (thin white lines). Note the similar structures detected at velocities around 80 km/s and longitudes 30 - 38 degrees that do not fit the model
    A plan view of the Milky Way with the locations of known sites of massive star formation outside the Galactic Centre region. The size of the red circles indicates the luminosity of the star-forming regions. The small white circle indicates the position of the Solar System and the various identified spiral arms are labelled. The Galactic central bar can be seen with sites of star formation at either end.

    Project 4: The transition from atomic to molecular gas, and the initial conditions for star formation

    Little work has been done on the transition from atomic to molecular hydrogen gas in the interstellar medium and the formation of molecular clouds so we know little about the process. The consequence of this is that we know little about the nature of molecular clouds, the origin of their supersonic turbulence and internal density structure, e.g., whether the dense clumps that form stars appear ready-made within a new cloud or whether they form on some longer timescale. We also have little information on the effect, if any, of the Galactic spiral arms on the process (do they trigger the phase change as the neutral gas falls into the arm potential?) or which hydrodynamic instabilities may be involved (the shear-related Kelvin-Helmholtz instability, for instance?). The result of this is that we do not understand the initial conditions of star formation, which lie in the physical nature and origin of molecular clouds.

    We now have the data to examine this question, in the form of large-scale surveys of molecular-gas-tracing CO from the CHIMPS and atomic HI from the THOR survey. But methods need to be developed to compare the two tracers and associate the neutral gas with the molecular clouds that form within it and to estimate density and temperature within the neutral gas. With estimates of these parameters and their variation across significant areas of the Galactic disc in different environments we can then compare, e.g., turbulent and thermal pressures in the two phases to reveal the physics of the phase transition.

    Project 5: The relationship between molecular-cloud turbulence and star formation efficiency within clouds

    Molecular clouds (and the interstellar medium in general) are highly turbulent and this turbulence both supports molecular clouds against gravitational collapse and is highly likely to provide the compression that produces the dense, gravitationally bound clumps that produce star formation, when turbulent flows collide. In this way, turbulence may both suppress and trigger star formation simultaneously, and thus it may be the key physical process that regulates the star-formation efficiency (SFE: the fraction of mass converted into stars) of clouds and in the Galaxy as a whole.

    There are hints from both observations and theoretical model predictions that the ratio of solenoidal (purely circulatory, or divergence-free) to compressive (curl-free) turbulence in a cloud may be the main effect influencing the SFE within clouds, which varies by factors of ~100 from cloud to cloud. Combining Herschel Space Telescope infrared data from the Hi-GAL survey with our CO Galactic-plane surveys, we have the data to measure both the turbulence ratio and the SFE in a large sample of clouds and prove whether this process is fundamental to regulating star formation in the Galaxy.