MAJORS: Massive, Active, JCMT-Observed Regions of Star formation

Determining the role of dense gas in star formation

We are observing a large, mass-selected sample of dust-continuum traced, star-forming molecular clouds in HCN J=3-2 and HCO+ J=3-2 with ‘Ū’ū. This sample includes clouds in the Central Molecular Zone (CMZ), the Inner Galaxy, and the Outer Galaxy. Dense gas is vital to the star-formation process, and high-resolution observations of this dense gas in a large sample of resolved star-forming sources is crucial to understanding its exact role in regulating star-formation efficiency.

Predictive, empirical relationships of star formation, such as the Kennicutt-Schmidt law, are able to link the scaling of the star-formation rate surface density with the surface density of the gas. However, this relationship only holds for normal and dwarf galaxies, and becomes super-linear in starburst systems and breaks down on the smallest scales of individual giant molecular clouds. However, when dense-gas observations are used, these relationships survive, once again indicating the apparent importance of dense gas in the star-formation process.

The key science outcomes and goals of this project are:

  1. Understand the impact of Galactic environment on the physics of dense gas, allowing for an understanding of how dense gas is produced and intrinsically linked to star formation.
  2. Distinguish between star-formation theories, and whether the star-formation rate is controlled by the free-fall time within bound structures or the amount of dense gas available for star formation.
  3. Produce LIR – Lgas relationships linking resolved Galactic clumps, Galactic molecular clouds, extragalactic systems and ULIRGS to study the universality of the star-formation process.
  4. Determine the cause of variations of the HCN/HCO+ ratio, and how it is linked to the physical conditions caused by Galactic environment.
  5. Find a sample of extreme star-forming sources using maps of dense-gas mass fraction and a sample of Galactic mini-starbursts using a LIR – Lgas relationship produced using CO maps.
  6. Link the clump-mass fraction to the star-formation efficiency and clump-formation efficiency.
  7. Identify outflows and active regions of star formation and determine the infall rates of the gas into individual clumps
  8. Provide a legacy sample matching those of extragalactic studies for future studies.

LIR – LHCN relationship from Tan et al. (2018) spanning ten orders of magnitude in luminosity from Galactic clumps to high-redshift ULIRGs.

Coordinators: David Eden (UK), Xue-Jian Jiang (EAO), James Di Francesco (Canada), Kee-Tae Kim (South Korea), Yu Gao (China), Masa Imanishi (Japan), and Raffaele Rani (Taiwan) 

– JCMT program code: M22AL002

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