ATLASGAL -- Relationship between dense star forming clumps and interstellar masers

We have used catalogues from several Galactic plane surveys and dedicated observations to investigate the relationship between various maser species and Galactic star forming clumps, as identified by the ATLASGAL survey. The maser transitions of interest are the 6.7 & 12.2 GHz methanol masers, 22.2 GHz water masers, and the masers emitting in the four ground-state hyperfine structure transitions of hydroxyl. We find clump association rates for the water, hydroxyl and methanol masers to be 56, 39 and 82 per cent respectively, within the Galactic longitude range of 60{deg} > $l$ > -60{deg}. We investigate the differences in physical parameters between maser associated clumps and the full ATLASGAL sample, and find that clumps coincident with maser emission are more compact with increased densities and luminosities. However, we find the physical conditions within the clumps are similar for the different maser species. A volume density threshold of $n$(H$_{2}$) > 10$^{4.1}$ cm$^{-3}$ for the 6.7 GHz methanol maser found in our previous study is shown to be consistent across for all maser species investigated. We find limits that are required for the production of maser emission to be 500 L$_{odot}$ and 6 M$_{odot}$ respectively. The evolutionary phase of maser associated clumps is investigated using the L/M ratio of clumps coincident with maser emission, and these have similar L/M ranges (~10$^{0.2}$ - 10$^{2.7}$ L$_{odot}$/M$_{odot}$) regardless of the associated transitions. This implies that the conditions required for the production of maser emission only occur during a relatively narrow period during a stars evolution. Lower limits of the statistical lifetimes for each maser species are derived, ranging from ~0.4 - 2 x 10$^{4}$ yrs and are in good agreement with the straw man evolutionary model previously presented.

Massive outflows associated with ATLASGAL clumps

We have undertaken the largest survey for outflows within the Galactic Plane using simultaneously observed 13CO and C18O data. 325 out of a total of 919 ATLASGAL clumps have data suitable to identify outflows, and 225 (69+-3%) of them show high velocity outflows. The clumps with detected outflows show significantly higher clump masses (M_{clump}), bolometric luminosities (L_{bol}), luminosity-to-mass ratios (L_{bol}/M_{clump}) and peak H_2 column densities (N_{H_2}) compared to those without outflows. Outflow activity has been detected within the youngest quiescent clump (i.e.,70um weak) in this sample and we find that the outflow detection rate increases with M_{clump},L_{bol},L_{bol}/M_{clump} and N_{H_2},approaching 90% in some cases(uchii regions=93+-3%;masers=86+-4%;hchii regions=100%). This high detection rate suggests that outflows are ubiquitous phenomena of massive star formation. The mean outflow mass entrainment rate implies a mean accretion rate of ~10^{-4}M_odot,yr^{-1}, in full agreement with the accretion rate predicted by theoretical models of massive star formation. Outflow properties are tightly correlated with M_{clump},L_{bol} and L_{bol}/M_{clump},and show the strongest relation with the bolometric clump luminosity. This suggests that outflows might be driven by the most massive and luminous source within the clump. The correlations are similar for both low-mass and high-mass outflows over 7 orders of magnitude, indicating that they may share a similar outflow mechanism. Outflow energy is comparable to the turbulent energy within the clump, however, we find no evidence that outflows increase the level of clump turbulence as the clumps evolve. This implies that the origin of turbulence within clumps is fixed before the onset of star formation.

Ionized Gas Towards Molecular Clumps: Physical Properties of Massive Star Forming Regions

We have conducted a search for ionized gas at 3.6 cm, using the Very Large Array, towards 31 Galactic intermediate- and high-mass clumps detected in previous millimeter continuum observations. In the 10 observed fields, 35 HII regions are identified, of which 20 are newly discovered. Many of the HII regions are multiply peaked indicating the presence of a cluster of massive stars. We find that the ionized gas tends to be associated towards the millimeter clumps; of the 31 millimeter clumps observed, 9 of these appear to be physically related to ionized gas, and a further 6 have ionized gas emission within 1. For clumps with associated ionized gas, the combined mass of the ionizing massive stars is compared to the clump masses to provide an estimate of the instantaneous star formation efficiency. These values range from a few percent to 25%, and have an average of 7 +/- 8%. We also find a correlation between the clump mass and the mass of the ionizing massive stars within it, which is consistent with a power law. This result is comparable to the prediction of star formation by competitive accretion that a power law relationship exists between the mass of the most massive star in a cluster and the total mass of the remaining stars.