Scientific Motivation

 

To study the formation of high-mass stars in our Galaxy, we follow two approaches: (i) large surveys of high-mass star-forming regions at cloud/clump scales (0.2-50 pc; see Figure 1), and (ii) detailed studies combining high-angular resolution observations and sophisticated modeling of characteristic regions at clump/core scales (< 0.5 pc; see Figures 2 and 3). In the following we report on our main results.

Large, statistically-significant studies of high-mass star-forming regions

We continued to exploit the ATLASGAL survey and related spectral-line follow-up projects. The APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) is the most extensive ground-based submillimeter dust continuum survey of the inner Galactic plane. In the last years, the entire dataset together with compact source catalogues have been released to the community. Some of the main results are:

  • ATLASGAL and Planck combination: We have successfully complemented ATLASGAL with the Planck all-sky survey. This combination allows recovering emission filtered out by the ground-based APEX observations, and thus helps to better identify large-structures such as extended filaments.
  • Catalogue of compact sources: ATLASGAL has provided a large and systematic inventory of all massive, dense clumps in the Galaxy, and includes representative samples of all the earliest embedded stages of high-mass star formation.
  • Spectral line studies of selected clumps: Observations of different molecular species were used to investigate how the warm-up process affects the physics and chemistry during star formation.
  • Galactocentric variation of physical and chemical properties: We extended our research of star-forming clumps into the outer Galaxy, and found the gas-to-dust ratio to increase with Galactocentric radius steeper than the increase of metallicity, suggesting that the dust-to-metal ratio decreases with distance from the Galactic center.

Figure 1.- Combination of the 870 micron emission from ATLASGAL with large-scale emission from the Planck all-sky survey leading to an unprecedented range of spatial scales in our submillimeter view of the Milky Way.

 

Detailed, high-resolution studies of selected sources

We acquired and analyzed high-quality observational data of selected high-mass star-forming regions with the goal of studying the properties of the clusters where high-mass stars form, together with the accretion and feedback processes at scales ranging from a few pc down to a few hundred au. The analysis and modelling of the observational data required the development of a number of tools that are now available to the community and summarized in the following.

  • Pandora, a 3D radiative transfer modelling framework: We develope the Pandora framework to model the physical and chemical structure of star-forming regions. We have successfully model, as an example, the dust and ionized gas content, together with the physical and chemical properties of molecular species such as CO, HCN and CH3CN in the SgrB2 star-forming complex.
  • Statistical tools for the analysis of complex, chemically rich datasets: The advent of new facilities like ALMA result in complex and extremely rich datasets for which new tools are necessary in order to decipher and answer the scientific questions residing inside them. For this we have develope a number of tools like STATCONT: a new method to determine the continuum and line content in sources where previous methods fail; XCLASS: a spectral line analysis tool to model complex spectra and obtain temperatures and molecular abundances; or StackLines: a suite of python scripts that take advantage of the large frequency ranges observed with the new facilities to stack hundreds of molecular lines and improve the sensitivity of the observations.

Making use of the developed tools, we have analyzed the properties of two prominent star-forming regions: SgrB2 and NGC6334, and prepared the path for further studies. We summarize some of the main results.

  • Distributed high-mass star formation throughout the whole SgrB2 complex: Our sensitive ALMA and VLA observations have revelaed the presence of hundreds of massive dense cores and ionized gas (from already formed high-mass stars) distributed all over the SgrB2 star forming cloud, and not only towards the central regions as previously thought.
  • Properties of rich star-forming clusters: We have found rich and extremely dense clusters in SgrB2 with an excess of massive dense cores, compareed to the clusters harboured in the NGC6334 star-forming cloud (a Galactic-disk typical star forming region).
  • Shocked gas from outflows and cloud-cloud collision events: Sensitive ALMA observations of the shock tracer SiO have revealed an unexpected complex network of collimated outflows in the NGC6334 star-forming complex, together with extended SiO emission distributed over large areas likely pinpointing the remnants of cloud-cloud collision events.
  • Transport of mass through filamentary structures: We have found filamentary structures connecting all the scales from a few ten pc down to a few hundred au in the NGC6334 region. The dynamics of the gas suggest that mass is transported from the large-scale molecular cloud reservoir down to the clusters and to the dense cores where filaments converge.

Figure 2.- ALMA and VLA views of SgrB2, the most massive cloud in the Galaxy. Distributed ionized gas (VLA at 5 GHz) and massive dense cores (ALMA at 100 GHz) are found throughout the whole cloud. The central regions are found to harbour rich and dense clusters with extremely chemically rich sources.

Figure 3.- ALMA view of the NGC6334 high-mass star-forming complex. We have identified three star-forming clusters with hundreds of members, and associated with extended SiO shocked gas emission as well as filamentary flows of mass.

 

Next steps in our research

Capitalizing on the success of the previous periods, we will continue our effort of characterizing the conditions and impact of high-mass star formation in our Galaxy. First, we will expand in our statistically-significant studies to better comprehend the conditions and properties of high-mass star formation throughout the Galaxy. Second, we will continue our detailed analysis of specific regions to understand the processes involved in the formation of high-mass stars. Third, we will close the gap between the statistically-significant but not detailed studies and the highly-detailes studies of specific sources, by analyzing a large sample of sources in great detail, making use of the tools already developed.

In the next period (2019-2022), we will pursue five work-packages to fulfill our scientific goals:

WP1 Statistical analysis based on ATLASGAL and its follow-ups
WP2 Carbon budged towards high-mass star forming regions
WP3 Fragmentation and cluster properteis at scales of a few 1000 au
WP4 Accretion process from large (0.5 pc) to small (500 au) scales
WP5 Chemical properties in the formation of high-mass stars