High-mass stars play a major role in the evolution of the Galaxy. They are the principal sources of heavy elements and UV radiation. However, high-mass star formation is still far from being well understood due to observational difficulties caused by large distances, large extinctions, and cluster formation behavior.
Using the observations from the Atacama Large Millimeter/submillimeter Array (ALMA), Tie Liu (SHAO), Ke Wang (KIAA) and an international team have initiated a new ALMA survey-type project: ALMA Three-millimeter Observations of Massive Star-forming regions (ATOMS). ATOMS have observed a large sample of 146 active star forming regions in the Galaxy with ALMA band 3 (see left panel of Figure 1).
Figure 1. Left panel: Spatial distribution of the ATOMS targets (red ‘+’) projected on to a top-down schematic view of the Milky Way (artist’s concept, R. Hurt: NASA/JPLCaltech/SSC). Right panel: Three color composite image of G9.62+0.19. red: 3 mm continuum emission (ionized gas and warm dust); blue: HCO+ J=1-0 emission (dense molecular gas); and green: SiO J=2-1 emission (shocked gas).
Recently, the ATOMS team has published two companion papers in Monthly Notices of the Royal Astronomical Society (Liu et al. 2020a, MNRAS, 496, 2790; Liu et al. 2020b, MNRAS, 496, 2821), reporting the first results from the survey.
“To deepen our understandings of high-mass star formation, we need to conduct a statistical study of a large sample of star forming regions with unpreceded resolution (<0.1 pc) and sensitivity. Only with ALMA such a survey is possible.” said Tie Liu of Shanghai Astronomical Observatory and principal investigator of the project.
The observations of ATOMS covered dozens of commonly used lines including the dense gas tracers (e.g. J = 1–0 of HCO+, HCN and their isotopes), hot core tracers
(e.g. CH3OH, HC3N), shock tracers (e.g. SiO, SO), and ionized gas tracers (H40α). “The ATOMS project has produced a large database of molecular line emission from species that are more highly weighted toward gas which is denser than the gas that emits most of the CO lines.” said Professor Neal J. Evans in University of Texas at Austin.
“These high angular resolution images of more than 140 star forming regions will allow study of the properties of the central condensations in which new stars form and to understand their relationship with the larger molecular clouds that comprise the reservoir of material for future generations of stars”, added by Professor Paul F. Goldsmith of JPL.
The main findings from these early results are:
- Previous studies have revealed a strong linear relationship between the recent star formation rate (SFR), as traced by the total infrared emission, and the dense molecular gas mass that is indicated by line luminosities of dense molecular gas tracers (e.g. HCN, HCO+, and CS). This so-called ‘dense gas star formation law’ may imply that the dense molecular gas rather than the total molecular gas is the direct fuel for star formation. However, the emission lines used in most of these previous studies such as J=1-0 transitions of HCN and HCO+ tend to be optical thick in dense parts of molecular clouds. They are poor tracers of the dense molecular cores, the birthplace of young stars. The ATOMS team firstly use isotopologue lines, which are much optically thinner, to calibrate the ‘dense gas star formation law’. Their studies suggest that statistically both the main lines and isotopologue lines are good tracers of the total masses of dense gas in Galactic molecular clumps. The large optical depths of main lines do not affect the interpretation of the slopes in star formation relations. They also find that the mean star formation efficiency (SFE) of massive Galactic clumps in the ‘ATOMS’ survey is reasonably consistent with other measures of the SFE for dense gas, even those using very different tracers or examining very different spatial scales.
- The high resolution ALMA observations revolve the spatial distributions of various kinds of gas (see right panel of Figure 1). Some transitions, commonly assumed to trace dense gas, including CS J = 2−1, HCO+ J = 1−0, and HCN J = 1−0, show extended gas emission in low-density regions within the clump; they are poor tracers of dense cores. In contrast, lines with high critical densities such as SO, CH3OH, H13CN, and HC3N reveal well the dense cores. Widespread narrow SiO emission may present (over ∼1 pc) in massive clumps like G9.62+0.19, which is likely caused by slow shocks induced by large–scale colliding flows or HII regions.
- High-mass stars tend to form with an age sequence in proto-clusters. The ATOMS team has identified at least five different evolutionary stages (high-mass protostellar object, hot molecular core, hyper-compact HII region, ultra-compact HII region and expanding cometary HII region) of high-mass proto-stars in the G9.62+0.19 clump, which has a clump size of ~1 pc. They find that stellar feedback from evolved HII regions could have greatly reshaped their natal clumps, significantly changed the spatial distribution of gas and may also account for the sequential high-mass star formation.
“In future, we will do more statistical studies on high-mass star formation with the ATOMS data,” said Tie Liu. “The ATOMS observational results would significantly improve our understanding of the roles of dense molecular gas, stellar feedback, and filaments in clustered star formation.” Added by Kee-Tae Kim, director of radio astronomy division in KASI.
Note: The ATOMS team includes 45 astronomers from China (Mainland and Taiwan), US, Korea, Japan, France, Finland, Chile, India, Hungary, Australia.