Massive Star Formation at the Edge of the Galaxy: The LZ-STAR Survey of Sh2-284
Stars are the fundamental building blocks of the universe, but how they form—especially in extreme environments—remains an open question in astronomy. Metallicity, or the amount of elements heavier than helium in a region, plays a key role in shaping how stars evolve. The early universe had very little metal content, so studying star formation in present-day regions with similarly low metallicity can provide insight into how the first stars may have formed.
The Low-Metallicity Star Formation Survey (LZ-STAR) focuses on Sh2-284 (S284), a massive star-forming region in the outer Galaxy. This region has about 30–50% of the metal content found in the Sun, making it one of the lowest-metallicity star-forming environments in the Milky Way. The survey utilizes data from multiple telescopes, including the James Webb Space Telescope (JWST), Atacama Large Millimeter/submillimeter Array (ALMA), and Hubble Space Telescope (HST). This paper presents the first results, focusing on one particularly massive young star, S284p1, and its remarkable outflow structure.
Observations: Seeing Through Dust with ALMA and JWST
The team observed S284 using ALMA and JWST to analyze the gas and dust where stars are forming. ALMA’s high-resolution radio observations revealed dense cores of gas and dust, while JWST’s infrared imaging uncovered young stars hidden behind thick clouds of interstellar material.
One region, S284-FIR1, stood out as a particularly active site of star formation. ALMA detected 1.3 mm dust continuum emission, which traces dense gas, and CO(2-1) emission, which highlights gas movements in the star-forming region. Meanwhile, JWST’s near-infrared images revealed a cluster of young stars surrounding the main protostar, S284p1.
Results: An Ordered, Massive Protostar with a Striking Outflow
S284p1 is one of the most massive young stars in the region, estimated to be about 11 times the mass of the Sun. It appears to be forming within a dense clump of gas, which originally had about 100 solar masses of material. The JWST images revealed a striking bipolar outflow, a structure where material is ejected in opposite directions from the forming star. The outflow spans more than two parsecs, making it one of the largest protostellar outflows ever observed.
The ALMA data confirmed this outflow through the detection of high-velocity gas moving away from S284p1. The outflow structure is remarkably symmetric, suggesting that the protostar has maintained a stable disk and accretion flow throughout its formation. This challenges some models of massive star formation that predict chaotic accretion in clustered environments.
Understanding S284p1: A New Look at Low-Metallicity Massive Star Formation
By modeling the protostar’s spectral energy distribution (SED), the researchers estimated that S284p1 is about 300,000 years old and is still actively growing. The results suggest that massive stars can form in an ordered manner, even in a low-metallicity environment like S284.
The protostar is embedded in a clump of dense gas with a mass surface density of 0.15 g/cm², a value similar to star-forming regions with higher metallicity. This suggests that the low-metallicity conditions of S284 do not necessarily prevent the formation of massive stars.
Implications: A Window into the Early Universe
The discovery of S284p1 and its well-structured outflow provides crucial insight into how massive stars form under conditions similar to those in the early universe. The results suggest that, even in environments with lower metal content, massive stars can still form in an organized way, following a process similar to what is seen in the Milky Way’s more metal-rich regions.
Future work in the LZ-STAR survey will expand on these results, exploring more forming stars in S284 to understand how low-metallicity environments influence the process of star birth. The combination of JWST’s unprecedented infrared sensitivity and ALMA’s detailed view of molecular gas will continue to push the boundaries of what we know about how the most massive stars in the universe come to be.
Source: Cheng
