Tracing the Origins of Alpha-Poor, Very Metal-Poor Stars
Alpha-poor very metal-poor stars (αPVMP stars) are rare celestial objects with unique chemical fingerprints. These stars, characterized by low levels of alpha elements (like magnesium) compared to iron, provide crucial clues about the early universe's star formation and chemical evolution. In a recent study, Jeena and Banerjee explore six different scenarios to explain the chemical origins of 17 such stars, analyzing whether their patterns align with ejecta from core-collapse supernovae (CCSNe), Type Ia supernovae (SNe Ia), or pair-instability supernovae (PISNe).
What Are Alpha-Poor Stars?
Stars are categorized as very metal-poor (VMP) when they have extremely low levels of iron ([Fe/H] ≤ -2), a characteristic linked to the universe's earliest phases. Most VMP stars are enriched in alpha elements, which are produced in supernova explosions. However, a subset, αPVMP stars, lack this enrichment. These peculiar stars are thought to originate from gas clouds affected by specific supernovae, offering an opportunity to study the roles of different supernova types in shaping the early galaxy.
The Methods: Six Scenarios Explored
The authors analyzed the elemental abundance patterns of the stars using advanced theoretical models, matching observed data against six scenarios:
Single PISN: Ejecta from one pair-instability supernova, known for producing heavy elements unevenly.
Single CCSN: A lone core-collapse supernova ejecta.
2CCSNe: Combined ejecta from two CCSNe.
CCSN + near-MCh SNe Ia: Mixing of CCSN ejecta with a near-Chandrasekhar-mass Type Ia supernova.
CCSN + sub-MCh SNe Ia: Mixing of CCSN ejecta with a sub-Chandrasekhar-mass Type Ia supernova.
CCSN + PISN: A mix of CCSN and PISN ejecta.
These models use advanced simulations, accounting for explosion energies and fallback processes, which determine how much material escapes during a supernova event.
Results and Key Findings
Dominance of Core-Collapse Supernovae: Most αPVMP stars (82%) were consistent with pure CCSN ejecta, suggesting these stars often form without requiring contributions from SNe Ia. However, certain stars, like SDSSJ0018-0939, show clear signs of sub-MCh SNe Ia involvement.
Role of Sub-MCh SNe Ia: A significant portion of the stars (76%) could also be explained by combining CCSN ejecta with sub-MCh SNe Ia, which have unique chemical signatures like enhanced titanium.
Limited Role of PISNe: The single PISN scenario was strongly ruled out for all stars, and CCSN + PISN combinations explained only 29% of the sample.
Variations in Star Groups: Stars were divided into categories based on their best-fit scenario, revealing diversity in their chemical origins. Some showed clear CCSN signatures, while others hinted at mixtures involving SNe Ia.
Conclusions
Jeena and Banerjee conclude that αPVMP stars mainly form from CCSN ejecta but can also arise from mixtures involving sub-MCh SNe Ia. This work emphasizes the variety of processes that shaped the early galaxy's chemical evolution and highlights the importance of rare supernova types in enriching interstellar gas. Future observations, particularly of less-studied elements, could help refine these conclusions and uncover more about these ancient stellar relics.
Source: Jeena
