09:15 〜 09:30
[PCG21-02] Unveiling the Role of Metallicity in Star Formation: JWST Observations of the Initial Mass Function in Low-metallicity Young Open Clusters
キーワード：褐色矮星、初期質量関数、James Webb Space Telescope、金属量
The Initial Mass Function (IMF) plays a crucial role in understanding star formation and the evolution of stellar populations. However, its dependence on the metallicity of the star-forming environment is not well understood. This study explores the IMF in a low-metallicity environment using the unique capabilities of the James Webb Space Telescope (JWST). We focus on two young open clusters located in the Digel Cloud 2, a star-forming region approximately 15 kpc away from the Sun in the outer Galaxy, with a metallicity of approximately 0.1 Z_sun. These clusters provide an exceptional opportunity to explore star formation and the evolution of stellar matter in chemically primitive regions, which is highly relevant to understanding the origins and evolution of material in the Solar System and beyond.
Using deep JWST/NIRCam and MIRI imaging, we have successfully resolved the cluster members down to a mass detection limit of 0.02 M_sun, enabling the first detection of brown dwarfs in low-metallicity clusters. Brown dwarfs, which occupy a mass range between stars and planets, play a crucial role in bridging the gap between stellar and planetary formation. Understanding the formation of brown dwarfs provides key insights into the processes that govern both star and planet formation. These findings contribute to a more refined understanding of the boundary between star and planet formation, particularly in low-metallicity environments where the formation of such objects may differ from solar neighborhood regions.
By analyzing these data, we extract mass-limited samples of 52 and 91 sources from each of the two clusters. From these samples, we derive the IMF by fitting the F200W band luminosity function using stellar isochrone models. The IMF describes the distribution of stellar masses in a cluster, with the peak mass, called the characteristic mass (Mc), representing the mass where the number of stars is at a maximum. The results show significantly lower IMF peak masses (log Mc/M_sun ~= -1.5 pm 0.5) compared to local clusters with similar ages but higher metallicities, which typically show peak masses around log Mc/M_sun ~ -0.5.
This finding suggests that metallicity plays a pivotal role in regulating the IMF, with lower metallicity environments favoring the formation of lower-mass stars. The relationship between metallicity and the IMF is of particular interest from a material science perspective, as it reflects how the availability of heavy elements in the interstellar medium influences the chemical and physical properties of the forming stars. This has direct implications for the chemical evolution of galaxies and the formation of planetary systems, particularly in low-metallicity environments where the formation of rocky planets and their potential habitability might be affected by the initial stellar populations.
In addition to stellar mass functions, our study also investigates the formation of brown dwarfs, providing important constraints on the substellar end of the IMF in such low-metallicity environments. These findings are consistent with theoretical models that predict lower thermal Jeans masses in regions with higher gas densities and lower metallicity, inhibiting the formation of high-mass stars while promoting the formation of brown dwarfs and low-mass stars. Although this effect has not been explicitly mentioned in theoretical studies under metallicities of ~-1 dex, it is observationally demonstrated here for the first time. The Mc values derived from observations in such an environment would place significant constraints on the understanding of star formation. Furthermore, comparisons with older, low-metallicity globular clusters suggest that the target clusters have not undergone significant dynamical evolution, which is likely due to the fact that these clusters have not yet experienced significant dynamical evolution and remain in their initial physical condition.
Using deep JWST/NIRCam and MIRI imaging, we have successfully resolved the cluster members down to a mass detection limit of 0.02 M_sun, enabling the first detection of brown dwarfs in low-metallicity clusters. Brown dwarfs, which occupy a mass range between stars and planets, play a crucial role in bridging the gap between stellar and planetary formation. Understanding the formation of brown dwarfs provides key insights into the processes that govern both star and planet formation. These findings contribute to a more refined understanding of the boundary between star and planet formation, particularly in low-metallicity environments where the formation of such objects may differ from solar neighborhood regions.
By analyzing these data, we extract mass-limited samples of 52 and 91 sources from each of the two clusters. From these samples, we derive the IMF by fitting the F200W band luminosity function using stellar isochrone models. The IMF describes the distribution of stellar masses in a cluster, with the peak mass, called the characteristic mass (Mc), representing the mass where the number of stars is at a maximum. The results show significantly lower IMF peak masses (log Mc/M_sun ~= -1.5 pm 0.5) compared to local clusters with similar ages but higher metallicities, which typically show peak masses around log Mc/M_sun ~ -0.5.
This finding suggests that metallicity plays a pivotal role in regulating the IMF, with lower metallicity environments favoring the formation of lower-mass stars. The relationship between metallicity and the IMF is of particular interest from a material science perspective, as it reflects how the availability of heavy elements in the interstellar medium influences the chemical and physical properties of the forming stars. This has direct implications for the chemical evolution of galaxies and the formation of planetary systems, particularly in low-metallicity environments where the formation of rocky planets and their potential habitability might be affected by the initial stellar populations.
In addition to stellar mass functions, our study also investigates the formation of brown dwarfs, providing important constraints on the substellar end of the IMF in such low-metallicity environments. These findings are consistent with theoretical models that predict lower thermal Jeans masses in regions with higher gas densities and lower metallicity, inhibiting the formation of high-mass stars while promoting the formation of brown dwarfs and low-mass stars. Although this effect has not been explicitly mentioned in theoretical studies under metallicities of ~-1 dex, it is observationally demonstrated here for the first time. The Mc values derived from observations in such an environment would place significant constraints on the understanding of star formation. Furthermore, comparisons with older, low-metallicity globular clusters suggest that the target clusters have not undergone significant dynamical evolution, which is likely due to the fact that these clusters have not yet experienced significant dynamical evolution and remain in their initial physical condition.