AI 中文总结
本研究结合JWST的NIRCam数据与NIRSpec光谱，对NGC 2024的亚恒星IMF进行约束，发现无证据支持12倍木星质量以下IMF存在拐点，明确了NIRCam数据的局限性。
AI 中文摘要
一项近期研究基于詹姆斯·韦伯空间望远镜（JWST）近红外相机（NIRCam）图像中的褐矮星候选体样本，报告了在致密星团NGC 2024（年龄小于1百万年）中，初始质量函数（IMF）在12倍木星质量（Mjup）以下出现拐点的探测结果，以及可能在3倍木星质量附近探测到最小质量的结论。本研究使用上述JWST的NIRCam数据，结合JWST近红外光谱仪（NIRSpec）的存档光谱，对NGC 2024的亚恒星IMF进行约束。研究提供了87个天体的NIRSpec数据，其中67个位于NIRCam视场范围内。基于这些光谱及前期研究数据（如X射线辐射），将45个NIRSpec目标归类为NGC 2024成员，其中17个的光谱型表明为褐矮星（M6.5-L型，13个位于NIRCam视场内）。根据理论演化模型，这些褐矮星的质量估计低至4倍木星质量。仍存在一些缺乏光谱的测光候选体，其质量延伸至图像的完整探测极限（消光校正后的K波段星等Ak≥1时，对应质量≥2倍木星质量），因此尚未探测到IMF的最小质量。由于NIRCam图像包含的褐矮星数量少、高消光导致对低质量天体的偏差，以及10倍木星质量以上因饱和极限导致的不完整性，无法从NIRCam图像中测量可靠的亚恒星IMF。因此，JWST数据中没有12倍木星质量以下存在拐点的证据。
英文摘要
A recent study has reported the detection of a turnover in the initial mass function (IMF) below 12 Mjup and the possible detection of a minimum mass near 3 Mjup in the heavily embedded cluster NGC 2024 (<1 Myr), which was based on a sample of brown dwarf candidates in NIRCam images from the James Webb Space Telescope (JWST). I have used those NIRCam data in conjunction with archival spectra from JWST's Near-Infrared Spectrograph (NIRSpec) to constrain the substellar IMF in NGC 2024. I present NIRSpec data for 87 sources, 67 of which are within the NIRCam field. Based on those spectra and data from previous studies (e.g., X-ray emission), I have classified 45 of the NIRSpec targets as members of NGC 2024, 17 of which have spectral types indicative of brown dwarfs (M6.5-L; 13 are within NIRCam). The latter have mass estimates as low as 4 Mjup according to theoretical evolutionary models. There remain a few photometric candidates lacking spectra that extend down to the completeness limit of the images (>=2 Mjup for Ak>=1), so the minimum mass of the IMF has not been detected. It is not possible to measure a reliable substellar IMF from the NIRCam images because of the small number of brown dwarfs encompassed by them, a bias against objects at lower masses due to the high extinctions, and incompleteness at >=10 Mjup due to the saturation limit. Thus, there is no evidence in the JWST data for a turnover below 12jup.
CommentsMNRAS, in press
