发表机构
National Tsing Hua University; Macquarie University; Polish Academy of Sciences; National Chung Hsing University（国立清华大学; 麦考瑞大学; 波兰科学院; 国立中兴大学）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
本研究通过JWST/NIRCam光谱和测光确认了Shabgard为一颗寒冷的贫金属褐矮星，有效温度约719K，距离约176pc，展示了深场巡天发现暗弱亚恒星天体的能力。
AI 中文摘要
褐矮星是研究冷大气、恒星形成的低质量端以及恒星与巨行星之间过渡的独特实验室。识别和表征最冷的褐矮星仍然具有挑战性，尤其是在深河外星系巡天中的暗弱星等情况下。我们研究了COSMOS2025 ID639188（以下简称Shabgard），这是一个先前根据COSMOS-Web测光被选为褐矮星候选体的致密源。我们利用了来自COSMOS-3D的偶然获得的JWST/NIRCam F444W宽场无缝光谱，结合COSMOS2025 NIRCam测光和多历元成像。我们提取了局部背景扣除、轮廓加权的$4.05$-$4.85\mu{\rm m}$光谱，并将数据与六个大气模型族进行比较。使用PCA插值和MCMC采样的LOWZ大气网格拟合给出了$T_{\rm eff}=718.6^{+10.6}_{-10.2}$ K和$[{\rm M/H}]=-1.00^{+0.07}_{-0.08}$，支持在晚T/Y转变附近存在寒冷、亚太阳金属丰度的大气。固定大气的分子不透明度测试强烈支持CH$_4$对观测到的$4$-$5 \mu{\rm m}$光谱形状的贡献，尽管受限的波长覆盖阻止了逐分子的丰度确定。模型相关的物理估计范围约为$23$-$52 {\rm M_{\rm Jup}}$和$140$-$180$ pc。我们测量了自行$|\mu|=89.2\pm0.4$ mas yr$^{-1}$，而LOWZ MCMC通量缩放给出的距离为$176.2^{+18.4}_{-17.8}$pc。综合这些结果，支持将Shabgard归类为一颗寒冷的贫金属银河系褐矮星，并展示了JWST深场巡天发现和表征暗弱亚恒星天体的能力。
英文摘要
Brown dwarfs are unique laboratories for studying cool atmospheres, the low-mass end of star formation, and the transition between stars and giant planets. Identifying and characterizing the coldest brown dwarfs remains challenging, particularly at faint magnitudes in deep extragalactic surveys. We investigate COSMOS2025 ID639188, hereafter Shabgard, a compact source previously selected as a brown-dwarf candidate from COSMOS-Web photometry. We use serendipitous JWST/NIRCam F444W wide-field slitless spectroscopy from COSMOS-3D, combined with COSMOS2025 NIRCam photometry and multi-epoch imaging. We extract a local-background-subtracted, profile-weighted $4.05$-$4.85μ{\rm m}$ spectrum and compare the data with six atmosphere-model families. The LOWZ atmosphere-grid fit, using PCA interpolation and MCMC sampling, gives $T_{\rm eff}=718.6^{+10.6}_{-10.2}$ K and $[{\rm M/H}]=-1.00^{+0.07}_{-0.08}$, favoring a cool, subsolar-metallicity atmosphere near the late-T/Y transition. A fixed-atmosphere molecular-opacity test strongly favors a contribution from CH$_4$ to the observed $4$-$5 μ{\rm m}$ spectral shape, although the restricted wavelength coverage prevents a molecule-by-molecule abundance determination. Model-dependent physical estimates span approximately $23$-$52 {\rm M_{\rm Jup}}$ and $140$-$180$ pc. We measure a proper motion of $|μ|=89.2\pm0.4$ mas yr$^{-1}$, while the LOWZ MCMC flux scaling gives a distance of $176.2^{+18.4}_{-17.8}$pc. Together, these results support the classification of Shabgard as a cool, metal-poor Galactic brown dwarf and demonstrate the ability of JWST deep-field surveys to discover and characterize faint substellar objects.
CommentsAccepted for publication in the Publications of the Astronomical Society of the Pacific (PASP)
