Towards precise ages and masses of Free Floating Planetary Mass Brown Dwarfs
Towards precise ages and masses of Free Floating Planetary Mass Brown Dwarfs
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寻找自由漂浮行星质量褐矮星的精确年龄和质量
DOI：
10.1093/mnras/stt1477
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发表时间：
2013
影响因子：
4.8
通讯作者：
Canty J
中科院分区：
文献类型：
--
作者：
Canty J
在非常年轻的星团中测量亚恒星初始质量函数（IMF）受到星团成员年龄扩散的可能性的阻碍。这对于候选行星质量物体（PMO）来说尤其严重，因为它们在赫罗图（HRD）上与更古老、质量更大的褐矮星的位置非常相似。这种简并性可以通过测量重力敏感光谱特征来消除。为此，我们获得了晚期M型/早期L型矮星样本的中分辨率（R≈5000）近红外积分场光谱仪（NIFS）K波段光谱。样本包括金牛座协会和猎户座 σ 星团中的古老矮星和非常年轻的褐矮星。我们证明了 2.21μm Naidoublet 的强度与物体的年龄之间呈正相关。我们证明了这些物体的年龄与其 K 波段光谱形状之间的进一步相关性。我们以新指数 H2(K) 指数的形式量化了这种相关性。该指数似乎比奈双峰对重力更敏感，并且具有可以计算重力敏感谱线未解析的光谱的优点，同时它在非常年轻的年龄（<10 Myr）比三角形H带峰对表面重力更敏感。两种相关性都将年轻天体与野外矮人区分开来，而 H2(K) 指数至少在统计上可以区分 ∼1 Myr 天体群体和 ∼10 Myr 天体群体。我们将 H2(K) 指数应用于猎户座星云团 (ONC) PMO 的 NIFS 数据，以及之前发布的其他几个 ONC PMO 的低分辨率光谱，其中 2.21μm Naidoublet 未解析，并得出结论，PMO 的平均年龄为 ∼1 Myr。
Measurement of the substellar initial mass function (IMF) in very young clusters is hampered by the possibility of the age spread of cluster members. This is particularly serious for candidate planetary mass objects (PMOs), which have a very similar location to older and more massive brown dwarfs on the Hertzsprung–Russell Diagram (HRD). This degeneracy can be lifted by the measurement of gravity-sensitive spectral features. To this end we have obtained medium-resolution (R≈ 5000) Near-infrared Integral Field Spectrometer (NIFS)K-band spectra of a sample of late M-/early L-type dwarfs. The sample comprises old field dwarfs and very young brown dwarfs in the Taurus association and in the σ Orionis cluster. We demonstrate a positive correlation between the strengths of the 2.21 μm Naidoublet and the objects’ ages. We demonstrate a further correlation between these objects’ ages and the shape of theirK-band spectra. We have quantified this correlation in the form of a new index, the H2(K) index. This index appears to be more gravity-sensitive than the Naidoublet and has the advantage that it can be computed for spectra where gravity-sensitive spectral lines are unresolved, while it is also more sensitive to surface gravity at very young ages (<10 Myr) than the triangularH-band peak. Both correlations differentiate young objects from field dwarfs, while the H2(K) index can distinguish, at least statistically, populations of ∼1 Myr objects from populations of ∼10 Myr objects. We applied the H2(K) index to NIFS data for one Orion nebula cluster (ONC) PMO and to previously published low-resolution spectra for several other ONC PMOs where the 2.21 μm Naidoublet was unresolved and concluded that the average age of the PMOs is ∼1 Myr.