Characterizing the nature of embedded young stellar objects through silicate, ice and millimeter observations
Characterizing the nature of embedded young stellar objects through silicate, ice and millimeter observations
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通过硅酸盐、冰和毫米波观测来表征嵌入式年轻恒星的性质
DOI：
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发表时间：
2008
期刊：
影响因子：
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通讯作者：
C. Dullemond
中科院分区：
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作者：
A. Crapsi;E. Dishoeck;M. Hogerheijde;K. Pontoppidan;C. Dullemond
上下文确定年轻恒星天体（YSO）的演化阶段对于检验星星形成理论具有重要意义。YSO的分类方案是基于对周围包层耗散程度的评估，其主要影响是来自中心YSO的光辐射的消光和电磁频谱远红外至毫米部分的再发射。由于消光是沿视线沿着的柱密度的一种性质，原行星盘的存在可能会导致从侧面观察时对具有盘的前主序星的错误分类。我们进行了辐射传输计算，以显示不同几何形状对YSO演化阶段主要指标的影响。特别是，我们不仅测试了对红外颜色的影响，如2.2和24 μ m之间通量的斜率α，而且还测试了对YSO演化阶段的其他流行指标的影响，如硅酸盐和冰的辐射热温度和光学深度。 方法.我们使用的轴对称三维辐射传输代码RADMC和RADCAL计算的光谱能量分布，包括硅酸盐和冰功能的网格模型，涵盖了嵌入式和前主要序列源的典型物理特性的范围。结果我们的一组模型与现有的观测结果进行了比较，支持假设的密度参数化和采用的尘埃不透明度。我们表明，对于从中间角度（25°-70°）观察的系统，“经典”演化指标能够对年轻恒星的演化程度进行分类，因为它们准确地跟踪了包络柱密度，并且它们都彼此一致。另一方面，对于大于~65° ± 5°的倾角，边缘系统被错误分类，其中扩散主要是由于质量范围和盘的张开程度。特别是具有盘状结构的前主序星的典型硅酸盐发射，当沿视线方向的盘柱密度沿着达到1 $时，就转变为硅酸盐吸收 imes $10 22 cm-2，对应于例如5 $ 在64°角观察到的直径为10 - 3倍的外展盘。在所研究的所有其他分类指标中注意到类似的效果：α、T bol以及H2 O和CO2冰吸收强度。这种错误的分类有一个很大的影响的性质的平谱源$（α simeq 0）$，其数量可以解释简单的几何参数，而无需调用演化。一个可靠的分类计划，使用最少的观测数构成的观测毫米通量与一个单一的盘和干涉仪。
Context. Determining the evolutionary stage of a Young Stellar Object (YSO) is of fundamental importance to test star formation theories. Classification schemes for YSOs are based on evaluating the degree of dissipation of the surrounding envelope, whose main effects are the extinction of the optical radiation from the central YSO and re-emission in the far-infrared to millimeter part of the electromagnetic spectrum. Since extinction is a property of column density along the line of sight, the presence of a protoplanetary disk may lead to a misclassification of pre-main sequence stars with disks when viewed edge-on. Aims. We performed radiative transfer calculations to show the effects of different geometries on the main indicators of YSO evolutionary stage. In particular we tested not only the effects on the infrared colors, like the slope α of the flux between 2.2 and 24 μ m, but also on other popular indicators of YSO evolutionary stage, such as the bolometric temperature and the optical depth of silicates and ices. Methods. We used the axisymmetric 3D radiative transfer codes RADMC and RADICAL to calculate the spectral energy distribution including silicates and ice features in a grid of models covering the range of physical properties typical of embedded and pre-main sequence sources. Results. Our set of models compares well with existing observations, supporting the assumed density parametrization and the adopted dust opacities. We show that for systems viewed at intermediate angles (25°-70°) the “classical” indicators of evolution are able to classify the degree of evolution of young stellar objects since they accurately trace the envelope column density, and they all agree with each other. On the other hand, edge-on system are misclassified for inclinations larger than ~65° ± 5°, where the spread is mostly due to the range of mass and the flaring degree of the disk. In particular, silicate emission, typical of pre-main sequence stars with disks, turns into silicate absorption when the disk column density along the line of sight reaches 1 $ imes$ 10 22 cm -2 , corresponding e.g. to a 5 $ imes$ 10 -3 $M_odot$ flaring disk viewed at 64°. A similar effect is noticed in all the other classification indicators studied: α , T bol , and the H 2 O and CO 2 ice absorption strengths. This misclassification has a large impact on the nature of the flat-spectrum sources $(alpha simeq 0)$, whose number can be explained by simple geometrical arguments without invoking evolution. A reliable classification scheme using a minimal number of observations is constituted by observations of the millimeter flux with both a single dish and an interferometer.