标题： L波段的13颗爆发性Be星光谱 显示英文标题

标题： L-band spectra of 13 outbursting Be stars

摘要： 我们展示了使用ISAAC在ESO帕拉纳尔天文台获得的13颗爆发Be星的新L波段光谱。 这些恒星可以根据发射线的存在/不存在以及Br$\alpha$和Pf$\gamma$发射线相对于Humphreys线（从6--14跃迁到系列末端）的强度分为三组。 这些组代表了具有不同光学深度的周恒星包层。 对于显示Br$\alpha$和Pf$\gamma$线比Humphreys线更强的组，Humphreys递减大致遵循Menzel情况-B在光学薄条件下的情况。 对于显示Br$\alpha$、Pf$\gamma$和Humphreys发射线强度相近的组，Humphreys递减在特征波长处从光学薄过渡到光学厚区域，该波长对每颗恒星来说是特有的，但通常位于3.65--3.75$\mu$m（跃迁6--19和6--17）。 高阶Humphreys线即使在光学更厚的包层中也能探测到光学薄的内部区域。 我们发现红外发射线相比光学线有更大的展宽，这可能反映了靠近恒星的更大垂直速度场。 上述组别的存在原则上与de Wit等人（2006）提出的关于Be星爆发的描述一致，即通过喷射一个光学厚的盘，该盘在消散到星际介质之前膨胀并变得光学薄。 时间分辨的L波段光谱采样爆发周期有望成为测试Be星盘演化的独特工具。 显示英文摘要

摘要： We present new L-band spectra of 13 outbursting Be stars obtained with ISAAC at the ESO Paranal observatory. These stars can be classified in three groups depending on the presence/absence of emission lines and the strength of Br$\alpha$ and Pf$\gamma$ emission lines relative to those of Humphreys lines from transitions 6--14 to the end of the series. These groups are representative of circumstellar envelopes with different optical depths. For the group showing Br$\alpha$ and Pf$\gamma$ lines stronger than Humphreys lines, the Humphreys decrement roughly follow the Menzel case-B for optically thin conditions. For the group showing comparable Br$\alpha$, Pf$\gamma$ and Humphreys emission line strengths, the Humphreys decrements moves from an optically thin to an optically thick regime at a transition wavelength which is characteristic for each star, but typically is located around 3.65--3.75 $\mu$m (transitions 6--19 and 6--17). Higher order Humphreys lines probe optically thin inner regions even in the optically thicker envelopes. We find evidence of larger broadening in the infrared emission lines compared with optical lines, probably reflecting larger vertical velocity fields near the star. The existence of the aforementioned groups is in principle consistent with the proposed description by de Wit et al. (2006) for Be star outbursts in terms of the ejection of an optically thick disk that expands and becomes optically thin before dissipation into the interstellar medium. Time resolved L-band spectroscopy sampling the outburst cycle promises to be an unique tool for testing Be star disk evolution.