Massive stars typically undergo short-lived post-main sequence evolutionary phases with strong mass loss and occasional mass eruptions. Many of such massive stars in transition phases have been identified based on their dusty envelopes. The ejected material often veils the stellar photospheres so that the central stars cannot be assigned proper spectral types and evolutionary stages. The infrared spectral range has proved to be ideal for the classification of evolved massive stars and for the characterization of their environments. To improve our knowledge on the central stars of four such dust enshrouded objects: [GKF2010] MN 83, [GKF2010] MN 108, [GKF2010] MN 109, and [GKF2010] MN 112, we collect and present their first medium resolution K-band spectra in the $2.3\,-\,2.47\,\mu$m region and discuss the location of the stars in the JHK color-color diagram. We find that the emission-line spectra of both MN 83 and MN 112 show characteristics typically seen in Luminous Blue Variable (LBV) stars. In addition, we propose that the presence and strength of the newly reported Mg II lines might be used as a new complementary criterion to identify LBV candidates. The spectra of the other two objects imply that MN 108 is an O-type supergiant, whereas MN 109 could be an LBV candidate in its active phase. We derive lower limits for the reddening toward the stars and find that three of all de-reddened fall into the region of confirmed LBVs.

中文翻译：

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四颗大质量恒星过渡阶段的近红外表征

大质量恒星通常经历短暂的主序后演化阶段，伴随着强烈的质量损失和偶尔的质量爆发。许多处于过渡阶段的大质量恒星已经根据它们的尘埃包络被识别出来。喷射出的物质常常掩盖恒星的光球层，因此无法为中央恒星分配适当的光谱类型和演化阶段。红外光谱范围已被证明是演化大质量恒星分类及其环境特征的理想选择。为了提高我们对四个这样的尘埃笼罩天体的中心恒星的了解：[GKF2010] MN 83、[GKF2010] MN 108、[GKF2010] MN 109 和 [GKF2010] MN 112，我们收集并展示了它们的第一个中等分辨率 K-带光谱在 $2.3\,-\,2.47\, \mu$m 区域并讨论星星在 JHK 颜色-颜色图中的位置。我们发现 MN 83 和 MN 112 的发射线光谱都显示出在夜光蓝变星 (LBV) 中常见的特征。此外，我们建议新报告的 Mg II 系的存在和强度可用作识别 LBV 候选者的新补充标准。其他两个天体的光谱表明 MN 108 是 O 型超巨星，而 MN 109 可能是处于活动阶段的 LBV 候选者。我们推导出恒星变红的下限，并发现所有去红化中的三个落入确认的 LBV 区域。我们建议新报告的 Mg II 系的存在和强度可用作识别 LBV 候选者的新补充标准。其他两个天体的光谱表明 MN 108 是 O 型超巨星，而 MN 109 可能是处于活动阶段的 LBV 候选者。我们推导出恒星变红的下限，并发现所有去红化中的三个落入确认的 LBV 区域。我们建议新报告的 Mg II 系的存在和强度可用作识别 LBV 候选者的新补充标准。其他两个天体的光谱表明 MN 108 是 O 型超巨星，而 MN 109 可能是处于活动阶段的 LBV 候选者。我们推导出恒星变红的下限，并发现所有去红化中的三个落入确认的 LBV 区域。