We investigated the chromospheric activity of 60 pre-main-sequence (PMS) stars in four molecular clouds and five moving groups. It is considered that strong chromospheric activity is driven by the dynamo processes generated by the stellar rotation. In contrast, several researchers have pointed out that the chromospheres of PMS stars are activated by mass accretion from their protoplanetary disks. In this study, the Ca II infrared triplet (IRT) emission lines were investigated utilizing medium- and high-resolution spectroscopy. The observations were conducted with Nayuta/MALLS and Subaru/HDS. Additionally, archive data obtained by Keck/HIRES, VLT/UVES, and VLT/X-Shooter was used. The small ratios of the equivalent widths indicate that Ca II IRT emission lines arise primarily in dense chromospheric regions. Seven PMS stars show broad emission lines. Among them, four PMS stars have more than one order of magnitude brighter emission line fluxes compared to the low-mass stars in young open clusters. The four PMS stars have a high mass accretion rate, which indicates that the broad and strong emission results from a large mass accretion. However, most PMS stars exhibit narrow emission lines. No significant correlation was found between the accretion rate and flux of the emission line. The ratios of the surface flux of the Ca II IRT lines to the stellar bolometric luminosity, R'_IRT, of the PMS stars with narrow emission lines are as large as the largest R'_IRT of the low-mass stars in the young open clusters. This result indicates that most PMS stars, even in the classical T Tauri star stage, have chromospheric activity similar to zero-age main-sequence stars.

中文翻译：

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前主序星Ca ii 红外三线态发射线的测量

我们研究了 4 个分子云和 5 个移动群中 60 颗前主序 (PMS) 恒星的色球活动。人们认为强烈的色球活动是由恒星自转产生的发电机过程驱动的。相比之下，几位研究人员指出，PMS 恒星的色球层是由其原行星盘的质量吸积激活的。在这项研究中，利用中分辨率和高分辨率光谱研究了 Ca II 红外三重 (IRT) 发射线。观察是由 Nayuta/MALLS 和 Subaru/HDS 进行的。此外，还使用了由 Keck/HIRES、VLT/UVES 和 VLT/X-Shooter 获得的存档数据。等效宽度的小比率表明 Ca II IRT 发射线主要出现在密集的色球区域。七颗 PMS 恒星显示出宽阔的发射线。其中，与年轻疏散星团中的低质量恒星相比，四颗 PMS 恒星的发射线通量要亮一个数量级以上。四颗 PMS 恒星的质量吸积率很高，这表明广泛而强烈的发射是由大质量吸积造成的。然而，大多数 PMS 恒星显示出狭窄的发射线。未发现吸积率与发射线通量之间存在显着相关性。Ca II IRT 线的表面通量与具有窄发射线的 PMS 恒星的恒星热光度 R'_IRT 之比与年轻疏散星团中低质量恒星的最大 R'_IRT 一样大. 这一结果表明，大多数 PMS 恒星，即使在经典的金牛 T 星阶段，也具有类似于零年龄主序星的色球活动。与年轻疏散星团中的低质量恒星相比，四颗 PMS 恒星的发射线通量要亮一个数量级以上。四颗 PMS 恒星的质量吸积率很高，这表明广泛而强烈的发射是由大质量吸积造成的。然而，大多数 PMS 恒星显示出狭窄的发射线。未发现吸积率与发射线通量之间存在显着相关性。Ca II IRT 线的表面通量与具有窄发射线的 PMS 恒星的恒星热光度 R'_IRT 之比与年轻疏散星团中低质量恒星的最大 R'_IRT 一样大. 这一结果表明，大多数 PMS 恒星，即使在经典的金牛 T 星阶段，也具有类似于零年龄主序星的色球活动。与年轻疏散星团中的低质量恒星相比，四颗 PMS 恒星的发射线通量要亮一个数量级以上。四颗 PMS 恒星的质量吸积率很高，这表明广泛而强烈的发射是由大质量吸积造成的。然而，大多数 PMS 恒星显示出狭窄的发射线。未发现吸积率与发射线通量之间存在显着相关性。Ca II IRT 线的表面通量与具有窄发射线的 PMS 恒星的恒星热光度 R'_IRT 之比与年轻疏散星团中低质量恒星的最大 R'_IRT 一样大. 这一结果表明，大多数 PMS 恒星，即使在经典的金牛 T 星阶段，也具有类似于零年龄主序星的色球活动。