探测龙鱼星形成复合体：年轻大质量星团Mercer 30的电离人口
Probing the Dragonfish star-forming complex: the ionizing population of the young massive cluster Mercer 30
摘要：
最近有人声称，龙鱼星云是由一个超亮但难以捉摸的OB协会提供动力的。相反，在近红外光度测量中的系统搜索在这个天空区域发现了许多其他的星团候选者。其中，第一个被证实的年轻大质量星团是Mercer 30，在那里发现了Wolf-Rayet星。我们进行了一个新的表征，前所未有的精度，结合NICMOS/HST和VVV测光数据与多历元ISAAC/VLT的H-和K-波段光谱。大多数光谱观测到的星团成员的恒星参数是通过精确的非LTE大气模型与CMFGEN代码。我们对这个星系团的分光光度研究得出了一个新的、修正后的距离d =（12.4 ± 1.7）kpc，总的莱曼电离光子数Q = 6.70 × 10^50。通过等时线拟合发现星团年龄为（4.0 ± 0.8）Myr，由于我们对后主序群的广泛了解，估计总质量为（1.6 ± 0.6）× 10^4 Msol。因此，Mercer 30的成员身份被确认为龙鱼星形成复合体，使我们能够使用这个星团作为整个复合体的探测器，结果是非常大的（400 pc宽），位于人马座-船底座旋臂的外缘（距离银河系中心11 kpc）。龙鱼复合体拥有19个年轻的星团或候选星团（包括Mercer 30和这项工作中提出的新候选星团），估计至少有9颗沃尔夫-拉叶星。所有这些贡献的总和占至少73%的龙鱼星云电离，并留下很少或根本没有所谓的超亮OB协会的空间;其他解释进行了讨论。
英文摘要：
The Dragonfish Nebula has been recently claimed to be powered by a superluminous but elusive OB association. Instead, systematic searches in near-infrared photometric surveys have found many other cluster candidates on this sky region. Among these, the first confirmed young massive cluster was Mercer 30, where Wolf-Rayet stars were found. We perform a new characterization of Mercer 30 with unprecedented accuracy, combining NICMOS/HST and VVV photometric data with multi-epoch ISAAC/VLT H- and K-band spectra. Stellar parameters for most of spectroscopically observed cluster members are found through precise non-LTE atmosphere modeling with the CMFGEN code. Our spectrophotometric study for this cluster yields a new, revised distance of d = (12.4 +- 1.7) kpc and a total of Q = 6.70 x 10^50 Lyman ionizing photons. A cluster age of (4.0 +- 0.8) Myr is found through isochrone fitting, and a total mass of (1.6 +- 0.6) x 10^4 Msol is estimated thanks to our extensive knowledge of the post-main-sequence population. As a consequence, membership of Mercer 30 to the Dragonfish star-forming complex is confirmed, allowing us to use this cluster as a probe for the whole complex, which turns out to be extremely large (400 pc across) and located at the outer edge of the Sagittarius-Carina spiral arm (11 kpc from the Galactic Center). The Dragonfish complex hosts 19 young clusters or cluster candidates (including Mercer 30 and a new candidate presented in this work) and an estimated minimum of 9 field Wolf-Rayet stars. The sum of all these contributions accounts for, at least, 73% of the Dragonfish Nebula ionization and leaves little or no room for the alleged superluminous OB association; alternative explanations are discussed.
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