约束 NGC 1977 中已知 proplyd 与 VLA 新识别候选 proplyd 的自由-自由辐射及光致蒸发质量损失率
Constraining free-free emission and photoevaporative mass loss rates for known proplyds and new VLA-identified candidate proplyds in NGC 1977
AI总结：
本研究利用 VLA 多频段观测识别 NGC 1977 中的候选 proplyd，并由自由-自由辐射约束其光致蒸发质量损失率，揭示中等 UV 环境下典型星团形成条件。
摘要：
我们展示了 Karl G. Jansky Very Large Array 在 3.0、6.4 和 15.0 GHz 下覆盖 NGC 1977 区域的观测。我们搜寻致密射电源，并探测到来自 34 个 NGC 1977 星团成员和 37 个背景天体的连续谱辐射。在这 34 个射电探测到的星团成员中，3 个与 NGC 1977 中已知的 proplyd 相关，22 个与 NGC 1977 中另外的年轻恒星天体相关，9 个是新识别的星团成员。我们检查了探测到的 NGC 1977 源的射电能谱分布、圆偏振和变化性，并识别出 10 个新的候选 proplyd，其射电流量由光学薄自由-自由辐射主导。我们利用自由-自由辐射测量来计算 NGC 1977 中已知 proplyd 和新候选 proplyd 的质量损失率，得到约 10^-9–10^-8 M⊙ yr^-1 的数值；这些值低于在 Orion Nebula Cluster 中朝向 proplyd 测得的质量损失率，但与外部光致蒸发模型对空间延展盘的预测一致，这些盘受到 NGC 1977 中遇到的典型外部 UV 场照射。最后，我们表明 NGC 1977 中的光致蒸发盘风可能由内部或外部电离源照亮，具体取决于它们在星团中的位置。本研究为一个成团恒星形成区域中的盘性质提供了新的约束；该区域的 UV 环境弱于 Orion Nebula Cluster，但强于 Taurus 等低质量恒星形成区域。这类中等 UV 环境代表了银河系恒星和行星形成的典型条件。
英文摘要：
We present Karl G. Jansky Very Large Array observations covering the NGC 1977 region at 3.0, 6.4, and 15.0 GHz. We search for compact radio sources and detect continuum emission from 34 NGC 1977 cluster members and 37 background objects. Of the 34 radio-detected cluster members, 3 are associated with known proplyds in NGC 1977, 22 are associated with additional young stellar objects in NGC 1977, and 9 are newly-identified cluster members. We examine the radio spectral energy distributions, circular polarization, and variability of the detected NGC 1977 sources, and identify 10 new candidate proplyds whose radio fluxes are dominated by optically thin free-free emission. We use measurements of free-free emission to calculate the mass-loss rates of known proplyds and new candidate proplyds in NGC 1977, and find values $\sim10^{-9}-10^{-8}$ M$_{\odot}$ yr$^{-1}$, which are lower than the mass-loss rates measured towards proplyds in the Orion Nebula Cluster, but consistent with the mass-loss rates predicted by external photoevaporation models for spatially-extended disks that are irradiated by the typical external UV fields encountered in NGC 1977. Finally, we show that photoevaporative disk winds in NGC 1977 may be illuminated by internal or external sources of ionization, depending on their positions within the cluster. This study provides new constraints on disk properties in a clustered star-forming region with a weaker UV environment than the Orion Nebula Cluster, but a stronger UV environment than low-mass star-forming regions like Taurus. Such intermediate UV environments represent the typical conditions of Galactic star and planet formation.
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