arXiv 2404.091122024-04-14astro-ph.GA

星团涌现的时间尺度：NGC 4449案例

The Timescales of Star Cluster Emergence: The Case of NGC 4449

Timothy McQuaid, Daniela Calzetti, Sean T. Linden, Matteo Messa, Angela Adamo, Bruce Elmegreen, Kathryn Grasha, Kelsey E. Johnson, Linda J. Smith, Varun Bajaj

DOI：: 10.3847/1538-4357/ad3e64

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AI总结：

本文基于HST多波段测光研究NGC 4449年轻星团，发现低质量星团嵌埋时间更长，清除时间尺度受初始质量函数高端随机采样及反馈强度的质量依赖影响。

摘要：

我们调研了矮星系NGC 4449中的年轻星团族群，目标是研究恒星反馈如何依赖于星团属性。利用哈勃空间望远镜获得的紫外（UV）-光学-近红外（NIR）测光，我们恢复出99个在Paβ氢复合线中表现出发射的致密源。分析表明，这些源的质量为10^2<M_⊙<10^5，年龄为1–20 Myr，色余E(B−V)处于0–1.4范围。为缓解恒星初始质量函数随机采样的影响，我们选择质量高于3,000 M_⊙的星团，发现这些红外选择星团的中位质量约为7×10^3 M_⊙，并在5–6 Myr内仍嵌埋于周围气体和尘埃中。相比之下，从现有UV/光学星表中选择的发射线源中位质量约为3.5×10^4 M_⊙，并在4 Myr时已清除周围物质。我们进一步发现，NGC 4449中的环境压力过低，不足以导致这些差异。我们将这些发现解释为证据，表明超新星前和超新星反馈的清除时间尺度依赖于星团质量。即使在质量约7,000 M_⊙的星团中，也存在恒星初始质量函数高端的随机采样，会随机减少可用于向周围介质注入能量和动量的大质量恒星数量。该效应可能通过降低超新星前和超新星反馈的有效性，延长这些星团的清除时间尺度；迄今模型和观测均未明确探索这种依赖关系。未来利用JWST等开展的研究和观测将填补这一空白。

英文摘要：

We survey the young star cluster population in the dwarf galaxy NGC4449 with the goal of investigating how stellar feedback may depend on the clusters' properties. Using Ultraviolet(UV)-optical-NearIR(NIR) photometry obtained from the Hubble Space Telescope, we have recovered 99 compact sources exhibiting emission in the Pa$β$ hydrogen recombination line. Our analysis reveals these sources possess masses $10^{2}<M_{\odot}<10^{5}$, ages 1-20 Myr, and color excess E(B - V) in the range 0-1.4. After selecting clusters with mass above 3,000M$_{\odot}$ to mitigate stochastic sampling of the stellar initial mass function, we find that our IR-selected clusters have a median mass of $\sim$7$\times{10^{3}\text{ M}_{\odot}}$ and remain embedded in their surrounding gas and dust for 5-6 Myr. In contrast, line-emitting sources selected from existing UV/optically catalogs have a median mass of $\sim$3.5$\times{10^{4}\text{ M}_{\odot}}$ and have cleared their surroundings by 4 Myr. We further find that the environment in NGC4449 is too low pressure to drive these differences. We interpret these findings as evidence that the clearing timescale from pre-supernova and supernova feedback is cluster mass-dependent. Even in clusters with mass$\sim$7,000~M$_{\odot}$, stochastic sampling of the upper end of the stellar initial mass function is present, randomly decreasing the number of massive stars available to inject energy and momentum into the surrounding medium. This effect may increase the clearing timescales in these clusters by decreasing the effectiveness of both pre-supernova and supernova feedback; neither models nor observations have so far explored such dependence explicitly. Future studies and observations with, e.g., the JWST, will fill this gap.

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