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arXiv 1106.3083astro-ph.GA
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大质量星系团中恒星的Bondi吸积

Star Formation in Massive Clusters via Bondi Accretion

Norman Murray, Philip Chang

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8 pages, 5 figures, submitted to ApJ
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摘要：

基本上所有的恒星都形成于巨大的分子云（GMC）中。然而，在GMC内部，大部分气体并不参与恒星的形成;相反，密度更大的气体聚集在GMC中的团块中，给定GMC中的大部分恒星形成在少数最大质量的团块中。在银河系中，这些团块具有GMC的质量$M_{\rm cl}\lesssim 5\times 10^{-2}$、半径$r_{\rm cl} \sim 1$ pc和自由落体时间$τ_{\rm cl} \sim 2\times 10^5\yr$。我们发现，巨大的分子云内的团块应在修改后的邦迪吸积率，这取决于团块质量$\dot M_{\rm cl}\sim M_{\rm cl}^{5/4}$。这个速率最初相当慢，通常比星团内部的初始恒星形成速率慢（我们采用通常的假设，即星团内部有$\dot M_*=ε_{\rm ff} M_{\rm cl}/τ_{\rm cl}$和$ε_{\rm ff} \approx 0.017$）。然而，在$\sim 2$ GMC自由落体乘以$τ_{\rm GMC}$之后，团块的吸积速率迅速加快;形式上，团块可以吸积$\sim 3τ_{\rm GMC}$中的整个GMC。与此同时，恒星的形成速度加快，跟踪邦迪吸积率。如果GMC被来自最大的星团的反馈所破坏，那么在GMC被破坏之前，该星团中的一半恒星在最终的$\taug$中形成。该理论预测，每GMC自由落体时间测量的有效恒星形成率的分布范围很广，从$\sim 0.001$到0.1或更大，星团的质谱比团块的质谱更平坦，与观测结果一致。

英文摘要：

Essentially all stars form in giant molecular clouds (GMCs). However, inside GMCs, most of the gas does not participate in star formation; rather, denser gas accumulates in clumps in the GMC, with the bulk of the stars in a given GMC forming in a few of the most massive clumps. In the Milky Way, these clumps have masses $M_{\rm cl}\lesssim 5\times 10^{-2}$ of the GMC, radii $r_{\rm cl} \sim 1$pc, and free-fall times $τ_{\rm cl} \sim 2\times 10^5\yr$. We show that clumps inside giant molecular clouds should accrete at a modified Bondi accretion rate, which depends on clump mass as $\dot M_{\rm cl}\sim M_{\rm cl}^{5/4}$. This rate is initially rather slow, usually slower than the initial star formation rate inside the clump (we adopt the common assumption that inside the clump, $\dot M_*=ε_{\rm ff} M_{\rm cl}/τ_{\rm cl}$, with $ε_{\rm ff} \approx 0.017$). However, after $\sim 2$ GMC free-fall times $τ_{\rm GMC}$, the clump accretion rate accelerates rapidly; formally, the clump can accrete the entire GMC in $\sim 3τ_{\rm GMC}$. At the same time, the star formation rate accelerates, tracking the Bondi accretion rate. If the GMC is disrupted by feedback from the largest clump, half the stars in that clump form in the final $\taug$ before the GMC is disrupted. The theory predicts that the distribution of effective star formation rates, measured per GMC free-fall time, is broad, ranging from $\sim 0.001$ up to 0.1 or larger and that the mass spectrum of star clusters is flatter than that of clumps, consistent with observations.

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