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arXiv 1904.01136astro-ph.GA
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非常年轻的无星核L1521E的化学结构

The chemical structure of the very young starless core L1521E

Z. Nagy, S. Spezzano, P. Caselli, A. Vasyunin, M. Tafalla, L. Bizzocchi, D. Prudenzano, E. Redaelli

备注：
Accepted for publication in A&A, abstract abridged
发表信息：
A&A 630, A136 (2019)
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摘要：

L1521 E是金牛座的一个致密的无星核心，早期的研究发现它的分子消耗相对较低，因此表明它是最近形成的。我们的目标是描述L1521 E的化学结构，并将其与更进化的L1544前星核进行比较。我们用IRAM-30 m望远镜获得了L1521 E附近的$\sim$ 2.5 $\times$ 2.5角分跃迁图。我们推导出该物种的丰度，并将其与L1544获得的丰度进行比较。我们估计了CO消耗因子。与L1544类似，$c$-C $_3$ H $_2$和CH $_3$ OH在不同位置处有峰.大多数物质的峰朝向$c$ -C $_3$ H $_2$峰.朝向$Herschel$尘埃峰导出的CO消耗因子为4.3 $\pm$ 1.6，这比朝向L1544的CO消耗因子低约三倍。含硫分子的丰度朝向L1521 E比朝向L1544高$\sim$ 2-20倍。甲醇的丰度在两个岩心上相似。L1521 E的含硫物质丰度高于L1544，表明在致密核的动力学演化过程中，从无星到星前阶段，发生了显著的硫贫化。在L1521 E和L1544之间类似的CH $_3$ OH丰度表明，金牛座在特定的物理条件下形成甲醇，其特征在于几个$\times$ 10 $^4$ cm $^{-3}$和$N$的密度（H $_2$）$\gtrsim$ 10 $^{22}$ cm $^{-2}$，当CO开始灾难性地冻结时，而水仍然可以被显著地光解，使得尘埃颗粒的表面变得富含固体CO和CH $_3$ OH，正如已经发现的L1544。因此，甲醇可以提供选择性的关键信息之间的过渡区致密的核心和周围的母云。

英文摘要：

L1521E is a dense starless core in Taurus that was found to have relatively low molecular depletion by earlier studies, thus suggesting a recent formation. We aim to characterize the chemical structure of L1521E and compare it to the more evolved L1544 pre-stellar core. We have obtained $\sim$2.5$\times$2.5 arcminute maps toward L1521E using the IRAM-30m telescope in transitions of various species. We derived abundances for the species and compared them to those obtained toward L1544. We estimated CO depletion factors. Similarly to L1544, $c$-C$_3$H$_2$ and CH$_3$OH peak at different positions. Most species peak toward the $c$-C$_3$H$_2$ peak. The CO depletion factor derived toward the $Herschel$ dust peak is 4.3$\pm$1.6, which is about a factor of three lower than that toward L1544. The abundances of sulfur-bearing molecules are higher toward L1521E than toward L1544 by factors of $\sim$2-20. The abundance of methanol is similar toward the two cores. The higher abundances of sulfur-bearing species toward L1521E than toward L1544 suggest that significant sulfur depletion takes place during the dynamical evolution of dense cores, from the starless to pre-stellar stage. The CO depletion factor measured toward L1521E suggests that CO is more depleted than previously found. Similar CH$_3$OH abundances between L1521E and L1544 hint that methanol is forming at specific physical conditions in Taurus, characterized by densities of a few $\times$10$^4$ cm$^{-3}$ and $N$(H$_2$)$\gtrsim$10$^{22}$ cm$^{-2}$, when CO starts to catastrophically freeze-out, while water can still be significantly photodissociated, so that the surfaces of dust grains become rich in solid CO and CH$_3$OH, as already found toward L1544. Methanol can thus provide selective crucial information about the transition region between dense cores and the surrounding parent cloud.

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