沿W43脊由扩展的SiO发射追踪的低速激波：见证年轻大质量星系团的形成
Low-velocity shocks traced by extended SiO emission along the W43 ridges: witnessing the formation of young massive clusters
摘要：
大质量恒星的形成与其母云的形成紧密相连。我们在这里集中在W 43的高密度部分，一个分子云正在经历一个有效的形成事件。云的结构进行了研究，从赫歇尔连续图在70，160，250，350和500微米的柱密度图像。我们发现了两个高柱密度的对流云，分别称为W 43-MM 1和W 43-MM 2脊，它们都能解释10^23 cm-2以上、5和14 pc ^2区域内的1.5x10^4 Msun气体质量。我们用N_2H^+ 1- 0线证实了W 43-MM 1和W 43-MM 2脊是速度相干和重力束缚的结构，尽管它们的速度色散很大，线宽约为5公里。W 43大型计划最令人感兴趣的结果是明亮的宽分布SiO2--1发射：1--11 K kms$，延伸面积约为28 pc^2。它集中在W 43-MM 1和W 43-MM 2脊及其附近，导致总光度为L_SiO2 -1 ~4 10^4 K kms kpc^2pc^2。我们测量了SiO~2- 1谱线的光度和速度范围之间的陡峭关系，并建议用它来区分这里观测到的低速激波和与大质量年轻恒星外流有关的更经典的高速激波。我们使用最先进的激波模型证明，低速（<10 km ^-1）激波中的少量（10%）Si原子最初处于气相或颗粒地幔中，可以解释W 43中观测到的SiO柱密度。高密度气体（n_H2>10^4-10^5 cm^-3）和受冲击的SiO气体在空间和速度上的重叠表明，洋脊可能是通过重力驱动的碰撞流和伴随的低速冲击形成的。这一机制可能是在这些脊中形成年轻大质量星团的初始条件。
英文摘要：
The formation of high-mass stars is tightly linked to that of their parental clouds. We here focus on the high-density parts of W43, a molecular cloud undergoing an efficient event of formation. The cloud structure is studied with a column density image derived from Herschel continuum maps obtained at 70, 160, 250, 350, and 500 micron. We identify two high-column density filamentary clouds, quoted as the W43-MM1 and W43-MM2 ridges, which both account for 1.5x10^4 Msun gas mass above 10^23 cm-2 and within areas of 5 and 14pc^2, respectively. We used the N_2H^+ 1--0 line to confirm that the W43-MM1 and W43-MM2 ridges are structures coherent in velocity and gravitationally bound, despite their large velocity dispersion and ~5 kms line widths. The most intriguing result of the W43 large program is the bright wide-spread SiO 2--1 emission: 1--11 K kms$ stretching an area of ~28 pc^2. Concentrated toward the W43-MM1 and W43-MM2 ridges and their immediate surroundings, it leads to a total luminosity of L_SiO 2-1 ~4 10^4 K kms kpc^2pc^2. We measured a steep relation between the luminosity and velocity extent of the SiO~2--1 lines and propose to use it to distinguish the low-velocity shocks observed here from the more classical high-velocity ones associated with outflows of high-mass young stellar objects. We used state-of-the-art shock models to demonstrate that low-velocity (<10 kms^-1) shocks with a small amount (10%) of Si atoms initially in gas phase or in grain mantles can explain the observed SiO column density in W43. The spatial and velocity overlaps between the ridges high-density gas (n_H2>10^4-10^5 cm^-3) and the shocked SiO gas suggests that ridges could be forming via colliding flows driven by gravity and accompanied by low-velocity shocks. This mechanism may be the initial conditions for the formation of young massive clusters in these ridges.
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