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arXiv 1605.09406astro-ph.GA
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亮I类甲醇脉泽的物理特性

Physical characteristics of bright Class I methanol masers

S. Leurini, K. M. Menten, C. M. Walmsley

备注：
Accepted for publication in A&A
发表信息：
A&A 592, A31 (2016)
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摘要：

I类CH $_3$ OH脉泽跟踪星际激波。它们很少受到关注，主要是因为它们的低光度;这种情况最近发生了变化，I类脉泽现在通常被用作外流的路标。最近对I类谱线极化的探测，使我们有可能获得有关激波中磁场的信息。我们利用新计算的碰撞率来研究I类脉泽的激发，并使其观察到的性质与模型结果相一致。我们进行LVG计算与平面平行板的几何形状来计算泵和损失率，调节不同的脉泽系统与脉泽水库的相互作用。本文研究了抽运速率、损失率和反转效率对I类脉泽抽运方案的依赖性。明亮的I类脉泽主要是高温高密度结构，其脉泽发射措施对应于高CH $_3$ OH丰度，接近碰撞猝灭的极限。我们的模型相当好地再现了I类脉泽的大部分观测特性。25 GHz脉泽谱线对密度和脉泽的灵敏度最高，且在密度较高时脉泽的灵敏度最高。此外，即使在高密度和高丰度下，它们的光度也低于44 GHz和36 GHz线。通过比较观测到的各向同性光子亮度和我们的模型，我们推断光束立体角~0.001球面度。一级脉泽可分为三个家族：$(J+1)_{-1}-J_{0}$ -E型、$(J+1)_0-J_1$ -A型和$J_2-J_1$ -E型。25 GHz谱线的行为与其他脉泽不同，因为与其他I类脉泽相比，它们仅在密度高于$10^6$ cm $^{-3}$时才反转。因此，在所有3个家庭中检测到微波激射活动是高密度的明确指示。

英文摘要：

Class I CH$_3$OH masers trace interstellar shocks. They have received little attention mostly as a consequence of their low luminosities; this situation has changed recently and Class I masers are now routinely used as signposts of outflows. The recent detection of polarisation in Class I lines now makes it possible to obtain information on magnetic fields in shocks. We make use of newly calculated collisional rates to investigate the excitation of Class I masers and to reconcile their observed properties with model results. We performed LVG calculations with a plane-parallel slab geometry to compute the pump and loss rates which regulate the interactions of the different maser systems with the maser reservoir. We study the dependence of the pump rate, the loss rate, and the inversion efficiency of the pumping scheme of Class I masers on the physics of the gas. Bright Class I masers are mainly high-temperature high-density structures with maser emission measures corresponding to high CH$_3$OH abundances close to the limits set by collisional quenching. Our model reproduces reasonably well most of the observed properties of Class I masers. The 25 GHz masers are the most sensitive to the density and mase at higher densities than other lines. Moreover, even at high density and high abundance, their luminosity is lower than that of the 44 GHz and 36 GHz lines. By comparison between observed isotropic photon luminosities and our model, we infer beam solid angles of ~0.001 steradian. Class I masers can be separated into 3 families: the $(J+1)_{-1}-J_{0}$-E type, the $(J+1)_0-J_1$-A type, and the $J_2-J_1$-E lines. The 25 GHz lines behave in a different fashion from the other masers as they are only inverted at densities above $10^6$ cm$^{-3}$ in contrast to other Class I masers. Therefore, the detection of maser activity in all 3 families is a clear indication of high densities.

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