Bottlenecks to interstellar sulfur chemistry: Sulfur-bearing hydrides in UV-illuminated gas and grains
Bottlenecks to interstellar sulfur chemistry: Sulfur-bearing hydrides in UV-illuminated gas and grains
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DOI：
10.1051/0004-6361/202039756
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发表时间：
2021-03-02
影响因子：
6.5
通讯作者：
Duran, C. A.
中科院分区：
文献类型：
--
作者：
Goicoechea, J. R.;Aguado, A.;Duran, C. A.
氢化物分子是星际化学的基础，但硫化氢化物的合成却知之甚少，其丰度往往受到粗略的限制。受猎户座棒光解离区（PDR）的新观测的启发-SH+的1 "分辨率阿尔马图像; IRAM 30 m明亮的H32 2 S，H-2（32）S和（H2S）-S-33线的探测; H3 S+（上限）;和索菲亚/GREAT观测SH（上限）-我们对含硫硫化物的化学进行了系统的研究。我们自洽地确定它们的柱密度使用耦合激发，辐射传输以及化学形成和破坏模型。我们修改了一些关键的气相反应，导致其化学合成。这包括振动态依赖反应SH+ + H-2（v）可逆箭头H2S+ + H和S + H-2（v）可逆箭头SH + H的从头计算。我们发现紫外泵浦的H-2（v >= 2）分子与S+离子的反应解释了在高热压气体组分中SH+的存在，Pth/k接近10（8）cm（-3）K，靠近H-2解离前沿（AV < 2 mag）。这些PDR层的特征在于没有或非常少地耗尽来自气体的元素硫。然而，随后SH+、H2S+和S原子与振动激发的H-2的夺氢反应未能形成足够的H2S+、H3 S+和SH来最终解释观察到的H2S柱密度（类似于2.5 x 10（14）cm（-2），邻位与帕拉比为2.9 +/- 0.3;与高温统计值一致）。为了克服这些瓶颈，我们建立了PDR模型，其中包括一个简单的网络，导致固体H2S（S-H2S）的形成的粮食表面反应。最近的研究表明，S和SH的较高吸附结合能意味着S原子在较温暖的尘埃温度（Td < 50 K）和更接近紫外线照射的分子云边缘处吸附在颗粒上（并形成s-H2S）。我们发现，无论s-H2S地幔形成（艾德），气相H2S发射线将是可检测的。光解吸和较小程度上的化学解吸产生大致相同的H2S柱密度（几个10（14）cm（-2））和丰度峰（几个10（-8）），几乎与n（H）和G（0）无关。这与在猎户座酒吧以及在乌云的边缘观察到的H2S柱密度一致，而不会引起元素硫丰度的大量消耗。
Hydride molecules lie at the base of interstellar chemistry, but the synthesis of sulfuretted hydrides is poorly understood and their abundances often crudely constrained. Motivated by new observations of the Orion Bar photodissociation region (PDR) - 1 '' resolution ALMA images of SH+; IRAM 30 m detections of bright H32 2 S, H-2(32) S, and (H2S)-S-33 lines; H3S+ (upper limits); and SOFIA/GREAT observations of SH (upper limits) - we perform a systematic study of the chemistry of sulfur-bearing hydrides. We self-consistently determine their column densities using coupled excitation, radiative transfer as well as chemical formation and destruction models. We revise some of the key gas-phase reactions that lead to their chemical synthesis. This includes ab initio quantum calculations of the vibrational-state-dependent reactions SH+ + H-2(v) reversible arrow H2S+ + H and S + H-2 (v) reversible arrow SH + H. We find that reactions of UV-pumped H-2( v >= 2) molecules with S+ ions explain the presence of SH+ in a high thermal-pressure gas component, Pth/k approximate to 10(8) cm(-3) K, close to the H-2 dissociation front (at AV < 2 mag). These PDR layers are characterized by no or very little depletion of elemental sulfur from the gas. However, subsequent hydrogen abstraction reactions of SH+, H2S+, and S atoms with vibrationally excited H-2, fail to form enough H2S+, H3S+, and SH to ultimately explain the observed H2S column density (similar to 2.5 x 10(14) cm(-2), with an ortho-to-para ratio of 2.9 +/- 0.3; consistent with the high-temperature statistical value). To overcome these bottlenecks, we build PDR models that include a simple network of grain surface reactions leading to the formation of solid H2S (s-H2S). The higher adsorption binding energies of S and SH suggested by recent studies imply that S atoms adsorb on grains (and form s-H2S) at warmer dust temperatures (T-d < 50 K) and closer to the UV-illuminated edges of molecular clouds. We show that everywhere s-H2S mantles form(ed), gas-phase H2S emission lines will be detectable. Photodesorption and, to a lesser extent, chemical desorption, produce roughly the same H2S column density (a few 10(14) cm(-2)) and abundance peak (a few 10 (-8)) nearly independently of n(H) and G(0). This agrees with the observed H2S column density in the Orion Bar as well as at the edges of dark clouds without invoking substantial depletion of elemental sulfur abundances.