标题： 年轻恒星天体周围的X射线化学 显示英文标题

标题： X-ray chemistry in envelopes around young stellar objects

摘要： 我们提出了年轻恒星天体（YSO）包层的化学模型，该包层暴露于中央X射线源下。 这些模型应用于具有不同X射线通量的大量恒星形成区AFGL 2591。 总X射线电离率主要由H2的`次级'电离率主导，这是由于快速电子引起的。 讨论了碳、硫和氮的化学过程。 发现He+和H3+被增强，并触发了一种特殊的化学反应。 发现了几种分子X射线示踪剂并与远紫外（FUV）场的示踪剂进行了比较。 与紫外线辐射场类似，X射线增强了简单的氢化物、离子和自由基。 与紫外线光子相反， X射线能够深入穿透包层并在远离源的大距离处影响化学过程。 而FUV增强的物种覆盖区域为200-300天文单位，X射线增强的区域>1000天文单位。 AFGL 2591的最佳拟合模型预测X射线亮度LX > 1e+31尔格/秒，具有硬X射线谱TX > 3e+07 K。此外，我们发现LX/Lbol ~ 1e-6。 对于X射线亮度LX < 1e+33尔格/秒，包层质量的化学过程主要由宇宙射线诱导的反应主导，而不是由X射线诱导的电离主导。 计算得到的HCO+和HCS+的线强度表明，由于较高的临界密度，高J线比低J线受到X射线更大的影响，并且这种差异即使使用大孔径单碟望远镜也可以检测到。 未来的仪器如赫歇尔-HIFI或SOFIA将能够观测X射线增强的氢化物，而ALMA的灵敏度和空间分辨率非常适合测量受X射线影响的区域的大小和几何形状。 显示英文摘要

摘要： We present chemical models of the envelope of a young stellar object (YSO) exposed to a central X-ray source. The models are applied to the massive star-forming region AFGL 2591 for different X-ray fluxes. The total X-ray ionization rate is dominated by the `secondary' ionization rate of H2 resulting from fast electrons. The carbon, sulphur and nitrogen chemistries are discussed. It is found that He+ and H3+ are enhanced and trigger a peculiar chemistry. Several molecular X-ray tracers are found and compared to tracers of the far ultraviolet (FUV) field. Like ultraviolet radiation fields, X-rays enhance simple hydrides, ions and radicals. In contrast to ultraviolet photons, X-rays can penetrate deep into the envelope and affect the chemistry even at large distances from the source. Whereas the FUV enhanced species cover a region of 200-300 AU, the region enhanced by X-rays is >1000 AU. Best-fit models for AFGL 2591 predict an X-ray luminosity LX > 1e+31 ergs/s with a hard X-ray spectrum TX > 3e+07 K. Furthermore, we find LX/Lbol ~ 1e-6. The chemistry of the bulk of the envelope mass is dominated by cosmic-ray induced reactions rather than by X-ray induced ionization for X-ray luminosities LX < 1e+33 ergs/s. The calculated line intensities of HCO+ and HCS+ show that high-J lines are more affected than lower J lines by the presence of X-rays due to their higher critical densities, and that such differences are detectable even with large aperture single-dish telescopes. Future instruments such as Herschel-HIFI or SOFIA will be able to observe X-ray enhanced hydrides whereas the sensitivity and spatial resolution of ALMA is well-suited to measure the size and geometry of the region affected by X-rays.