标题： 使用赫歇尔/HIFI探测弥漫分子云中的氟化氢吸收：分子气体的普遍示踪剂 显示英文标题

标题： Detection of hydrogen fluoride absorption in diffuse molecular clouds with Herschel/HIFI: a ubiquitous tracer of molecular gas

摘要： 我们讨论了沿视线方向对星际氢氟化物（HF）吸收的探测，该视线指向亚毫米连续谱源W49N和W51。 我们使用赫歇尔望远镜HIFI仪器的双波束切换模式观测了接收器5a频段上边带中的1232.4762 GHz J = 1 - 0 HF跃迁。我们在两个源的方向上检测到了宽速度范围内的前景吸收。在两条视线中均检测到了光学薄吸收组分，使我们能够首次测量而不是获得HF柱密度的下限。 与之前在G10.6-0.4源方向上HF的观测结果一致，推导出的HF柱密度通常与水蒸气的柱密度相当，尽管氧元素丰度比氟元素高四个数量级。 我们使用之前在弥散分子云中推导出的相当不确定的N(CH)-N(H2)关系来推断在显示HF吸收的云中的分子氢柱密度。 在不确定性范围内，我们发现HF相对于H2的丰度与理论预测一致，即HF是这些云中气相氟的主要储库。 因此，氢氟化物有可能成为分子氢的极佳示踪剂，并提供对小H2柱密度云的灵敏探针。 确实，这里报告的氢氟化物观测揭示了在W51视线方向上存在一个低柱密度弥散分子云，在赫歇尔发射前的分子吸收线研究中未被识别，其LSR速度约为24公里/秒。 显示英文摘要

摘要： We discuss the detection of absorption by interstellar hydrogen fluoride (HF) along the sight line to the submillimeter continuum sources W49N and W51. We have used Herschel's HIFI instrument in dual beam switch mode to observe the 1232.4762 GHz J = 1 - 0 HF transition in the upper sideband of the band 5a receiver. We detected foreground absorption by HF toward both sources over a wide range of velocities. Optically thin absorption components were detected on both sight lines, allowing us to measure - as opposed to obtain a lower limit on - the column density of HF for the first time. As in previous observations of HF toward the source G10.6-0.4, the derived HF column density is typically comparable to that of water vapor, even though the elemental abundance of oxygen is greater than that of fluorine by four orders of magnitude. We used the rather uncertain N(CH)-N(H2) relationship derived previously toward diffuse molecular clouds to infer the molecular hydrogen column density in the clouds exhibiting HF absorption. Within the uncertainties, we find that the abundance of HF with respect to H2 is consistent with the theoretical prediction that HF is the main reservoir of gas-phase fluorine for these clouds. Thus, hydrogen fluoride has the potential to become an excellent tracer of molecular hydrogen, and provides a sensitive probe of clouds of small H2 column density. Indeed, the observations of hydrogen fluoride reported here reveal the presence of a low column density diffuse molecular cloud along the W51 sight line, at an LSR velocity of ~ 24kms-1, that had not been identified in molecular absorption line studies prior to the launch of Herschel.