arXiv 2403.070752024-03-11astro-ph.GA

弥漫云中的H2CO和CS：激发与丰度

H2CO and CS in diffuse clouds: Excitation and abundance

Maryvonne Gerin, Harvey Liszt, Jerome Pety, Alexandre Faure

备注：: 13 pages, abridged abstract
发表信息：: A&A 686, A49 (2024)
DOI：: 10.1051/0004-6361/202449152

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AI总结：

利用NOEMA观测弥漫云中H2CO和CS的激发与丰度，通过非LTE计算揭示不同速度成分的密度差异及同位素比，约束有机物种的化学过程。

摘要：

为了对产生观测到的有机物种柱密度的化学过程提供约束，我们使用NOEMA在2毫米谱段对NRAO150方向的视线进行了观测。我们瞄准了o-H2CO 2(1,1)-1(1,0)和p-H2CO 2(0,2)-1(0,1)的低激发线，以及邻近的CS(3-2)和c-C3H2跃迁。我们将这些数据与之前的观测相结合，以确定不同速度成分中的激发条件、柱密度以及相对于H2的丰度。我们进行了非局部热力学平衡（non-LTE）辐射转移计算，包括与邻位和对位H2以及电子的碰撞截面。我们为邻位和对位甲醛计算了新的与电子的碰撞截面。c-C3H2的谱线轮廓与HCO+和CCH的非常相似，而CS的吸收特征更窄，主要集中在V = -17和-10 km/s的两个主要速度成分中。H2CO吸收线呈现中间模式，在所有速度成分中都有吸收，但在两个主要速度成分中具有更大的不透明度。H2CO和c-C3H2的邻位-对位比与统计值3一致。虽然所有c-C3H2速度成分的激发温度与宇宙微波背景（CMB）一致，但在CS中检测到的两个强成分显示出明显的CMB超额，表明CS沿着这条特定视线存在于比其他物种更高的密度区域，n(H2) ~ 2500 cm-3，而其他速度成分的n(H2) < 500 cm-3。我们检测到来自o-H213CO和C34S的微弱吸收，从而推导出同位素比：o-H2CO/o-H213CO = 61，C32S/C34S = 24。甲醛的4.8GHz线的激发对电子分数敏感，其激发温度预计在低和中等的电子分数（x(e)< 6E-5）下低于CMB，而在高电子分数（> 1e-4）下升高到CMB之上。

英文摘要：

To provide constraints on the chemical processes responsible for the observed columns of organic species, we used NOEMA to observe the sight line toward NRAO150 in the 2mm spectral window. We targeted the low excitation lines of o-H2CO 2(1,1)-1(1,0) and p-H2CO 2(0,2)-1(0,1) as well as the nearby transitions of CS(3-2) and c-C3H2. We combined these data with previous observations to determine the excitation conditions, column densities, and abundances relative to H2 in the different velocity components. We performed non-LTE radiative transfer calculations including collision cross sections with ortho and para H2 and with electrons. New collision cross sections with electrons were computed for ortho and para formaldehyde. The c-C3H2 line profiles are very similar to those of HCO+ and CCH, while the CS absorption features are narrower and mostly concentrated in two main velocity components at V = -17 and -10 km/s. H2CO absorption lines present an intermediate pattern with absorption in all velocity components but larger opacities in the two main velocity components. The ortho-to-para ratios of H2CO and c-C3H2 are consistent with the statistical value of 3. While the excitation temperature of all c-C3H2 velocity components is consistent with the CMB, the two strong components detected in CS show a clear excess over the CMB indicating that CS resides at higher densities than other species along this particular sightline, n(H2) ~ 2500 cm-3 while n(H2) < 500 cm-3 for the other velocity components. We detected faint absorption from o-H213CO and C34S allowing us to derive isotopic ratios: o-H2CO/o-H213CO = 61 and C32S/C34S = 24. The excitation of the 4.8GHz line of formaldehyde is sensitive to the electron fraction and its excitation temperature is predicted to be lower than the CMB at low and moderate electron fractions, x(e)< 6E-5, and to rise above the CMB at high electron fractions, > 1e-4.

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