硫和碳同位素朝向银河系中心云
Sulphur and carbon isotopes towards Galactic centre clouds
摘要：
测量同位素比率是一种敏感的技术，用于获得恒星核合成和化学演化的信息。我们目前的碳和硫的丰度在我们的银河系中心区域的星际介质的测量。选定的目标是+50 km/s的云和几个l.o.s.云向Sgr B2（N）移动。对于+50km/s云，我们用IRAM-30 m望远镜观测到CS，C34 S，13 CS，C33 S和13 C34 S的J=2-1转动跃迁，CS和C34 S的J=3-2跃迁，以及用APEX 12 m望远镜观测到C34 S和13 CS的J=6-5跃迁，都是在发射中。用ALMA在Sgr B2（N）的包层中观察到CS、C34 S、13 CS和13 C34 S的J=2-1转动跃迁，在1.o.s中也观察到CS和C34 S的J=2-1转动跃迁。云向Sgr B2（N），都在吸收中。在+50km/s的云中，我们得到12 C13 C同位素比约为22.1，这导致，与测量的13 CS/C34 S线强度比，32 S/34 S比为16.3+3.0-2.4。我们还更直接地从13 CS和13 C34 S这两种同位素体推导出32 S/34 S同位素比值，这导致+50km/s云和Sgr B2（N）的32 S/34 S独立估计值分别为16.3+2.1-1.7和17.9+-5.0。在+50 km/s的云中，我们还获得了~（34）S/~（33）S比值~4.3。先前的研究观察到，当接近银河系中心时，32 S/34 S同位素比率呈下降趋势。我们的结果表明，这种趋势至少在130-30+60 pc的银河系中心距离终止。这与基于12 C/13 C、14 N/15 N和18 O/17 O同位素比值的结果不同，在这些比值中，观察到上述趋势一直延续到中心分子带。这可能表明银河系中心大质量恒星的产生量下降，与最近的金属丰度梯度研究一致，并开启了与银河系和恒星演化模型进行比较的工作。
英文摘要：
Measuring isotopic ratios is a sensitive technique used to obtain information on stellar nucleosynthesis and chemical evolution. We present measurements of the carbon and sulphur abundances in the interstellar medium of the central region of our Galaxy. The selected targets are the +50km/s Cloud and several l.o.s. clouds towards Sgr B2(N). Towards the +50km/s Cloud, we observed the J=2-1 rotational transitions of CS, C34S, 13CS, C33S, and 13C34S, and the J=3-2 transitions of CS and C34S with the IRAM-30m telescope, as well as the J=6-5 transitions of C34S and 13CS with the APEX 12m telescope, all in emission. The J=2-1 rotational transitions of CS, C34S, 13CS, and 13C34S were observed with ALMA in the envelope of Sgr B2(N), with those of CS and C34S also observed in the l.o.s. clouds towards Sgr B2(N), all in absorption. In the +50km/s Cloud we derive a 12C13C isotopic ratio of ~22.1, that leads, with the measured 13CS/C34S line intensity ratio, to a 32S/34S ratio of 16.3+3.0-2.4. We also derive the 32S/34S isotopic ratio more directly from the two isotopologues 13CS and 13C34S, which leads to an independent 32S/34S estimation of 16.3+2.1-1.7 and 17.9+-5.0 for the +50km/s Cloud and Sgr B2(N), respectively. We also obtain a 34S/33S ratio of ~4.3 in the +50 km/s Cloud. Previous studies observed a decreasing trend in the 32S/34S isotopic ratios when approaching the Galactic centre. Our result indicates a termination of this tendency at least at a galactocentric distance of 130-30+60 pc. This is at variance with findings based on 12C/13C, 14N/15N and 18O/17O isotope ratios, where the above-mentioned trend is observed to continue right to the central molecular zone. This can indicate a drop in the production of massive stars at the Galactic centre, in the same line as recent metallicity gradient studies, and opens the work towards a comparison with Galactic and stellar evolution models.
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