Астрономы впервые обнаружили РРЛ ионов углерода/кислорода

Исследовательская группа из Шанхайской астрономической обсерватории (SHAO) Китайской академии наук впервые успешно обнаружила линии радиорекомбинации (РРЛ) ионов тяжелее гелия с помощью 65-метрового радиотелескопа TianMa (TMRT). Эти линии были идентифицированы как принадлежащие ионам углерода и/или кислорода.

Результаты недавно были опубликованы в журнале Астрономия и астрофизика.

Ионизированный газ является наиболее распространенным компонентом межзвездного газа и ценным инструментом для определения содержания элементов. Радиорекомбинационные линии (РРЛ) позволяют избежать трудностей наблюдения оптических линий, поскольку РРЛ обычно оптически тонкие и имеют хорошо понятные механизмы излучения.

Однако до сих пор почти все обнаруженные излучатели РРЛ были нейтральными атомами. Ранее сообщалось о RRL-переходах (121α и 115α) ионов гелия в планетарных туманностях. Смешение линий затрудняет спектроскопическое разрешение РРЛ атомов тяжелее гелия. Напротив, RRL ионов обычно не смешивают с RRL нейтральных атомов, что делает первые более мощным инструментом для измерения численности.

Исследователи сделали это новое открытие во время поиска межзвездных эмиссионных линий в рамках продолжающегося исследования спектральных линий TMRT в направлении Orion KL. Спектроскопически охарактеризовав Ka-диапазон (26–35 ГГц) Orion KL, они обнаружили несколько широколинейных особенностей, которые нельзя было отнести ни к одной молекуле.[{» attribute=»»>species nor to the RRLs of atoms.

“These line features have weak intensities but are already significant enough to be distinguished due to the high sensitivity of the spectrum. Because their line widths are similar to those of H/He RRLs, we realized that those line features could be RRLs of ions,” said Dr. Liu Xunchuan from SHAO, corresponding and first author of the study.

To confirm this, the astronomers conducted follow-up Ku-band (12-18 GHz) observations using TMRT to search for signals of ion RRLs at the expected frequencies, and eight more alpha lines (RRLs with Δn=1) of ions were detected.

In addition, they found marginal signals of alpha lines in the Q band and beta lines (Δn=2) in the Ka band. They compared the spectra obtained on different days and found that the frequencies of the line features remained unchanged when corrected for the motion of the Earth, confirming that the ion RRLs originated from space.

In total, tens of RRLs of interstellar ions were detected by TMRT, and many of them are not blended with any transitions of molecules nor with RRLs of atoms. The lines detected by TMRT are more than 20 kilometers per second bluer than the expected frequencies of helium ion RRLs and were thus assigned to ions heavier than helium. The abundance of the doubly ionized elements associated with those ion RRLs was accurately determined to be 8.8 parts per 10,000, which is consistent with the value of carbon/oxygen estimated from optical/infrared observations.

Previously, RRLs were commonly defined as radio spectral lines, caused by transitions of high-n levels of atoms, that appeared after the recombination of singly ionized ions and electrons. But now, the researchers have detected tens of unblended ion RRLs simultaneously.

“Such a new technique would be very valuable to study the abundances of carbon and oxygen, the most important constituents of carbon monoxide and interstellar complex organic molecules, in the inner Galaxy, where optical observations are very difficult,” said Prof. Neal J. Evans II from the University of Texas at Austin.

SHAO researchers see this new discovery by TMRT as the first of many. “The ongoing TMRT spectral line survey towards Orion KL and other Galactic objects will reach an unprecedented line sensitivity, which will lead to more new discoveries such as RRLs of heavy ions, new transitions of molecular lines, and even new molecule species,” said Liu Tie, a researcher in SHAO and co-corresponding author of the study.

Reference: “First detection of radio recombination lines of ions heavier than helium” by Xunchuan Liu, Tie Liu, Zhiqiang Shen, Paul F. Goldsmith, Neal J. Evans II, Sheng-Li Qin, Qiuyi Luo, Yu Cheng6, Sheng-Yuan Liu, Fengyao Zhu, Ken’ichi Tatematsu, Meizhu Liu, Dongting Yang, Chuanshou Li, Li Chen, Juan Li, Xing Lu, Qilao Gu, Rongbing Zhao, Bin Li, Yajun Wu, Weiye Zhong, Zhang Zhao, Jinqing Wang, Qinghui Liu, Bo Xia, Li Fu, Zhen Yan, Chao Zhang, Lingling Wang, Qian Ye, Hongli Liu, Chao Zhang, Fengwei Xu and Dipen Sahu, 7 February 2023, Astronomy and Astrophysics.
DOI: 10.1051/0004-6361/202345904