Tianma 65m Radio Telescope Helps Unravel Mysteries of Orion Kleinman-Low Nebula

A research team from the Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences recently completed a deep-line Q-band scanning survey of the Orion Kleinmann-Low Nebula (Orion KL) using the Tianma 65-m radio telescope (TMRT). , achieving the widest frequency coverage (35–50 GHz) and highest sensitivity (~mK) spectra so far.

In total, about 600 emission lines were detected, including 177 radiorecombination lines (RRLs) and 371 molecular line transitions from 53 molecular species, among which 21 species were consistently detected for the first time in the Q band. This research highlights the ability of TMRT to search for weak lines in extremely long broadband spectra.

The study was published in The Astrophysical Journal Supplement Series.

Interstellar complex organic molecules play an important role in the origin of life. A broadband scanning survey of typical star-forming regions is one of the best methods for understanding their physical and chemical properties and for studying the origin of complex organic molecules, especially prebiotics.

Orion KL is the closest high-mass star-forming region to Earth, about 1,300 light-years away. It is a molecular complex interacting with an extended HII region (M42) and composed of several gas components with various temperatures, including a cold ambient gas component (500 K).

These gas components have different chemical properties and contribute to numerous emission lines at a wide range of wavelengths, many of which are the brightest in the sky. Thus, Orion KL is well suited for a broadband line scan survey. However, previous Orion KL line surveys were mostly conducted in the submillimeter, submillimeter, or infrared bands. Line surveys in low frequency bands such as the Q band (35–50 GHz) are very rare.

In fact, the Q-band has irreplaceable advantages in the search for high Δn RRLs, new molecular transitions, and even new molecules.

TMRT is located in the western suburbs of Shanghai, China. It is currently the largest fully steerable radio telescope in Asia. L to Q band receivers are available, covering the full frequency range of 1–50 GHz. The Q-band receiver developed by the group of Dr. Zhong Weiye from SHAO provides the highest frequency coverage on TMRT.

“TMRT is very suitable for broadband line surveys. The Orion KL TMRT Q-band line survey detected a huge number of emission lines, and two-thirds of them have not been detected in previous line surveys,” said Liu Xunchuan. first author of the study.

When an electron is captured by an ion, it will continue to jump down and create a series of emission lines. These transmission lines are called RRLs if they appear in the radio regime. In the TMRT line survey, high Δn RRLs such as H135π and C81γ were detected. These RRLs have the highest Δn detected in the Q band.

In addition, the survey identified (15, 15) and (16, 16) NH lines₃. This is the first time that NH has been detected₃ emission lines with upper state energies greater than 2000 K in a survey of Orion KL, indicating the existence of an extremely hot gas component in the Orion KL region.

Many complex organic molecules, such as NH₂CHO, CH₃OCHO, C₂H₅CN, CH₃OCH₃CH₃CH₂OH and CH₃KOH₃, were also detected. These complex organic molecules form the basis of more complex prebiotic molecules.

In addition, the researchers detected the emission lines of the vibrationally excited states of ethyl cyanide (C₂H₅CN v13/v21) in the Q band. This is the first time to detect and locate the Q band emission of C₂H₅CN v13/v21 from the interstellar medium.

The detected complex organic molecules can be assigned to different Orion KL gas components by comparing their line widths and central velocities. Aldehyde-containing species and species with sulfur and oxygen elements tend to have plateau-derived spectral components, which supports a possible enhancement of the abundances of these species by shocks.

“This research reaches an unprecedented line sensitivity that demonstrates the enormous potential of TMRT in astrochemistry studies,” said Liu Tie, a researcher at SHAO and co-corresponding author of the study.

“This deep Q-band line survey provides excellent guidance for similar line-scanning surveys in the Q-band of other targets,” added Professor Qin Shengli from Yunnan University and co-author of the study.

In the future, the research team will perform a deeper line survey of the Orion KL, aiming to achieve sensitivity better than mK and extend the frequency coverage to the full Q/Ka band (26–50 GHz). In addition, the research team also plans to perform line surveys in the Q/Ka band of a much larger sample of complex, molecule-rich organic sources using TMRT.

“The follow-up surveys will create an unprecedented collection of Q/Ka band spectra of typical complex organic sources rich in molecules and help identify new complex organic molecules, especially prebiotic species,” said Shen Zhiqiang, director of SHAO and a co- corresponding author of the study.