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MOLLId: software for automatic identification of spectral molecular lines in the sub-millimeter and millimeter bands and its application to the spectra of protostars from the region RCW 120

A. A. Farafontova, M. S. Kirsanova, S. V. Salii

TL;DR

This work introduces MOLLId, an automated tool for identifying molecular spectral lines in submillimeter/millimeter observations by combining Gaussian line-profile fitting with multi-level frequency matching and LTE-based parameter estimation. The method is validated on RCW 120 YSO S1 and S2, identifying 100 lines across 41 molecules for S1 and 407 lines across 79 molecules for S2, with CH$_3$OH, CH$_3$CN, and CH$_3$CCH among the most prominent species. Rotation-diagram analysis reveals two-temperature components, indicative of hot-core chemistry in RCW 120 YSO S2, while S1 shows fewer high-energy features, consistent with a younger evolutionary stage. The approach achieves low unidentified-line fractions (0.7–1%) and runs rapidly on standard CPUs (~6–8 minutes per spectrum), illustrating a practical, scalable framework for quantitative chemical and physical diagnostics in star-forming regions.

Abstract

In this work, we present the program MOLLId (MOLecular Line Identification) for automated molecular lines approximation with gaussian profile. Molecular identification was performed using multi-level comparison of the lines' center frequencies and rest frequencies from the spectroscopic database. The program was tested using identification of the molecular lines in observational spectra of young stellar objects RCW 120 YSO S1 and RCW 120 YSO S2, located near the border of the RCW 120 PDR. In the spectra of the RCW 120 YSO S1 source, 100 lines of 41 molecules were identified over the level of 4-6 sigma. In the spectra of the RCW 120 YSO S2 source, 407 lines of 79 molecules were identified over the level 3-5 sigma. Using Intel Core i7-12700K CPU, identification time is equal to 6 and 8 minutes per spectral range for the YSOs S1 and S2, respectively. From the analysis of CH3OH, CH3CN, CH3CCH molecules identified in RCW 120 YSO S2 we found a two-component structure and estimated the physical parameters in the LTE approximation for each of the components.

MOLLId: software for automatic identification of spectral molecular lines in the sub-millimeter and millimeter bands and its application to the spectra of protostars from the region RCW 120

TL;DR

This work introduces MOLLId, an automated tool for identifying molecular spectral lines in submillimeter/millimeter observations by combining Gaussian line-profile fitting with multi-level frequency matching and LTE-based parameter estimation. The method is validated on RCW 120 YSO S1 and S2, identifying 100 lines across 41 molecules for S1 and 407 lines across 79 molecules for S2, with CHOH, CHCN, and CHCCH among the most prominent species. Rotation-diagram analysis reveals two-temperature components, indicative of hot-core chemistry in RCW 120 YSO S2, while S1 shows fewer high-energy features, consistent with a younger evolutionary stage. The approach achieves low unidentified-line fractions (0.7–1%) and runs rapidly on standard CPUs (~6–8 minutes per spectrum), illustrating a practical, scalable framework for quantitative chemical and physical diagnostics in star-forming regions.

Abstract

In this work, we present the program MOLLId (MOLecular Line Identification) for automated molecular lines approximation with gaussian profile. Molecular identification was performed using multi-level comparison of the lines' center frequencies and rest frequencies from the spectroscopic database. The program was tested using identification of the molecular lines in observational spectra of young stellar objects RCW 120 YSO S1 and RCW 120 YSO S2, located near the border of the RCW 120 PDR. In the spectra of the RCW 120 YSO S1 source, 100 lines of 41 molecules were identified over the level of 4-6 sigma. In the spectra of the RCW 120 YSO S2 source, 407 lines of 79 molecules were identified over the level 3-5 sigma. Using Intel Core i7-12700K CPU, identification time is equal to 6 and 8 minutes per spectral range for the YSOs S1 and S2, respectively. From the analysis of CH3OH, CH3CN, CH3CCH molecules identified in RCW 120 YSO S2 we found a two-component structure and estimated the physical parameters in the LTE approximation for each of the components.

Paper Structure

This paper contains 12 sections, 8 equations, 7 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Observations
  3. Methods
  4. Approximation of line profiles
  5. Identification of molecules
  6. Estimates of physical parameters in the LTE approximation
  7. Results
  8. Approximations obtained for line profiles
  9. Identified molecules
  10. High excitation lines in RCW 120 YSO S2
  11. Discussion
  12. Conclusion

Figures (7)

  • Figure 1: Spectra of RCW 120 YSO S1 (two top panels) and RCW 120 YSO S2 (two bottom panels). Data corresponding to spectral bands B1 $-$ B6 from Table \ref{['tab:obs_bands']} are color-coded. The most intense lines are labeled with the names of the identified molecules
  • Figure 2: Examples of line profile approximations for the RCW 120 YSO S2 source. The Gaussian function is shown in red in the figure. The corresponding area under the Gaussian profile is shown in gray
  • Figure 3: Classification by groups for RCW 120 YSO S1 (top panel) and RCW 120 YSO S2 (bottom panel)
  • Figure 4: Rotation diagram for CH$_{3}$OH (top panel), CH$_{3}$CN (middle panel), and CH$_{3}$CCH (bottom panel) molecules for the RCW 120 YSO S2 source. The dashed line shows the approximation of the observational data by a linear function
  • Figure 5: CH$_{3}$CN line profiles for RCW 120 YSO S2. The LSR velocity of -7 km s$^{-1}$ is indicated by a vertical solid gray line. When approximating using the Gaussian function (red), for lines with E$_{\rm up}$$\leq$ 157 K, the line width was fixed at $\Delta$v=5 km s$^{-1}$. For lines with E$_{\rm up}$ > 157 K, $\Delta$v=7 km s$^{-1}$
  • ...and 2 more figures