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The LMT 2 millimeter receiver system (B4R). II. Science demonstration observations toward Orion-KL/OMC-1

Teppei Yonetsu, Ryohei Kawabe, Yuki Yoshimura, Kotomi Taniguchi, Yoshito Shimajiri, Omar Sergio Rojas-García, Arturo I. Gómez-Ruiz, Takeshi Sakai, Kunihiko Tanaka, Bunyo Hatsukade, Akio Taniguchi, Yoichi Tamura, Tatsuya Takekoshi, Tai Oshima, Kotaro Kohno, Masato Hagimoto, David H. Hughes, Peter F. Schloerb, David Sánchez-Argüelles, Kamal Souccar, Gopal Narayanan, Min S. Yun, Víctor Gómez-Rivera, Iván Rodríguez-Montoya, Edgar Colín-Beltrán, Miguel Chávez Dagostino, Javier Zaragoza-Cardiel, Shinji Fujita, Hiroyuki Maezawa

TL;DR

This study demonstrates the Band 4 Receiver (B4R) on the Large Millimeter Telescope-50 m as a capable instrument for wideband, high-resolution spectral mapping in the 2 mm band. Using on-the-fly mapping toward Orion-KL/OMC-1, the authors obtained 5′×5′ maps over two 10 GHz settings with ~11–12″ beams and ~0.15–0.17 km s$^{-1}$ resolution, identifying ~400 emission lines from 29 molecular species, including COMs. Rotation-diagram analyses of C$_2$H$_5$CN and CH$_3$OCHO show distinct HC and CR distributions and temperatures, highlighting the impact of spatial resolution on inferred abundances and chemistry. The results validate B4R’s suitability for astrochemical surveys and motivate future upgrades (LO tuning, expanded IF bandwidth, FMLO, and data-science methods) to enable deeper surveys of COMs and prebiotic species in the Milky Way and beyond.

Abstract

We present the results of mapping and single-point spectral scans toward Orion-KL/OMC-1 performed as science demonstrations of a 2 mm SIS receiver, the Band 4 Receiver (B4R), installed on the 50 m Large Millimeter Telescope (LMT). To prove the capabilities of mapping and spectral scans with the B4R on the LMT, commissioning observations were conducted employing the on-the-fly mapping technique toward Orion-KL/OMC-1, which covers a map size of 5$'\times$5$'$. These mapping observations were performed with two frequency settings providing 10 GHz in total (131.4-133.9 GHz and 145.1-147.6 GHz; 136.2-138.7 GHz and 149.9-152.4 GHz) with a frequency resolution of 76.293 kHz. We conducted spectral line identification analysis for the hot core and compact ridge regions in the Orion-KL with a beam size of 11-12$''$. We detected nearly 400 emission lines and identified two recombination lines and 29 molecular species, including isotopologues, deuterated molecules, and vibrational excited states, despite the short integration time. These results are consistent with those of previous studies. The 29 molecular species include nitrogen (N)-bearing complex organic molecules (COMs) and oxygen (O)-bearing COMs. To demonstrate the capability of the B4R in astrochemistry, we conducted detailed analyses of column densities, rotational temperatures, and relative abundances with respect to H$_2$ on two representative COMs, N-bearing C$_2$H$_5$CN and O-bearing CH$_3$OCHO in the central 40$''\times$40$''$ area of the map. The wide bandwidth of 10 GHz enabled the use of 8 and 34 emission lines, respectively. The spatial differences in the physical and chemical properties between these molecules were derived at a spatial resolution of $\sim$12$''$. The B4R on the LMT was successfully demonstrated to be powerful for mapping and spectral scans and to have high potential for the study of interstellar chemistry.

The LMT 2 millimeter receiver system (B4R). II. Science demonstration observations toward Orion-KL/OMC-1

TL;DR

This study demonstrates the Band 4 Receiver (B4R) on the Large Millimeter Telescope-50 m as a capable instrument for wideband, high-resolution spectral mapping in the 2 mm band. Using on-the-fly mapping toward Orion-KL/OMC-1, the authors obtained 5′×5′ maps over two 10 GHz settings with ~11–12″ beams and ~0.15–0.17 km s resolution, identifying ~400 emission lines from 29 molecular species, including COMs. Rotation-diagram analyses of CHCN and CHOCHO show distinct HC and CR distributions and temperatures, highlighting the impact of spatial resolution on inferred abundances and chemistry. The results validate B4R’s suitability for astrochemical surveys and motivate future upgrades (LO tuning, expanded IF bandwidth, FMLO, and data-science methods) to enable deeper surveys of COMs and prebiotic species in the Milky Way and beyond.

Abstract

We present the results of mapping and single-point spectral scans toward Orion-KL/OMC-1 performed as science demonstrations of a 2 mm SIS receiver, the Band 4 Receiver (B4R), installed on the 50 m Large Millimeter Telescope (LMT). To prove the capabilities of mapping and spectral scans with the B4R on the LMT, commissioning observations were conducted employing the on-the-fly mapping technique toward Orion-KL/OMC-1, which covers a map size of 55. These mapping observations were performed with two frequency settings providing 10 GHz in total (131.4-133.9 GHz and 145.1-147.6 GHz; 136.2-138.7 GHz and 149.9-152.4 GHz) with a frequency resolution of 76.293 kHz. We conducted spectral line identification analysis for the hot core and compact ridge regions in the Orion-KL with a beam size of 11-12. We detected nearly 400 emission lines and identified two recombination lines and 29 molecular species, including isotopologues, deuterated molecules, and vibrational excited states, despite the short integration time. These results are consistent with those of previous studies. The 29 molecular species include nitrogen (N)-bearing complex organic molecules (COMs) and oxygen (O)-bearing COMs. To demonstrate the capability of the B4R in astrochemistry, we conducted detailed analyses of column densities, rotational temperatures, and relative abundances with respect to H on two representative COMs, N-bearing CHCN and O-bearing CHOCHO in the central 4040 area of the map. The wide bandwidth of 10 GHz enabled the use of 8 and 34 emission lines, respectively. The spatial differences in the physical and chemical properties between these molecules were derived at a spatial resolution of 12. The B4R on the LMT was successfully demonstrated to be powerful for mapping and spectral scans and to have high potential for the study of interstellar chemistry.

Paper Structure

This paper contains 16 sections, 1 equation, 11 figures, 6 tables.

Table of Contents

  1. Introduction
  2. Observations and data reductions
  3. Band 4 receiver (B4R)
  4. Observations
  5. Data reductions
  6. Results
  7. Line identification and OTF maps
  8. Chemical analysis of C$_2$H$_5$CN and CH$_3$OCHO
  9. Discussion
  10. Comparison of line identification with previous studies
  11. Comparison of chemical properties with results from previous studies
  12. Orion-KL
  13. Other high-mass star-forming regions
  14. Future upgrades and prospects of B4R system
  15. Conclusion
  16. ...and 1 more sections

Figures (11)

  • Figure 1: Integrated intensity maps of HDO (left panel) and HC$_3$N ($v_7=2$) (center) in the 136.2--138.7 GHz band and HC$_3$N ($v_7=2$) (right) in the 146.1--147.6 GHz band, after correcting for the main beam efficiency and pointing. The velocity ranges of integrals are $-$5.8--22 km s$^{-1}$, $-$23--11 km s$^{-1}$, and $-$2.3--22 km s$^{-1}$, respectively. The red cross (+) denotes the hot core and compact ridge. The white cross (x) and star indicate the position of BN and Source I, respectively. The coordinates of each substructure are summarized in table \ref{['tab:cord']}. The lower left circle in each panel denotes the beam size in table \ref{['tab:obsinfo']}. Alt text: The integrated intensity maps of the three panels. The x axis is the right ascension, and the y axis is the declination at the map size of approximately 40$\arcsec\times$40$\arcsec$.
  • Figure 2: Spectra of C$_2$H$_5$CN, CH$_3$OCHO ($v=0,v_{18}=1$), and CH$_3$OCH$_3$ in the 145.1--147.6 GHz band. The blue and red lines represent spectra with 10-channel binning, averaging in circles with diameters corresponding to the beam size of 10.9$\arcsec$ centered at the HC and CR Feng2015-au, respectively. Alt text: The molecular emission line spectra of the two panels. The x axis is the frequency at gigahertz, and the y axis is the intensity at kelvin on the $T_{\rm{MB}}$ scale.
  • Figure 3: Integrated intensity maps of C$_2$H$_5$CN ($v=0$) (upper panel) in the 145.1--147.6 GHz band and CH$_3$OCHO ($v=0$) (lower) in the 136.2--138.7 GHz band, after correcting for the main beam efficiency and pointing. The velocity ranges of integrals are $-$40--40 km s$^{-1}$ and 0--20 km s$^{-1}$, respectively. The various sources indicated on each map are described in figure \ref{['fig:point_integmap']} and table \ref{['tab:cord']}. The lower left circle in each panel denotes the beam size in table \ref{['tab:obsinfo']}. Alt text: Integrated intensity maps of the two panels. The x axis is the right ascension, and the y axis is the declination at the map size of approximately 40$\arcsec\times$40$\arcsec$.
  • Figure 4: Column density $N$ (upper row), rotational temperature $T_{\rm{rot}}$ (middle), and relative abundance with respect to $\rm {H_2}$$X$ (lower) maps of C$_2$H$_5$CN (left column) and CH$_3$OCHO ($v=0$ and $v_{18}=1$) (right). The various sources indicated on each map are described in figure \ref{['fig:point_integmap']} and table \ref{['tab:cord']}. The lower left circle in each panel denotes the beam size (12$\arcsec$). The column density, the rotational temperature, and the abundance maps are masked above the maximum around the HC and CR. Alt text: The column density, rotational temperature, and relative abundance with respect to $\rm {H_2}$ maps of the six panels. The x axis is the right ascension, and the y axis is the declination at map size of approximately 40$\arcsec\times$40$\arcsec$.
  • Figure 5: Rotation diagram plots of C$_2$H$_5$CN and CH$_3$OCHO ($v=0$ and $v_{18}=1$) for the HC and CR (12$\arcsec$). The error bars indicate the 1$\sigma$ noise of $W$. The upper panels show the plots of C$_2$H$_5$CN. In the lower panels, the black plots are CH$_3$OCHO ($v=0$) and the blue plots are CH$_3$OCHO ($v_{18}=1$). The black line shows the fitting result using only ($v=0$) states in each panel. The red line represents the fitting result using both torsional states CH$_3$OCHO ($v=0$ and $v_{18}=1$). Alt text: The rotation diagram plots of the four panels. The x axis is the upper-level energy at kelvin, and the y axis is the left side of equation \ref{['rd']}.
  • ...and 6 more figures