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Observation of CH$_{3}$$^{17}$OH and CH$_{3}$$^{18}$OH in Orion KL: A New Tool to Study Star-Formation History

Yoshimasa Watanabe, Takahiro Oyama, Akemi Tamanai, Shaoshan Zeng, Nami Sakai

TL;DR

This work confirms CH$_3$^{17}$OH in the Orion KL region by identifying six lines in ALMA data and showing line profiles and distributions that match CH$_3$^{18}$OH. Using LTE with fixed $T_{rot}$, the authors derive CH$_3$^{18}$OH/CH$_3$^{17}$OH ratios of about 3.3–3.5, consistent with the canonical $^{18}$O/$^{17}$O value, and they obtain CH$_3$^{16}$OH/CH$_3$^{17}$OH ≈ 2300–2500 that aligns with the local ISM $^{16}$O/$^{17}$O ratio. They argue that CH$_3$OH isotopologues provide robust tracers of oxygen isotope ratios in star-forming regions, offering a method to study the Galaxy’s star-formation history through Galactic chemical evolution. The approach benefits from observing multiple CH$_3$OH transitions within a single ALMA setup, enabling accurate optical-depth corrections and LTE or non-LTE analyses, and it highlights the potential of CH$_3$OH isotopologues as tools for isotopic studies in dense gas.

Abstract

Methanol is a seed species of complex organic molecules that is of fundamental importance in astrochemistry. Although various isotopologues of CH$_3$OH have been detected in the interstellar medium (ISM), CH$_{3}$$^{17}$OH is only tentatively detected in Sgr~B2. To confirm the presence of CH$_{3}$$^{17}$OH in the ISM and to investigate its abundance, we search for its emission lines in the Orion~KL region. We have obtained image cubes covering the frequency ranges 236.40~GHz-236.65~GHz and 231.68~GHz-231.88~GHz using ALMA archival data observed toward the Orion~KL region. The column densities of CH$_3$$^{17}$OH and CH$_3$$^{18}$OH are estimated under the assumption of local thermodynamic equilibrium condition with fixed excitation temperatures at the two CH$_3$$^{18}$OH peaks, MeOH1 and MeOH2,. We have identified six emission lines of CH$_{3}$$^{17}$OH in MeOH1 and MeOH2 and confirmed that the line profiles and spatial distributions are consistent with those of CH$_3$$^{18}$OH. The abundance ratios of CH$_3$$^{18}$OH/CH$_3$$^{17}$OH are evaluated to be $\sim 3.4-3.5$ and are similar to the canonical value of $^{18}$O/$^{17}$O $\sim 3-4$ derived from CO observations in the Orion~KL region. We have compared the results with the previous study of CH$_3$OH and evaluated CH$_3$$^{16}$OH/CH$_3$$^{17}$OH ratios to be $\sim 2300-2500$ at a resolution of $\sim 4$~arcsec. The ratios are close to the $^{16}$O/$^{17}$O ratio in the local ISM. This result indicates that the CH$_3$OH isotopologues can serve as new tracers of oxygen isotope ratios in star-forming regions because the opacity of CH$_3$OH can be evaluated using transition lines spanning a wide range of line intensities. Moreover, this method enables us to study the star-formation history of our Galaxy with the aid of the Galactic chemical evolution models.

Observation of CH$_{3}$$^{17}$OH and CH$_{3}$$^{18}$OH in Orion KL: A New Tool to Study Star-Formation History

TL;DR

This work confirms CH^{17}_3OH. Using LTE with fixed , the authors derive CH^{18}_3OH ratios of about 3.3–3.5, consistent with the canonical O/O value, and they obtain CH^{16}_3OH ≈ 2300–2500 that aligns with the local ISM O/O ratio. They argue that CHOH isotopologues provide robust tracers of oxygen isotope ratios in star-forming regions, offering a method to study the Galaxy’s star-formation history through Galactic chemical evolution. The approach benefits from observing multiple CHOH transitions within a single ALMA setup, enabling accurate optical-depth corrections and LTE or non-LTE analyses, and it highlights the potential of CHOH isotopologues as tools for isotopic studies in dense gas.

Abstract

Methanol is a seed species of complex organic molecules that is of fundamental importance in astrochemistry. Although various isotopologues of CHOH have been detected in the interstellar medium (ISM), CHOH is only tentatively detected in Sgr~B2. To confirm the presence of CHOH in the ISM and to investigate its abundance, we search for its emission lines in the Orion~KL region. We have obtained image cubes covering the frequency ranges 236.40~GHz-236.65~GHz and 231.68~GHz-231.88~GHz using ALMA archival data observed toward the Orion~KL region. The column densities of CHOH and CHOH are estimated under the assumption of local thermodynamic equilibrium condition with fixed excitation temperatures at the two CHOH peaks, MeOH1 and MeOH2,. We have identified six emission lines of CHOH in MeOH1 and MeOH2 and confirmed that the line profiles and spatial distributions are consistent with those of CHOH. The abundance ratios of CHOH/CHOH are evaluated to be and are similar to the canonical value of O/O derived from CO observations in the Orion~KL region. We have compared the results with the previous study of CHOH and evaluated CHOH/CHOH ratios to be at a resolution of ~arcsec. The ratios are close to the O/O ratio in the local ISM. This result indicates that the CHOH isotopologues can serve as new tracers of oxygen isotope ratios in star-forming regions because the opacity of CHOH can be evaluated using transition lines spanning a wide range of line intensities. Moreover, this method enables us to study the star-formation history of our Galaxy with the aid of the Galactic chemical evolution models.

Paper Structure

This paper contains 9 sections, 4 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Data and Reductions
  3. Results
  4. Detections of CH$_3$$^{18}$OH and CH$_3$$^{17}$OH
  5. Column densities
  6. Discussion
  7. Comparisons of oxygen isotope ratios with those of other molecules
  8. CH$_3$OH as a tool of isotope ratio measurement
  9. Conclusions

Figures (4)

  • Figure 1: (a) 1.3 mm continuum image of Orion KL observed with ALMA (#2013.1.00553.S). Contour levels are from 50 mJy beam$^{-1}$ ($5\sigma$) to 950 mJy beam$^{-1}$ with a 100 mJy beam$^{-1}$ step. (b) Integrated intensity image of CH$_3$$^{18}$OH ($5_0-4_0$$E$) obtained by the ALMA SV. Contour levels are from 4.8 K km s$^{-1}$ ($3\sigma$) to 28.8 K km s$^{-1}$ with a 4.8 K km s$^{-1}$ step. White ellipses at the left bottom corner indicate the synthesized beams. The CH$_3$$^{18}$OH peaks (MeOH1 and MeOH2) are shown by the white squares. The positions of Hot Core (HC), IRc7, and the HCOOCH$_3$ peaks (MF) identified by Favre2011 are shown by the star, circle, and gray cross marks, respectively. The black cross marks show the positions of dM-1 and dM-3.
  • Figure 2: CH$_3$$^{18}$OH spectra (black line) observed at the CH$_3$$^{18}$OH peaks (a) MeOH1 and (b) MeOH2 in the rest frame, and CH$_3$$^{17}$OH spectra (black line) observed at (c) MeOH1 and (d) MeOH2. The system velocities are assumed to be $V_{\rm LSR} = 7.50$ km s$^{-1}$ and 7.55 km s$^{-1}$ for MeOH1 and MeOH2, respectively. The spectra are obtained after the angular resolutions of observation data are convolved to be the $2".0 \times 2".0$ beam. The solid red line in (c) and (d) is the experimentally measured CH$_3$$^{17}$OH spectrum obtained in a laboratory Tamanai2025 with an arbitrary intensity scale. Red dashed lines are the positions of CH$_3$$^{18}$OH and CH$_3$$^{17}$OH and black dashed lines are those of other molecules. Molecular names with a question mark "?" indicate the potential molecular species. "U" indicates an unidentified emission line. Blue lines are model spectra of CH$_3$$^{18}$OH and CH$_3$$^{17}$OH predicted with the LTE model by using the column densities estimated in Table \ref{['tab3']} and the line parameters listed in Table \ref{['tab2']}. The line width is assumed to be 2.5 km s$^{-1}$ and 2.1 km s$^{-1}$ for MeOH1 and MeOH2, respectively, which are evaluated from the Gaussian fitting to the CH$_3$$^{18}$OH ($5_0-4_0,\,E$) line. The $S\mu^2$ and $E_{\rm u}$ values are taken from Tamanai2025.
  • Figure 3: Line profiles of CH$_3$$^{18}$OH and CH$_3$$^{17}$OH (Table \ref{['tab1']}) at the two methanol peaks MeOH1 (left: a - h) and MeOH2 (right: i - o). The spectra are obtained after the angular resolutions of observation data are convolved to be the $2".0 \times 2".0$ beam for fair comparisons. The vertical scale is the brightness temperature. The vertical dashed lines indicate the system velocities of $V_{\rm LSR} = 7.50$ km s$^{-1}$ and 7.55 km s$^{-1}$ for MeOH1 and MeOH2, respectively.
  • Figure 4: Integrated intensity maps of (a) CH$_3$$^{18}$OH ($5_0 - 4_0$$E$), (b) CH$_3$$^{18}$OH ($5_1 - 4_1$$E$), (c) CH$_3$$^{17}$OH ($5_0 - 4_0$$E$), (d) CH$_3$$^{17}$OH ($5_{-1} - 4_{-1}$$E$), (e) CH$_3$$^{17}$OH ($5_3 - 4_3$$E$), (f) CH$_3$$^{17}$OH ($5_1 - 4_1$$E$), (g) CH$_3$$^{17}$OH ($5_{-2} - 4_{-2}$$E$), and (h) CH$_3$$^{17}$OH ($5_2 - 4_2$$E$). The contour levels are (a) from 4.8 K km s$^{-1}$ ($3\sigma$) to 19.2 K km s$^{-1}$ with a 4.8 K km s$^{-1}$ step, (b) from 4.8 K km s$^{-1}$ ($3\sigma$) to 24.0 K km s$^{-1}$ with a 4.8 K km s$^{-1}$ step, (c) from 2.3 K km s$^{-1}$ ($3\sigma$) to 11.3 K km s$^{-1}$ with a 2.3 K km s$^{-1}$ step, (d) from 2.3 K km s$^{-1}$ ($3\sigma$) to 8.4 K km s$^{-1}$ with a 1.5 K km s$^{-1}$ step, (e) from 2.3 K km s$^{-1}$ ($3\sigma$) to 21.3 K km s$^{-1}$ with a 3.8 K km s$^{-1}$ step, (f) from 4.8 K km s$^{-1}$ ($3\sigma$) to 8.4 K km s$^{-1}$ with a 1.5 K km s$^{-1}$ step, (g) from 4.8 K km s$^{-1}$ ($3\sigma$) to 18.2 K km s$^{-1}$ with a 2.3 K km s$^{-1}$ step, and (h) from 2.3 K km s$^{-1}$ ($3\sigma$) to 13.6 K km s$^{-1}$ with a 2.3 K km s$^{-1}$ step. White ellipses at the left bottom corner indicate the synthesized beams. The symbols shown in the maps are the same as those in Figure \ref{['fig1']}.