First detection of <sup>13</sup>CH in the interstellar medium

Jacob, A. M.; Menten, K. M.; Wiesemeyer, H.; Guêsten, R.; Wyrowski, F.; Klein, Bernd

doi:10.1051/0004-6361/201937385

Cited by 26 publications

(22 citation statements)

References 61 publications

(81 reference statements)

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“…Our estimates for the values of N(CH) are in agreement with those previously determined byWiesemeyer et al (2018);Jacob et al (2019). We note the caveat regarding the determination of the continuum level of the 2006 GHz line spectrum of CH towards W49 (N) as discussed inJacob et al (2020).…”

supporting

confidence: 90%

The CH radical at radio wavelengths: revisiting emission in the 3.3 GHz ground-state lines

Jacob

Menten

Wiesemeyer

et al. 2021

A&A

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Context. The intensities of the three widely observed radio-wavelength hyperfine structure (HFS) lines between the Λ-doublet components of the rotational ground state of CH are inconsistent with local thermodynamic equilibrium (LTE) and indicate ubiquitous population inversion. While this can be qualitatively understood assuming a pumping cycle that involves collisional excitation processes, the relative intensities of the lines and in particular the dominance of the lowest frequency satellite line are not well understood. This has limited the use of CH radio emission as a tracer of the molecular interstellar medium. Aims. We aim to investigate the nature of the (generally) weak CH ground-state masers by employing synergies between the ground-state HFS transitions themselves and the far-infrared lines near 149 μm (2 THz) that connect these levels to the first HFS-split, rotationally excited level of the 2Π1∕2 spin–orbital manifold. Methods. We present the first interferometric observations of the CH 9 cm ground-state HFS transitions at 3.264 GHz, 3.335 GHz, and 3.349 GHz towards the four high-mass star-forming regions (SFRs) Sgr B2 (M), G34.26+0.15, W49 (N), and W51 made with the Karl G. Jansky Very Large Array. We combine this data set with our high-spectral-resolution observations of the N, J = 2, 3∕2 → 1, 1∕2 transitions of CH near 149 μm observed towards the same sources made with the upGREAT receiver on SOFIA, which share common lower energy levels with the HFS transitions within the rotational ground state. Results. Towards all four sources, we observe the 3.264 GHz lower satellite line in enhanced emission with a higher relative intensity than is expected at LTE, by a factor of between 4 and 20. Employing recently calculated collisional rate coefficients, we perform statistical equilibrium calculations with the non-LTE radiative-transfer code MOLPOP-CEP in order to model the excitation conditions traced by the ground-state HFS lines of CH and to infer the physical conditions in the emitting regions. The models account for effects of far-infrared line overlap with additional constraints provided by reliable column densities of CH estimated from the 149 μm lines. Conclusions. The derived gas densities indicate that the CH radio emission lines (and the far-infrared absorption) arise from the diffuse and translucent outer regions of the envelopes of the SFRs as well as in such clouds located along the lines of sight. We infer temperatures ranging from 50 to 125 K. These elevated temperatures, together with astrochemical considerations, may indicate that CH is formed in material heated by the dissipation of interstellar turbulence, which has been invoked for other molecules. The excitation conditions we derive reproduce the observed level inversion in all three of the ground-state HFS lines of CH over a wide range of gas densities with an excitation temperature of ~−0.3 K, consistent with previous theoretical predictions.

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supporting

confidence: 90%

The CH radical at radio wavelengths: revisiting emission in the 3.3 GHz ground-state lines

Jacob

Menten

Wiesemeyer

et al. 2021

A&A

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“…Our estimates for the values of N(CH) are in agreement with those previously determined byWiesemeyer et al (2018);Jacob et al (2019). Note the caveat regarding the determination of the continuum level of the 2006 GHz line spectrum of CH toward W49 (N) as discussed inJacob et al (2020).…”

supporting

confidence: 90%

The CH radical at radio wavelengths: Revisiting emission in the 3.3GHz ground state lines

Jacob,

Menten,

Wiesemeyer

et al. 2021

Preprint

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Context. The intensities of the three widely observed radio-wavelength hyperfine structure (HFS) lines between the Λ-doublet components of the rotational ground state of CH are inconsistent with local thermodynamic equilibrium (LTE) and indicate ubiquitous population inversion. While this can be qualitatively understood assuming a pumping cycle that involves collisional excitation processes, the relative intensities of the lines and in particular the dominance of the lowest frequency satellite line has not been well understood. This has limited the use of CH radio emission as a tracer of the molecular interstellar medium. Aims. We aim to investigate the nature of the (generally) weak CH ground state masers by employing synergies between the ground state HFS transitions themselves and with the far-infrared lines, near 149 µm (2 THz), that connect these levels to an also HFS split rotationally excited level. Methods. We present the first interferometric observations, with the Karl G. Jansky Very Large Array, of the CH 9 cm ground state HFS transitions at 3.264 GHz, 3.335 GHz, and 3.349 GHz toward the four high mass star-forming regions (SFRs) Sgr B2 (M), G34.26+0.15, W49 (N), and W51. We combine this data set with our high spectral resolution observations of the N, J = 2, 3/2 → 1, 1/2 transitions of CH near 149 µm observed toward the same sources made with the upGREAT receiver on SOFIA, which share a common lower energy levels with the HFS transitions within the rotational ground state.Results. Toward all four sources, we observe the 3.264 GHz lower satellite line in enhanced emission with its relative intensity higher than its expected value at LTE by a factor between 4 and 20. Employing recently calculated collisional rate coefficients, we perform statistical equilibrium calculations with the non-LTE radiative transfer code MOLPOP-CEP in order to model the excitation conditions traced by the ground state HFS lines of CH and to infer the physical conditions in the emitting regions. The models account for effects of far-infrared line overlap with additional constraints provided by reliable column densities of CH estimated from the 149 µm lines. Conclusions. The derived gas densities indicate that the CH radio emission lines (and the far-infrared absorption) arise from the diffuse and translucent outer regions of the SFRs' envelopes as well as in such clouds located along the lines of sight. We infer temperatures ranging from 50 to 125 K. These elevated temperatures, together with astrochemical considerations, may indicate that CH is formed in material heated by the dissipation of interstellar turbulence, which has been invoked for other molecules. The excitation conditions we derive reproduce the observed level inversion in all three of the ground state HFS lines of CH over a wide range of gas densities with an excitation temperature of ∼ −0.3 K, consistent with previous theoretical predictions.

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“…However, there are sources with higher degrees of deuteration that may be considered if their column densities of c-H 2 C 3 O are sufficiently large. A search for the cyclopropenone isotopomers with one 13 C may be more promising in the envelope of Sagittarius B2(N) because the 12 C to 13 C ratio in the Galactic center is as low as 20 to 30 (Müller et al 2008(Müller et al , 2016Halfen et al 2017;Jacob et al 2020).…”

Section: Discussionmentioning

confidence: 99%

Rotational spectroscopy of isotopic cyclopropenone, c-H₂C₃O, and determination of its equilibrium structure

Müller

Brahmi

Guillemin

et al. 2021

A&A

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Context. Cyclopropenone was first detected in the cold and less dense envelope of the giant molecular cloud Sagittarius B2(N). It was found later in several cold dark clouds and it may be possible to detect its minor isotopic species in these environments. In addition, the main species may well be identified in warmer environments. Aims. We aim to extend existing line lists of isotopologs of c-H 2 C 3 O from the microwave to the millimeter region and create one for the singly deuterated isotopolog to facilitate their detections in space. Furthermore, we aim to extend the line list of the main isotopic species to the submillimeter region and to evaluate an equilibrium structure of the molecule. Methods. We employed a cyclopropenone sample in natural isotopic composition to investigate the rotational spectra of the main and 18 O-containing isotopologs as well as the two isotopomers containing one 13 C atom. Spectral recordings of the singly and doubly deuterated isotopic species were obtained using a cyclopropenone sample highly enriched in deuterium. We recorded rotational transitions in the 70−126 GHz and 160−245 GHz regions for all isotopologs and also in the 342−505 GHz range for the main species. Quantum-chemical calculations were carried out to evaluate initial spectroscopic parameters and the differences between ground-state and equilibrium rotational parameters in order to derive semi-empirical equilibrium structural parameters. Results. We determined new or improved spectroscopic parameters for six isotopologs and structural parameters according to different structure models. Conclusions. The spectroscopic parameters are accurate enough to identify minor isotopic species at centimeter and millimeter wavelengths while those of the main species are deemed to be reliable up to 1 THz. Our structural parameters differ from earlier ones. The deviations are attributed to misassignments in the earlier spectrum of one isotopic species.

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First detection of ¹³CH in the interstellar medium

Cited by 26 publications

References 61 publications

The CH radical at radio wavelengths: revisiting emission in the 3.3 GHz ground-state lines

The CH radical at radio wavelengths: revisiting emission in the 3.3 GHz ground-state lines

The CH radical at radio wavelengths: Revisiting emission in the 3.3GHz ground state lines

Rotational spectroscopy of isotopic cyclopropenone, c-H₂C₃O, and determination of its equilibrium structure

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First detection of 13CH in the interstellar medium

Cited by 26 publications

References 61 publications

The CH radical at radio wavelengths: revisiting emission in the 3.3 GHz ground-state lines

The CH radical at radio wavelengths: revisiting emission in the 3.3 GHz ground-state lines

The CH radical at radio wavelengths: Revisiting emission in the 3.3GHz ground state lines

Rotational spectroscopy of isotopic cyclopropenone, c-H2C3O, and determination of its equilibrium structure

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First detection of ¹³CH in the interstellar medium

Rotational spectroscopy of isotopic cyclopropenone, c-H₂C₃O, and determination of its equilibrium structure