Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

The evolution of CH in Planck Galactic Cold Clumps

Gan Luo, Arshia M. Jacob, Marco Padovani, Daniele Galli, Ana López-Sepulcre, Ningyu Tang, Di Li, Jing Zhou, Pei Zuo

TL;DR

This study uses CH 3.3 GHz observations toward 27 Planck Galactic Cold Clumps (PGCCs) with the Arecibo telescope, augmented by archival $^{13}$CO(1-0), HI (HINSA), and dust-derived $N_{ m H_2}$ to examine CH excitation, kinematics, and chemistry in cold, dense clumps. Radiative transfer via RADEX yields CH column densities and excitation temperatures with a median $T_{ m ex}\approx -6.5$ K, while HINSA-based estimates place upper limits on the cosmic-ray ionization rate $\zeta_2$ that favor CR attenuation model $\mathscr{L}$. The CH non-thermal velocity dispersions are typically subsonic, unlike $^{13}$CO, implying CH traces coherent, dense gas; CH abundance declines with $N_{ m H_2}$ but correlates positively with $\zeta_2$, indicating CH formation is tied to C$^+$ abundance and CR ionization, modulated by atomic O availability. These results constrain atomic O and C$^+$ abundances and highlight the role of CRs in shaping CH chemistry in dense PGCC environments, with implications for using CH as a tracer of H$_2$ in cold clouds and for CR transport in the densest ISM regimes.

Abstract

Methylidyne (CH) has long been considered a reliable tracer of molecular gas in the low-to-intermediate extinction range. Although extended CH 3.3 GHz emission is commonly observed in diffuse and translucent clouds, observations in cold, dense clumps are rare. In this work, we conducted high-sensitivity CH observations toward 27 PGCCs with the Arecibo 305m telescope. Toward each source, the CH data were analyzed in conjunction with $^{13}$CO (1--0), HINSA, and H$_2$ column densities. Our results revealed ubiquitous subsonic velocity dispersions of CH, in contrast to $^{13}$CO, which is predominantly supersonic. The findings suggest that subsonic CH emissions may trace dense, low-turbulent gas structures in PGCCs. To investigate environmental effects, particularly the cosmic-ray ionization rate (CRIR), we estimated CRIR upper limits from HINSA, yielding values from $(8.1\pm4.7)\times10^{-18}$ to $(2.0\pm0.8)\times10^{-16}$ s$^{-1}$ ($N_{H_2}$ from $(1.7\pm0.2)\times10^{21}$ to $(3.6\pm0.4)\times10^{22}$~cm$^{-2}$). This result favors theoretical predictions of a cosmic-ray attenuation model, in which the interstellar spectra of low-energy CR protons and electrons match {\it Voyager} measurements, although alternative models cannot yet be ruled out. The abundance of CH decreases with increasing column density, while showing a positive dependence on the CRIR, which requires atomic oxygen not heavily depleted to dominate CH destruction in PGCCs. By fitting the abundance of CH with an analytic formula, we place constraints on atomic O abundance ($2.4\pm0.4\times10^{-4}$ with respect to total H) and C$^+$ abundance ($7.4\pm0.7\times10^{13}ζ_2/n_{\rm H_2}$). These findings indicate that CH formation is closely linked to the C$^+$ abundance, regulated by cosmic-ray ionization, while other processes, such as turbulent diffusive transport, might also contribute a non-negligible effect.

The evolution of CH in Planck Galactic Cold Clumps

TL;DR

This study uses CH 3.3 GHz observations toward 27 Planck Galactic Cold Clumps (PGCCs) with the Arecibo telescope, augmented by archival CO(1-0), HI (HINSA), and dust-derived to examine CH excitation, kinematics, and chemistry in cold, dense clumps. Radiative transfer via RADEX yields CH column densities and excitation temperatures with a median K, while HINSA-based estimates place upper limits on the cosmic-ray ionization rate that favor CR attenuation model . The CH non-thermal velocity dispersions are typically subsonic, unlike CO, implying CH traces coherent, dense gas; CH abundance declines with but correlates positively with , indicating CH formation is tied to C abundance and CR ionization, modulated by atomic O availability. These results constrain atomic O and C abundances and highlight the role of CRs in shaping CH chemistry in dense PGCC environments, with implications for using CH as a tracer of H in cold clouds and for CR transport in the densest ISM regimes.

Abstract

Methylidyne (CH) has long been considered a reliable tracer of molecular gas in the low-to-intermediate extinction range. Although extended CH 3.3 GHz emission is commonly observed in diffuse and translucent clouds, observations in cold, dense clumps are rare. In this work, we conducted high-sensitivity CH observations toward 27 PGCCs with the Arecibo 305m telescope. Toward each source, the CH data were analyzed in conjunction with CO (1--0), HINSA, and H column densities. Our results revealed ubiquitous subsonic velocity dispersions of CH, in contrast to CO, which is predominantly supersonic. The findings suggest that subsonic CH emissions may trace dense, low-turbulent gas structures in PGCCs. To investigate environmental effects, particularly the cosmic-ray ionization rate (CRIR), we estimated CRIR upper limits from HINSA, yielding values from to s ( from to ~cm). This result favors theoretical predictions of a cosmic-ray attenuation model, in which the interstellar spectra of low-energy CR protons and electrons match {\it Voyager} measurements, although alternative models cannot yet be ruled out. The abundance of CH decreases with increasing column density, while showing a positive dependence on the CRIR, which requires atomic oxygen not heavily depleted to dominate CH destruction in PGCCs. By fitting the abundance of CH with an analytic formula, we place constraints on atomic O abundance ( with respect to total H) and C abundance (). These findings indicate that CH formation is closely linked to the C abundance, regulated by cosmic-ray ionization, while other processes, such as turbulent diffusive transport, might also contribute a non-negligible effect.

Paper Structure

This paper contains 26 sections, 15 equations, 13 figures, 5 tables.

Table of Contents

  1. Introduction
  2. Observations
  3. Arecibo CH observations
  4. $^{13}$CO (1--0) observations
  5. Archival H i data
  6. Archival continuum data
  7. Results
  8. Spectra line fitting
  9. Non-thermal velocity dispersion
  10. Column densities of $^{13}$CO
  11. radex modeling of CH
  12. Column densities of HINSA
  13. Analysis and discussion
  14. Uncertainties
  15. Excitation of CH
  16. ...and 11 more sections

Figures (13)

  • Figure 1: Example presenting the (a): $^{13}$CO, (b) CH 3335 MHz, and (c) H i spectra toward G168.13-16.39. In subplots (a) and (b), the red dashed curve represents the Gaussian fitting results, while the green dotted curves represent each component. In subplot (c), the red dashed curve denotes the recovered background H i emission without absorption ($T_{\rm b}$). The green dotted and black vertical lines denote the decomposed $\tau$ and $V_{\rm lsr}$ of HINSA, respectively.
  • Figure 2: Comparison between the $V_{\rm lsr}$'s of $^{13}$CO emission with that of HINSA (left) and CH (right). The red dashed lines mark a slope of unity.
  • Figure 3: Comparison between the non-thermal velocity dispersion ($\sigma_{\rm NT}$) of $^{13}$CO with that of HINSA (left) and CH (right). The red dashed lines mark a slope of unity, blue dashed lines mark the sound speed ($c_{\rm s}$ = 0.21 km s$^{-1}$) at $T_{\rm k}$ = 12 K.
  • Figure 4: The posterior probability distribution of $n_{\rm H_2}$ and the column density of CH ($N_{\rm CH}$) toward (a) G168.13-16.39 at $V_{\rm lsr} = 6.53$ km s$^{-1}$ and (b) G158.77-33.30 at $V_{\rm lsr} = -2.44$ km s$^{-1}$. The kinetic temperatures used in the models are 11.6 K for G168.13-16.39 and 12.5 K for G158.77-33.30.
  • Figure 5: Deviations in $N_{\rm ^{13}CO}$ computed assuming $T_{\rm ex} = 12$ K for $T_{\rm mb}$ = 0.1 (in black) and 1 K (in blue).
  • ...and 8 more figures