The effect of varying cosmic ray ionization rates on the chemistry of interstellar clouds

Farquhar, P. R. A.; Millar, T. J.; Herbst, Eric

doi:10.1063/1.46612

Cited by 2 publications

(1 citation statement)

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“…Bayet et al 2006). The greatly enhanced supernova rate by several orders of magnitude relative to the solar system value leads to an enhanced cosmic ray flux, providing another source of gas heating in the centers (Farquhar et al 1994;Bradford et al 2003). Another mechanism for heating is provided by shocks.…”

Section: Emission Of Co [C I] (And [C Ii]) Traces the Bulk Of Car-mentioning

confidence: 99%

$^\mathsf{{12}}$CO 4–3 and [CI] 1–0 at the centers of NGC 4945 and Circinus

Hitschfeld

Aravena²,

Krämer

et al. 2007

A&A

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Context. Studying molecular gas in the central regions of the star burst galaxies NGC 4945 and Circinus enables us to characterize the physical conditions and compare them to previous local and high-z studies. Aims. We estimate temperature, molecular density and column densities of CO and atomic carbon. Using model predictions we give a range of estimated CO/C abundance ratios. Methods. Using the new NANTEN2 4 m sub-millimeter telescope in Pampa La Bola, Chile, we observed for the first time CO 4-3 and [C i] 3 P 1 − 3 P 0 at the centers of both galaxies at linear scale of 682 pc and 732 pc respectively. We compute the cooling curves of 12 CO and 13 CO using radiative transfer models and estimate the physical conditions of CO and [CI].Results. 3 P 1 − 3 P 0 /CO 4-3 ratio of integrated intensities are large at 1.2 in NGC 4945 and 2.8 in Circinus. Combining previous CO J = 1−0, 2-1 and 3-2 and 13 CO J = 1−0, 2-1 studies with our new observations, the radiative transfer calculations give a range of densities, n(H 2 ) = 10 3 −3 × 10 4 cm −3 , and a wide range of kinetic temperatures, T kin = 20−100 K, depending on the density. To discuss the degeneracy in density and temperature, we study two representative solutions. In both galaxies the estimated total [CI] cooling intensity is stronger by factors of ∼1−3 compared to the total CO cooling intensity. The CO/C abundance ratios are 0.2−2, similar to values found in Galactic translucent clouds. Conclusions. Our new observations enable us to further constrain the excitation conditions and estimate the line emission of higher-J CO-and the upper [CI]-lines. For the first time we give estimates for the CO/C abundance ratio in the center regions of these galaxies. Future CO J = 7−6 and [CI] 2-1 observations will be important to resolve the ambiguity in the physical conditions and confirm the model predictions.

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Section: Emission Of Co [C I] (And [C Ii]) Traces the Bulk Of Car-mentioning

confidence: 99%

$^\mathsf{{12}}$CO 4–3 and [CI] 1–0 at the centers of NGC 4945 and Circinus

Hitschfeld

Aravena²,

Krämer

et al. 2007

A&A

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A Study of the Physics and Chemistry of TMC‐1

Pratap¹,

Dickens²,

Snell³

et al. 1997

ApJ

254

243

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We present a comprehensive study of the physical and chemical conditions along the TMC-1 ridge. Temperatures were estimated from observations of CH3CCH, NH3, and CO. Densities were obtained from a multitransition study of HC3N. The values of the density and temperature allow column densities for 13 molecular species to be estimated from statistical equilibrium calculations, using observations of rarer isotopomers where possible, to minimize opacity effects. The most striking abundance variations relative to HCO+ along the ridge were seen for HC3N, CH3CCH, and SO, while smaller variations were seen in CS, C2H, and HCN. On the other hand, the NH3, HNC, and N2H+ abundances relative to HCO+ were determined to be constant, indicating that the so-called NH3 peak in TMC-1 is probably a peak in the ammonia column density rather than a relative abundance peak. In contrast, the well-studied cyanopolyyne peak is most likely due to an enhancement in the abundance of long-chain carbon species. Comparisons of the derived abundances to the results of time-dependent chemical models show good overall agreement for chemical timescales around 10(5) yr. We find that the observed abundance gradients can be explained either by a small variation in the chemical timescale from 1.2 x 10(5) to 1.8 x 10(5) yr or by a factor of 2 change in the density along the ridge. Alternatively, a variation in the C/O ratio from 0.4 to 0.5 along the ridge produces an abundance gradient similar to that observed.

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The effect of varying cosmic ray ionization rates on the chemistry of interstellar clouds

Cited by 2 publications

References 0 publications

$^\mathsf{{12}}$CO 4–3 and [CI] 1–0 at the centers of NGC 4945 and Circinus

$^\mathsf{{12}}$CO 4–3 and [CI] 1–0 at the centers of NGC 4945 and Circinus

A Study of the Physics and Chemistry of TMC‐1

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