Fractional abundances of molecules in dense interstellar clouds: 
 A compendium of recent model results

Lee, H.-H.; Bettens, Ryan P. A.; Herbst, Eric

doi:10.1051/aas:1996232

Cited by 243 publications

(207 citation statements)

References 31 publications

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“…When these complex molecules are dissociated by the strong UV fields of M 82, their abundances decrease, while a similar decrease for CH 3 CCH is balanced by ion molecule reactions favoring its formation. On the other hand, a strong anti correlation between N CH3CCH and T kin has been observed in galactic sources (Lee et al 1996), and the gas in M 82 is indeed as cold as ∼30 K. Methyl acetylene is not observed in the regions of densities < 10 5 cm −3 since it would be dissociated, but in regions of intermediate densities (Region II), where it is well shielded from the dissociating radiation. Furthermore, we do not see it in the Region I, presumably because its abundance in such places is almost negligible, although it could exist there (Chrurchwell & Hollis 1983).…”

Section: Evolved Starburst: M 82mentioning

confidence: 98%

CS, HC₃N, and CH₃CCH multi-line analyses toward starburst galaxies

Aladro

Martı́n-Pintado

Martín

et al. 2010

A&A

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Aims. We aim to study the properties of the dense molecular gas towards the inner few 100 pc of four nearby starburst galaxies dominated both by photo dissociation regions (M 82) and large-scale shocks (NGC 253, IC 342 and Maffei 2), and to relate the chemical and physical properties of the molecular clouds with the evolutionary stage of the nuclear starbursts. Methods. We have carried out multi-transitional observations and analyses of three dense gas molecular tracers, CS, HC 3 N (cyanoacetylene) and CH 3 CCH (methyl acetylene), using Boltzmann diagrams in order to determine the rotational temperatures and column densities of the dense gas, and using a Large Velocity Gradients model to calculate the H 2 density structure in the molecular clouds.Results. The CS and HC 3 N data indicate the presence of density gradients in the molecular clouds. These two molecules show similar excitation conditions, suggesting that they arise from the same gas components. In M 82, CH 3 CCH has the highest fractional abundance determined in a extragalactic source (1.1 × 10 −8 ). Conclusions. The density and the chemical gradients we have found in all galaxies can be explained in the framework of the starburst evolution, which affects the chemistry and the structure of molecular clouds around the galactic nuclei. The young shock-dominated starburst galaxies, like presumably Maffei 2, show a cloud structure with a rather uniform density and chemical composition which suggests low star formation activity. Molecular clouds in galaxies with starburst in an intermediate stage of evolution, such as NGC 253 and IC 342, show clouds with a large density contrast (two orders of magnitude) between the denser regions (cores) and the less dense regions (halos) of the molecular clouds and relatively constant chemical abundance. Finally, the galaxy with the most evolved starburst, M 82, has clouds with a rather uniform density structure, large envelopes of atomic/molecular gas subjected to UV photodissociating radiation from young star clusters, and very different chemical abundances of HC 3 N and CH 3 CCH.

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Section: Evolved Starburst: M 82mentioning

confidence: 98%

CS, HC₃N, and CH₃CCH multi-line analyses toward starburst galaxies

Aladro

Martı́n-Pintado

Martín

et al. 2010

A&A

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“…The initial chemical abundances used for the Horsehead PDR (Lee et al 1996a;Morata & Herbst 2008) are listed in Table 2 and represent abundances for a dark cloud prior to the onset of a nearby star. These abundances comprise observed values for small (less-than-six-atom) species in TMC-1, as well as calculated early-time values from Smith et al (2004) for atoms and small molecules that have not been observed, based on socalled "low-metal" elemental abundances.…”

Section: Physical Conditions and Initial Chemical Abundancesmentioning

confidence: 99%

Observing a column-dependentζin dense interstellar sources: the case of the Horsehead nebula

Rimmer

Herbst

Morata

et al. 2011

A&A

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Context. Observations of small carbon-bearing molecules such as CCH, C 4 H, c-C 3 H 2 , and HCO in the Horsehead nebula have shown these species to have higher abundances towards the edge of the source than towards the center. Aims. Given the determination of a wide range of values for ζ (s −1 ), the total ionization rate of hydrogen atoms, and the proposal of a column-dependent ζ(N H ), where N H is the total column of hydrogen nuclei, we desire to determine if the effects of ζ(N H ) in a single object with spatial variation can be observable. We chose the Horsehead nebula because of its geometry and high density. Methods. We model the Horsehead nebula as a near edge-on photon dominated region (PDR), using several choices for ζ, both constant and as a function of column. The column-dependent ζ functions are determined by a Monte Carlo model of cosmic ray penetration, using a steep power-law spectrum and accounting for ionization and magnetic field effects. We consider a case with low-metal elemental abundances as well as a sulfur-rich case.Results. We show that use of a column-dependent ζ(N H ) of 5 × 10 −15 s −1 at the surface and 7.5 × 10 −16 s −1 at A V = 10 on balance improves agreement between measured and theoretical molecular abundances, compared with constant values of ζ.

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“…where k HD is the rate coefficient of reaction 3 (see below), Oliveira et al 2003), k CO = 3.3 × 10 −9 (T/10 K) −0.5 cm 3 s −1 is the rate coefficient of the reaction H 2 D + + CO → products, k N 2 = 1.7 × 10 −9 cm 3 s −1 is the rate coefficient for the reaction Lee et al 1996), k e = 5.5 × 10 −7 (T/10 K) −0.65 cm 3 s −1 is the dissociative recombination rate of H 2 D + (see Caselli et al 1998), and k gr is the rate of recombination of H 2 D + on negatively charged dust grains, which, following Draine & Sutin (1987), is given by:…”

Section: A Simple Analytical Modelmentioning

confidence: 99%

Observations of L1521F: A highly evolved starless core

Crapsi

Caselli

Walmsley

et al. 2004

A&A

161

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Abstract. We observed the pre-stellar core L1521F in dust emission at 1.2 mm and in two transitions each of N 2 H + , N 2 D + , C 18 O and C 17 O in order to increase the sample of well studied centrally concentrated and chemically evolved starless cores, likely on the verge of star formation, and to determine the initial conditions for low-mass star formation in the Taurus Molecular Cloud. The dust observation allows us to infer the density structure of the core and together with measurements of CO isotopomers gives us the CO depletion. N 2 H + and N 2 D + lines are good tracers of the dust continuum and thus they give kinematic information on the core nucleus. We derived in this object a molecular hydrogen number density n(H 2 ) ∼ 10 6 cm −3 and a CO depletion factor, integrated along the line of sight, f D ≡ 9.5 × 10 −5 /x obs (CO) ∼ 15 in the central 20 , similar to the pre-stellar core L1544. However, the N(N 2 D + )/N(N 2 H + ) column density ratio is ∼0.1, a factor of about 2 lower than that found in L1544. The observed relation between the deuterium fractionation and the integrated CO depletion factor across the core can be reproduced by chemical models if N 2 H + is slightly (factor of ∼2 in fractional abundance) depleted in the central 3000 AU. The N 2 H + and N 2 D + linewidths in the core center are ∼0.3 km s −1 , significantly larger than in other more quiescent Taurus starless cores but similar to those observed in the center of L1544. The kinematical behaviour of L1521F is more complex than seen in L1544, and a model of contraction due to ambipolar diffusion is only marginally consistent with the present data. Other velocity fields, perhaps produced by accretion of the surrounding material onto the core and/or unresolved substructure, are present. Both chemical and kinematical analyses suggest that L1521F is less evolved than L1544, but, in analogy with L1544, it is approaching the "critical" state.

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Fractional abundances of molecules in dense interstellar clouds: A compendium of recent model results

Cited by 243 publications

References 31 publications

CS, HC₃N, and CH₃CCH multi-line analyses toward starburst galaxies

CS, HC₃N, and CH₃CCH multi-line analyses toward starburst galaxies

Observing a column-dependentζin dense interstellar sources: the case of the Horsehead nebula

Observations of L1521F: A highly evolved starless core

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Fractional abundances of molecules in dense interstellar clouds: A compendium of recent model results

Cited by 243 publications

References 31 publications

CS, HC3N, and CH3CCH multi-line analyses toward starburst galaxies

CS, HC3N, and CH3CCH multi-line analyses toward starburst galaxies

Observing a column-dependentζin dense interstellar sources: the case of the Horsehead nebula

Observations of L1521F: A highly evolved starless core

Contact Info

Product

Resources

About

CS, HC₃N, and CH₃CCH multi-line analyses toward starburst galaxies

CS, HC₃N, and CH₃CCH multi-line analyses toward starburst galaxies