The fractional ionization in dark molecular clouds

Flower, D. R.; Forêts, G. Pineau des; Walmsley, C. M.

doi:10.1051/0004-6361:20078138

Cited by 32 publications

(51 citation statements)

References 38 publications

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“…Hence, lower ionization fractions (and a much weaker dependence on the assumed metal elemental abundance) are predicted by the model with PAHs. These results agrees with theoretical predictions for UV shielded gas (Lepp & Dalgarno 1988;Flower et al 2007;Wakelam & Herbst 2008).…”

Section: The Ionization Fraction Gradientsupporting

confidence: 90%

“…In particular, we take into account PAHgas processes such as neutralization reactions of atomic and molecular cations on PAH − , PAH electron attachment and photodetachment of PAH − and PAH by UV photons. Such processes can play a significant role in the ionization balance of dense molecular clouds (e.g., Lepp & Dalgarno 1988;Bakes & Tielens 1998;Flower et al 2007, Wakelam & Herbst 2008Wolfire et al 2008). We have not included larger grains in the network in order to isolate the role of PAHs in the gasphase chemistry.…”

Section: Chemical Network and Elemental Abundancesmentioning

confidence: 99%

“…is high (≥10 −7 cm 3 s −1 ), although probably dependent on the PAH size (Omont 1986;Allamandola et al 1989;Flower et al 2007;Wakelam & Herbst 2008 ). For our choice of PAH parameters, this means that PAH − can be the most abundant negatively charged species, more than electrons for A V ≥ 5.…”

Section: The Role Of Pahsmentioning

confidence: 99%

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The ionization fraction gradient across the Horsehead edge: an archetype for molecular clouds

Goicoechea¹,

Pety²,

Gérin³

et al. 2009

A&A

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Context. The ionization fraction (i.e., the electron abundance) plays a key role in the chemistry and dynamics of molecular clouds. Aims. We study the H 13 CO + , DCO + and HOC + line emission towards the Horsehead, from the shielded core to the UV irradiated cloud edge, i.e., the photodissociation region (PDR), as a template to investigate the ionization fraction gradient in molecular clouds. Methods. We analyze an IRAM Plateau de Bure Interferometer map of the H 13 CO + J = 1-0 line at a 6.8 × 4.7 resolution, complemented with IRAM-30 m H 13 CO + and DCO + higher-J line maps and new HOC + and CO + observations. We compare self-consistently the observed spatial distribution and line intensities with detailed depth-dependent predictions of a PDR model coupled with a nonlocal radiative transfer calculation. The chemical network includes deuterated species, 13 C fractionation reactions and HCO + /HOC + isomerization reactions. The role of neutral and charged PAHs in the cloud chemistry and ionization balance is investigated. Results. The detection of the HOC + reactive ion towards the Horsehead PDR proves the high ionization fraction of the outer UV irradiated regions, where we derive a low [HCO + ]/[HOC + ] 75-200 abundance ratio. In the absence of PAHs, we reproduce the observations with gas-phase metal abundances, [Fe+Mg+...], lower than 4 × 10 −9 (with respect to H), and a cosmic-ray ionization rate of ζ = (5 ± 3) × 10 −17 s −1 . The inclusion of PAHs modifies the ionization fraction gradient and increases the required metal abundance. Conclusions. The ionization fraction in the Horsehead edge follows a steep gradient, with a scale length of ∼0.05 pc (or ∼25 ), from [e − ] 10 −4 (or n e ∼ 1-5 cm −3 ) in the PDR to a few times ∼10 −9 in the core. PAH − anions play a role in the charge balance of the cold and neutral gas if substantial amounts of free PAHs are present ([PAH] > 10 −8 ).

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Section: The Ionization Fraction Gradientsupporting

confidence: 90%

Section: Chemical Network and Elemental Abundancesmentioning

confidence: 99%

See 1 more Smart Citation

The ionization fraction gradient across the Horsehead edge: an archetype for molecular clouds

Goicoechea¹,

Pety²,

Gérin³

et al. 2009

A&A

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“…The electron abundance obtained assuming an o/p-H 2 of 1.0 × 10 −4 is probably an upper limit. This o/p ratio corresponds to a steady state of highly depleted dense gas with high abundances of H + and H + 3 capable of efficient proton exchange with H 2 (Flower et al 2007). Their replacement by other ions in less extreme situations can sustain higher o/p-H 2 ratios.…”

Section: Ionization Degree and Cosmic-ray Ionization Ratementioning

confidence: 99%

Prestellar and protostellar cores in Orion B9

Miettinen

Harju

Haikala

et al. 2009

A&A

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Context. Dense molecular cores are studied to gain insight into the processes causing clouds to fragment and form stars. In this study, we concentrate on a region that is assumed to represent an early stage of clustered star-formation in a giant molecular cloud. Aims. We aim to determine the properties and spatial distribution of dense cores in the relatively quiescent Ori B9 cloud, and to estimate their ages and dynamical timescales. Methods. The cloud was mapped in the 870 μm continuum with APEX/LABOCA, and selected positions were observed in the lines of N 2 H + and N 2 D + using IRAM-30 m. These were used together with our previous H 2 D + observations to derive the degree of deuteration and some other chemical characteristics. Archival far-infrared Spitzer/MIPS maps were combined with the LABOCA map to distinguish between prestellar and protostellar cores, and to estimate the evolutionary stages of protostars. Results. Twelve dense cores were detected at 870 μm continuum in the Ori B9 cloud. The submm cores constitute ∼4% of the total mass of the Ori B9 region. There is an equal number of prestellar and protostellar cores. Two of the submm sources, which we call SMM 3 and SMM 4, are previously unknown Class 0 candidates. There is a high likelihood of the core masses and mutual separations representing the same distributions as observed in other parts of Orion. We found a moderate degree of deuteration in N 2 H + (0.03-0.04). There is, furthermore, evidence of N 2 H + depletion in the core SMM 4. These features suggest that the cores have reached chemical maturity. We derive a relatively high degree of ionization (∼10 −7 ) in the clump associated with IRAS 05405-0117. The ambipolar diffusion timescales for two of the cores are ∼70-100 times longer than the free-fall time.Conclusions. The distribution and masses of dense cores in Ori B9 are similar to those observed in more active regions of Orion, where the statistical core properties have been explained by turbulent fragmentation. The 50/50 proportions of prestellar and protostellar cores imply that the duration of the prestellar phase is comparable to the free-fall time. However, on the basis of chemical data of the IRAS 05405-0117 region, this timescale could be questioned. A possible explanation is that this survey samples only the densest, i.e., dynamically most advanced cores.

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“…We adopt the dissociation factors p M calculated by Gredel et al (1989) for various species, leading to a rate per unit volume of (Flower et al 2007), where we adopt a grain albedo ω = 0.5. For CO, we adopt the highabundance, high-temperature limit of p CO = 28 for ω = 0.5 (Gredel et al 1987).…”

Section: X-ray Induced Dissociationmentioning

confidence: 99%

Molecule survival in magnetized protostellar disk winds

Panoglou¹,

Cabrit²,

Forêts³

et al. 2012

A&A

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144

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Context. Molecular counterparts to atomic jets have recently been detected within 1000 AU of young stars at early evolutionary stages. Reproducing these counterparts is an important new challenge for proposed ejection models. Aims. We explore whether molecules may survive in the magneto-hydrodynamic (MHD) disk wind solution currently invoked to reproduce the kinematics and tentative rotation signatures of atomic jets in T Tauri stars. Methods. The coupled ionization, chemical, and thermal evolution along dusty flow streamlines is computed for the prescribed MHD disk wind solution, using a method developed for magnetized shocks in the interstellar medium. Irradiation by (wind-attenuated) coronal X-rays and far-ultraviolet photons from accretion hot spots is included, with an approximate self-shielding of H 2 and CO. Disk accretion rates of 5 × 10 −6 , 10 −6 and 10 −7 M yr −1 are considered, representative of low-mass young protostars (so-called "Class 0"), evolved protostars ("Class I") and very active T Tauri stars ("Class II") respectively. Results. The disk wind has an "onion-like" thermo-chemical structure, with streamlines launched from larger radii having lower temperature and ionization, and higher H 2 abundance. The coupling between charged and neutral fluids is sufficient to eject molecules from the disk out to at least 9 AU. The launch radius beyond which most H 2 survives moves outward with evolutionary stage, from 0.2 AU (sublimation radius) in the Class 0 disk wind, to 1 AU in the Class I, and >1 AU in the Class II. In this molecular wind region, CO survives in the Class 0 but is significantly photodissociated in the Class I/II. Balance between ambipolar heating and molecular cooling establishes a moderate asymptotic temperature 700−3000 K, with cooler jets at earlier protostellar stages. As a result, endothermic formation of H 2 O is efficient, with abundances up to 10 −4 , while CH + and SH + can reach ≥10 −6 in the hotter and more ionised Class I/II winds. Conclusions. A centrifugal MHD disk wind launched from beyond 0.2−1 AU can produce molecular jets/winds up to speeds 100 km s −1 in young low-mass stars ranging from Class 0 to active Class II. The model predicts a high abundance ratio of H 2 to CO and an increase of molecular launch radius, temperature, and flow width as the source evolves, in promising agreement with current observed trends. Calculations of synthetic maps and line profiles in H 2 , CO and H 2 O will allow detailed tests of the model against observations.

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The fractional ionization in dark molecular clouds

Cited by 32 publications

References 38 publications

The ionization fraction gradient across the Horsehead edge: an archetype for molecular clouds

The ionization fraction gradient across the Horsehead edge: an archetype for molecular clouds

Prestellar and protostellar cores in Orion B9

Molecule survival in magnetized protostellar disk winds

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