Sustained oscillations in interstellar chemistry models

Roueff, E.; Bourlot, J. Le

doi:10.1051/0004-6361/202039085

Cited by 10 publications

(6 citation statements)

References 30 publications

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“…The actual gas phase abundances of CO versus time for these models are shown in Figure 3c, along with similar evolutions of HCO + and N 2 H + . In this figure, we sometimes see oscillations in the abundances of CO, HCO + and N 2 H + , i.e., in the models with high CRIR and where the condition 10 18 cm −3 s < n H /ζ < 2.2 × 10 18 cm −3 s. Such behaviour has been noted in the previous study of Maffucci et al (2018), corresponding to the High Ionization Phase (HIP) case discussed by Roueff & Le Bourlot (2020). More generally, the results of Figures 1c and 2c indicate that as ζ increases, CO depletion begins earlier in time, but reaches a lower equilibrium value.…”

Section: Dependence On Cosmic Ray Ionization Ratesupporting

confidence: 74%

Astrochemical modelling of infrared dark clouds

Entekhabi,

Tan,

Cosentino

et al. 2021

Preprint

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Context. Infrared Dark Clouds (IRDCs) are cold, dense regions of the interstellar medium (ISM) that are likely to represent the initial conditions for massive star and star cluster formation. It is thus important to study the physical and chemical conditions of IRDCs to provide constraints and inputs for theoretical models of these processes. Aims. We aim to determine the astrochemical conditions, especially cosmic ray ionization rate (CRIR) and chemical age, in different regions of the massive IRDC G28.37+00.07 by comparing observed abundances of multiple molecules and molecular ions with the predictions of astrochemical models. Methods. We have computed a series of single-zone, time-dependent, astrochemical models with a gas-grain network that systematically explores the parameter space of density, temperature, CRIR, and visual extinction. We have also investigated the effects of choices of CO ice binding energy and temperatures achieved in transient heating of grains when struck by cosmic rays. We selected 10 positions across the IRDC that are known to have a variety of star formation activity. We utilised mid-infrared (MIR) extinction maps and sub-mm emission maps to measure the mass surface densities of these regions, needed for abundance and volume density estimates. The sub-mm emission maps were also used to measure temperatures. We then used IRAM-30m observations of various tracers, especially C 18 O(1-0), H 13 CO + (1-0), HC 18 O + (1-0), and N 2 H + (1-0), to estimate column densities and thus abundances. Finally, we investigated the range of astrochemical conditions that are consistent with the observed abundances.Results. The typical physical conditions of the IRDC regions are n H ∼ 3 × 10 4 to 10 5 cm −3 and T 10 to 15 K. Strong emission of H 13 CO + (1-0) and N 2 H + (1-0) is detected towards all the positions and these species are used to define relatively narrow velocity ranges of the IRDC regions, which are used for estimates of CO abundances, via C 18 O(1-0). CO depletion factors are estimated to be in the range f D ∼ 3 to 10. Using estimates of the abundances of CO, HCO + and N 2 H + we find consistency with astrochemical models that have relatively low CRIRs of ζ ∼ 10 −18 to ∼ 10 −17 s −1 , with no evidence for systematic variation with the level of star formation activity. Astrochemical ages, defined with reference to an initial condition of all H in H 2 , all C in CO, and all other species in atomic form, are found to be < 1 Myr. We also explore the effects of using other detected species, i.e., HCN, HNC, HNCO, CH 3 OH, and H 2 CO, to constrain the models. These generally lead to implied conditions with higher levels of CRIRs and older chemical ages. Considering the observed f D versus n H relation of the 10 positions, which we find to have relatively little scatter, we discuss potential ways in which the astrochemical models can match such a relation as a quasi-equilibrium limit valid at ages of at least a few free-fall times, i.e., 0.3 Myr, including the effect of CO envelope contamination...

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Section: Dependence On Cosmic Ray Ionization Ratesupporting

confidence: 74%

Astrochemical modelling of infrared dark clouds

Entekhabi,

Tan,

Cosentino

et al. 2021

Preprint

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“…The ice chemistry can reasonably well be approximated with a stationary chemistry for A V ≲ 5 − 10 and even deeper in the cloud for some ice species, such as J(CO) (see also Hollenbach et al 2009). On the other hand, time-dependent computations by Esplugues et al (2019) show that J(H 2 O) abun-dance at strong FUV illumination and n = 10 5 cm −3 reaches a plateau at A V > 7 after 10 6 −10 7 yr. We note that previous studies showed the existence of multiple chemical solutions (Le Bourlot et al 1993bBourlot et al , 1995Lee et al 1998;Charnley & Markwick 2003;Boger & Sternberg 2006;Dufour & Charnley 2019) as well as the existence of sustained chemical oscillations (Roueff & Le Bourlot 2020) in gas-phase models of the ISM.…”

Section: Chemical Time Scalesmentioning

confidence: 48%

The KOSMA-τPDR model

Röllig

Ossenkopf-Okada

2022

A&A

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Numerical models of photodissociation regions (PDRs) are an essential tool to quantitatively understand observations of massive star forming regions through simulations. Few mature PDR models are available and the Cologne KOSMA-τ PDR model is the only sophisticated model that uses a spherical cloud geometry, thereby allowing us to simulate clumpy PDRs. We present the current status of the code as a reference for modelers and for observers that plan to apply KOSMA-τ to interpret their data. For the numerical solution of the chemical problem, we present a superior Newton-Raphson stepping algorithm and discuss strategies to numerically stabilize the problem and speed up the iterations. The chemistry in KOSMA-τ is upgraded to include the full surface chemistry in an up-to-date formulation and we discuss a novel computation of branching ratios in chemical desorption reactions. The high dust temperature in PDRs leads to a selective freeze-out of oxygen-bearing ice species due to their higher condensation temperatures and we study changes in the ice mantle structures depending on the PDR parameters, in particular the impinging ultraviolet field. Selective freeze-out can produce enhanced C abundances and higher gas temperatures, resulting in a fine-structure line emission of atomic carbon [C I] enhanced by up to 50% if surface reactions are considered. We show how recent Atacama Large Millimeter Array (ALMA) observations of HCO + emission in the Orion Bar with high spatial resolution on the scale of individual clumps can be interpreted in the context of nonstationary, clumpy PDR ensembles. Additionally, we introduce WL-PDR, a simple plane-parallel PDR model written in Mathematica to act as a numerical testing environment of PDR modeling aspects.

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“…On the other hand, time-dependent computations by Esplugues et al (2019) show that J(H 2 O) abundance at strong FUV illumination and n = 10 5 cm −3 reaches a plateau at A V > 7 after 10 6 − 10 7 yr. Note, that previous studies showed the existence of multiple chemical solutions (Le Bourlot et al 1993bBourlot et al , 1995Lee et al 1998;Charnley & Markwick 2003;Boger & Sternberg 2006;Dufour & Charnley 2019) as well as the existence of sustained chemical oscillations (Roueff & Le Bourlot 2020) in gas-phase models of the ISM.…”

Section: Chemical Time Scalesmentioning

confidence: 87%

The KOSMA-$τ$ PDR Model -- I. Recent updates to the numerical model of photo-dissociated regions

Röllig,

Ossenkopf-Okada

2022

Preprint

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Numerical models of Photodissociation Regions (PDRs) are an essential tool to quantitatively understand observations of massive star forming regions through simulations. Few mature PDR models are available and the Cologne KOSMA-τ PDR model is the only sophisticated model that uses a spherical cloud geometry thereby allowing us to simulate clumpy PDRs. We present the current status of the code as reference for modelers and for observers that plan to apply KOSMA-τ to interpret their data. For the numerical solution of the chemical problem we present a superior Newton-Raphson stepping algorithm and discuss strategies to numerically stabilize the problem and speed up the iterations. The chemistry in KOSMA-τ is upgraded to include the full surface chemistry in an up-to-date formulation and we discuss a novel computation of branching ratios in chemical desorption reactions. The high dust temperature in PDRs leads to a selective freeze-out of oxygen-bearing ice species due to their higher condensation temperatures and we study changes in the ice mantle structures depending on the PDR parameters, in particular the impinging UV field. Selective freeze-out can produce enhanced C abundances and higher gas temperatures resulting in a fine-structure line emission of atomic carbon [C I] enhanced by up to 50% if surface reactions are considered. We show how recent ALMA observations of HCO + emission in the Orion Bar with high spatial resolution on the scale of individual clumps can be interpreted in the context of non-stationary, clumpy PDR ensembles. Additionally, we introduce WL-PDR, a simple plane-parallel PDR model written in Mathematica to act as numerical testing environment of PDR modeling aspects.

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Sustained oscillations in interstellar chemistry models

Cited by 10 publications

References 30 publications

Astrochemical modelling of infrared dark clouds

Astrochemical modelling of infrared dark clouds

The KOSMA-τPDR model

The KOSMA-$τ$ PDR Model -- I. Recent updates to the numerical model of photo-dissociated regions

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