In studies of the interstellar medium in galaxies, radiative transfer models of molecular emission are useful for relating molecular line observations back to the physical conditions of the gas they ...trace. However, doing this requires solving a highly degenerate inverse problem. In order to alleviate these degeneracies, the abundances derived from astrochemical models can be converted into column densities and fed into radiative transfer models. This ensures that the molecular gas composition used by the radiative transfer models is chemically realistic. However, because of the complexity and long running time of astrochemical models, it can be difficult to incorporate chemical models into the radiative transfer framework. In this paper, we introduce a statistical emulator of the UCLCHEM astrochemical model, built using neural networks. We then illustrate, through examples of parameter estimations, how such an emulator can be applied to real and synthetic observations.
Phosphorus-bearing molecules in the Galactic Center Rivilla, V M; Jiménez-Serra, I; Zeng, S ...
Monthly notices of the Royal Astronomical Society. Letters,
03/2018, Letnik:
475, Številka:
1
Journal Article
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Abstract
Phosphorus (P) is one of the essential elements for life due to its central role in biochemical processes. Recent searches have shown that P-bearing molecules (in particular PN and PO) are ...present in star-forming regions, although their formation routes remain poorly understood. In this letter, we report observations of PN and PO towards seven molecular clouds located in the Galactic Center, which are characterized by different types of chemistry. PN is detected in five out of seven sources, whose chemistry is thought to be shock-dominated. The two sources with PN non-detections correspond to clouds exposed to intense UV/X-rays/cosmic ray (CR) radiation. PO is detected only towards the cloud G+0.693−0.03, with a PO/PN abundance ratio of ∼1.5. We conclude that P-bearing molecules likely form in shocked gas as a result of dust grain sputtering, while are destroyed by intense UV/X-ray/CR radiation.
Context. We present a study of the sulphur chemistry evolution in the region Orion KL along the gas and grain phases of the cloud. Aims. Our aim is to investigate the processes that dominate the ...sulphur chemistry in Orion KL and to determine how physical and chemical parameters, such as the final mass of the star and the initial elemental abundances, influence the evolution of the hot core and of the surrounding outflows and shocked gas (the plateau). Methods. We independently modelled the chemistry evolution of the hot core and the plateau using the time-dependent gas-grain model UCL_CHEM and considering two different phase calculations. Phase I starts with the collapsing cloud and the depletion of atoms and molecules onto grain surfaces. Phase II starts when a central protostar is formed and the evaporation from grains takes place. We show how the stellar mass, the gas density, the gas depletion efficiency, the initial sulphur abundance, the shocked gas temperature, and the different chemical paths on the grains leading to different reservoirs of sulphur on the mantles affect sulphur-bearing molecules at different evolutionary stages for both components. We also compare the predicted column densities with those inferred from observations of the species SO, SO2, CS, OCS, H2S, and H2CS. Results. The models that reproduce the observations of the largest number of sulphur-bearing species in both components are those with an initial sulphur abundance of 0.1 times the sulphur solar abundance (0.1 S⊙) and a density of at least nH = 5 × 106 cm-3 in the shocked gas region. Conclusions. We conclude that most of the sulphur atoms were ionised during Phase I, consistent with an inhomogeneous and clumpy region where the UV interstellar radiation penetrates and leading to sulphur ionisation. We also conclude that the main sulphur reservoir on the ice mantles was H2S. In addition, we deduce that a chemical transition currently takes place in the plateau shocked gas, where SO and SO2 gas-phase formation reactions change from being dominated by O2 to being dominated by OH.
We report on Atacama Large Millimeter Array (ALMA) observations of CO(3−2) emission in the Seyfert2/starburst galaxy NGC 1808, at a spatial resolution of 4 pc. Our aim is to investigate the ...morphology and dynamics of the gas inside the central 0.5 kpc and to probe the nuclear feeding and feedback phenomena. We discovered a nuclear spiral of radius 1″ = 45 pc. Within it, we found a decoupled circumnuclear disk or molecular torus of a radius of 0.13″ = 6 pc. The HCN(4−3) and HCO
+
(4−3) and CS(7−6) dense gas line tracers were simultaneously mapped and detected in the nuclear spiral and they present the same misalignment in the molecular torus. At the nucleus, the HCN/HCO
+
and HCN/CS ratios indicate the presence of an active galactic nucleus (AGN). The molecular gas shows regular rotation, within a radius of 400 pc, except for the misaligned disk inside the nuclear spiral arms. The computations of the torques exerted on the gas by the barred stellar potential reveal that the gas within a radius of 100 pc is feeding the nucleus on a timescale of five rotations or on an average timescale of ∼60 Myr. Some non-circular motions are observed towards the center, corresponding to the nuclear spiral arms. We cannot rule out that small extra kinematic perturbations could be interpreted as a weak outflow attributed to AGN feedback. The molecular outflow detected at ≥250 pc in the NE direction is likely due to supernovae feedback and it is connected to the kpc-scale superwind.
Aims. The circumnuclear disk (CND) of the Galactic center is exposed to many energetic phenomena coming from the supermassive black hole Sgr A* and from stellar activities. These energetic activities ...can affect the chemical composition in the CND through interaction with UV photons, cosmic rays, X-rays, and shock waves. We aim to constrain the physical conditions present in the CND through chemical modeling of observed molecular species detected toward it. Methods. We analyzed a selected set of molecular line data taken toward a position in the southwest lobe of the CND with the IRAM 30m and APEX 12-m telescopes and derived the column density of each molecule via a large velocity gradient (LVG) analysis. The determined chemical composition is compared with a time-dependent, gas-grain chemical model based on the UCL_CHEM code,which includes the effects of shock waves with varying physical parameters. Results. We detect molecules, such as CO, HCN, HCO+, HNC, CS, SO, SiO, NO, CN, H2CO, HC3N, N2H+, and H3O+, and obtain their column densities. Total hydrogen densities obtained from LVG analysis range between 2 × 104 and 1 × 106cm-3 and most species indicate values around several × 105cm-3. These values are lower than those corresponding to the Roche limit, which shows that the CND is tidally unstable. The chemical models show good agreement with the observations in cases where the density is ~104cm-3, the cosmic-ray ionization rate is high, > 10-15s-1, or shocks with velocities > 40 km s-1 have occurred. Conclusions. Comparison of models and observations favors a scenario where the cosmic-ray ionization rate in the CND is high, but precise effects of other factors, such as shocks, density structures, UV photons, and X-rays from the Sgr A*, must be examined with higher spatial resolution data.
Abstract
We have performed combined photoionization and photodissociation region (PDR) modelling of a Crab nebula filament subjected to the synchrotron radiation from the central pulsar wind nebula, ...and to a high flux of charged particles; a greatly enhanced cosmic-ray ionization rate over the standard interstellar value, ζ0, is required to account for the lack of detected C i emission in published Herschel SPIRE FTS observations of the Crab nebula. The observed line surface brightness ratios of the OH+ and ArH+ transitions seen in the SPIRE FTS frequency range can only be explained with both a high cosmic-ray ionization rate and a reduced ArH+ dissociative recombination rate compared to that used by previous authors, although consistent with experimental upper limits. We find that the ArH+/OH+ line strengths and the observed H2 vibration–rotation emission can be reproduced by model filaments with nH = 2 × 104 cm−3, ζ = 107ζ0 and visual extinctions within the range found for dusty globules in the Crab nebula, although far-infrared emission from O i and C ii is higher than the observational constraints. Models with nH = 1900 cm−3 underpredict the H2 surface brightness, but agree with the ArH+ and OH+ surface brightnesses and predict O i and C ii line ratios consistent with observations. These models predict HeH+ rotational emission above detection thresholds, but consideration of the formation time-scale suggests that the abundance of this molecule in the Crab nebula should be lower than the equilibrium values obtained in our analysis.
Grain surface chemistry and its treatment in gas-grain chemical models is an area of large uncertainty. While laboratory experiments are making progress, there is still much that is unknown about ...grain surface chemistry. Further, the results and parameters produced by experiments are often not easily translated to the rate equation approach most commonly used in astrochemical modeling. It is possible that statistical methods can reduce the uncertainty in grain surface chemical networks. In this work, a simple model of grain surface chemistry in a molecular cloud is developed and a Bayesian inference of the reactions rates is performed through Markov Chain Monte Carlo sampling. Using observational data of the solid state abundances of major chemical species in molecular clouds, the posterior distributions for the rates of seven reactions producing CO, CO2, CH3OH, and H2O are calculated in a form that is suitable for rate equation models. This represents a vital first step in the development of a method to infer reaction rates from observations of chemical abundances in astrophysical environments.
The interaction of ionizing and far-ultraviolet radiation with the interstellar medium is of great importance. It results in the formation of regions in which the gas is ionized, beyond which are ...photodissociation regions (PDRs) in which the gas transitions to its atomic and molecular form. Several numerical codes have been implemented to study these two main phases of the interstellar medium either dynamically or chemically. In this paper we present torus-3dpdr, a new self-consistent code for treating the chemistry of three-dimensional photoionization and photodissociation regions. It is an integrated code coupling the two codes torus, a hydrodynamics and Monte Carlo radiation transport code, and 3d-pdr, a PDRs code. The new code uses a Monte Carlo radiative transfer scheme to account for the propagation of the ionizing radiation including the diffusive component as well as a ray-tracing scheme based on the healpix package in order to account for the escape probability and column density calculations. Here, we present the numerical techniques we followed and we show the capabilities of the new code in modelling three-dimensional objects including single or multiple sources. We discuss the effects introduced by the diffusive component of the ultraviolet field in determining the thermal balance of PDRs as well as the effects introduced by a multiple sources treatment of the radiation field. With this new code, three-dimensional synthetic observations for the major cooling lines are possible, for making feasible a detailed comparison between hydrodynamical simulations and observations.
Aims. We present and compare the distribution of two shock tracers, SiO and HNCO, in the circumnuclear disk (CND) of NGC 1068. We aim to determine the causes of the variation in emission across the ...CND. Methods. SiO(3−2) and HNCO(6−5) emission has been imaged in NGC 1068 with the Plateau de Bure Interferometer (PdBI). We perform an LTE and RADEX analysis to determine the column densities and physical characteristics of the gas emitting these two lines. We then use a chemical model to determine the origin of the emission. Results. There is a strong SiO peak to the east of the AGN, with weak detections to the west. This distribution contrasts that of HNCO, which is detected more strongly to the west. The SiO emission peak in the east is similar to the peak of the molecular gas mass traced by CO. HNCO emission is offset from this peak by as much as ~80 pc (\hbox{$\leqslant$}⩽1′′). We compare velocity integrated line ratios in the east and west. We confirm that SiO emission strongly dominates in the east, while the reverse is true in the west. We use RADEX to analyse the possible gas conditions that could produce such emission. We find that, in both east and west, we cannot constrain a single temperature for the gas. We run a grid of chemical models of potential shock processes in the CND and find that SiO is significantly enhanced during a fast (60 km s-1) shock but not during a slow (20 km s-1) shock, nor in a gas not subjected to shocks at all. We find the inverse for HNCO, whose abundance increases during slow shocks and in warm non-shocked gas. Conclusions. High SiO and low HNCO indicated a fast shock, while high HNCO and low SiO indicates either a slow shock or warm, dense, non-shocked gas. The East Knot is therefore likely to contain gas that is heavily shocked. From chemical modelling, gas in the West Knot may be non-shocked, or maybe undergoing a much milder shock event. When taking into account RADEX results, the milder shock event is the more likely of the two scenarios.
Context. Cyanopolyynes are chains of carbon atoms with an atom of hydrogen and a CN group on either side. They are detected almost everywhere in the interstellar medium (ISM), as well as in comets. ...In the past, they have been used to constrain the age of some molecular clouds, since their abundance is predicted to be a strong function of time. Finally, cyanopolyynes can potentially contain a large portion of molecular carbon. Aims. We present an extensive study of the cyanopolyynes distribution in the solar-type protostar IRAS 16293-2422. The goals are (i) to obtain a census of the cyanopolyynes in this source and of their isotopologues; (ii) to derive how their abundance varies across the protostar envelope; and (iii) to obtain constraints on the history of IRAS 16293-2422 by comparing the observations with the predictions of a chemical model. Methods. We analysed the data from the IRAM-30 m unbiased millimeter and submillimeter spectral survey towards IRAS 16293-2422 named TIMASSS. The derived spectral line energy distribution (SLED) of each detected cyanopolyyne was compared with the predictions from the radiative transfer code GRenoble Analysis of Protostellar Envelope Spectra (GRAPES) to derive the cyanopolyyne abundances across the envelope of IRAS 16293-2422. Finally, the derived abundances were compared with the predictions of the chemical model UCL_CHEM. Results. We detect several lines from cyanoacetylene (HC3N) and cyanodiacetylene (HC5N), and report the first detection of deuterated cyanoacetylene, DC3N, in a solar-type protostar. We found that the HC3N abundance is roughly constant (~1.3 × 10-11) in the outer cold envelope of IRAS 16293-2422, and it increases by about a factor 100 in the inner region where the dust temperature exceeds 80 K, namely when the volcano ice desorption is predicted to occur. The HC5N has an abundance similar to HC3N in the outer envelope and about a factor of ten lower in the inner region. The comparison with the chemical model predictions provides constraints on the oxygen and carbon gaseous abundance in the outer envelope and, most importantly, on the age of the source. The HC3N abundance derived in the inner region, and where the increase occurs, also provide strong constraints on the time taken for the dust to warm up to 80 K, which has to be shorter than ~103−104 yr. Finally, the cyanoacetylene deuteration is about 50% in the outer envelope and ≤5% in the warm inner region. The relatively low deuteration in the warm region suggests that we are witnessing a fossil of the HC3N abundantly formed in the tenuous phase of the pre-collapse and then frozen into the grain mantles at a later phase. Conclusions. The accurate analysis of the cyanopolyynes in IRAS 16293-2422 unveils an important part of its past story. It tells us that IRAS 16293-2422 underwent a relatively fast (≤105 yr) collapse and a very fast (≤103−104 yr) warming up of the cold material to 80 K.
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