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Register a new userChemistry in Infrared Dark Clouds
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Massive stars play an important role in shaping the structure of galaxies. Infrared dark clouds (IRDCs), with their low temperatures and high densities, have been identified as the potential birthplaces of massive stars. In order to understand the formation processes of massive stars the physical and chemical conditions in infrared dark clouds have to be characterized. The goal of this paper is to investigate the chemical composition of a sample of southern infrared dark clouds. One important aspect of the observations is to check, if the molecular abuncances in IRDCs are similar to the low-mass pre-stellar cores, or whether they show signatures of more evolved evolutionary stages. We performed observations toward 15 IRDCs in the frequency range between 86 and 93 GHz using the 22-m Mopra radio telescope. We detect HNC, HCO$^+$ and HNC emission in all clouds and N$_2$H$^+$ in all IRDCs except one. In some clouds we detect SiO emission. Complicated shapes of the HCO$^+$ emission line profile are found in all IRDCs. Both signatures indicates the presence of infall and outflow motions and beginning of star formation activity, at least in some parts of the IRDCs. Where possible, we calculate molecular abundances and make a comparison with previously obtained values for low-mass pre-stellar cores and high-mass protostellar objects (HMPOs). We show a tendency for IRDCs to have molecular abundances similar to low-mass pre-stellar cores rather than to HMPOs abundances on the scale of our single-dish observations.
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We simulate the chemistry of infrared dark clouds (IRDCs) with a model in which the physical conditions are homogeneous and time-independent. The chemistry is solved as a function of time with three networks: one purely gas-phase, one that includes accretion and desorption, and one, the complete gas-grain network, that includes surface chemistry in addition. We compare our results with observed molecular abundances for two representative IRDCs -- IRDC013.90-1 and IRDC321.73-1 -- using the molecular species N$_2$H$^+$, HC$_3$N, HNC, HCO$^+$, HCN, C$_2$H, NH$_3$ and CS. IRDC013.90-1 is a cold IRDC, with a temperature below 20 K, while IRDC321.73-1 is somewhat warmer, in the range 20 - 30 K. We find that the complete gas-grain model fits the data very well, but that the goodness-of-fit is not sharply peaked at a particular temperature. Surface processes are important for the explanation of the high gas-phase abundance of N$_2$H$^+$ in IRDC321.73-1. The general success of the 0-D model in reproducing single-dish observations of our limited sample of 8 species shows that it is probably sufficient for an explanation of this type of data. To build and justify more complicated models, including spatial temperature and density structure, contraction, and heating, we require high-resolution interferometric data.
It is currently assumed that infrared dark clouds (IRDCs) represent the earliest evolutionary stages of high-mass stars ($>$ 8 M$_{odot}$). Submillimeter and millimeter-wave studies performed over the past 15 years show that IRDCs possess a broad variety of properties, and hence a wide range of problems and questions that can be tackled. In this paper, we report an investigation of the molecular composition and chemical processes in two groups of IRDCs. Using the Mopra, APEX, and IRAM radio telescopes over the last four years, we have collected molecular line data for CO, H$_2$CO, HNCO, CH$_3$CCH, CH$_3$OH, CH$_3$CHO, CH$_3$OCHO, and CH$_3$OCH$_3$. For all of these species we estimated molecular abundances. We then undertook chemical modeling studies, concentrating on the source IRDC028.34+0.06, and compared observed and modeled abundances. This comparison showed that to reproduce observed abundances of complex organic molecules (COMs), a 0-D gas-grain model with constant physical conditions is not sufficient. We achieved greater success with the use of a warm-up model, in which warm-up from 10 K to 30 K occurs following a cold phase.
It is commonly assumed that cold and dense Infrared Dark Clouds (IRDCs) likely represent the birth sites massive stars. Therefore, this class of objects gets increasing attention. To enlarge the sample of well-characterised IRDCs in the southern hemisphere, we have set up a program to study the gas and dust of southern IRDCs. The present paper aims at characterizing the continuuum properties of this sample of objects. We cross-correlated 1.2 mm continuum data from SIMBA@SEST with Spitzer/GLIMPSE images to establish the connection between emission sources at millimeter wavelengths and the IRDCs we see at 8 $mu$m in absorption against the bright PAH background. Analysing the dust emission and extinction leads to a determination of masses and column densities, which are important quantities in characterizing the initial conditions of massive star formation. The total masses of the IRDCs were found to range from 150 to 1150 $rm M_odot$ (emission data) and from 300 to 1750 $rm M_odot$ (extinction data). We derived peak column densities between 0.9 and 4.6 $times 10^{22}$ cm$^{-2}$ (emission data) and 2.1 and 5.4 $times 10^{22}$ cm$^{-2}$ (extinction data). We demonstrate that the extinction method fails for very high extinction values (and column densities) beyond A$_{rm V}$ values of roughly 75 mag according to the Weingartner & Draine (2001) extinction relation $R_{rm V} = 5.5$ model B. The derived column densities, taking into account the spatial resolution effects, are beyond the column density threshold of 3.0 $times 10^{23}$ cm$^{-2}$ required by theoretical considerations for massive star formation. We conclude that the values for column densities derived for the selected IRDC sample make these objects excellent candidates for objects in the earliest stages of massive star formation.
Infrared Dark Clouds appear to be the long sought population of cold and dense aggregations with the potential of harbouring the earliest stages of massive star formation. Up to now there has been no systematic study on the temperature distribution, velocity fields, chemical and physical state toward this new cloud population. Knowing these properties is crucial for understanding the presence, absence and the very potential of star formation. The present paper aims at addressing these questions. We analyse temperature structures and velocity fields and gain information on their chemical evolution. The gas emission is remarkably coextensive with the extinction seen at infrared wavelengths and with the submillimeter dust emission. Our results show that IRDCs are on average cold (T < 20 K) and have variations among the different cores. IRDC cores are in virial equilibrium, are massive (M > 100 M_sun), highly turbulent (1 -- 3 km/s) and exhibit significant velocity structure (variations around 1 -- 2 km/s over the cloud). We find an increasing trend in temperature from IRDCs with high ammonia column density to high mass protostellar objects and hot core/Ultracompact Hii regions stages of early warm high-mass star formation while linewidths of IRDCs are smaller. On the basis of this sample, we infer that while active star formation is not yet pervasive in most IRDCs, local condensations might collapse in the future or have already begun forming stars.
Context. The chemistry of the diffuse interstellar medium rests upon three pillars: exothermic ion-neutral reactions ( cold chemistry ), endothermic neutral-neutral reactions with significant activation barriers ( warm chemistry ), and reactions on the surfaces of dust grains. While warm chemistry becomes important in the shocks associated with turbulent dissipation regions, the main path for the formation of interstellar OH and H2O is that of cold chemistry. Aims. The aim of this study is to observationally confirm the association of atomic oxygen with both atomic and molecular gas phases, and to understand the measured abundances of OH and OH + as a function of the available reservoir of H2. Methods. We obtained absorption spectra of the ground states of OH, OH+ and OI with high-velocity resolution, with GREAT on-board SOFIA, and with the THz receiver at the APEX. We analyzed them along with ancillary spectra of HF and CH from HIFI. To deconvolve them from the hyperfine structure and to separate the blend that is due to various velocity components on the sightline, we fit model spectra consisting of an appropriate number of Gaussian profiles using a method combining simulated annealing with downhill simplex minimization. Together with HF and/or CH as a surrogate for H2, and HI $lambda$21 cm data, the molecular hydrogen fraction f^N_H2 = N(H 2)/(N(H) + 2N(H 2)) can be determined. We then investigated abundance ratios as a function of f^N_H2. Results. The column density of OI is correlated at a high significance with the amount of available molecular and atomic hydrogen, with an atomic oxygen abundance of $3 times 10 ^{-4}$ relative to H nuclei. While the velocities of the absorption features of OH and OH+ are loosely correlated and reflect the spiral arm crossings on the sightline, upon closer inspection they display an anticorrespondence. The arm-to-interarm density contrast is found to be higher in OH than in OH+. While both species can coexist, with a higher abundance in OH than in OH+, the latter is found less frequently in absence of OH than the other way around, which is a direct consequence of the rapid destruction of OH+ by dissociative recombination when not enough H2 is available. This conjecture has been substantiated by a comparison between the OH/OH+ ratio with f^N_H2, showing a clear correlation. The hydrogen abstraction reaction chain OH+ (H2,H) H2O+ (H2,H)H3O+ is confirmed as the pathway for the production of OH and H 2 O. Our estimate of the branching ratio of the dissociative recombination of H3O+ to OH and H2O is confined within the interval of 84 to 91%, which matches laboratory measurements (74 to 83%). -- A correlation between the linewidths and column densities of OH+ features is found to be significant with a false-alarm probability below 5%. Such a correlation is predicted by models of interstellar MHD turbulence. For OH the same correlation is found to be insignificant because there are more narrow absorption features. Conclusions. While it is difficult to assess the contributions of warm neutral-neutral chemistry to the observed abundances, it seems fair to conclude that the predictions of cold ion-neutral chemistry match the abundance patterns we observed.
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