Dense Interstellar Clouds Research Articles

The Effect of an Inert Solid Reservoir on Molecular Abundances in Dense Interstellar Clouds

Abstract The question, what is the role of freeze-out of chemical species in determining the molecular abundances in the interstellar gas is a matter of debate. We investigate a theoretical case of a dense interstellar molecular cloud core by time-dependent modeling of chemical kinetics, where grain surface reactions deliberately are not included. That means, the gas-phase and solid-phase abundances are influenced only by gas reactions, accretion on grains and desorption. We compare the results to a reference model where no accretion occurs, and only gas-phase reactions are included. We can trace that the purely physical processes of molecule accretion and desorption have major chemical consequences on the gas-phase chemistry. The main effect of introduction of the gas-grain interaction is long-term molecule abundance changes that come nowhere near an equilibrium during the typical lifetime of a prestellar core.

Using deuterated H 3 + and other molecular species to understand the formation of stars and planets

The ion plays a key role in the chemistry of dense interstellar gas clouds where stars and planets are forming. The low temperatures and high extinctions of such clouds make direct observations of impossible, but lead to large abundances of H 2 D + and D 2 H + that are very useful probes of the early stages of star and planet formation. Maps of H 2 D + and D 2 H + pure rotational line emission towards star-forming regions show that the strong deuteration of is the result of near-complete molecular depletion of CNO-bearing molecules onto grain surfaces, which quickly disappears as cores warm up after stars have formed. In the warmer parts of interstellar gas clouds, transfers its proton to other neutrals such as CO and N 2 , leading to a rich ionic chemistry. The abundances of such species are useful tracers of physical conditions such as the radiation field and the electron fraction. Recent observations of HF line emission towards the Orion Bar imply a high electron fraction, and we suggest that observations of OH + and H 2 O + emission may be used to probe the electron density in the nuclei of external galaxies.

Two Isomers of Protonated Isocyanic Acid: Evidence for an Ion-Molecule Pathway for HNCO ↔ HOCN Isomerization.

Ion-molecule reactions are thought to play a crucial role in the formation of metastable isomers, but relatively few protonated intermediates beyond HNCH(+) have been characterized at high spectral resolution. We present here laboratory measurements of the rotational spectra of protonated isocyanic acid in two isomeric forms, the ground state H2NCO(+) with C2v symmetry and a low-lying bent chain HNCOH(+), guided by coupled cluster calculations of their molecular structure. Somewhat surprisingly, HNCOH(+) is found to be more abundant than H2NCO(+), even though this metastable isomer is calculated to lie approximately 15-20 kcal/mol higher in energy. In the same way that HCNH(+) serves as a key intermediate in ion-molecule reactions that form HNC via dissociative electron recombination in cold dense interstellar molecular clouds, HNCOH(+) may play an analogous role in the conversion of HNCO to HOCN.

FIRST DETECTION OF WATER VAPOR IN A PRE-STELLAR CORE

Water is a crucial molecule in molecular astrophysics as it controls much of the gas/grain chemistry, including the formation and evolution of more complex organic molecules in ices. Pre-stellar cores provide the original reservoir of material from which future planetary systems are built, but few observational constraints exist on the formation of water and its partitioning between gas and ice in the densest cores. Thanks to the high sensitivity of the Herschel Space Observatory, we report on the first detection of water vapor at high spectral resolution toward a dense cloud on the verge of star formation, the pre-stellar core L1544. The line shows an inverse P-Cygni profile, characteristic of gravitational contraction. To reproduce the observations, water vapor has to be present in the cold and dense central few thousand AU of L1544, where species heavier than Helium are expected to freeze-out onto dust grains, and the ortho:para H2 ratio has to be around 1:1 or larger. The observed amount of water vapor within the core (about 1.5x10^{-6} Msun) can be maintained by Far-UV photons locally produced by the impact of galactic cosmic rays with H2 molecules. Such FUV photons irradiate the icy mantles, liberating water wapor in the core center. Our Herschel data, combined with radiative transfer and chemical/dynamical models, shed light on the interplay between gas and solids in dense interstellar clouds and provide the first measurement of the water vapor abundance profile across the parent cloud of a future solar-type star and its potential planetary system.

Upper limit for the D2H+ortho-to-para ratio in the prestellar core 16293E (CHESS)

The H3+ ion plays a key role in the chemistry of dense interstellar gas clouds where stars and planets are forming. The low temperatures and high extinctions of such clouds make direct observations of H3+ impossible, but lead to large abundances of H2D+ and D2H+, which are very useful probes of the early stages of star and planet formation. The ground-state rotational ortho-D2H+ 111-000 transition at 1476.6 GHz in the prestellar core 16293E has been searched for with the Herschel/HIFI instrument, within the CHESS (Chemical HErschel Surveys of Star forming regions) Key Program. The line has not been detected at the 21 mK km/s level (3 sigma integrated line intensity). We used the ortho-H2D+ 110-111 transition and para-D2H+ 110-101 transition detected in this source to determine an upper limit on the ortho-to-para D2H+ ratio as well as the para-D2H+/ortho-H2D+ ratio from a non-LTE analysis. The comparison between our chemical modeling and the observations suggests that the CO depletion must be high (larger than 100), with a density between 5e5 and 1e6 cm-3. Also the upper limit on the ortho-D2H+ line is consistent with a low gas temperature (~ 11 K) with a ortho-to-para ratio of 6 to 9, i.e. 2 to 3 times higher than the value estimated from the chemical modeling, making it impossible to detect this high frequency transition with the present state of the art receivers.

A new way to produce and isolate the OH-(H2O) complex

In interstellar media, in which water is present as a major species, OH radicals can form following irradiation by energetic cosmic rays. Laboratory experiments have proved that when water ices are irradiated with energetic particles, OH–(H2O) are formed. Additionally, ground-state OH radicals may exist in the interstellar media, especially in dense interstellar clouds whose outer regions stop energetic particles. To this extent, the understanding of the reactivity of OH radicals in their ground state is of primary importance in the astrochemical context. We experimentally show that the co-injection of an OH radical beam with dilute water molecules leads to the formation of OH–(H2O) complexes on copper rhodiated mirrors maintained at 3.5 K, under a pressure of 10−5 mbar.

Thermal reactions of oxygen atoms with CS2 at low temperatures on interstellar dust

The heterogeneous thermal reaction between carbon disulphide (CS2) and oxygen (O) atoms, producing carbonyl sulphide (OCS), has been investigated under astrophysically relevant conditions in laboratory experiments. We study this reaction at a range of fixed surface temperatures between 15 and 70 K. Following the interaction of the reactants, on a highly oriented pyrolytic graphite surface at a particular surface temperature, we use temperature-programmed desorption coupled with time-of-flight mass spectrometry to determine the yield of OCS. Consequently, our experimental data reveal the temperature dependence of the OCS yield over the aforementioned range of surface temperatures. Our experimental results indicate that thermal O atoms can readily convert CS2 contained in the icy mantles on interstellar grains to OCS, without the need for energetic processing of the ice. Our data set shows that this reaction proceeds at surface temperatures as low as 15 K. Therefore, contrary to previous suggestions, CS2 molecules in icy mantles are perhaps unlikely to account for a substantial proportion of the sulphur content of dense interstellar clouds. Reaction barriers and Arrhenius pre-exponential factors, and hence rate constants, are derived for this surface reaction by fitting our experimental data using a kinetic model. Additionally, this modelling procedure yields values for the desorption energies of O atoms and OCS.

Efficient surface formation route of interstellar hydroxylamine through NO hydrogenation. I. The submonolayer regime on interstellar relevant substrates

Dust grains in the interstellar medium are known to serve as the first chemical laboratory where the rich inventory of interstellar molecules are synthesized. Here we present a study of the formation of hydroxylamine--NH(2)OH--via the non-energetic route NO + H (D) on crystalline H(2)O and amorphous silicate under conditions relevant to interstellar dense clouds. Formation of nitrous oxide (N(2)O) and water (H(2)O, D(2)O) is also observed and the reaction network is discussed. Hydroxylamine and water results are detected in temperature-programmed desorption (TPD) experiments, while N(2)O is detected by both reflection-absorption IR spectroscopy and TPD techniques. The solid state NO + H reaction channel proves to be a very efficient pathway to NH(2)OH formation in space and may be a potential starting point for prebiotic species in dark interstellar clouds. The present findings are an important step forward in understanding the inclusion of interstellar nitrogen into a non-volatile aminated species since NH(2)OH provides a solid state nitrogen reservoir along the whole evolutionary process of interstellar ices from dark clouds to planetary systems.

Radiolysis of astrophysical ices by heavy ion irradiation: Destruction cross section measurement

Many solar system objects, such as planets and their satellites, dust grains in rings, and comets, are known to either be made of ices or to have icy surfaces. These ices are exposed to ionizing radiation including keV, MeV and GeV ions from solar wind or cosmic rays. Moreover, icy dust grains are present in interstellar space and, in particular, in dense molecular clouds. Radiation effects include radiolysis (the destruction of molecules leading to formation of radicals), the formation of new molecules following radiolysis, the desorption or sputtering of atoms or molecules from the surface, compaction of porous ices, and phase changes. This review discusses the application of infrared spectroscopy FTIR to study the evolution of the chemical composition of ices containing the most abundant molecular species found in the solar system and interstellar medium, such as H2O, CO, CO2 and hydrocarbons. We focus on the evolution of chemical composition with ion fluence in order to deduce the corresponding destruction and formation cross sections. Although initial approach focused on product identification, it became increasingly necessary to work toward a comprehensive understanding of ice chemistry. The abundances of these molecules in different phases of ice mantles provide important clues to the chemical processes in dense interstellar clouds, and therefore it is of importance to accurately measure the quantities such as dissociation and formation cross sections of the infrared features of these molecules. We also are able to obtain the scaling of these cross sections with deposited energy.

Star Formation in the Long Filamentary Infrared Dark Cloud at l ∼ 53°.2

AbstractMassive stars govern the evolution of galaxies by providing ionizing photons and energy as well as enriching heavy elements into interstellar medium; however, their formation is still poorly understood. Infrared dark clouds (IRDCs) are cold (< 25 K) and very dense (> 105 cm−3) interstellar clouds which are seen silhouette against the bright Galactic background in mid-IR. With very high column densities (∼ 1023–1025cm−2), IRDCs are believed to be the precursors to massive stars and star clusters (Simon et al. 2006).We report a remarkable IRDC at (l, b) ∼ (53°.2, 0°.0) which shows a number of bright mid-IR stellar sources along the cloud that are likely young stellar objects (YSOs). There are also several H2 (at 2.122 μm) outflow features in the cloud revealed by UWISH2 (Ukirt Widefield Infrared Survey for H2, Froebrich et al. 2011), in particular where earlier evolutionary stage of YSOs are located. The IRDC was previously partly identified as three separate IRDCs in the MSXDC catalog (Simon et al. 2006), whereas we have found that a long, filamentary cloud extending ∼ 30 pc including these three IRDCs is very well coincident with a CO cloud at v ∼ 23.5 km/s (or at d ∼ 2 kpc) which is clearly distinct from the other velocity components. Therefore, in this study, we investigate the overall star formation activity in this IRDC (IRDC G53.2, hereafter).We perform the PRF photometry of Spitzer MIPSGAL 24 μm data using MOPEX and build a catalog of YSOs by matching the detected 24 μm sources with published catalogs. The limiting magnitude in 24 μm is ∼ 7.8 mag, and YSO candidates which have counterparts in GLIMPSE I catalog are 354. The YSO candidates are classified using spectral index derived between 2 and 24 μm, following Greene et al. (1994). We also remove the field-star contamination using reference fields where there is no CO cloud; the fraction of each class after reference field analysis is 18, 22, 45, 10, and 5% for Class I, Flat, Class II, Class III, and sources which cannot be classified due to the lack of data. The spatial distribution that earlier classes (i.e., Class I and Flat) are concentrated where far-IR or millimeter emission is strong and larger fraction of Flat objects compared to other low-mass star forming regions (e.g., Evans et al. 2009 and Billot et al. 2010) may imply that the IRDC G53.2 is indeed an active star-forming region in rather early evolutionary stage. Further investigation of each YSO such as SED modeling will reveal detailed information on star formation activity in this intriguing IRDC.

Near-infrared spectroscopy of a nitrogen-loud quasar SDSS J1707+6443

We present near-infrared spectroscopy of the z=3.2 quasar SDSS J1707+6443, obtained with MOIRCS on the Subaru Telescope. This quasar is classified as a "nitrogen-loud" quasar because of the fairly strong NIII] and NIV] semi-forbidden emission lines from the broad-line region (BLR) observed in its rest-frame UV spectrum. However, our rest-frame optical spectrum from MOIRCS shows strong [OIII] emission from the narrow-line region (NLR) suggesting that, at variance with the BLR, NLR gas is not metal-rich. In order to reconcile these contradictory results, there may be two alternative possibilities; (1) the strong nitrogen lines from the BLR are simply due to a very high relative abundance of nitrogen rather than to a very high BLR metallicity, or (2) the BLR metallicity is not representative of the metallicity of the host galaxy, better traced by the NLR. In either case, the strong broad nitrogen lines in the UV spectrum are not indication of a chemically enriched host galaxy. We estimated the black hole mass and Eddington ratio of this quasar from the velocity width of both CIV and H_beta, that results in log(M_BH/M_sun) = 9.50 and log(L_bol/L_Edd) = -0.34. The relatively high Eddington ratio is consistent with our earlier result that strong nitrogen emission from BLRs is associated with high Eddington ratios. Finally, we detected significant [NeIII] emission from the NLR, implying a quite high gas density of n~10^6 cm^-3 and suggesting a strong coupling between quasar activity and dense interstellar clouds in the host galaxy.

Elemental nitrogen partitioning in dense interstellar clouds

Many chemical models of dense interstellar clouds predict that the majority of gas-phase elemental nitrogen should be present as N(2), with an abundance approximately five orders of magnitude less than that of hydrogen. As a homonuclear diatomic molecule, N(2) is difficult to detect spectroscopically through infrared or millimeter-wavelength transitions. Therefore, its abundance is often inferred indirectly through its reaction product N(2)H(+). Two main formation mechanisms, each involving two radical-radical reactions, are the source of N(2) in such environments. Here we report measurements of the low temperature rate constants for one of these processes, the N + CN reaction, down to 56 K. The measured rate constants for this reaction, and those recently determined for two other reactions implicated in N(2) formation, are tested using a gas-grain model employing a critically evaluated chemical network. We show that the amount of interstellar nitrogen present as N(2) depends on the competition between its gas-phase formation and the depletion of atomic nitrogen onto grains. As the reactions controlling N(2) formation are inefficient, we argue that N(2) does not represent the main reservoir species for interstellar nitrogen. Instead, elevated abundances of more labile forms of nitrogen such as NH(3) should be present on interstellar ices, promoting the eventual formation of nitrogen-bearing organic molecules.

Гидростатическая модель само-гравитирующего оптически плотного меж-звездного облака

Гидростатическая модель само-гравитирующего оптически плотного меж-звездного облака

Modelling the dust emission from dense interstellar clouds: disentangling the effects of radiative transfer and dust properties

With Planck and Herschel, we now have the spectral coverage and angular resolution required to observe dense and cold molecular clouds. As these clouds are optically thick at short wavelength but optically thin at long wavelength, it is tricky to conclude anything about dust properties without a proper treatment of the radiative transfer (RT). Our aim is to disentangle the effects of RT and of dust properties on the variations in the dust emission to provide observers with keys to analyse the emission arising from dense clouds. We model cylindrical clouds, illuminated by the ISRF, and carry out full RT calculations. Dust temperatures are solved using DustEM for amorphous carbons and silicates, grains coated with carbon mantles, and mixed aggregates of carbon and silicate. We allow variations of the grain optical properties with wavelength and temperature. We determine observed colour temperatures, T, and emissivity spectral indices, beta, by fitting the dust emission with modified blackbodies, to compare our models with observations. RT effects can neither explain the low T nor the increased submm emissivity measured at the centre of dense clouds, nor the observed beta-T anti-correlation. Adding noise to the modelled data, we show that it is not likely to be the unique explanation for the beta-T anti-correlation observed in starless clouds. It may be explained by intrinsic variations in the grain optical properties with temperature. As for the increased submm emissivity and the low T, they have to originate in variations in the grain optical properties, probably caused by their growth to form porous aggregates. We find it difficult to track back the nature of grains from the spectral variations in their emission. Finally, the column density is underestimated when determined with blackbody fitting because of the discrepancy between T and the true dust temperature at the cloud centre.

Does the H+5 hydrogen cluster exist in dense interstellar clouds?

AbstractThe clustering of hydrogen molecules around a cation in dense interstellar clouds is examined taking into account recent accurate information about the structure and thermochemistry of these clusters. In general, these clustering processes are not important due to the small probability of occurrence of radiative association at low densities. However, the long lived metastable complex (H)* makes the concentration of the H clusters to be of the same order of the concentration of H in these media. This result may have considerable implications for the chemistry of dense interstellar clouds. © 2012 Wiley Periodicals, Inc.

Formation of planets during the evolution of single and binary stars

Current views of the origin and evolution of single and binary stars suggest that the planets can form aroundmain-sequence single and binary stars, degenerate dwarfs, neutron stars, and stellarmass black holes according to several scenarios. Planets can arise during the formation of a star mainly due to excess angular momentum leading to the formation of an accretion-decretion disk of gas and dust around a single star or the components of a binary. It is the evolution of such disks that gives rise to planetary systems. A disk can arise around a star during its evolution due to the accretion of matter from dense interstellar clouds of gas and dust onto the star, the accretion of mass froma companion in a binary system, and the loss of matter during the contraction of a rapidly rotating star, in particular, if the star rotates as a rigid body and the rotation accelerates with its evolution along the main sequence. The fraction of stars with planetary systems is theoretically estimated as 30–40%, which is close to the current observational estimate of ∼34%.

A KINETIC DATABASE FOR ASTROCHEMISTRY (KIDA)

We present a novel chemical database for gas-phase astrochemistry. Named the KInetic Database for Astrochemistry (KIDA), this database consists of gas-phase reactions with rate coefficients and uncertainties that will be vetted to the greatest extent possible. Submissions of measured and calculated rate coefficients are welcome, and will be studied by experts before inclusion into the database. Besides providing kinetic information for the interstellar medium, KIDA is planned to contain such data for planetary atmospheres and for circumstellar envelopes. Each year, a subset of the reactions in the database (kida.uva) will be provided as a network for the simulation of the chemistry of dense interstellar clouds with temperatures between 10 K and 300 K. We also provide a code, named Nahoon, to study the time-dependent gas-phase chemistry of 0D and 1D interstellar sources.

SOFT X-RAY IRRADIATION OF PURE CARBON MONOXIDE INTERSTELLAR ICE ANALOGUES

There is an increasing evidence for the existence of large organic molecules in the interstellar and circumstellar medium. Very few among such species are readily formed in conventional gas-phase chemistry under typical conditions of interstellar clouds. Attention has therefore focused on interstellar ices as a potential source of these relatively complex species. Laboratory experiments show that irradiation of interstellar ice analogues by fast particles or ultraviolet radiation can induce significant chemical complexity. However, stars are sources of intense X-rays at almost every stage of their formation and evolution. Such radiation may thus provide chemical changes in regions where ultraviolet radiation is severely inhibited. After H2O, CO is often the most abundant component of icy grain mantles in dense interstellar clouds and circumstellar disks. In this work we present irradiation of a pure carbon monoxide ice using a soft X-ray spectrum peaked at 0.3 keV. Analysis of irradiated samples shows formation of CO2, C2O, C3O2, C3, C4O, and CO3/C5. Comparison of X-rays and ultraviolet irradiation experiments, of the same energy dose, shows that X-rays are more efficient than ultraviolet radiation in producing new species. With the exception of CO2, X-ray photolysis induces formation of a larger number of products with higher abundances, e.g., C3O2 column density is about one order of magnitude higher in the X-ray experiment. To our knowledge this is the first report on X-ray photolysis of CO ices. The present results show that X-ray irradiation represents an efficient photo-chemical way to convert simple ices to more complex species.

A Theoretical Investigation of the Plausibility of Reactions between Ammonia and Carbonyl Species (Formaldehyde, Acetaldehyde, and Acetone) in Interstellar Ice Analogs at Ultracold Temperatures

We have reexamined the reaction between formaldehyde and ammonia, which was previously studied by us and other workers in modestly sized cluster calculations. Larger model systems with up to 12H(2)O were employed, and reactions of two more carbonyl species, acetaldehyde and acetone, were also carried out. Calculations were performed at the B3LYP/6-31+G** level with bulk solvent effects treated with a polarizable continuum model; limited MP2/6-31+G** calculations were also performed. We found that while the barrier for the concerted proton relay mechanism described in previous work remains modest, it is still prohibitively high for the reaction to occur under the ultracold conditions that prevail in dense interstellar clouds. However, a new pathway emerged in more realistic clusters that involves at least one barrierless step for two of the carbonyl species considered here: ammonia reacts with formaldehyde and acetaldehyde to form a partial charge transfer species in small clusters (4H(2)O) and a protonated hydroxyamino intermediate species in large clusters (9H(2)O, 12H(2)O); modest barriers that decrease sharply with cluster size are found for the analogous processes for the acetone-NH(3) reaction. Furthermore, if a second ammonia replaces one of the water molecules in calculations in the 9H(2)O clusters, deprotonation can occur to yield the same neutral hydroxyamino species that is formed via the original concerted proton relay mechanism. In at least one position, deprotonation is barrierless when zero-point energy is included. In addition to describing the structures and energetics of the reactions between formaldehyde, acetaldehyde, and acetone with ammonia, we report spectroscopic predictions of the observable vibrational features that are expected to be present in ice mixtures of different composition.

THE EFFECTS OF GRAIN SIZE AND GRAIN GROWTH ON THE CHEMICAL EVOLUTION OF COLD DENSE CLOUDS

We investigate the formation of molecules during the chemical evolution of a cold dense interstellar cloud using a gas-grain numerical code in order to study the effects of grain-size distribution and grain growth on molecular abundances. Three initial size distributions have been used, based on earlier models. To incorporate different granular sizes, we divided the distribution of sizes utilized into five logarithmically equally spaced ranges, integrated over each range to find its total granular number density, and assigned that number density to an average size in that range. We utilized rate coefficients for surface reactions, accretion, and desorption as functions of grain size. We then followed the chemical evolution of the surface populations of the five average-sized grains along with the gas-phase chemistry. We find that the total effective granular surface area of a distribution is an important parameter in the determination of surface abundances, with and without grain growth. The effect on gas-phase abundances can also be sizable. Grain growth with time increases the rate of depletion of molecules, such as CO, produced in the gas phase. Use of a size distribution for grains in gas-grain models does not improve the agreement of calculated and observed abundances, in the gas or on grains, as compared with models containing grains of a fixed radius of 0.1 μm. This result helps to verify the quality of the classical grain approximation for cold cloud models. Further, it provides an important basis for future gas-grain models.