scholarly journals Chemical Signatures of the Evolutionary State of Cores

2004 ◽  
Vol 221 ◽  
pp. 67-74 ◽  
Author(s):  
Yuri Aikawa
Keyword(s):  
Gas Phase ◽  
Angular Resolution ◽  
Reaction Network ◽  
High Angular Resolution ◽  
Gas Phase Reactions ◽  
Parent Molecule ◽  
The Core ◽  
Chemical Signatures ◽  
Prestellar Cores ◽  
Molecular Abundances

Recent observations with high angular resolution revealed chemical differentiation in several prestellar cores; while N2H+ emission peaks at the core center, CO, CS and CCS emission peaks are offset from the center. Molecular abundances also vary among cores; some cores have high CCS abundance and low N2H+ abundance, but others have less CCS and more N2H+. Numerical calculations of a chemical-reaction network in contracting cores show that these differentiations and variations can be diagnostics of physical evolution of cores, because molecular abundances and distributions are determined by the balance between the dynamical and chemical time scales. In prestellar cores, low temperatures and high densities cause adsorption of molecules onto grains. Depletion time scale varies among species; early-phase species deplete first because of destruction via gas-phase reactions in addition to the adsorption. N2H+ is the last to deplete because of the low adsorption energy of its parent molecule N2 and depletion of main reactants such as CO. Molecular D/H ratio is another probe of core evolution, since it increases as the adsorption proceeds.

scholarly journals Hunting for intermediate-mass black holes in globular clusters: an astrometric study of NGC 6441

2021 ◽  
Vol 503 (1) ◽  
pp. 1490-1506
Author(s):  
Maximilian Häberle ◽  
Mattia Libralato ◽  
Andrea Bellini ◽  
Laura L Watkins ◽  
Jörg-Uwe Pott ◽  
...  
Keyword(s):  
Globular Clusters ◽  
Velocity Dispersion ◽  
Angular Resolution ◽  
Hubble Space Telescope ◽  
High Mass ◽  
High Angular Resolution ◽  
Stellar Kinematics ◽  
Intermediate Mass ◽  
The Core ◽  
Large Telescopes

ABSTRACT We present an astrometric study of the proper motions (PMs) in the core of the globular cluster NGC 6441. The core of this cluster has a high density and observations with current instrumentation are very challenging. We combine ground-based, high-angular-resolution NACO@VLT images with Hubble Space Telescope ACS/HRC data and measure PMs with a temporal baseline of 15 yr for about 1400 stars in the centremost 15 arcsec of the cluster. We reach a PM precision of ∼30 µas yr−1 for bright, well-measured stars. Our results for the velocity dispersion are in good agreement with other studies and extend already existing analyses of the stellar kinematics of NGC 6441 to its centremost region never probed before. In the innermost arcsecond of the cluster, we measure a velocity dispersion of (19.1 ± 2.0) km s−1 for evolved stars. Because of its high mass, NGC 6441 is a promising candidate for harbouring an intermediate-mass black hole (IMBH). We combine our measurements with additional data from the literature and compute dynamical models of the cluster. We find an upper limit of $M_{\rm IMBH} \lt 1.32 \times 10^4\, \textrm{M}_\odot$ but we can neither confirm nor rule out its presence. We also refine the dynamical distance of the cluster to $12.74^{+0.16}_{-0.15}$ kpc. Although the hunt for an IMBH in NGC 6441 is not yet concluded, our results show how future observations with extremely large telescopes will benefit from the long temporal baseline offered by existing high-angular-resolution data.

The chemistry in clumpy AGB outflows

2018 ◽  
Vol 14 (S343) ◽  
pp. 531-532
Author(s):  
M. Van de Sande ◽  
J. O. Sundqvist ◽  
T. J. Millar ◽  
D. Keller ◽  
L. Decin
Keyword(s):  
Density Distribution ◽  
Gas Phase ◽  
Thermodynamic Equilibrium ◽  
Angular Resolution ◽  
Abundance Ratio ◽  
Chemical Model ◽  
High Angular Resolution ◽  
Gas Phase Chemistry ◽  
Phase Chemistry ◽  
Smooth Density

AbstractThe chemistry within the outflow of an AGB star is determined by its elemental C/O abundance ratio. Thanks to the advent of high angular resolution observations, it is clear that most outflows do not have a smooth density distribution, but are inhomogeneous or “clumpy”. We have developed a chemical model that takes into account the effect of a clumpy outflow on its gas-phase chemistry by using a theoretical porosity formalism. The clumpiness of the model increases the inner wind abundances of all so-called unexpected species, i.e. species that are not predicted to be present assuming an initial thermodynamic equilibrium chemistry. By applying the model to the distribution of cyanopolyynes and hydrocarbon radicals within the outflow of IRC+10216, we find that the chemistry traces the underlying density distribution.

scholarly journals HIGH-ANGULAR-RESOLUTION OBSERVATIONS AT 7 MM OF THE CORE OF THE QUADRUPOLAR HH 111/121 OUTFLOW

2008 ◽  
Vol 136 (5) ◽  
pp. 1852-1856 ◽  
Author(s):  
Luis F. Rodríguez ◽  
José M. Torrelles ◽  
Guillem Anglada ◽  
Bo Reipurth
Keyword(s):  
Angular Resolution ◽  
High Angular Resolution ◽  
The Core

scholarly journals Molecular evolution in star-forming cores: From prestellar cores to protostellar cores

2008 ◽  
Vol 4 (S251) ◽  
pp. 129-136 ◽  
Author(s):  
Yuri Aikawa ◽  
Valentine Wakelam ◽  
Nami Sakai ◽  
R. T. Garrod ◽  
E. Herbst ◽  
...  
Keyword(s):  
Gas Phase ◽  
Surface Reactions ◽  
Reaction Network ◽  
Central Star ◽  
Carbon Chain ◽  
Complex Species ◽  
Star Forming ◽  
Organic Species ◽  
Molecular Abundances ◽  
Grain Surface

AbstractWe investigate the molecular abundances in protostellar cores by solving the gas-grain chemical reaction network. As a physical model of the core, we adopt a result of one-dimensional radiation-hydrodynamics calculation, which follows the contraction of an initially hydrostatic prestellar core to form a protostellar core. Temporal variation of molecular abundances is solved in multiple infalling shells, which enable us to investigate the spatial distribution of molecules in the evolving core. The shells pass through the warm region of T ~ 20–100 K in several 104 yr and falls onto the central star in ~100 yr after they enter the region of T > 100 K. We found that the complex organic species such as HCOOCH3 are formed mainly via grain-surface reactions at T ~ 20–40 K, and then sublimated to the gas phase when the shell temperature reaches their sublimation temperatures (T ≥ 100 K). Carbon-chain species can be re-generated from sublimated CH4 via gas-phase and grain-surface reactions. HCO2+, which is recently detected towards L1527, are abundant at r = 100–2,000 AU, and its column density reaches ~1011 cm−2 in our model. If a core is isolated and irradiated directly by interstellar UV radiation, photo-dissociation of water ice produces OH, which reacts with CO to form CO2 efficiently. Complex species then become less abundant compared with the case of embedded core in ambient clouds. Although a circumstellar (protoplanetary) disk is not included in our core model, we can expect similar chemical reactions (i.e., production of large organic species, carbon-chains and HCO2+) to proceed in disk regions with T ~ 20–100 K.

scholarly journals Molecular Evolution in Dense Clouds

1980 ◽  
Vol 87 ◽  
pp. 337-338
Author(s):  
H. Suzuki
Keyword(s):  
Molecular Evolution ◽  
Gas Phase ◽  
Gas Phase Reactions ◽  
Exothermic Reactions ◽  
Heavy Atoms ◽  
Condensation Reactions ◽  
Chemical Scheme ◽  
Molecular Abundances

Molecular abundances were calculated time-dependently using a chemical scheme which was carefully constructed to represent the molecular evolution in dense clouds. All ion-neutral reactions of species containing up to 2 heavy atoms (C, N, O, S and Si) were surveyed and a set of possible reactions was selected from numerous exothermic reactions. As molecules containing 3 to 4 heavy atoms are produced by condensation reactions, the scheme was extended. As a result, 234 species, their 2884 gas-phase reactions, and H recombination on grain surfaces were included in our chemical scheme.

scholarly journals Chemistry of a newly detected circumbinary disk in Ophiuchus

2018 ◽  
Vol 614 ◽  
pp. A26 ◽  
Author(s):  
Elizabeth Artur de la Villarmois ◽  
Lars E. Kristensen ◽  
Jes K. Jørgensen ◽  
Edwin A. Bergin ◽  
Christian Brinch ◽  
...  
Keyword(s):  
Binary Systems ◽  
Angular Resolution ◽  
Circumstellar Disks ◽  
Carbon Chain ◽  
Physical Structure ◽  
Continuum Emission ◽  
High Angular Resolution ◽  
Density Region ◽  
Spatially Resolved ◽  
Molecular Abundances

Context. Astronomers recently started discovering exoplanets around binary systems. Therefore, understanding the formation and evolution of circumbinary disks and their environment is crucial for a complete scenario of planet formation. Aims. The purpose of this paper is to present the detection of a circumbinary disk around the system Oph-IRS67 and analyse its chemical and physical structure. Methods. We present high-angular-resolution (0.′′4, ~60 AU) observations of C17O, H13CO+, C34S, SO2, C2H and c−C3H2 molecular transitions with the Atacama Large Millimeter/submillimeter Array (ALMA) at wavelengths of 0.8 mm. The spectrally and spatially resolved maps reveal the kinematics of the circumbinary disk as well as its chemistry. Molecular abundances are estimated using the non-local thermodynamic equilibrium (LTE) radiative-transfer tool RADEX. Results. The continuum emission agrees with the position of Oph-IRS67 A and B, and reveals the presence of a circumbinary disk around the two sources. The circumbinary disk has a diameter of ~620 AU and is well traced by C17O and H13CO+ emission. Two further molecular species, C2H and c−C3H2, trace a higher-density region which is spatially offset from the sources (~430 AU). Finally, SO2 shows compact and broad emission around only one of the sources, Oph-IRS67 B. The molecular transitions which trace the circumbinary disk are consistent with a Keplerian profile on smaller disk scales (≲200 AU) and an infalling profile for larger envelope scales (≳200 AU). The Keplerian fit leads to an enclosed mass of 2.2 M⊙. Inferred CO abundances with respect to H2 are comparable to the canonical ISM value of 2.7 × 10−4, reflecting that freeze-out of CO in the disk midplane is not significant. Conclusions. Molecular emission and kinematic studies prove the existence and first detection of the circumbinary disk associated with the system Oph-IRS67. The high-density region shows a different chemistry than the disk, being enriched in carbon chain molecules. The lack of methanol emission agrees with the scenario where the extended disk dominates the mass budget in the innermost regions of the protostellar envelope, generating a flat density profile where less material is exposed to high temperatures, and thus, complex organic molecules would be associated with lower column densities. Finally, Oph-IRS67 is a promising candidate for proper motion studies and the detection of both circumstellar disks with higher-angular-resolution observations.

scholarly journals Theoretical Studies of Dense Cloud Chemistry

1987 ◽  
Vol 120 ◽  
pp. 235-244 ◽  
Author(s):  
Eric Herbst
Keyword(s):  
Gas Phase ◽  
Low Temperatures ◽  
Rate Coefficients ◽  
Theoretical Studies ◽  
Cloud Chemistry ◽  
Physical Processes ◽  
Gas Phase Reactions ◽  
Interstellar Clouds ◽  
Molecular Abundances ◽  
Quantitative Calculations

Based on analyses by a variety of investigators, it has become understood that gas phase reactions can account for much of the chemistry observed in dense interstellar clouds. However, quantitative calculations of molecular abundances utilizing gas phase reactions are beset with difficulties. These difficulties include uncertainties in needed rate coefficients at the low temperatures of interstellar clouds, uncertainties in the dynamics of physical processes such as cloud collapse and clumping, and uncertainties in our understanding of gasgrain interactions. New work in some of these areas and its impact on modelling is emphasized.

scholarly journals Clumpy Structure in the W3 Molecular Core

1987 ◽  
Vol 115 ◽  
pp. 163-166
Author(s):  
Hideyuki Kobayashi ◽  
Masahiko Hayashi ◽  
Tetsuo Hasegawa
Keyword(s):  
Angular Resolution ◽  
Young Star ◽  
High Angular Resolution ◽  
H Ii Regions ◽  
Molecular Line ◽  
The Core ◽  
Molecular Core ◽  
Resolution Observation ◽  
Infrared Sources ◽  
Co Emission

The core region of the W3 molecular cloud has high molecular line luminosities (Dickel et al. 1980; Brackman and Scoville 1980). This region contains luminous infrared sources and ultra compact H II regions. A young star cluster may be forming in this region. Recent interferometric high angular-resolution observation revealed a bipolar outflow toward IRS 5 in the CO emission (Claussen et al. 1984) and a mass condensation in the HCN emission (Wright et al. 1984) in this region.

scholarly journals Modeling sulfur depletion in interstellar clouds

2019 ◽  
Vol 624 ◽  
pp. A108 ◽  
Author(s):  
Jacob C. Laas ◽  
Paola Caselli
Keyword(s):  
Gas Phase ◽  
Chemical Elements ◽  
Reaction Network ◽  
Interstellar Cloud ◽  
Molecular Gas ◽  
Dark Cloud ◽  
Interstellar Clouds ◽  
Simple Physical Model ◽  
Molecular Abundances ◽  
Elemental Depletion

Context. The elemental depletion of interstellar sulfur from the gas phase has been a recurring challenge for astrochemical models. Observations show that sulfur remains relatively non-depleted with respect to its cosmic value throughout the diffuse and translucent stages of an interstellar molecular cloud, but its atomic and molecular gas-phase constituents cannot account for this cosmic value toward lines of sight containing higher-density environments. Aims. We have attempted to address this issue by modeling the evolution of an interstellar cloud from its pristine state as a diffuse atomic cloud to a molecular environment of much higher density, using a gas-grain astrochemical code and an enhanced sulfur reaction network. Methods. A common gas-grain astrochemical reaction network has been systematically updated and greatly extended based on previous literature and previous sulfur models, with a focus on the grain chemistry and processes. A simple astrochemical model was used to benchmark the resulting network updates, and the results of the model were compared to typical astronomical observations sourced from the literature. Results. Our new gas-grain astrochemical model is able to reproduce the elemental depletion of sulfur, whereby sulfur can be depleted from the gas-phase by two orders of magnitude, and that this process may occur under dark cloud conditions if the cloud has a chemical age of at least 106 years. The resulting mix of sulfur-bearing species on the grain ranges across all the most common chemical elements (H/C/N/O), not dissimilar to the molecules observed in cometary environments. Notably, this mixture is not dominated simply by H2S, unlike all other current astrochemical models. Conclusions. Despite our relatively simple physical model, most of the known gas-phase S-bearing molecular abundances are accurately reproduced under dense conditions, however they are not expected to be the primary molecular sinks of sulfur. Our model predicts that most of the “missing” sulfur is in the form of organo-sulfur species that are trapped on grains.

scholarly journals High angular resolution observations of the gas phase composition of young stellar objects

1997 ◽  
Vol 178 ◽  
pp. 31-44 ◽  
Author(s):  
Geoffrey A. Blake
Keyword(s):  
Gas Phase ◽  
Angular Resolution ◽  
Young Stellar Objects ◽  
High Angular Resolution ◽  
Stellar Objects ◽  
Star Forming ◽  
Circumstellar Material ◽  
The Past ◽  
Cloud Cores

Recent advances in the observational characterization of young stellar objects (YSOs) with millimeter-wave aperture synthesis arrays and (sub)millimeter single dish telescopes are reviewed. Studies of circumstellar material with ∼arc second resolution, which have only become possible at these frequencies within the past few years and which can potentially probe all stages of the star formation process, are emphasized. Molecules that are sensitive to different routes of formation and modification are outlined, including comments about their utility in distinguishing between a variety of environments and histories in star-forming cloud cores.

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