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 Hydrodynamical-Chemical Models from Prestellar Cores to Protostellar Cores

2011 ◽  
Vol 7 (S280) ◽  
pp. 33-42 ◽  
Author(s):  
Yuri Aikawa ◽  
Kenji Furuya ◽  
Valentine Wakelam ◽  
Frank Hersant ◽  
Tomoaki Matsumoto ◽  
...  
Keyword(s):  
Molecular Evolution ◽  
Gas Phase ◽  
Organic Molecules ◽  
Carbon Chain ◽  
Phase Reaction ◽  
Fall Velocity ◽  
Disk Radius ◽  
Star Forming ◽  
Organic Species ◽  
Collapse Model

AbstractWe investigate the molecular evolution in star forming cores from dense cloud cores (nH ~ 104 cm−3, T ~ 10 K) to protostellar cores. A detailed gas-grain reaction network is solved in infalling fluid parcels in 1-D radiation hydrodynamic model. Large organic molecules are mainly formed via grain-surface reaction at T ~ several 10 K and sublimated to the gas-phase at ~ 100 K, while carbon-chain species are formed at a few 10 K after the sublimation of CH4 ice. The former accounts for the high abundance of large organic molecules in hot corinos such as IRAS16293, and the latter accounts for the carbon chain species observed toward L1527. The relative abundance of carbon chain species and large organic species would depend on the collapse time scale and/or temperature in the dense core stage. The large organic molecules and carbon chains in the protostellar cores are heavily deuterated; although they are formed in the warm temperatures, their ingredients have high D/H ratios, which are set in the cold core phase and isothermal collapse phase. HCOOH is formed by the gas-phase reaction of OH with the sublimated H2CO, and is further enriched in Deuterium due to the exothermic exchange reaction of OH + D → OD + H.In the fluid parcels of the 1-D collapse model, warm temperature T. ~ several 10 K lasts for only ~ 104 yr, and the fluid parcels fall to the central star in ~ 100 yr after the temperature of the parcel rises to T ≥ 100 K. These timescales are determined by the size of the warm region and infall (~ free-fall) velocity: rwarm/tff. In reality, circum stellar disk is formed, in which fluid parcels stay for a longer timescale than the infall timescale. We investigate the molecular evolution in the disk by simply assuming that a fluid parcel stays at a constant temperature and density (i.e. a fixed disk radius) for 104 − 105 yrs. We found that some organic species which are underestimated in our 1-D collapse model, such as CH3OCH3 and HCOOCH3, become abundant in the disk. We also found that these disk species have very high D/H ratio as well, since their ingredients are highly deuterated.Finally we investigate molecular evolution in a 3D hydrodynamic simulation of star forming core. We found CH3OH are abundant in the vicinity of the first core. The abundances of large organic species are determined mainly by the local temperature (sublimation), because of the short lifetime of the first core and the efficient mass accretion via angular momentum transfer.

scholarly journals Surface Reactions in Interstellar Space

2002 ◽  
Vol 12 ◽  
pp. 55-57 ◽  
Author(s):  
Eric Herbst
Keyword(s):  
Surface Chemistry ◽  
Gas Phase ◽  
Condensed Phase ◽  
Current Knowledge ◽  
Surface Reactions ◽  
Future Research ◽  
Interstellar Space ◽  
Interstellar Molecules ◽  
Grain Surface

AbstractIt is impossible to explain the abundances of some gas-phase and most condensed-phase interstellar molecules without the use of grain chemistry. Nevertheless, grain-surface chemistry is relatively poorly understood for a variety of reasons. Our current knowledge of this chemistry and its use in interstellar models is discussed along with specific needs for future research.

scholarly journals The ALMA-PILS survey: propyne (CH3CCH) in IRAS 16293–2422

2019 ◽  
Vol 631 ◽  
pp. A137 ◽  
Author(s):  
H. Calcutt ◽  
E. R. Willis ◽  
J. K. Jørgensen ◽  
P. Bjerkeli ◽  
N. F. W. Ligterink ◽  
...  
Keyword(s):  
Gas Phase ◽  
Spatial Scales ◽  
Excitation Temperature ◽  
Gas Phase Reactions ◽  
Star Forming ◽  
Physical Conditions ◽  
Star Forming Regions ◽  
Chemical Modelling ◽  
Low Mass ◽  
Grain Surface

Context. Propyne (CH3CCH), also known as methyl acetylene, has been detected in a variety of environments, from Galactic star-forming regions to extragalactic sources. These molecules are excellent tracers of the physical conditions in star-forming regions, allowing the temperature and density conditions surrounding a forming star to be determined. Aims. This study explores the emission of CH3CCH in the low-mass protostellar binary, IRAS 16293–2422, and examines the spatial scales traced by this molecule, as well as its formation and destruction pathways. Methods. Atacama Large Millimeter/submillimeter Array (ALMA) observations from the Protostellar Interferometric Line Survey (PILS) were used to determine the abundances and excitation temperatures of CH3CCH towards both protostars. This data allows us to explore spatial scales from 70 to 2400 au. This data is also compared with the three-phase chemical kinetics model MAGICKAL, to explore the chemical reactions of this molecule. Results. CH3CCH is detected towards both IRAS 16293A and IRAS 16293B, and is found the hot corino components, one around each source, in the PILS dataset. Eighteen transitions above 3σ are detected, enabling robust excitation temperatures and column densities to be determined in each source. In IRAS 16293A, an excitation temperature of 90 K and a column density of 7.8 × 1015 cm−2 best fits the spectra. In IRAS 16293B, an excitation temperature of 100 K and 6.8 × 1015 cm−2 best fits the spectra. The chemical modelling finds that in order to reproduce the observed abundances, both gas-phase and grain-surface reactions are needed. The gas-phase reactions are particularly sensitive to the temperature at which CH4 desorbs from the grains. Conclusions. CH3CCH is a molecule whose brightness and abundance in many different regions can be utilised to provide a benchmark of molecular variation with the physical properties of star-forming regions. It is essential when making such comparisons, that the abundances are determined with a good understanding of the spatial scale of the emitting region, to ensure that accurate abundances are derived.

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

2012 ◽  
Vol 21 (4) ◽  
Author(s):  
Juris Kalvāns ◽  
Ivar Shmeld
Keyword(s):  
Gas Phase ◽  
Reference Model ◽  
Solid Phase ◽  
Chemical Species ◽  
Interstellar Gas ◽  
Interstellar Clouds ◽  
Major Chemical ◽  
Molecular Abundances ◽  
Phase Chemistry ◽  
Grain Surface

AbstractThe 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.

scholarly journals Chemical Evolution of a Protoplanetary Disk

2011 ◽  
Vol 7 (S280) ◽  
pp. 114-126
Author(s):  
Dmitry A. Semenov
Keyword(s):  
Chemical Composition ◽  
Gas Phase ◽  
Chemical Evolution ◽  
Molecular Layer ◽  
High Energy ◽  
Complex Species ◽  
High Energy Radiation ◽  
Energy Radiation ◽  
Atomic Ions ◽  
Organic Species

AbstractIn this paper we review recent progress in our understanding of the chemical evolution of protoplanetary disks. Current observational constraints and theoretical modeling on the chemical composition of gas and dust in these systems are presented. Strong variations of temperature, density, high-energy radiation intensities in these disks, both radially and vertically, result in a peculiar disk chemical structure, where a variety of processes are active. In hot, dilute and heavily irradiated atmosphere only the most photostable simple radicals and atoms and atomic ions exist, formed by gas-phase processes. Beneath the atmosphere a partly UV-shielded, warm molecular layer is located, where high-energy radiation drives rich ion-molecule and radical-radical chemistry, both in the gas phase and on dust surfaces. In a cold, dense, dark disk midplane many molecules are frozen out, forming thick icy mantles where surface chemistry is active and where complex polyatomic (organic) species are synthesized. Dynamical processes affect disk chemical composition by enriching it in abundances of complex species produced via slow surface processes, which will become detectable with ALMA.

scholarly journals An unbiased spectral line survey observation toward the low-mass star-forming region L1527

2019 ◽  
Vol 71 (Supplement_1) ◽  
Author(s):  
Kento Yoshida ◽  
Nami Sakai ◽  
Yuri Nishimura ◽  
Tomoya Tokudome ◽  
Yoshimasa Watanabe ◽  
...  
Keyword(s):  
Spectral Line ◽  
Carbon Chain ◽  
Mass Star ◽  
Star Forming ◽  
Organic Species ◽  
Good Template ◽  
Solar Type ◽  
Low Mass ◽  
Line Survey ◽  
Supplementary Observation

Abstract An unbiased spectral line survey toward a solar-type Class 0/I protostar, IRAS 04368+2557, in L1527 has been carried out in the 3 mm band with the Nobeyama 45 m telescope. L1527 is known as a warm carbon-chain chemistry (WCCC) source, which harbors abundant unsaturated organic species such as CnH (n = 3, 4, 5, …) in a warm and dense region near the protostar. The observation covers the frequency range from 80 to 116 GHz. A supplementary observation has also been conducted in the 70 GHz band to observe fundamental transitions of deuterated species. In total, 69 molecular species are identified, among which 27 species are carbon-chain species and their isomers, including their minor isotopologues. This spectral line survey provides us with a good template of the chemical composition of the WCCC source.

scholarly journals The GUAPOS project: G31.41+0.31 Unbiased ALMA sPectral Observational Survey

2020 ◽  
Vol 644 ◽  
pp. A84
Author(s):  
C. Mininni ◽  
M. T. Beltrán ◽  
V. M. Rivilla ◽  
A. Sánchez-Monge ◽  
F. Fontani ◽  
...  
Keyword(s):  
Acetic Acid ◽  
Gas Phase ◽  
Organic Molecules ◽  
Methyl Formate ◽  
Galactic Center ◽  
Physical Parameters ◽  
Star Forming ◽  
Grain Surface ◽  
Spectral Survey ◽  
Complex Organic

Context. One of the goals of astrochemistry is to understand the degree of chemical complexity that can be reached in star-forming regions, along with the identification of precursors of the building blocks of life in the interstellar medium. To answer such questions, unbiased spectral surveys with large bandwidth and high spectral resolution are needed, in particular, to resolve line blending in chemically rich sources and identify each molecule (especially for complex organic molecules). These kinds of observations have already been successfully carried out, primarily towards the Galactic Center, a region that shows peculiar environmental conditions. Aims. We present an unbiased spectral survey of one of the most chemically rich hot molecular cores located outside the Galactic Center, in the high-mass star-forming region G31.41+0.31. The aim of this 3mm spectral survey is to identify and characterize the physical parameters of the gas emission in different molecular species, focusing on complex organic molecules. In this first paper, we present the survey and discuss the detection and relative abundances of the three isomers of C2H4O2: methyl formate, glycolaldehyde, and acetic acid. Methods. Observations were carried out with the ALMA interferometer, covering all of band 3 from 84 to 116 GHz (~32 GHz bandwidth) with an angular resolution of 1.2′′ × 1.2′′ (~ 4400 au × 4400 au) and a spectral resolution of ~0.488 MHz (~1.3−1.7 km s−1). The transitions of the three molecules have been analyzed with the software XCLASS to determine the physical parameters of the emitted gas. Results. All three isomers were detected with abundances of (2 ± 0.6) × 10−7, (4.3−8) × 10−8, and (5.0 ± 1.4) × 10−9 for methyl formate, acetic acid, and glycolaldehyde, respectively. Methyl formate and acetic acid abundances are the highest detected up to now, if compared to sources in the literature. The size of the emission varies among the three isomers with acetic acid showing the most compact emission while methyl formate exhibits the most extended emission. Different chemical pathways, involving both grain-surface chemistry and cold or hot gas-phase reactions, have been proposed for the formation of these molecules, but the small number of detections, especially of acetic acid and glycolaldehyde, have made it very difficult to confirm or discard the predictions of the models. The comparison with chemical models in literature suggests the necessity of grain-surface routes for the formation of methyl formate in G31, while for glycolaldehyde both scenarios could be feasible. The proposed grain-surface reaction for acetic acid is not capable of reproducing the observed abundance in this work, while the gas-phase scenario should be further tested, given the large uncertainties involved.

scholarly journals Chemical Models of Circumstellar Disks

2000 ◽  
Vol 197 ◽  
pp. 425-434
Author(s):  
Yuri Aikawa ◽  
Eric Herbst
Keyword(s):  
Molecular Evolution ◽  
Gas Phase ◽  
Uv Radiation ◽  
Reasonable Agreement ◽  
Circumstellar Disks ◽  
Disk Surface ◽  
Central Star ◽  
X Rays ◽  
Chemical Models ◽  
Molecular Abundances

We investigate the evolution of molecular abundances in circumstellar disks around young stars via numerical simulations. The results are compared with the composition of comets and radio observations of protoplanetary disks. First, we consider the molecular evolution in the midplane of the disk. Because of ionization by cosmic-rays or decay of radioactive nuclei, the molecular evolution in the region R ≳ 10 AU bears some similarity to that in molecular clouds. Considerable amounts of CO and N2, which come from the cloud core, are transformed, however, into CO2, CH4, NH3, and HCN. In regions where the temperature is low enough for these products to freeze onto grains, they accumulate in ice mantles. In the inner warmer regions of the disk, these molecules are desorbed from the mantles and transformed by gas-phase reactions into less volatile molecules, such as larger hydrocarbons, which then freeze out. Molecular abundances both in the gas phase and in ice mantles crucially depend on the temperature and thus vary significantly with the distance from the central star. Molecular abundances in ice mantles show reasonable agreement with the composition of comets. Our model suggests that comets formed in different regions of the disk having different molecular compositions.Secondly, we report two-dimensional (R, Z) distributions of molecules in circumstellar disks. In the Z-direction, which is perpendicular to the midplane, there is a steep gradient of density and radiation field. Since the density is lower in the regions removed from the midplane, there are significant amounts of gas-phase species in these regions, while most species are adsorbed onto grains in the midplane. Chemistry in the surface regions is also affected by the X-rays and UV radiation from the central star, and UV radiation from the interstellar field. The molecular abundances in these regions differ significantly from those in the midplane. Radicals such as CN are especially abundant near the disk surface. We obtain radial distributions of molecular column densities and averaged molecular abundances, which show reasonable agreement with the observational data at millimeter wavelengths.

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 Complex organic molecules along the accretion flow in isolated and externally irradiated protoplanetary disks

2014 ◽  
Vol 168 ◽  
pp. 389-421 ◽  
Author(s):  
Catherine Walsh ◽  
Eric Herbst ◽  
Hideko Nomura ◽  
T. J. Millar ◽  
Susanna Widicus Weaver
Keyword(s):  
Surface Chemistry ◽  
Gas Phase ◽  
Organic Molecules ◽  
Molecular Layer ◽  
Accretion Flow ◽  
Chemical Complexity ◽  
Outer Disk ◽  
Molecular Abundances ◽  
Grain Surface ◽  
Complex Organic

The birth environment of the Sun will have influenced the physical and chemical structure of the pre-solar nebula, including the attainable chemical complexity reached in the disk, important for prebiotic chemistry. The formation and distribution of complex organic molecules (COMs) in a disk around a T Tauri star is investigated for two scenarios: (i) an isolated disk, and (ii) a disk irradiated externally by a nearby massive star. The chemistry is calculated along the accretion flow from the outer disk inwards using a comprehensive network which includes gas-phase reactions, gas-grain interactions, and thermal grain-surface chemistry. Two simulations are performed, one beginning with complex ices and one with simple ices only. For the isolated disk, COMs are transported without major chemical alteration into the inner disk where they thermally desorb into the gas reaching an abundance representative of the initial assumed ice abundance. For simple ices, COMs can efficiently form on grain surfaces under the conditions in the outer disk. Gas-phase COMs are released into the molecular layer via photodesorption. For the irradiated disk, complex ices are also transported inwards; however, they undergo thermal processing caused by the warmer conditions in the irradiated disk which tends to reduce their abundance along the accretion flow. For simple ices, grain-surface chemistry cannot efficiently synthesise COMs in the outer disk because the necessary grain-surface radicals, which tend to be particularly volatile, are not sufficiently abundant on the grain surfaces. Gas-phase COMs are formed in the inner region of the irradiated disk via gas-phase chemistry induced by the desorption of strongly bound molecules such as methanol; hence, the abundances are not representative of the initial molecular abundances injected into the outer disk. These results suggest that the composition of comets formed in isolated disks may differ from those formed in externally irradiated disks with the latter composed of more simple ices.

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