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ABSTRACT: Both grain surface and gas phase chemistry have been invoked to explain the disparate relative abundances of methyl formate and its structural isomers acetic acid and glycolaldehyde in the Sgr B2(N) star-forming region. While a network of grain surface chemistry involving radical–radical reactions during the warm-up phase of a hot core is the most chemically viable option proposed to date, neither qualitative nor quantitative agreement between modeling and observation has yet been obtained. In this study, we seek to test additional grain surface and gas phase processes to further investigate methyl formate-related chemistry by implementing several modifications to the Ohio State University gas/grain chemical network. We added two new gas phase chemical pathways leading to methyl formate, one involving an exothermic, barrierless reaction of protonated methanol with neutral formic acid; and one involving the reaction of protonated formic acid with neutral methanol to form both the cis and trans forms of protonated methyl formate. In addition to these gas phase processes, we have also investigated whether the relative product branching ratios for methanol photodissociation on grains influence the relative abundances of methyl formate and its structural isomers. We find that while the new gas phase formation pathways do not alter the relative abundances of methyl formate and its structural isomers, changes in the photodissociation branching ratios and adjustment of the overall timescale for warm-up can be used to explain their relative ratios in Sgr B2(N).
The Astrophysical Journal. 09/2015; 728:71-9.
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ABSTRACT: We use a high-temperature chemical network to derive the molecular abundances
in axisymmetric accretion disk models around active galactic nuclei (AGNs)
within 100 pc using simple radial and vertical density and temperature
distributions motivated by more detailed physical models. We explore the
effects of X-ray irradiation and cosmic ray ionization on the spatial
distribution of the molecular abundances of CO, CN, CS, HCN, HCO+, HC3N, C2H,
and c-C3H2 using a variety of plausible disk structures. These simple models
have molecular regions with a layer of X-ray dominated regions, a midplane
without the strong influence of X-rays, and a high-temperature region in the
inner portion with moderate X-ray flux where families of polyynes (C$_{\rm
n}$H$_{2}$) and cyanopolyynes can be enhanced. For the high midplane density
disks we explore, we find that cosmic rays produced by supernovae do not
significantly affect the regions unless the star formation efficiency
significantly exceeds that of the Milky Way. We highlight molecular abundance
observations and ratios that may distinguish among theoretical models of the
density distribution in AGN disks. Finally, we assess the importance of the
shock crossing time and the accretion time relative to the formation time for
various chemical species. Vertical column densities are tabulated for a number
of molecular species at both the characteristic shock crossing time and steady
state. Although we do not attempt to fit any particular system or set of
observations, we discuss our models and results in the context of the nearby
AGN NGC 1068.
01/2013;
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ABSTRACT: We investigate the molecular evolution and D/H abundance ratios that develop
as star formation proceeds from a dense-cloud core to a protostellar core, by
solving a gas-grain reaction network applied to a 1-D radiative hydrodynamic
model with infalling fluid parcels. Spatial distributions of gas and ice-mantle
species are calculated at the first-core stage, and at times after the birth of
a protostar. Gas-phase methanol and methane are more abundant than CO at radii
$r\lesssim 100$ AU in the first-core stage, but gradually decrease with time,
while abundances of larger organic species increase. The warm-up phase, when
complex organic molecules are efficiently formed, is longer-lived for those
fluid parcels in-falling at later stages. The formation of unsaturated carbon
chains (warm carbon-chain chemistry) is also more effective in later stages;
C$^+$, which reacts with CH$_4$ to form carbon chains, increases in abundance
as the envelope density decreases. The large organic molecules and carbon
chains are strongly deuterated, mainly due to high D/H ratios in the parent
molecules, determined in the cold phase. We also extend our model to simulate
simply the chemistry in circumstellar disks, by suspending the 1-D infall of a
fluid parcel at constant disk radii. The species CH$_3$OCH$_3$ and HCOOCH$_3$
increase in abundance in $10^4-10^5$ yr at the fixed warm temperature; both
also have high D/H ratios.
10/2012;
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ABSTRACT: In previous warm-up chemical models of the low-mass star-forming region
L1527, we investigated the evolution of carbon-chain unsaturated hydrocarbon
species when the envelope temperature is slightly elevated to $T\approx 30$ K.
These models demonstrated that enhanced abundances of such species can be
explained by gas-phase ion-molecule chemistry following the partial sublimation
of methane from grain surfaces. We also concluded that the abundances of
hydrocarbon radicals such as the C$_{\rm n}$H family should be further enhanced
as the temperatures increase to higher values, but this conclusion stood in
contrast with the lack of unambiguous detection of these species toward hot
core and corino sources. Meanwhile, observational surveys have identified
C$_2$H, C$_4$H, CH$_3$CCH, and CH$_3$OH toward hot corinos (especially IRAS
16293-2422) as well as towards L1527, with lower abundances for the carbon
chain radicals and higher abundances for the other two species toward the hot
corinos. In addition, the {\it Herschel Space Telescope} has detected the bare
linear chain C$_3$ in 50 K material surrounding young high-mass stellar
objects. To understand these new results, we revisit previous warm-up models
with an augmented gas-grain network that incorporated reactions from a
gas-phase network that was constructed for use with increased temperature up to
800 K. Some of the newly adopted reactions between carbon-chain species and
abundant H$_2$ possess chemical activation energy barriers. The revised model
results now better reproduce the observed abundances of unsaturated carbon
chains under hot-corino (100 K) conditions and make predictions for the
abundances of bare carbon chains in the 50 K regions observed by Herschel HIFI.
12/2011;
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ABSTRACT: Recent models of hot cores have incorporated previously-uninvestigated chemical pathways that lead to the formation of complex organic molecules (COMs; i.e. species containing six or more atoms). In addition to the gas-phase ion-molecule reactions long thought to dominate the organic chemistry in these regions, these models now include photodissociation-driven grain surface reaction pathways that can also lead to COMs. Here, simple grain surface ice species photodissociate to form small radicals such as OH, CH3, CH2OH, CH3O, HCO, and NH2. These species become mobile at temperatures above 30 K during the warm-up phase of star formation. Radical-radical addition reactions on grain surfaces can then form an array of COMs that are ejected into the gas phase at higher temperatures. Photodissociation experiments on pure and mixed ices also show that these complex molecules can indeed form from simple species. The molecules predicted to form from this type of chemistry reasonably match the organic inventory observed in high mass hot cores such as Sgr B2(N) and Orion-KL. However, the relative abundances of the observed molecules differ from the predicted values, and also differ between sources. Given this disparity, it remains unclear whether grain surface chemistry governed by photodissociation is the dominant mechanism for the formation of COMs, or whether other unexplored gas-phase reaction pathways could also contribute significantly to their formation. The influence that the physical conditions of the source have on the chemical inventory also remains unclear. Here we overview the chemical pathways for COM formation in hot cores. We also present new modeling results that begin to narrow down the possible routes for production of COMs based on the observed relative abundances of methyl formate (HCOOCH3) and its C2H4O2 structural isomers.
Proceedings of the International Astronomical Union 05/2011; 7:79 - 87.
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ABSTRACT: We 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.
Proceedings of the International Astronomical Union 05/2011; 7:33 - 42.
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ABSTRACT: We present a new interstellar chemical gas-phase reaction network for time-dependent kinetics that can be used for modeling high-temperature sources up to 800 K. This network contains an extended set of reactions based on the Ohio State University (OSU) gas-phase chemical network. The additional reactions include processes with significant activation energies, reverse reactions, proton exchange reactions, charge exchange reactions, and collisional dissociation. Rate coefficients already in the OSU network are modified for H2 formation on grains, ion-neutral dipole reactions, and some radiative association reactions. The abundance of H2O is enhanced at high temperature by hydrogenation of atomic O. Much of the elemental oxygen is in the form of water at T ≥ 300 K, leading to effective carbon-rich conditions, which can efficiently produce carbon-chain species such as C2H2. At higher temperatures, HCN and NH3 are also produced much more efficiently. We have applied the extended network to a simplified model of the accretion disk of an active galactic nucleus.
The Astrophysical Journal 09/2010; 721(2):1570. · 6.02 Impact Factor
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ABSTRACT: A gas-phase mechanism is studied to explain the production of propylene (C3H6) in cold interstellar clouds. In this mechanism, propylene is formed by dissociative recombination of the precursor ion, ( ), with electrons. The protonated precursor ion is itself formed via two radiative association reactions involving ions and molecular hydrogen, starting with the propargyl ion, H2CCCH+, and continuing with the 2-propenyl ion, . Calculations show that these radiative association reactions are very efficient at 10 K, which allows the synthesis of propylene to occur in cold clouds if the product channel C3H6+H is a sizable one for the dissociative recombination. The results also show, however, that our calculated ternary association rate coefficients for the reactions between H2CCCH+ and H2 and and H2 are apparently not in agreement with earlier experimental work at room temperature.
Molecular Physics. 09/2010; 108(17):2171-2177.
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ABSTRACT: We use a gas-grain chemical network to investigate the initial synthesis
of molecular ices as a cold molecular cloud forms behind a shock in the
diffuse interstellar medium. The reaction network includes newly
measured rates of photodesorption. The results show that CO is first
produced in the gas phase in early stages of cloud birth. This is
followed by concurrent formation of water ice on the grain and CO
accretion to the grain surface from the gas, at intermediate values of
the visual extinction. The production of CO_2 occurs on grains, via
both diffusive processes and the Eley-Rideal mechanism. The formation
of CH_4 ice is inhibited by the gas phase formation of CO. These
results show reasonable agreement with detection thresholds for the
major ice species, and show best agreement with the observed ice
composition along quiescent lines of sight in the Taurus dark cloud for
values of A_{V} of 2-3 mag. When the dense core begins to condense from
the cloud, the initial state is not dominated by a gas rich in ionized
C, as typically assumed.
05/2009; -1.
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ABSTRACT: We have investigated the role of molecular anion chemistry in pseudo-time dependent chemical models of dark clouds. With oxygen-rich elemental abundances, the addition of anions results in a slight improvement in the overall agreement between model results and observations of molecular abundances in TMC-1 (CP). More importantly, with the inclusion of anions, we see an enhanced production efficiency of unsaturated carbon-chain neutral molecules, especially in the longer members of the families CnH, CnH2, and HCnN. The use of carbon-rich elemental abundances in models of TMC-1 (CP) with anion chemistry worsens the agreement with observations obtained in the absence of anions. Comment: Accepted for publication in ApJ, 24 pages, 5 figures
05/2009;
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ABSTRACT: We investigate the recombination of the HCO+ ion on grain surfaces. This process is a key reaction in dense regions such as protoplanetary disks. The branching fraction among intact desorption, dissociation, and desorption with dissociation has been analyzed by a classical trajectory approach. It is found that dissociation (with or without desorption) dominates over intact desorption.
The Astrophysical Journal 01/2009; 527(1):262. · 6.02 Impact Factor
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ABSTRACT: Chemical networks used for models of interstellar clouds contain many reactions, some of them with poorly determined rate coefficients and/or products. In this work, we report a method for improving the predictions of molecular abundances using sensitivity methods and ab initio calculations. Based on the chemical network osu.2003, we used two different sensitivity methods to determine the most important reactions as a function of time for models of dense cold clouds. Of these reactions, we concentrated on those between C and C3 and between C and C5, both for their effect on specific important species such as CO and for their general effect on large numbers of species. We then used ab initio and kinetic methods to determine an improved rate coefficient for the former reaction and a new set of products, plus a slightly changed rate coefficient for the latter. Putting our new results in a pseudo-time-dependent model of cold dense clouds, we found that the abundances of many species are altered at early times, based on large changes in the abundances of CO and atomic C. We compared the effect of these new rate coefficients/products on the comparison with observed abundances and found that they shift the best agreement from 3e4 yr to (1-3)e5 yr.
01/2009;
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ABSTRACT: The low abundance of molecular oxygen in cold cores of interstellar clouds poses a continuing problem to modelers of the chemistry of these regions. In chemical models O2 is formed principally by the reaction between O and OH, which has been studied experimentally down to 39 K. It remains possible that the rate coefficient of this reaction at 10 K is considerably less than its measured value at 39 K, which might inhibit the production of O2 and possibly bring theory and observation closer together over a wider range of times. Two theoretical determinations of the rate coefficient for the O + OH reaction at temperatures down to 10 K have been undertaken recently; both results show that the rate coefficient is indeed lower at 10 K than at 39 K, although they differ in the magnitude of the decrease. Here we show, using gas-phase models, how the calculated interstellar O2 abundance in cold cores is affected by a decrease in the rate coefficient. We also consider its effect on other species. Our major finding is that for standard O-rich abundances, the calculated abundance of O2 in cold cores is sufficiently low to explain observations only at early times regardless of the value of k1 in the range investigated here. For C-rich abundances, on the other hand, late-time solutions can also be possible.
The Astrophysical Journal 12/2008; 681(2):1318. · 6.02 Impact Factor
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ABSTRACT: Virtually all detailed gas-phase models of the chemistry of dense interstellar clouds exclude polycyclic aromatic hydrocarbons (PAHs). This omission is unfortunate because from the few studies that have been done on the subject, it is known that the inclusion of PAHs can affect the gas-phase chemistry strongly. We have added PAHs to our network to determine the role they play in the chemistry of cold dense cores. Initially, only the chemistry of neutral and negatively charged PAH species was considered, since it was assumed that positively charged PAHs are of little importance. Subsequently, this assumption was checked and confirmed. In the models presented here, we include radiative attachment to form PAH−, mutual neutralization between PAH anions and small positively charged ions, and photodetachment. We also test the sensitivity of our results to changes in the size and abundance of the PAHs. Our results confirm that the inclusion of PAHs changes many of the calculated abundances of smaller species considerably. In TMC-1, the general agreement with observations is significantly improved, unlike in L134N. This may indicate a difference in PAH properties between the two regions. With the inclusion of PAHs in dense cloud chemistry, high-metal elemental abundances give a satisfactory agreement with observations. As a result, we do not need to decrease the observed elemental abundances of all metals, and we do not need to vary the elemental C/O ratio in order to produce large abundances of carbon species in TMC-1 (CP).
The Astrophysical Journal 12/2008; 680(1):371. · 6.02 Impact Factor
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ABSTRACT: We have investigated the evolution and distribution of molecules in collapsing prestellar cores via numerical chemical models, adopting the Larson-Penston solution and its delayed analogs to study collapse. Molecular abundances and distributions in a collapsing core are determined by the balance among the dynamical, chemical, and adsorption timescales. When the central density nH of a prestellar core with the Larson-Penston flow rises to 3 × 106 cm-3, the CCS and CO column densities are calculated to show central holes of radius 7000 and 4000 AU, respectively, while the column density of N2H+ is centrally peaked. These predictions are consistent with observations of L1544. If the dynamical timescale of the core is larger than that of the Larson-Penston solution owing to magnetic fields, rotation, or turbulence, the column densities of CO and CCS are smaller, and their holes are larger than in the Larson-Penston core with the same central gas density. On the other hand, N2H+ and NH3 are more abundant in the more slowly collapsing core. Therefore, molecular distributions can probe the collapse timescale of prestellar cores. Deuterium fractionation has also been studied via numerical calculations. The deuterium fraction in molecules increases as a core evolves and molecular depletion onto grains proceeds. When the central density of the core is nH = 3 × 106 cm-3, the ratio DCO+/HCO+ at the center is in the range 0.06-0.27, depending on the collapse timescale and adsorption energy; this range is in reasonable agreement with the observed value in L1544.
The Astrophysical Journal 12/2008; 552(2):639. · 6.02 Impact Factor
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ABSTRACT: The chemical evolutionary age of dark cloud cores is estimated from the degree of deuterium fractionation, on the assumption that the chemical model used is reasonable. The method is applied to dark cloud cores along the TMC-1 ridge, using the abundance ratios between DCO+ and H13CO+ combined with the new standard model network of gas-phase chemical reactions. The difference in deuterium fractionation between the ammonia peak and the cyanopolyyne peak, though its error transferred from the observed data is relatively large, is explained by a time difference in the evolutionary age of more than 105 yr, or by a small change in the depletion factor of carbon and oxygen, which also indicates the degree of core evolution. The method of determining the evolutionary age of dark cloud cores is somewhat free from the details of chemical reaction not directly related to deuterium fractionation. The present result is compared with those from other methods.
The Astrophysical Journal 12/2008; 569(2):836. · 6.02 Impact Factor
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ABSTRACT: We present a study of the chemistry of a dense photon-dominated region (PDR) using a time-dependent chemical model. Our major interest is to study the spatial distribution of complex molecules such as hydrocarbons and cyanopolyynes in the cool dense material bordering regions where star formation has taken place. Our standard model uses a homogeneous cloud of density 2 × 104 cm−3 and temperature T= 40 K, which is irradiated by a far-ultraviolet radiation field of intermediate intensity, given by χ= 100. We find that over a range of times unsaturated hydrocarbons (e.g. C2H, C4H, C3H2) have relatively high fractional abundances in the more external layers of the PDR, whereas their abundances in the innermost layers are several orders of magnitudes lower. On the other hand, molecules that are typical of late-time chemistry are usually more abundant in the inner parts of the PDR. We also present results for models with different density, temperature, intensity of the radiation field and initial fractional abundances. Our results are compared with both high and moderate angular resolution observations of the Horsehead nebula. Our standard model is partially successful in reproducing the observations. Additional models run with different physical parameters are able to reproduce the abundance of many of the observed molecules, but we do not find a single model that fits all the observations at the same time. We discuss the suitability of a time-dependent model of a dense PDR such as ours as an estimator of the age of a PDR, provided that enough observational data exist.
Monthly Notices of the Royal Astronomical Society 11/2008; 390(4):1549 - 1561. · 4.90 Impact Factor
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ABSTRACT: The low-mass protostellar region L1527 is unusual because it contains observable abundances of unsaturated carbon-chain molecules including CnH radicals, H2Cn carbenes, cyanopolyynes, and the negative ions C4H- and C6H-, all of which are more associated with cold cores than with protostellar regions. Sakai et al. suggested that these molecules are formed in L1527 from the chemical precursor methane, which evaporates from the grains during the heat-up of the region. With the gas-phase osu.03.2008 network extended to include negative ions of the families Cn-, and CnH-, as well as the newly detected C3N-, we modeled the chemistry that occurs following methane evaporation at T~ 25-30 K. We are able to reproduce most of the observed molecular abundances in L1527 at a time of ~5000 yr. At later times, the overall abundance of anions become greater than that of electrons, which has an impact on many organic species and ions. The anion-to-neutral ratio in our calculation is in good agreement with observation for C6H- but exceeds the observed ratio by more than three orders of magnitude for C4H-. In order to explain this difference, further investigation is needed on the rate coefficients for electron attachment and other reactions regarding anions. Comment: 28 pages, 8 figures, ApJ accepted
08/2008;
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ABSTRACT: Sakai et al. have observed long-chain unsaturated hydrocarbons and cyanopolyynes in the low-mass star-forming region L1527, and have attributed this result to a gas-phase ion-molecule chemistry, termed ``Warm Carbon Chain Chemistry'', which occurs during and after the evaporation of methane from warming grains. The source L1527 is an envelope surrounding a Class 0/I protostar with regions that possess a slightly elevated temperature of ~30 K. The molecules detected by Sakai et al. are typically associated only with dark molecular clouds, and not with the more evolved hot corino phase. In order to determine if L1527 is chemically distinct from a dark cloud, we compute models including various degrees of heating. The results indicate that the composition of L1527 is somewhat more likely to be due to ``Warm Carbon Chain Chemistry'' than to be a remnant of a colder phase. If so, the molecular products provide a signature of a previously uncharacterized early phase of low mass star formation, which can be characterized as a ``lukewarm'' corino. We also include predictions for other molecular species that might be observed toward candidate lukewarm corino sources. Although our calculations show that unsaturated hydrocarbons and cyanopolyynes can be produced in the gas phase as the grains warm up to 30 K, they also show that such species do not disappear rapidly from the gas as the temperature reaches 200 K, implying that such species might be detected in hot corinos and hot cores. Comment: 31 pages, 7 figures, Accepted for publication in ApJ
03/2008;
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ABSTRACT: Gas-phase processes were long thought to be the key formation mechanisms for complex organic molecules in star-forming regions. However, recent experimental and theoretical evidence has cast doubt on the efficiency of such processes. Grain-surface chemistry is frequently invoked as a solution, but until now there have been no quantitative models taking into account both the high degree of chemical complexity and the evolving physical conditions of star-forming regions. Here, we introduce a new gas-grain chemical network, wherein a wide array of complex species may be formed by reactions involving radicals. The radicals we consider (H, OH, CO, HCO, CH3, CH3O, CH2OH, NH and NH2) are produced primarily by cosmic ray-induced photodissociation of the granular ices formed during the colder, earlier stages of evolution. The gradual warm-up of the hot core is crucial to the formation of complex molecules, allowing the more strongly-bound radicals to become mobile on grain surfaces. This type of chemistry is capable of reproducing the high degree of complexity seen in Sgr B2(N), and can explain the observed abundances and temperatures of a variety of previously detected complex organic molecules, including structural isomers. Many other complex species are predicted by this model, and several of these species may be detectable in hot cores. Differences in the chemistry of high- and low-mass star-formation are also addressed; greater chemical complexity is expected where evolution timescales are longer. Comment: Accepted for publication in ApJ (57 pages, 9 figures)
03/2008;
