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1.
Magdy Y. Amin 《Astrophysics and Space Science》2003,284(4):1143-1157
A chemical evolution of W3 IRS5 have been studied during J-type shock. We modified our code of for one-dimensional J-type
shock to satisfy the physical conditions of W3 IRS5. The chemical rate equations and the hydrodynamics of shocks are integrated
simultaneously. We find that, a shock of speed V
s = 10 km s-1 and magnetic induction B=4.1×10-6 Gauss, propagate into molecular gas density of n=104cm-3, and with initial temperature of 30 K (pre shock) satisfy the physical conditions of W3 IRS5. The results using the extensive
chemical network with the most updating reaction rates show that the calculated fractional abundances are in agreement with
observation.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
2.
The evolution of the different chemical species are followed in a model of contracting interstellar cloud. The central density increases from n = 10 cm–3 diffuse initial cloud model to a dense cloud with central density number of n >- 105 cm–3 after a time of 1.2 × 107 yr. A network of 622 reactions has been involved. The chemistry of the cloud is integrated simultaneously with the hydrodynamic equations of contraction.The results predict that the different molecular species increase in abundance as the contraction proceeds. The species which enhance significantly are CO, HCO, CS and NO. The fractional abundances of many of the other molecular species increase distinctly with contraction, e.g. CH, C2H, CN, SO2, CO2, H2O, C2, NH3, HCN, SO, OCS and SN. The transformation of the initial diffuse cloud model with small abundances of molecular species to a dense molecular cloud with enhancement of the different molecular species is confirmed. The results predict good agreements of our results with both the observations and other theoretical studies. 相似文献
3.
We have constructed models for a region of low mass star formation where stellar winds ablate material from dark dense cores and return it to a translucent intercore medium from which subsequent generations of cores condense. Depletion of gas phase species onto grains plays a major role in the chemistry. For reasonable agreement between model core chemical fractional abundances and measured TMC-1 fractional abundances to obtain, the core collapse, once started, must be relatively uninhibited by turbulence or magnetic fields and the core lifetime must fall in a limited range determined by the assumed depletion rates. In a core with the TMC-1 fractional abundances, CH, OH, C2H, H2CO, HCN, HNC, and CN are the only simple species that have been detected in TMC-1 at radio and millimeter wavelengths to have fractional abundances that are roughly constant or increasing with time; this result bears considerably on previous work concerned with searches for spectroscopic evidence for and the diagnosis of collapse during protostellar formation, but depends on the fractions of the OH and CH emissions that are associated with the core centre rather than more extended gas or a core-stellar wind boundary layer. Model results for the abundance ratios of H2O, CH4, and NH3 ices are in good agreement with those inferred for Halley's Comet. 相似文献
4.
We present a generalization of the multiphase chemical evolution model (CEM) applied to a wide set of theoretical galaxies with different masses and evolutionary rates. This generalized set of models has been computed using the so-called universal rotation curve from Persic, Salucci & Steel to calculate the radial mass distribution of 44 theoretical protogalaxies. This distribution is a fundamental input which, besides its own effect on the galaxy evolution, defines the characteristic collapse time-scale or gas infall rate on to the disc. We have adopted 10 sets of values, between 0 and 1, for the molecular cloud and star formation efficiencies, as corresponding to their probability nature, for each one of the radial distributions of total mass. Thus, we have constructed a biparametric grid of models, depending on those efficiency sets and on the rotation velocity, whose results are valid in principle for any spiral or irregular galaxy. The model results provide the time-evolution of different regions of the disc and the halo along galactocentric distance, measured by the gas (atomic and molecular) and stellar masses, the star formation rate (SFR) and chemical abundances of 14 elements, for a total of 440 models. This grid may be used to estimate the evolution of a given galaxy for which only present time information, such as radial distributions of elemental abundances, gas densities and/or star formation, which are the usual observational constraints of chemical evolution models (CEMs), is available. 相似文献
5.
New multifrequency spatial and spectral studies of the hot molecular core associated with the ultracompact HII region G34.3+0.15 have demonstrated an extremely rich chemistry in this archetypal hot core and revealed differing spatial structure between certain species which may be a dynamical effect of chemical evolution. The structure of the hot core has been studied with the JCMT in the high excitation J=19-18 and J=18-17 lines of CH3CN and with the Nobeyama Millimetre Array at 4 arc resolution in the J=6-5 transition. Comparison with a VLA NH3(3,3) map shows a displacement between peak emission in the two chemical species which is consistent with chemical processing on a time scale comparable to the dynamical time scale of 105 yrs.A 330-360 GHz spectral survey of the hot core with the JCMT has detected 358 spectral lines from at least 46 distinct chemical species, including many typical of shocked chemistry while other species indicate abundances that reflect the chemistry of a previous cold phase. The first unambiguous detection of ethanol in hot gas has been made. Observations of 14 rotational transitions of this molecule yield a temperature of 125 K and column density 2×1015 cm–2. This large abundance cannot be made by purely gas-phase processes and it is concluded that ethanol must have formed by grain surface chemistry. 相似文献
6.
7.
We have constructed a chemical reaction model in a contracting interstellar cloud including 104 species which are involved in a network of 557 reactions. The chemical kinetic equations were integrated as a function of time by using gear package. The evolution of the system was followed in the density range 10–107 particles cm-3.The calculated fractional abundances of the charged species are in good agreement with those given by other investigators. The charge density has been followed in diffuse, intermediate and dense regions. The most dominant ionic species are metallic ions, HCO+ and H
3
+
in the shielded regions and atomic ions H+, C+, Si+, He+, S+ and metal ions in the diffuse and intermediate regions. The abundances of negatively charged ions were found to be negligible. The results of the calculations on the different metallic ions are interpreted. 相似文献
8.
E. A. Semenko D. O. Kudryavtsev T. A. Ryabchikova I. I. Romanyuk 《Astrophysical Bulletin》2008,63(2):128-138
The results of a complex study of the chemically peculiar star HD 45583 are reported. Observations were made using the Main Stellar Spectrograph equipped with a circular polarization analyzer and NES echelle spectrograph of the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences. Our measurements of Zeeman spectra show that the star exhibits unusual variations of the longitudinal component of magnetic field with a secondary minimum. The period of spectral and magnetic variability coincides with the rotation period, which is equal to 1.d177000. Two possible causes of the secondary minimum are discussed: spots with higher than ambient content of some chemical elements on the star’s surface or complex structure of the stellar magnetic field. The parameters of the star’s atmosphere are determined (T eff = 13000 K, log g = 4.0), as well as the abundances of some elements: the star shows a 1–2 dex overabundance of Fe, Si, and Cr, helium is underabundant by about 2 dex with respect to the Sun. 相似文献
9.
We studied 14 presolar SiC mainstream grains for C‐, Si‐, and S‐isotopic compositions and S elemental abundances. Ten grains have low levels of S contamination and CI chondrite‐normalized S/Si ratios between 2 × 10?5 and 2 × 10?4. All grains have S‐isotopic compositions compatible within 2σ of solar values. Their mean S isotope composition deviates from solar by at most a few percent, and is consistent with values observed for the carbon star IRC+10216, believed to be a representative source star of the grains, and the interstellar medium. The isotopic data are also consistent with stellar model predictions of low‐mass asymptotic giant branch (AGB) stars. In a δ33S versus δ34S plot the data fit along a line with a slope of 1.8 ± 0.7, suggesting imprints from galactic chemical evolution. The observed S abundances are lower than expected from equilibrium condensation of CaS in solid solution with SiC under pressure and temperature conditions inferred from the abundances of more refractory elements in SiC. Calcium to S abundance ratios are generally above unity, contrary to expectations for stoichiometric CaS solution in the grains, possibly due to condensation of CaC2 into SiC. We observed a correlation between Mg and S abundances suggesting solid solution of MgS in SiC. The low abundances of S in mainstream grains support the view that the significantly higher abundances of excess 32S found in some Type AB SiC grains are the result of in situ decay of radioactive 32Si from born‐again AGB stars that condensed into AB grains. 相似文献
10.
We have constructed a reaction system containing the chemical families of H, C, O, N, S, Si, Cl, metals (Me) and grains. A total of 104 species have been included and a network of 557 reactions has been studied. The chemical kinetic equations were integrated as a function of time by using gear program. The chemical reaction system was followed at low, intermediate and high cloud densities i.e. from 10–107 particles cm-3. The calculated fractional abundances of N2, CN, HCN, and CH which are in good agreement with the results of observations and with those of previous theoretical studies. 相似文献
11.
T. Şahin 《Astrophysical Bulletin》2018,73(1):35-51
We present an LTE analysis of high resolution echelle optical spectra obtained with the 3.9-m Anglo-Australian Telescope (AAT) and the UCLES spectrograph for a B1Ib high galactic latitude supergiant HD119608. A fresh determination of the atmospheric parameters using line-blanketed LTE model atmospheres and spectral synthesis provided Teff = 23 300 ± 1000 K, log g = 3.0 ± 0.3, and the microturbulent velocity ξ = 6.0 ± 1.0 kms?1 and [Fe/H] = 0.16. The rotational velocity of the star was derived fromC, O, N, Al, and Fe lines as v sin i = 55.8 ± 1.3 kms?1. Elemental abundances were obtained for 10 different species. He, Al, and P abundances of the star were determined for the first time. In the spectra, hot post-AGB status as well as the Pop I characteristics of the star were examined. The approximately solar carbon and oxygen abundances, along with mild excess in helium and nitrogen abundances do not stipulate a CNO processed surface composition, hence a hot post-AGB status. The LTE abundances analysis also indicates solar sulphur and moderately enriched magnesium abundances. The average abundances of B dwarfs of well studied OB associations and Population I stars show a striking resemblance to abundances obtained for HD119608 in this study. This may imply a runaway status for the star. 相似文献
12.
Model atmosphere analysis, based on Kurucz models has been applied to study the F6V star π3 Ori (=BS1543=HD30652). The following values of the effective temperature, surface gravity and microturbulence velocity were
obtained: = 6270±200 K, log g = 3.80.2, ξt =3.5±0.5 km/s. The abundances of 10 elements were determined. The resulting element abundances for the π3
Ori were found to be about three times lower with respect to the Sun. From evolutionary calculations we derived a mass, radius
and luminosity for π3
Ori of M =1.3 M⊙, R =2.38 R⊙, L =7.9 L⊙. Hence this star should be classified F6IV instead of F6 V.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
13.
S. Hyung L. H. Aller W. A. Feibelman W. B. Lee A. de Koter 《Monthly notices of the Royal Astronomical Society》2000,318(1):77-91
With the Hamilton echelle spectrograph at the Lick Observatory, emission-rich spectral lines of the planetary nebula NGC 6543 were secured in the wavelength range from 3550 to 10 100 Å. We chose two bright regions, ∼8 arcsec east and ∼13 arcsec north of the central star, the physical conditions and chemical abundances of which may differ as a result of the different physical characteristics involving the mass ejection of different epochs. By combining Hamilton echelle observations with archive UV data secured with the International Ultraviolet Explorer ( IUE ), we obtain improved diagnostics and chemical compositions for the two observed regions. The diagnostic diagram gives the average value of T e =8000∼8300 K, and the electron number density near N e ∼5000 cm−3 for most ions, while some low-excitation lines indicate much higher temperatures, i.e. T e ∼10 000 K. With the construction of a photoionization model, we try to fit the observed spectra in a self-consistent way: thus, for most elements, we employ the same chemical abundances in the nebular shell; and we adopt an improved Sobolev approximation model atmosphere for the hydrogen-deficient Wolf–Rayet type central star. Within the observational errors, the chemical abundances do not seem to show any positional variation except for helium. The chemical abundances of NGC 6543 appear to be the same as in average planetary nebulae. The progenitor star may have been an object of one solar mass, most of the heavier elements of which were less plentiful than in the Sun. 相似文献
14.
A. Ali 《Journal of Astrophysics and Astronomy》2006,27(4):399-409
We have constructed two gas-phase models to study the chemistry of circumstellar envelope surrounding the carbon-rich variable
star IRAS 15194-5115. The network used consists of 3893 reactions involving 397 gas-phase species. The derived fractional
abundances for many molecules are in excellent agreement with values obtained from observations. The predicted column densities
from the two models go well with the observed values of carbon star IRC + 10216. The dominant formation routes for three groups
of species are discussed through the inner and outer envelopes. 相似文献
15.
Leonard C. Rosen 《Astrophysics and Space Science》1969,5(2):150-170
The composition of neutron star atmospheres is calculated as a function of time including effects of diffusion, cooling and thermonuclear reactions. A seven-component nuclear reaction network with includes He4, C12, O16, Ne20, Mg24, Si28 and Fe56 is utilized. Neutron star models with different initial nuclear abundances are compared as to subsequent nucleosynthesis. It is found that the final abundances are independent of original composition assuming He4 as the major initial constituent. The final composition of the atmosphere is predominantly Fe56. Mass loss from an evolving neutron star is examined as a possible source of cosmic rays. It is found that a neutron star contributes only Fe56 significantly to the cosmic-ray spectrum. 相似文献
16.
R. T. Garrod D. A. Williams T. W. Hartquist J. M. C. Rawlings S. Viti 《Monthly notices of the Royal Astronomical Society》2005,356(2):654-664
We investigate the chemistry of a transient density fluctuation, with properties similar to those of a dense core within a molecular cloud. We run a multipoint chemical code through a core's condensation from a diffuse medium to its eventual dispersion, over a period of ∼1 Myr. We find a significant enhancement of the chemical composition of the core material on its return to diffuse conditions, whilst the expansion of the core as it disperses moves this material out to large distances from the core centre. This process transports molecular species formed in the high-density regions out into the diffuse medium. Chemical enrichment of the cloud as a whole also occurs, as other cores of various sizes, life-spans and separations evolve throughout. Enrichment is strongly affected by freeze-out on to dust grains, which takes place in high-density, high visual extinction regions. As the core disperses after reaching its peak density and the visual extinction drops below a critical value, grain mantles are evaporated back into the gas phase, initiating more chemistry. The influence of the sizes, masses and cycle periods of cores will be large both for the level of chemical enrichment of a dark cloud and ultimately for the low-mass star formation rate. We also consider the case of a self-gravitating core, by holding its peak density conditions for a further 0.4 Myr. We find that the differences are generally small, and the resultant column densities do not provide definitive criteria for detection of this condition. However, increases in fractional abundances due to reinjection of mantle-borne species may provide a criterion for a negative detection. 相似文献
17.
T. N. Tarasova 《Astronomy Letters》2014,40(6):309-319
Based on our spectrophotometric observations, we have studied the envelope of the HeN Nova Mon 2012. The abundances of some chemical elements in the envelope and its mass have been estimated. Our results show that the helium, nitrogen, oxygen, and neon abundances in the Nova envelope exceed the solar ones by a factor of 1.5, 33, 9, and 95, respectively. The envelope mass has been found to be 2.3 × 10?4 M ⊙. 相似文献
18.
The relative abundances of seven constitutent nuclei, He4, C12, O16, Ne20, Mg24, Si28 and Fe56, are calculated as a function of time for neutron star atmospheres within which exist magnetic fields of the order of 1013G. The opacity, equation of state of the electrons, and cooling rate of the magnetic star are discussed, and it is shown to be a reasonable approximation to assume an atmosphere to be isothermal. The effects of particle diffusion are included in the nuclear reaction network. Computations are performed both for a constant mass atmosphere and for an atmosphere in which mass is being ejected. It is found that the final abundances are model independent as long as the initial model contains predominantly He4. The relative abundances are compared to the cosmic ray spectrum. For both the constant mass and mass loss atmospheres, nucleosynthesis proceeds virtually completely to Fe56. However the outermost layers of the envelope, in which no mass is being ejected, are composed almost entirely of He4 with trace amounts of Fe56. After the loss of about 1021 g, only Fe56 is ejected from atmospheres expelling mass.A portion of the research on which this paper was based was performed while L. C. Rosen was present at the Lawrence Radiation Laboratory, Livermore, California. 相似文献
19.
We study the evolution of elemental abundances in an ensemble of Galactic planetary nebulae as a function of the masses of the central stars (M cs) and their progenitors (M ini). We derive the dependences of the C, N, Ne, Cl, Ar, and S abundances on M cs and M ini for a large sample of nebulae. We calculate the theoretical elemental abundances in nebulae under the assumption of complete mixing of the progenitor’s matter ejected at different stages of its evolution. The theoretical dependences of the C and N abundances on M ini have been found to correspond to the observed ones. At the same time, the observed mean O abundance is approximately half its theoretical value. The Ne, Cl, Ar, and S abundances monotonically increase with increasing mass of the progenitor star, which reflects an increase in the mean abundances of heavy elements during the chemical evolution of the Galaxy. We have derived the relation between the abundances of the elements under consideration in planetary nebulae and the masses of their central stars. This relation is used to construct the mass function for the nuclei of planetary nebulae. 相似文献
20.
We have computed a set of multiphase chemical evolution models in which the radial mass distributions of each theoretical
galaxy is calculated using the universal rotation curve from Persic, Salucci and Steel (1996). We obtain the chemical evolution
for galaxies of different masses and morphological types by changing the efficiencies to form molecular clouds and stars according
with these types. We obtain the radial distributions of diffuse and molecular gas densities, the star formation rate and abundances
for 15 elements for each galaxy.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献