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1.
The apparition of Comet C/1996 B2 (Hyakutake) offered an unexpected and rare opportunity to probe the inner atmosphere of a comet with high spatial resolution and to investigate with unprecedented sensitivity its chemical composition. We present observations of over 30 submillimeter transitions of HCN, H13CN, HNC, HNCO, CO, CH3OH, and H2CO in Comet Hyakutake carried out between 1996 March 18 and April 9 at the Caltech Submillimeter Observatory. Detections of the H13CN (4–3) and HNCO (160,16–150,15) transitions represent the first observations of these species in a comet. In addition, several other transitions, including HCN (8–7), CO (4–3), and CO (6–5) are detected for the first time in a comet as is the hyperfine structure of the HCN (4–3) line. The observed intensities of the HCN (4–3) hyperfine components indicate a line center optical depth of 0.9 ± 0.2 on March 22.5 UT. The HCN/HNC abundance ratio in Comet Hyakutake at a heliocentric distance of 1 AU is similar to that measured in the Orion extended ridge— a warm, quiescent molecular cloud. The HCN/H13CN abundance ratio implied by our observations is 34 ± 12, similar to that measured in giant molecular clouds in the galactic disk but significantly lower than the Solar System12C/13C ratio. The low HCN/H13CN abundance ratio may be in part due to contamination by an SO2line blended with the H13CN (4–3) line. In addition, chemical models suggest that the HCN/H13CN ratio can be affected by fractionation during the collapse phase of the protosolar nebula; hence a low HCN/H13CN ratio observed in a comet is not inconsistent with the solar system12C/13C isotopic ratio. The abundance of HNCO relative to water derived from our observations is (7 ± 3) × 10−4. The HCN/HNCO abundance ratio is similar to that measured in the core of Sagittarius B2 molecular cloud. Although a photo-dissociative channel of HNCO leads to CO, the CO produced by HNCO is a negligible component of cometary atmospheres. Production rates of HCN, CO, H2CO, and CH3OH are presented. Inferred molecular abundances relative to water are typical of those measured in comets at 1 AU from the Sun. The exception is CO, for which we derive a large relative abundance of 30%. The evolution of the HCN production rate between March 20 and March 30 suggests that the increased activity of the comet was the cause of the fragmentation of the nucleus. The time evolution of the H2CO emission suggests production of this species from dust grains.  相似文献   

2.
We have analyzed infrared spectra of Titan recorded by the Cassini Composite Infrared Spectrometer (CIRS) to measure the isotopic ratio 12C/13C in each of three chemical species in Titan's stratosphere: CH4, C2H2 and C2H6. This is the first measurement of 12C/13C in any C2 molecule on Titan, and the first measurement of 12CH4/13CH4 (non-deuterated) on Titan by remote sensing. Our spectra cover five widely-spaced latitudes, 65° S to 71° N and we have searched for both latitude variability of 12C/13C within a given species, and also for differences between the 12C/13C in the three gases. For CH4 alone, we find (1-σ), essentially in agreement with the 12CH4/13CH4 measured by the Huygens Gas Chromatograph/Mass Spectrometer instrument (GCMS) [Niemann, H.B., and 17 colleagues, 2005. Nature 438, 779-784]: 82.3±1.0, and also with measured values in H13CN and 13CH3D by CIRS at lower precision [Bézard, B., Nixon, C., Kleiner, I., Jennings, D., 2007. Icarus 191, 397-400; Vinatier, S., Bézard, B., Nixon, C., 2007. Icarus 191, 712-721]. For the C2 species, we find in C2H2 and 89.8±7.3 in C2H6, a possible trend of increasingly value with molecular mass, although these values are both compatible with the Huygens GCMS value to within error bars. There are no convincing trends in latitude. Combining all fifteen measurements, we obtain a value of , also compatible with GCMS. Therefore, the evidence is mounting that 12C/13C is some 8% lower on Titan than on the Earth (88.9, inorganic standard), and lower than typical for the outer planets (88±7 [Sada, P.V., McCabe, G.H., Bjoraker, G.L., Jennings, D.E., Reuter, D.C., 1996. Astrophys. J. 472, 903-907]). There is no current model for this enrichment, and we discuss several mechanisms that may be at work.  相似文献   

3.
Despite the low elemental abundance of atomic deuterium in the interstellar medium (ISM), observational evidence suggests that several species, both in the gas phase and in ices, could be heavily fractionated. We explore various aspects of deuterium enrichment by constructing a chemical evolution model in both gaseous and granular phases. Depending on various physical parameters, gases and grains are allowed to interact with each other through the exchange of their chemical species. It is known that HCO+ and N2H+ are two abundant gas phase ions in the ISM and, their deuterium fractionation is generally used to predict the degree of ionization in the various regions of a molecular cloud. For a more accurate estimation, we consider the density profile of a collapsing cloud. The radial distributions of important interstellar molecules, along with their deuterated isotopomers, are presented. Quantum chemical simulations are computed to study the effects of isotopic substitution on the spectral properties of these interstellar species. We calculate the vibrational (harmonic) frequencies of the most important deuterated species (neutral and ions). The rotational and distortional constants of these molecules are also computed in order to predict the rotational transitions of these species. We compare vibrational (harmonic) and rotational transitions as computed by us with existing experimental and theoretical results. It is hope that our results will assist observers in detecting several hitherto unobserved deuterated species.  相似文献   

4.
C/2006 P1 McNaught is a dynamically new comet from the Oort cloud that passed very close to the Sun, driving overall volatile production rates up to about 1031 molecules s−1. Post-perihelion observations were obtained in a target-of-opportunity campaign using the CSHELL instrument at the NASA Infrared Telescope Facility atop Mauna Kea, Hawaii, on UT 2007 January 27 and 28. Eight parent volatiles (H2O, CH4, C2H2, C2H6, HCN, CO, NH3, H2CO) and two daughter fragments (OH and NH2) were detected, enabling the determination of a rotational temperature and production rate for H2O on UT January 27 and absolute and relative production rates for all the detected parent species on UT January 28. The chemical composition measured in the coma suggests that this close perihelion passage stripped off processed outer surface layers, likely exposing relatively fresh primordial material during these observations. The post-perihelion abundances we measure for CO and CH4 (relative to H2O) are slightly depleted while C2H2, NH2 and possibly NH3 are enhanced when compared to the overall comet population. Measured abundances for other detected molecular species were within the range typically observed in comets.  相似文献   

5.
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   

6.
A full radiative transfer model is presented for the ultraviolet (UV) radiation impinging on an interstellar cloud of spherical or finite plane-parallel slab geometry containing gas and dust. The penetration of the UV photons is coupled to detailed chemical processes. Photodestruction rates of atomic and molecular species are calculated from the corresponding cross-sections. We show that CO line intensities are quite sensitive to geometrical effects and to the extinction curve in the far-UV.  相似文献   

7.
The12C/13C isotope ratio in the interstellar medium and in stellar atmospheres is discussed and compared to the value found in the solar system and especially in comets. The cometary value (100) tends to be slightly above the terrestrial value and is definitively higher than that for interstellar molecular clouds (about 30 to 50).This result implies that comets are not of interstellar origin; that the original isotopic abundances of the primitive solar nebula has been preserved in the cometary material; and that due to an enrichment of the interstellar medium in13C, the12C/13C isotope ratio has decreased by a factor of about 2.5 since the formation of the solar system (i.e., during the past 4.5×109 years) — a result which is roughly in agreement with present theories of the chemical evolution of our Galaxy. The relatively high cometary carbon isotope ratio (as compared to the terrestrial value) indicates that some correction should be applied to the semi-empirical models describing the13C enrichment in the Galaxy.Paper dedicated to Professor Hannes Alfvén, on the occasion of his 70th birthday, 30 May, 1978.  相似文献   

8.
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   

9.
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   

10.
We have followed the chemistry of W3 IRS4, as one of the most famous regions with a chemistry similar to that found in quiescent molecular clouds. Here the time scale, since star formation, has probably been long enough for the gas-phase ion-molecule chemistry to become dominant again. We have constructed a reaction system containing the chemical families of H, C, O, N, S and metals (Me:Fe, Mg, Na and Si). A total of 282 species have been included and a network of 1270 reactions has been used. The chemical kinetic equations were integrated as a function of pseudo-time-dependent approach. The evolution of the different chemical species are followed in models of density n equal to 106 cm-3 and temperature T equal to 55 K. Different initial abundances have been assumed. Our results are in reasonable agreement with the observation, especially in Model 6. The resulting molecular abundances are compared with those obtained in the intermediate mass star NGC 2264 IRS1, as well as the low mass IRAS 16293-2422. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

11.
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   

12.
The paper considers the question after the circumstances in which kinematic mean-field dynamos can have oscillatory magnetic field modes. The conducting fluid body is allowed to be of almost arbitrary shape; its surroundings are vacuum. A general relation for the frequency of oscillation is derived. This relation is discussed more closely for models with pure α2-mechanism. Proof is given that no oscillations can occur for constant α. The investigations published so far on spherical models with pure α2-mechanism call up the question whether there is a chance for axisymmetric modes to be oscillatory. For both spherical models and disk models the possibility of oscillatory axisymmetric modes is demonstrated by examples.  相似文献   

13.
Abstract– Insight into the chemical history of an ungrouped type 2 carbonaceous chondrite meteorite, Wisconsin Range (WIS) 91600, is gained through molecular analyses of insoluble organic matter (IOM) using solid‐state 13C nuclear magnetic resonance (NMR) spectroscopy, X‐ray absorption near edge structure spectroscopy (XANES), and pyrolysis‐gas chromatography coupled with mass spectrometry (pyr‐GC/MS), and our previous bulk elemental and isotopic data. The IOM from WIS 91600 exhibits similarities in its abundance and bulk δ15N value with IOM from another ungrouped carbonaceous chondrite Tagish Lake, while it exhibits H/C, δ13C, and δD values that are more similar to IOM from the heated CM, Pecora Escarpment (PCA) 91008. The 13C NMR spectra of IOM of WIS 91600 and Tagish Lake are similar, except for a greater abundance of CHxO species in the latter and sharper carbonyl absorption in the former. Unusual cross‐polarization (CP) dynamics is observed for WIS 91600 that indicate the presence of two physically distinct organic domains, in which the degrees of aromatic condensation are distinctly different. The presence of two different organic domains in WIS 91600 is consistent with its brecciated nature. The formation of more condensed aromatics is the likely result of short duration thermal excursions during impacts. The fact that both WIS 91600 and PCA 91008 were subjected to short duration heating that is distinct from the thermal history of type 3 chondrites is confirmed by Carbon‐XANES. Finally, after being briefly heated (400 °C for 10 s), the pyrolysis behavior of Tagish Lake IOM is similar to that of WIS 91600 and PCA 91008. We conclude that WIS 91600 experienced very moderate, short duration heating at low temperatures (<500 °C) after an episode of aqueous alteration under conditions that were similar to those experienced by Tagish Lake.  相似文献   

14.
The five hundred meter aperture spherical radio telescope is will use an active spherical reflector. When the zenith scan angle is changed, the illuminated part of the reflecting surface is made to fit a paraboloid of revolution in real time by active control. The maximum zenith scan angle |ψmax| of FAST is 30° under conditions of the geometry selected in order that the feed does not illuminate the ground. The result of this paper shows that the maximum zenith scan angle |ψmax| can be extended to 69° by offsetting the feed. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

15.
We have investigated 128 14C‐dated ordinary chondrites from Oman for macroscopically visible weathering parameters, for thin section‐based weathering degrees, and for chemical weathering parameters as analyzed with handheld X‐ray fluorescence. These 128 14C‐dated meteorites show an abundance maximum of terrestrial age at 19.9 ka, with a mean of 21.0 ka and a pronounced lack of samples between 0 and 10 ka. The weathering degree is evaluated in thin section using a refined weathering scale based on the current W0 to W6 classification of Wlotzka (1993), with five newly included intermediate steps resulting in a total of nine (formerly six) steps. We find significant correlations between terrestrial ages and several macroscopic weathering parameters. The correlation of various chemical parameters including Sr and Ba with terrestrial age is not very pronounced. The microscopic weathering degree of metal and sulfides with newly added intermediate steps shows the best correlation with 14C terrestrial ages, demonstrating the significance of the newly defined weathering steps. We demonstrate that the observed 14C terrestrial age distribution can be modeled from the abundance of meteorites with different weathering degrees, allowing the evaluation of an age‐frequency distribution for the whole meteorite population.  相似文献   

16.
Despois  D. 《Earth, Moon, and Planets》1997,79(1-3):103-124
We present here a review of the radio observations of the remarkable comet Hale-Bopp C/1995 O1 in which most major radio astronomical facilities have been involved. These observations started in August 1995, soon after the discovery of the comet (it was then at ∼7 AU from the sun), and well before its perihelion on April 1st, 1997; they are still going on, hopefully up to end of 1998. Extended cartographies have been obtained using multibeam receivers and on-the-fly techniques. High spatial resolution (a few ″) has been achieved with interferometers. Submillimetric observations are playing an increasing role, and high resolution (R ∼ 106−107) spectroscopy of cometary lines is now performed from decimetric to submillimetric wavelengths. The number of species observed at radio wavelengths now reaches ∼28,when it was ∼14 for comet C/1996 B2 Hyakutake. Most of these species are parent molecules. However, ions have been observed for the first time at radio wavelengths, and their velocities measured. Several isotopic species (involving D,13C,34S,15N) have been sought, allowing isotopic enrichment determinations. The abundances of cometary molecules present many similarities and some differences with the abundances of interstellar molecules in regions where grain mantles are believed to be evaporated to the gas phase (hot cores, bipolar flows). They will be discussed for their implications on the origin of cometary ices and of comets themselves. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

17.
Numerical simulations are performed to study the tidal effects of non-merging rapid head-on collision between a disk galaxy and a spherical galaxy. The disk consists of three components – a disk, a bulge and a halo – and the spherical galaxy is a Plummer model. The galaxies have the same dimensions with different mass ratios viz., 2, 1 and 0.5. They move in a rectilinear orbit with a relative velocity of 1000 km s−1. None of the simulations leads to the merger of the galaxies by tidal capture. The results of our simulations indicate that although tidal effects are sensitive to both the mass ratio and the inclination of the disk to the orbital plane, it is the mass ratio which is more important in producing tidal damage to the less massive galaxy. The spherical galaxy undergoes considerable tidal effects if the mass of the disk is same or larger. On the other hand the collisions in which the mass of the spherical galaxy is more, result in the formation of a ring structure after the closest approach and the structure disappears by the end of the simulations.  相似文献   

18.
Kinoshita  D.  Watanabe  J.  Fukushima  H.  Sekiguchi  T.  Yamamoto  N.  Abe  S. 《Earth, Moon, and Planets》1997,77(3):265-269
A sequential imaging observation of the ion tail of Comet C/Hale-Bopp 1995O1 was carried out in February–March 1997 with a wide-field CCD imaging camera using narrow band filters for two ion species; CO+ and H2O+ along with those for blue and red continuum. From the surface photometry of the ion tail of two species, we derived a relationship between plasma density and distance from the nucleus. The local velocity of the ion flow as a function of the distance from the nucleus was also estimated on the basis of some assumptions. We report preliminary results of our analysis, and discuss some characteristics of cometary plasma and its interaction with interplanetary magnetic field (IMF). May the source be with you! This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

19.
The chemistry of both nitrogen and sulfur presents interesting problems in comets.In this paper, we use a model of cometary comae with gas-phase chemical kineticsand gas dynamics to predict molecular abundances in the inner coma region for twoof the brightest comets in the past 20 years, Hyakutake (C/1996 B2) and Hale–Bopp(C/1995 O1). In this progress report we concentrate on the gas-phase chemistry of thenitrogen sulfide (NS) radical at a heliocentric distance of 1 AU to study the abundanceof NS using a detailed photo and chemical reaction network with over 100 species andabout 1000 reactions. The results are compared with recent observations of CometHale–Bopp and reveal that conventional gas-phase reactions schemes do not produceNS in sufficient quantities to explain the observations. We plan to continue therefinement of the model to improve agreement with observational constraints.  相似文献   

20.
A new evaluation of chemical evolution coefficients has been made using recent stellar evolution and nucleosynthesis data. The role of the low and intermediate mass stars in galactic nucleosynthesis has been emphasized. A significant amount of4He,12C and neutron-rich species is found to be contributed by these stars. Comparison with observed abundances suggests a primary origin of14N. The simple model of galactic evolution with the new coefficients has been used to derive the ratio of helium to heavy element enrichment in the Galaxy. The new stellar evolution data do not explain the large value of this ratio that has been determined observationally.  相似文献   

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