首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 65 毫秒
1.
Wink  J.  Bockelée-Morvan  D.  Despois  D.  Colom  P.  Biver  N.  Crovisier  J.  Gérard  E.  Lellouch  E.  Davies  J. K.  Dent  W. R. F.  Jorda  L. 《Earth, Moon, and Planets》1997,78(1-3):63-63
Comet C/1995 O1 (Hale-Bopp) has been observed on October 5 and 25, 1996 and from March 6 to March 22, 1997 with the Institut de Radioastronomie Millimétrique (IRAM) interferometer at Plateau de Bure (France). Millimetre lines of HCN,HNC, CO, H2CO, CH3OH, H2S, CS and SO were mapped with spatial resolutions of 1.5–3.5 arc sec. These observations allow us to investigate whether these species are released by the nucleus or produced in the coma by extended sources or photo-processes. The brightness distribution of the HCN J (1-0) line is consistent with release from the nucleus. The HNC J (1-0) distribution deviates from that of HCN in the innermost coma, and indicates production of HNC in the coma. This is in agreement with the heliocentric variation of the HNC/HCN ratio (Biver et al., 1997, Science 275, 1915; Irvine et al., 1998, this issue) and formation by chemical reactions (Rodgers and Charnley, 1998, Ap. J. 501, L227; Irvine et al., 1998, Nature 393, 547). There is clear evidence that SO is a photo dissociation product. The observations also confirm that H2CO is mainly produced by an extended source, as first evidenced in comet P/Halley. The contribution of the nucleus to the total H2CO production rate does not exceed 6%. The molecular lines have also been monitored hourly with the five antennas of the interferometer in single-dish mode. The line velocity shifts show aperiodic modulation linked to the nucleus rotation. The amplitude of the modulation differs from one species to another. The periodic modulation seen for the CO J (2-1) line on March 11 suggests that a significant fraction of CO is released continuously night and day by an active source situated at equatorial latitudes on the nucleus surface. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

2.
T.Y Brooke  H.A Weaver  G Chin  S.J Kim 《Icarus》2003,166(1):167-187
High resolution infrared spectra of Comet C/1995 O1 (Hale-Bopp) were obtained during 2-5 March 1997 UT from the NASA Infrared Telescope Facility on Mauna Kea, Hawaii, when the comet was at r≈1.0 AU from the Sun pre-perihelion. Emission lines of CH4, C2H6, HCN, C2H2, CH3OH, H2O, CO, and OH were detected. The rotational temperature of CH4 in the inner coma was Trot=110±20 K. Spatial profiles of CH4, C2H6, and H2O were consistent with release solely from the nucleus. The centroid of the CO emission was offset from that of the dust continuum and H2O. Spatial profiles of the CO lines were much broader than those of the other molecules and asymmetric. We estimate the CO production rate using a simplified outflow model: constant, symmetric outflow from the peak position. A model of the excitation of CO that includes optical depth effects using an escape probability method is presented. Optical depth effects are not sufficient to explain the broad spatial extent. Using a parent+extended-source model, the broad extent of the CO lines can be explained by CO being produced mostly (∼90% on 5 March) from an extended source in the coma. The CO rotational temperature was near 100 K. Abundances relative to H2O (in percent) were 1.1±0.3 (CH4), 0.39±0.10 (C2H6), 0.18±0.04 (HCN), 0.17±0.04 (C2H2), 1.7±0.5 (CH3OH), and 37-41 (CO, parent+extended source). These are roughly comparable to those obtained for other long-period comets also observed in the infrared, though CO appears to vary.  相似文献   

3.
High resolution interferometric COJ=1–0 observations of the outflows from two young embedded sources, TMC1 and TMC1A, show the high-velocity gas to have a conical structure, with a constant opening angle of 45° extending to within 1000 AU of the central stars. The correspondence of near-infrared reflection nebulosity atK band with blueshifted CO emission in both objects suggests the lobes are partially evacuated, as do position-velocity diagrams from single-dish COJ=2–1 data. We suggest that the outflows are driven by jets which impart momentum to the ambient medium through shocks, rather than through the entrainment of molecular material along the edges of the jet.The NRAO is operated by Associated Universities, Inc., under cooperative agreement with the National Science Foundation  相似文献   

4.
The H2CO production rates measured in Comet C/1995 O1 (Hale-Bopp) from radio wavelength observations [Biver, N., and 22 colleagues, 2002a. Earth Moon Planets 90, 5-14] showed a steep increase with decreasing heliocentric distance. We studied the heliocentric evolution of the degradation of polyoxymethylene (formaldehyde polymers: (CH2O)n, also called POM) into gaseous H2CO. POM decomposition can indeed explain the H2CO density profile measured in situ by Giotto spacecraft in the coma of Comet 1P/Halley, which is not compatible with direct release from the nucleus [Cottin, H., Bénilan, Y., Gazeau, M.-C., Raulin, F., 2004. Icarus 167, 397-416]. We show that the H2CO production curve measured in Comet C/1995 O1 (Hale-Bopp) can be accurately reproduced by this mechanism with a few percents by mass of solid POM in grains. The steep heliocentric evolution is explained by the thermal degradation of POM at distances less than 3.5 AU. This study demonstrates that refractory organics present in cometary dust can significantly contribute to the composition of the gaseous coma. POM, or POM-like polymers, might be present in cometary grains. Other molecules, like CO and HNC, might also be produced by a similar process.  相似文献   

5.
C.M. Lisse  K.E. Kraemer  A. Li 《Icarus》2007,187(1):69-86
Spitzer Infrared Spectrograph observations of the Deep Impact experiment in July 2005 have created a new paradigm for understanding the infrared spectroscopy of primitive solar nebular (PSN) material—the ejecta spectrum is the most detailed ever observed in cometary material. Here we take the composition model for the material excavated from Comet 9P/Tempel 1's interior and successfully apply it to Infrared Space Observatory spectra of material emitted from Comet C/1995 O1 (Hale-Bopp) and the circumstellar material found around the young stellar object HD 100546. Comparison of our results with analyses of the cometary material returned by the Stardust spacecraft from Comet 81P/Wild 2, the in situ Halley flyby measurements, and the Deep Impact data return provides a fundamental cross-check for the spectral decomposition models presented here. We find similar emission signatures due to silicates, carbonates, phyllosilicates, water ice, amorphous carbon, and sulfides in the two ISO-observed systems but there are significant differences as well. Compared to Tempel 1, no Fe-rich olivines and few crystalline pyroxenes are found in Hale-Bopp and HD 100546. The YSO also lacks amorphous olivine, while being super-rich in amorphous pyroxene. All three systems show substantial emission due to polycyclic aromatic hydrocarbons. The silicate and PAH material in Hale-Bopp is clearly less processed than in Tempel 1, indicating an earlier age of formation for Hale-Bopp. The observed material around HD 100546 is located ∼13 AU from the central source, and demonstrates an unusual composition due to either a very different, non-solar starting mix of silicates or due to disk material processing during formation of the interior disk cavity and planet(s) in the system.  相似文献   

6.
Biver  N.  Bockelée-Morvan  D.  Colom  P.  Crovisier  J.  Germain  B.  Lellouch  E.  Davies  J. K.  Dent  W. R. F.  Moreno  R.  Paubert  G.  Wink  J.  Despois  D.  Lis  D. C.  Mehringer  D.  Benford  D.  Gardner  M.  Phillips  T. G.  Gunnarsson  M.  Rickman  H.  Winnberg  A.  Bergman  P.  Johansson  L. E. B.  Rauer  H. 《Earth, Moon, and Planets》1997,78(1-3):5-11
C/1995 O1 (Hale-Bopp) has been observed on a regular basis since August 1995 at millimetre and submillimetre wavelengths using IRAM, JCMT, CSO and SEST radio telescopes. The production rates of eight molecular species (CO, HCN, CH3OH, H2CO,H2S, CS, CH3CN,HNC) have been monitored as a function of heliocentric distance(rh from 7 AU pre-perihelion to 4 AU post-perihelion. As comet Hale-Bopp approached and receded from the Sun, these species displayed different behaviours. Far from the Sun, the most volatile species were found in general relatively more abundant in the coma. In comparison to other species, HNC, H2CO and CS showed a much steeper increase of the production rate with decreasing rh. Less than 1.5 AU from the Sun, the relative abundances were fairly stable and approached those found in other comets near 1 AU. The kinetic temperature of the coma, estimated from the relative intensities of the CH3OH and CO lines, increased with decreasing rh, from about10 K at 7 AU to 110 K around perihelion. The expansion velocity of the gaseous species, derived from the line shapes, also increased with a law close torh 3. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

7.
Volatile organic emissions were detected post-perihelion in the long-period Comet C/2006 M4 (SWAN) in October and November 2006. Our study combines target-of-opportunity infrared observations using the Cryogenic Echelle Spectrometer (CSHELL) at the NASA-IRTF 3-m telescope, and millimeter wavelength observations using the Arizona Radio Observatory (ARO) 12-m telescope. Five parent volatiles were measured with CSHELL (H2O, CO, CH3OH, CH4, and C2H6), and two additional species (HCN and CS) were measured with the ARO 12-m. These revealed highly depleted CO and somewhat enriched CH3OH compared with abundances observed in the dominant group of long-period (Oort cloud) comets in our sample and similar to those observed recently in Comet 8P/Tuttle. This may indicate highly efficient H-atom addition to CO at very low temperature (∼10-20 K) on the surfaces of interstellar (pre-cometary) grains. Comet C/2006 M4 had nearly “normal” C2H6 and CH4, suggesting a processing history similar to that experienced by the dominant group. When compared with estimated water production at the time of the millimeter observations, HCN was slightly depleted compared with the normal abundance in comets based on IR observations but was consistent with the majority of values from the millimeter. The ratio CS/HCN in C/2006 M4 was within the range measured in ten comets at millimeter wavelengths. The higher apparent H-atom conversion efficiency compared with most comets may indicate that the icy grains incorporated into C/2006 M4 were exposed to higher H-atom densities, or alternatively to similar densities but for a longer period of time.  相似文献   

8.
Hofstadter  M. D.  Hartogh  P.  McMullin  J. P.  Martin  R. N.  Jarchow  C.  Peters  W. 《Earth, Moon, and Planets》1997,78(1-3):53-61
We observed submillimeter lines of H2CO and HCN in comet Hale-Bopp near perihelion. One of our goals was to search for short term variability. Our observations are suggestive, but not conclusive, of temporal and/or spatial changes in the coma's HCN/H2CO abundance ratio of ∼25%. If due to spatial variability, the ratio on the sunward side of the coma is enhanced over other regions. If due to temporal variability, we find the bulk ratio in the coma changed in less than 16 hours. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

9.
We discuss the composition and size distribution of the dust in the coma of Comet Hale-Bopp. We do this using a model fit for the infrared emission measured by the Infrared Space Observatory (ISO) and the measured degree of linear polarization of scattered light at various phase angles and wavelengths. The effects of particle shape on the modeled optical properties of the dust grains are taken into account. Both the short wavelength (7-44 μm) and the long wavelength (44-120 μm) infrared spectrum are fitted using the same dust parameters, as well as the degree of linear polarization at twelve different wavelengths in the optical to near-infrared domains. We constrain our fit by forcing the abundances of the major rock forming chemical elements to be equal to those observed in meteorites. The infrared spectrum at long wavelengths reveals that large grains are needed in order to fit the spectral slope. The size and shape distribution we employ allows us to estimate the sizes of the crystalline silicates. The ratios of the strength of various forsterite features show that the crystalline silicate grains in Hale-Bopp must be submicrometer-sized. On the basis of our analysis the presence of large crystalline silicate grains in the coma can be excluded. Because of this lack of large crystalline grains combined with the fact that we do need large amorphous grains to fit the emission spectrum at long wavelengths, we need only approximately 4% of crystalline silicates by mass (forsterite and enstatite) to reproduce the observed spectral features. After correcting for possible hidden crystalline material included in large amorphous grains, our best estimate of the total mass fraction of crystalline material is ∼7.5%, which is significantly lower than deduced in previous studies in which the typical derived crystallinity is ∼20-30%. The implications of this low abundance of crystalline material on the possible origin and evolution of the comet are discussed. We conclude that the crystallinity we observe in Hale-Bopp is consistent with the production of crystalline silicates in the inner Solar System by thermal annealing and subsequent radial mixing to the comet forming region (∼30 AU).  相似文献   

10.
We have performed millimeter- and submilli- meter-wave survey observations using the Nobeyama millimeter array (NMA) and the Atacama Submillimeter Telescope Experiment (ASTE) in one of the nearest intermediate-mass (IM) star-forming regions: Orion Molecular Cloud-2/3 (OMC-2/3). Using the high-resolution capabilities offered by the NMA (∼several arcsec), we observed dust continuum and H13CO+(1–0) emission in 12 pre- and proto-stellar candidates identified previously in single-dish millimeter observations. We unveiled the evolutionary changes with variations of the morphology and velocity structure of the dense envelopes traced by the H13CO+(1–0) emission. Furthermore, using the high-sensitivity capabilities offered by the ASTE, we searched for large-scale molecular outflows associated with these pre- and proto-stellar candidates observed with the NMA. As a result of the CO(3–2) observations, we detected six molecular outflows associated with the dense gas envelopes traced by H13CO+(1–0) and 3.3 mm continuum emission. The estimated CO outflow momentum increases with the evolutionary sequence from early to late type of the protostellar cores. We also found that the 24 μm flux increases as the dense gas evolutionary sequence. We propose that the enhancement of the 24 μm flux is caused by the growth of the cavity (i.e. the CO outflow destroys the envelope) as the evolutionary sequence. Our results show that the dissipation of the dense gas envelope plays an essential role in the evolution of the IM protostars. The extremely high-sensitivity and high-angular resolution offered by ALMA will reveal unprecedented details of the inner ∼50 AU of these protostars, which will provide us a break through in the classic scenario of IM star/disk formation.  相似文献   

11.
We observed submillimeter lines of H2CO and HCN in comet Hale-Bopp near perihelion. One of our goals was to search for short term variability. Our observations are suggestive, but not conclusive, of temporal and/or spatial changes in the coma's HCN/H2CO abundance ratio of ～25%. If due to spatial variability, the ratio on the sunward side of the coma is enhanced over other regions. If due to temporal variability, we find the bulk ratio in the coma changed in less than 16 hours.  相似文献   

12.
S.M. Lederer  H. Campins  D.J. Osip 《Icarus》2009,199(2):477-843
We describe a 3-dimensional, time-dependent Monte Carlo model developed to analyze the chemical and physical nature of a cometary gas coma. Our model includes the necessary physics and chemistry to recreate the conditions applicable to Comet Hale-Bopp when the comet was near 1 AU from the Sun. Two base models were designed and are described here. The first is an isotropic model that emits particles (parents of the observed gases) from the entire nucleus; the second is a jet model that ejects parent particles solely from discrete active areas on the surface of the comet nucleus, resulting in coma jets. The two models are combined to produce the final model, which is compared with observations. The physical processes incorporated in both base models include: (1) isotropic ejection of daughter molecules (the observed gases) in the parent's frame of reference, (2) solar radiation pressure, (3) solar insolation effects, (4) collisions of daughter products with other molecules in the coma, and (5) acceleration of the gas in the coma. The observed daughter molecules are produced when a parent decays, which is represented by either an exponential decay distribution (photodissociation of the parent gas) or a triangular distribution (production from a grain extended source). Application of this model to the analysis the OH, C2 and CN gas jets observed in the coma of Comet Hale-Bopp is the focus of the accompanying paper [Lederer, S.M., Campins, H., Osip, D.J., 2008. Icarus, in press (this issue)].  相似文献   

13.
Many new cometary molecules — both parents and daughters — were detected in the exceptionally productive comet C/1995 O1 (Hale-Bopp).The space distribution of several of these species could be investigated from radio interferometry or from long-slit spectroscopy in the infrared. The distinction between parent species — directly sublimated from nucleus ices — and secondary species — resulting from chemical processing in the coma or produced by a secondary source — is not always clear. It is important to assess whether or not observed minor species (HCOOCH3, HCOOH...) could be synthesized by chemical reactions favoured by the high density of the coma of comet Hale-Bopp. Chemical modelling by Rodgers and Charnley suggests that this is notthe case. CO and H2CO are abundant cometary species which partly come from distributed sources. The nature of these sources is still a mystery. A special case, now well documented, is that of HNC, for which the abundance evolution with heliocentric distance could be observed in comet Hale-Bopp and which was observed in several much less productive comets.  相似文献   

14.
We report on spectroscopic observations of periodic Comet 9P/Tempel 1 by the Alice ultraviolet spectrograph on the Rosetta spacecraft in conjunction with NASA's Deep Impact mission. Our objectives were to measure an increase in atomic and molecular emissions produced by the excavation of volatile sub-surface material. We unambiguously detected atomic oxygen emission from the quiescent coma but no enhancement at the 10% (1-σ) level following the impact. We derive a quiescent H2O production rate of 9×1027 molecules s−1 with an estimated uncertainty of ∼30%. Our upper limits to the volatiles produced by the impact are consistent with other estimates.  相似文献   

15.
On UT 2000 August 21 we obtained simultaneous visible and mid-infrared observations of Comet 9P/Tempel 1, the target of the upcoming NASA Discovery Program mission Deep Impact. The comet was still quite active while 2.55 AU from the Sun (post-perihelion). Two independent analyses of our data, one parameterizing the coma morphology and the other modeling infrared spectrophotometry, show that the nucleus's cross section at the time the data were taken corresponds to an effective radius of 3.0±0.2 km. Based on visible-wavelength photometry of the comet taken during this observing run and others in the summer of 2000, all of which show the rotational modulation of the nucleus's brightness, we find that the infrared data were obtained near the maximum of the light curve. If we assume that the nucleus's light curve had a peak-to-valley range of 0.6±0.2 mag, then the mean effective radius is 2.6±0.2 km. Visible-wavelength photometry of the nucleus, including data published by other groups, lets us constrain the nucleus's R-band geometric albedo: 0.072±0.016. The nucleus's flux contributed about 85% of the light in the mid-infrared images.  相似文献   

16.
Comet 73P-B/Schwassmann-Wachmann 3 was observed with IRCS/Subaru at geocentric distance of 0.074 AU on UT 10 May 2006. Multiple H2O emission lines were detected in non-resonant fluorescence near 2.9 μm. No significant variation in total H2O production rate was found during the (3 h) duration of our observations. H2O rotational temperatures and ortho-to-para abundance ratios were measured for several positions in the coma. The temperatures extracted from two different time intervals show very similar spatial distributions. For both, the rotational temperature decreased from ∼110 to ∼90 K as the projected distance from the nucleus increased from ∼5 to ∼30 km. We see no evidence for OPR change in the coma. The H2O ortho-para ratio is consistent with the statistical equilibrium value (3.0) for all spatially resolved measurements. This implies a nuclear spin temperature higher than ∼45 K.  相似文献   

17.
The analysis of the polarized light scattered by cometary dust particles provides information on the physical properties of the solid component of cometary comae for C/1995 O1 Hale-Bopp and 1P/Halley. A model of light scattering by a size distribution of aggregates of up to 256 submicron-sized grains (spherical or spheroidal) mixed with single spheroidal particles has been developed, with its parameters adjusted to fit the phase angle and wavelength dependence of the polarization observations. The particles are built of two materials: a non-absorbing silicates-type material and a more absorbing organic-type material. The model reproduces accurately the inversion angle and the positive branch of the polarization phase curves from the visible to the near-infrared spectral domains. A negative branch of the polarization phase curves appears in our model, although the negative branch is not deep enough to reproduce accurately the observations. Significant differences are shown between the two comets, with dominance of small grains in the coma of Comet C/1995 O1 Hale-Bopp, well fitted by a distribution of the volume-equivalent diameter, a, following a−3.0 with a lower cutoff around 0.20 μm and an upper cutoff of at least 40 μm. For 1P/Halley, the size distribution follows a−2.8 with a lower cutoff around 0.26 μm and an upper cutoff of about 38 μm. The relative amount of organic-type particles is larger for 1P/Halley while the amount of aggregates, significant for both comets, is larger for C/1995 O1 Hale-Bopp.  相似文献   

18.
Comet Hale-Bopp was imaged at wavelengths from 1.87 to 2.22 μm by HST/NICMOS in post-perihelion observations starting on UT 1997 August 27.95. Diffraction-limited (∼02) images were obtained at high signal-to-noise (∼1500) to probe the composition and dynamics of the inner coma and also the size and activity of the nucleus. The velocities of several unusual morphological features over a 1.7 h period, indicate that a significant outburst occurred 7.4 h prior to these images while the comet was at a heliocentric distance of 2.49 AU. Similar features are also apparent after re-analysis of pre-perihelion ground-based images. The inner coma (radius ?2500 km) is dominated by an “arc” feature, which expanded and became more diffuse with time. This feature can be modeled as the bright central portion of a “jet of outburst” from a near-equatorial region of the nucleus. Less prominent, time-variable linear and circular morphologies are also apparent. The expansion rates of both the arc feature and the circular morphologies imply a common origin and also suggest a grain size distribution with two broad maxima. In addition, several static linear features extend to the edge of the field of view (21,100 km). Radial brightness profiles are highly asymmetric and only approach a ρ−1 decline at distances ?15,000 km. Images in a narrow-band filter at 2.04 μm exhibit a ∼4% absorption feature relative to nearly simultaneous images at wavelengths of 2.22, 1.90, and 1.87 μm. This absorption is attributed to H2O ice in the coma grains. The spatial distribution and expansion velocity of the absorption at 2.04 μm indicate that these grains are associated with the outburst. The constancy of the absorption feature indicates no appreciable sublimation over 1.7 h. The unresolved nucleus has a flux density consistent with a 40±10 km diameter assuming a 4% geometric albedo.  相似文献   

19.
Comet 29P/Schwassmann-Wachmann 1 has been studied during seven days in August 1998 with the SEST submillimeter telescope at ESO, La Silla, Chile. The CO (J=2−1) emission at 230 GHz was mapped by directing the telescope beam at the nucleus and six off-nucleus positions. The CO line profiles exhibit the blue- and redshifted components previously observed by various observers. The strength of the observed lines does not decrease with projected distance to the nucleus as expected if CO molecules were coming from the nucleus only. Instead, the line area is nearly constant throughout the map. This can be explained if CO molecules are being released from both the sunlit side of the nucleus and CO-bearing particles distributed in a shell-like cloud. The extended source must consist of icy grains globally moving toward the Sun at ∼50 m s−1 released ∼30 days before the observations were made. The nuclear and extended sources produce (7±1)×1027 and 2.4×1028 molecules s−1, respectively. Our 1996 observations of the comet (Festou, M., M. Gunnarsson, A. Winnberg, H. Rickman, and G. Tancredi 2001. Icarus150, 140-150) were reexamined using this new two-source model. In this case, the nuclear and extended CO sources produced 10±1×1027 and 2.9×1028 CO molecules s−1, respectively. It is not necessary to postulate night side outgassing, but a large quantity of solid grains has to be expelled into the coma.  相似文献   

20.
E.L. Gibb  M.J. Mumma  M.A. DiSanti 《Icarus》2003,165(2):391-406
We detected CH4 in eight Oort cloud comets using high-dispersion (λ/Δλ∼2×104) infrared spectra acquired with CSHELL at NASA's IRTF and NIRSPEC at the W.M. Keck Observatory. The observed comets were C/1995 O1 (Hale-Bopp), C/1996 B2 (Hyakutake), C/1999 H1 (Lee), C/1999 T1 (McNaught-Hartley), C/1999 S4 (LINEAR), C/2000 WM1 (LINEAR), C/2001 A2 (LINEAR), and 153/P Ikeya-Zhang (C/2002 C1). We detected the R0 and R1 lines of the ν3 vibrational band of CH4 near 3.3 μm in each comet, with the exception of McNaught-Hartley where only the R0 line was measured. In order to obtain production rates, a fluorescence model has been developed for this band of CH4. We report g-factors for the R0 and R1 transitions at several rotational temperatures typically found in comet comae and relevant to our observations. Using g-factors appropriate to Trot as determined from HCN, CO and/or H2O and C2H6, CH4 production rates and mixing ratios are presented. Abundances of CH4/H2O are compared among our existing sample of comets, in the context of establishing their place of origin. In addition, CH4 is compared to native CO, another hypervolatile species, and no correlation is found among the comets observed.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号