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521.
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
522.
Aerogel collectors have been deployed in low-Earth orbit to collect orbital debris and micrometeorites. An array of silica aerogel collectors is currently en-route back to Earth following an encounter with the Comet Wild-2 on board the Stardust spacecraft. Stardust is returning, for laboratory analysis, cometary and interstellar dust grains which impacted into the aerogel collectors at hypervelocities. While the morphology of impact craters in aerogels has been studied empirically, a theoretical understanding of the physical mechanisms responsible for the formation of impact craters in these solids is lacking. Here we propose and test a model of compaction driven impact cratering in aerogels. Our model derives impact crater dimensions directly from energy and momentum deposition. 相似文献
523.
524.
525.
J. R. Flenley A. Sarah M. King Joan Jackson C. Chew J. T. Teller M. E. Prentice 《第四纪科学杂志》1991,6(2):85-115
Easter Island occupies an exceptionally isolated position in the south Pacific Ocean. It is entirely volcanic, and is famous for its giant statues. Late Quaternary sediments have been investigated in three craters: Rano Raraku, Rano Aroi and Rano Kao, giving a continuous record over the past 30 Ka. Pollen records indicate that the island was formerly forested. Palynological and sedimentological evidence suggests a Late Pleistocene climate slightly cooler and drier than the present. Deforestation by people occurred mainly between 1200 and 800 yr BP. This may have led to an ecological disaster and to the decline of the megalithic civilization. The depauperation of the present native flora owes more to human activity than to isolation. 相似文献
526.
We present three sets of observations of n = 1 to n = 2 lines due to helium-like aluminium (Alxii), made during two solar flares (25 August, 1980 and 19 October, 1986), using the X-Ray Polychromator on the SMM satellite. The observed temperature-sensitive line ratio G is shown to be consistent with the close-coupling calculations of Keenan and McCann (1987), although the ratio R, which is both temperature and density-sensitive for lower-Z elements, is not sufficiently well determined from these data to say more than that the observed values of R are not inconsistent with the theoretical calculations. This region of the spectrum also includes the helium-like magnesium (Mgxi) 11
S - 31
P line, and it is shown that the ratio of this line to the Alxii resonance (11
S - 21
P) line is a more sensitive indicator of electron temperature than are the Alxii
G and R ratios. We demonstrate that the three ratios may be used together in order to derive values of emission measure, electron temperature and electron density during these flares. 相似文献
527.
The discovery last year of a planetary system orbiting a millisecond pulsar raises important questions in pulsar evolution, planet formation, and planetary dynamics. We review the literature concerning pulsar-planetary systems, emphasizing particularly the contributions to the meetingPlanets around Pulsars held at Caltech in 1992.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at Caltech, Pasadena, California, U.S.A. 相似文献
528.
529.
Studies of the flare-produced line feature at 1.9 Å due to highly ionized iron show that it is emitted in conditions closely approximating steady-state ionization equilibrium. Calculations of the line flux per unit emission measure from time-dependent and steady-state ionization equilibria are compared with observed values during four flares in particular. Only for electron densities N
e 1010 cm-3 do the time-dependent equilibrium values give as good an approximation to the observed values as the steady-state equilibrium. This lower limit is compared with values of N
e derived from analyses of the temperature decline in each of these events, and with estimates of N
e given by other workers.NAS/NRC Research Associate. 相似文献
530.