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1.
We examine the potential contamination of cometary nuclei through impacts from asteroidal origin meteoroids. The paper uses
a simple model and has the goal of determining whether asteroidal contamination is potentially significant. We assume a meteoroid
power law mass distribution with index values in the range from s=1.83 to s=2.09. We used maximum and minimum models which we believe will bracket the true meteoroid mass distribution. We identify
those comets which are expected to be most significantly contaminated, and find values of up to 3.6 kg of asteroidal meteoroid
impact per square meter of the cometary surface per orbital revolution. This is less than the expected mass loss per perihelion
passage for most comets. Therefore any remnant effects of the contamination will depend on the penetration depth of the meteoroids
in the cometary nucleus, and possibly on the distribution of active and inactive areas on cometary nuclei. We present a simple
model which suggests that even small meteoroids will embed relatively deeply into a cometary nucleus. 相似文献
2.
本文讨论了彗星中尘埃粒子的充电机制 ,带电特性和平衡电势的变化规律 ,分析了彗星尘埃的破碎特性和临界半径 ,得出了很有意义的结果 相似文献
3.
Polarimetric observations of the light scattered by dust have been carried out at Pic-du-Midi Observatory with the 2 m telescope
in June and September–October 1996, and at Haute-Provence Observatory with the 0.80 m telescope in April 1997. They cover
a total number of 11 nights and a large (6.9°–47.7°) phase angle range. The spatial resolution allows to underline structures
in the coma, as well in the brightness images as in the polarization maps, with a correlation between the regions of bright
structures and the regions of higher polarization. A clear difference appears between the sunward and antisunward side, with
higher polarization on the antisunward side. The phase angle coverage allows us to obtain a polarimetric phase curve for the
whole coma and to compare it with other cometary phase curves. The degree of polarization is higher for Hale-Bopp than for
the comets previously observed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
4.
天体物理环境中的硅酸盐尘粒 总被引：1，自引：0，他引：1
硅酸盐尘粒是宇宙尘埃的主要成分之一,它广泛存在于许多天体物理环境中,其特性随环境而变化。由于近年来观测数据的不断增加和红外光谱质量的逐步提高,宇宙空间中的硅酸盐尘粒正受到越来越多的关注．该文详细地介绍了在各种天体环境(星际空间、演化晚期恒星的星周尘埃包层、绕年轻恒星和主序星的星周尘埃盘、彗星的彗发和行星际空间)中的硅酸盐尘粒的观测特征,并分别对其物理和化学性质进行了综合比较．观测已经证实在星际尘埃演化的前身(演化晚期恒星的星周尘埃包层)和其遗迹(彗星)均有可观数量的结晶硅酸盐存在。但是至今还没有在其中间态(弥散星际介质)找到结晶硅酸盐存在的证据。这一尚未解决的难题突出了结晶态硅酸盐在天体物理研究中的重要意义。 相似文献
5.
Shulman L. M. Kidger M. Serrat-Ricart M. Torres-Chico R. 《Earth, Moon, and Planets》2002,90(1-4):109-117
At the First International Conference on Comet Hale–Bopp(Canaries, 1998) many astronomers agreed with the value of theperiod
of axial rotation of its nucleus, which was obtained byobservers by a comparison of the two sequential patterns of thedust
arcs. It was noted that the pattern had been repeated each∼12 hours. The value 11.35h was taken as the periodof axial rotation. However, alternative periods of 4 and 8 days havealso been suggested. The possible
reasons for thisdiscrepancy are discussed. It is shown that a short period could be obtained by a misidentification of the
same arc on twosequential images and, as a result, an overestimate of theradial velocity of dust in the arcs. To obtain the
half day period one must take the projected velocity of the arc ∼ 1.6 km/s,which exceeds the H2O gas expansion velocity. If one takes a lower expansion velocity a longer period would be obtained. 相似文献
6.
H.-U. Kaeufl T. Bonev H. Boehnhardt Y. R. Fernandez C. Lisse 《Earth, Moon, and Planets》2005,97(3-4):331-339
Comet 9P/Tempel 1, the target of the Deep Impact mission, has been intensively observed for a long time period before the encounter. Pre-impact ground based monitoring of the comet was an important prerequisite for the success of the first space experiment in which a comet is treated by an artificial impact. It provided the background data needed to disentangle the features caused by the impact from variations caused by the natural activity of the comet. In this paper we present results from the ESO-monitoring of the comet, conducted in the thermal infrared and optical spectral ranges during several months before the Deep Impact encounter with the comet. 相似文献
7.
BRAUNSTEIN M. COMSTOCK R. HOFFMAN P. WOMACK M. DEGLMAN F. PINNICK D. AAKER G. GOLDSCHEN M. JACOBSON A. ZILKA J. FAITH D. MOORE S. RICOTTA J. WEIST A. MODI C. 《Earth, Moon, and Planets》1997,78(1-3):219-227
An optical archive of comet Hale-Bopp was obtained over 28 nights from June 1996 to May 1997 covering a range of heliocentric
distances of r = 4.12 − 0.91. The CCD images were obtained with the 0.8-m University of Washington Manastash Ridge Observatory
and the 0.25-m Pennsylvania State University at Eriemobile observatory. The reduced images clearly document the evolution
of features of the coma, which were dominated by radial jetsfrom r = 4.1 − 2.3 AU, curved fan-shaped jets from r = 1.2 − 1.0
AU, and arcs or shells for 0.9 < r < 1.0 AU. We present the images and derive an average dust expansion velocity of Vdust = 0.67 ± 0.07 km s−1 from measurements of the arc features. The images in thearchive will be made available to other researchers for further study.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
8.
W.M. Napier 《Celestial Mechanics and Dynamical Astronomy》1997,69(1-2):59-75
The orbit of the solar system within the Galaxy is subject both to adiabatic tidal forces and to irregular buffeting caused
by encounters with molecular clouds and spiral arms, while the long-period comet flux varies with the differential of these
forces. Quasi-periodic episodes of Earth bombardment by Oort cloud comets are expected, with the half-period of the solar
vertical oscillations being prominent. Various terrestrial datasets are examined and show evidence of a periodicity ∼27±1
Myr at a confidence level ≳ 99.5%, consistently with the expectations. Bombardment episodes thus appear to exert a controlling
influence on global terrestrial processes. Conversely, the long-term terrestrial record provides an empirical constraint on
the source regions of NEOs as well as on the dark matter content of the Galactic disc. During such episodes, climatic effects
due to stratospheric dusting by very large comets are likely to be a major factor in geological and biological change.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
9.
The discovery of C/1995 O1 (Hale-Bopp) at 7 AU from the Sun provided the first opportunity to follow the activity of a bright
comet over a large range of heliocentric distances rh. Production rates of a number of parent molecules and daughter species have been monitored both pre- and postperihelion.
CO was found to be the major driver of the activity far from the Sun, surpassed by water within 3 AU whose production rate
reached 1031 s−1 at perihelion. Gas production curves obtained for various species show several behaviours with rh.
Gas production curves contain important information concerning the physical state of cometary ices, the structure of the nucleus
and all the processes taking place inside the nucleus leading to outgassing. They are relevant to the study of several other
phenomena such as the sublimation from icy grains, dust mantling or seasonal effects. For some species, such as H2CO or HNC, they permit to constrain their origin in the coma.
We discuss models of subsurface gas production in distant comets and predictions of how such a source may vary as the comet
moves along its orbit, approaching perihelion and receding again. Features in the observed gas production curves of comet
Hale-Bopp are generally interpretable in terms of either subsurface production (typical example: CO at large rh) or free sublimation (typical example: H2O). Possible implications for the vertical stratification of the cometary ices are reviewed, and preference is found for a
model with crystallization of amorphous ice close to the nuclear surface.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
10.
An active region on the surface of a cometarynucleus is considered as a conic hole in the surface dust mantle with icy bottom
and dusty side-walls. This conic structureshould concentrate solar energy onto the bottom andtherefore enhance sublimation.
Preliminary results of thecalculation of this effect are given. The temperature distributionat the bottom of the crater is
calculated for different sets ofits geometrical parameters. Effects of intensified sublimation depending on the geometrical
parameters areconsidered for the specific case when a single active region islocated exactly at the pole of the nucleus and
the pole is directed tothe Sun. 相似文献