Laboratory Modeling of Standing Shocks and Radiatively Cooled Jets with Angular Momentum     

D. J. Ampleford  S. V. Lebedev  A. Ciardi  S. N. Bland  S. C. Bott  G. N. Hall  N. Naz  C. A. Jennings  M. Sherlock  J. P. Chittenden  A. Frank  E. Blackman 《Astrophysics and Space Science》2007,307(1-3):51-56

Collimated flows ejected from young stars are believed to play a vital role in the star formation process by extracting angular momentum from the accretion disk. We discuss the first experiments to simulate rotating radiatively cooled, hypersonic jets in the laboratory. A modification of the conical wire array $z$-pinch is used to introduce angular momentum into convergent flows of plasma, a jet-forming standing shock and into the jet itself. The rotation of the jet is evident in laser imaging through the presence of discrete filaments which trace the rotational history of the jet. The presence of angular momentum results in a hollow density profile in both the standing conical shock and the jet.  相似文献   

The origin and distribution of the Centaur population     

Romina P. Di Sisto  Adrián Brunini 《Icarus》2007,190(1):224-235

We analyze the Centaur population as a group of objects with perihelion distances (q) of less than 30 AU and heliocentric distances outside the orbit of Jupiter, formed by objects entering this region from the Scattered Disk (SD). We perform a numerical integration of 95 real Scattered Disk Objects (SDOs) extracted from the Minor Planet Center database and of 905 synthetic SDOs compensating for observational biases. SDOs have in the Centaur zone a mean lifetime of 72 Myr, though this number falls with a decrease of q. After this incursion, 30% of them enter the zone interior to Jupiter's orbit. We find that the contribution to the Centaur population from the SD gives a total of ∼2.8×¹⁰⁸ Centaurs with a radius R>1 km. We also propose a model for the intrinsic distribution of orbital elements of Centaurs and their distance and apparent magnitude distribution.  相似文献   

Albedo and size determination of potentially hazardous asteroids: (99942) Apophis     

M. Delbò  A. Cellino 《Icarus》2007,188(1):266-269

The near-Earth object (99942) Apophis will make an extremely close approach to the Earth in 2029, and currently has approximately a one-in-45,000 chance of impacting our planet in 2036 (JPL Sentry, November 2006). Computation of the orbital evolution of this object is limited by insufficient knowledge of physical properties required to determine the role played by non-gravitational effects. Using polarimetric observations, we have obtained the first reliable determination of the albedo of Apophis, obtaining 0.33±0.08. We also derive an updated estimate of the asteroid's absolute magnitude: H=19.7±0.4. Using this albedo and H, we find that Apophis has a diameter of 270±60 m, slightly smaller than preliminary estimates based upon an assumed albedo. Our observations demonstrate the feasibility of polarimetric observations aimed at obtaining albedos and sizes of small, potentially hazardous asteroids.  相似文献   

利用MUSIC算法分析BL Lac天体S5 0716+714光变周期     

唐洁 《天文学报》2012,53(1):1-8

将基于多重信号分类的MUSIC谱估计算法引入BL Lac天体光变周期分析中.给出了MUSIC算法的基本原理,利用模拟信号检测了算法的频谱分辨率.从大量文献中收集了BL Lac天体S5 0716+714光学V、R、I 3个波段从1994年到2008年的有效观测数据,用MUSIC算法和平均周期图算法分别计算了它们的光变周期,发现存在两个主要光变周期:一个是(3.33±0.08)yr的周期,另一个是(1.24±0.01)yr的周期.对这两种算法的周期估计性能进行了比较,结果表明,MUSIC谱估计算法对样本长度要求较低,具有良好的分辨特性和抗噪声能力,能提高在样本长度较短情况下光变周期分析的准确性.  相似文献   

Thermal inertia of near-Earth asteroids and implications for the magnitude of the Yarkovsky effect     

Marco Delbo'  Aldo dell'Oro  Stefano Mottola 《Icarus》2007,190(1):236-249

Thermal inertia determines the temperature distribution over the surface of an asteroid and therefore governs the magnitude the Yarkovsky effect. The latter causes gradual drifting of the orbits of km-sized asteroids and plays an important role in the delivery of near-Earth asteroids (NEAs) from the main belt and in the dynamical spreading of asteroid families. At present, very little is known about the thermal inertia of asteroids in the km size range. Here we show that the average thermal inertia of a sample of NEAs in the km-size range is . Furthermore, we identify a trend of increasing thermal inertia with decreasing asteroid diameter, D. This indicates that the dependence of the drift rate of the orbital semimajor axis on the size of asteroids due to the Yarkovsky effect is a more complex function than the generally adopted D⁻¹ dependence, and that the size distribution of objects injected by Yarkovsky-driven orbital mobility into the NEA source regions is less skewed to smaller sizes than generally assumed. We discuss how this fact may help to explain the small difference in the slope of the size distribution of km-sized NEAs and main-belt asteroids.  相似文献   

The orbit, mass, size, albedo, and density of (65489) Ceto/Phorcys: A tidally-evolved binary Centaur     

W.M. Grundy  J.A. Stansberry  D.C. Stephens  D.E. Trilling  J.R. Spencer  H.F. Levison 《Icarus》2007,191(1):286-297

Hubble Space Telescope observations of Uranus- and Neptune-crossing object (65489) Ceto/Phorcys (provisionally designated 2003 FX₁₂₈) reveal it to be a close binary system. The mutual orbit has a period of 9.554±0.011 days and a semimajor axis of 1840±48 km. These values enable computation of a system mass of (5.41±0.42)×¹⁰¹⁸ kg. Spitzer Space Telescope observations of thermal emission at 24 and 70 μm are combined with visible photometry to constrain the system's effective radius and geometric albedo . We estimate the average bulk density to be , consistent with ice plus rocky and/or carbonaceous materials. This density contrasts with lower densities recently measured with the same technique for three other comparably-sized outer Solar System binaries (617) Patroclus, (26308) 1998 SM₁₆₅, and (47171) 1999 TC₃₆, and is closer to the density of the saturnian irregular satellite Phoebe. The mutual orbit of Ceto and Phorcys is nearly circular, with an eccentricity ?0.015. This observation is consistent with calculations suggesting that the system should tidally evolve on a timescale shorter than the age of the Solar System.  相似文献   

Digging into the surface of the icy dwarf planet Eris     

M.R. Abernathy  W.M. Grundy  D. Cornelison 《Icarus》2009,199(2):520-1305

We describe optical spectroscopic observations of the icy dwarf planet Eris with the 6.5-m MMT telescope and the Red Channel Spectrograph. We report a correlation, that is at the edge of statistical significance, between blue shift and albedo at maximum absorption for five methane ice bands. We interpret the correlation as an increasing dilution of methane ice with another ice component, probably nitrogen, with increasing depth into the surface. We suggest a mechanism to explain the apparent increase in nitrogen with depth. Specifically, if we are seeing Eris 50 degrees from pole-on [Brown, M.E., Schaller, L., 2008. Science 316, 1585], the pole we are seeing now at aphelion was in winter darkness at perihelion. Near perihelion, sublimation could have built up atmospheric pressure on the sunlit (summer) hemisphere sufficient to drive winds toward the dark (winter) hemisphere, where the winds would condense. Because nitrogen is more volatile and scarcer than methane, it sublimated from the sunlit hemisphere relatively early in the season, so the early summer atmosphere was nitrogen rich, and so was the ice deposited on the winter pole. Later in the season, much of the nitrogen was exhausted from the summer pole, but there was plenty of methane, which continued to sublimate. At this point, the atmosphere was more depleted in nitrogen, as was the ice freezing out on top of the earlier deposited nitrogen rich ice. Our increasing nitrogen abundance with depth apparently contradicts the Licandro et al. [Licandro, J., Grundy, W.M., Pinilla-Alonso, N., Leisy, P., 2006. Astron. Astrophys. 458, L5-L8] result of a decreasing nitrogen abundance with depth. A comparison of observational, data reduction, and analysis techniques between the two works, suggests the difference between the two works is real. If so, we may be witnessing the signature of weather on Eris. The work reported here is intended to trigger further observational effort by the community.  相似文献   

Corona-like atmospheric escape from KBOs: I. Gas dynamics     

Amit Levi 《Icarus》2009,202(2):681-693

We show that for low temperatures (T∼30 K) and small, but non-negligible, gravitational fields the hydrodynamic escape of gas can be treated by Parker's theory of coronal expansion [Parker, E.N., 1963. Interplanetary Dynamical Processes. Interscience Publishers, New York]. We apply this theory to gas escape from Kuiper belt objects. We derive limits on the density and radius of the bodies for which this theory is applicable, and show how the flow depends on the mean molecular weight and internal degrees of freedom of the gas molecules. We use these results to explain the CH₄ dichotomy seen on KBOs [Schaller, E.L., Brown, M.E., 2007. Astrophys. J., 659, L61-L64].  相似文献   

Thermal evolution of Kuiper belt objects, with implications for cryovolcanism     

Steven J. Desch  Jason C. Cook  Simon B. Porter 《Icarus》2009,202(2):694-714

We investigate the internal thermal evolution of Kuiper belt objects (KBOs), small (radii <1000 km), icy (mean densities ) bodies orbiting beyond Neptune, focusing on Pluto's moon Charon in particular. Our calculations are time-dependent and account for differentiation. We review evidence for ammonia hydrates in the ices of KBOs, and include their effects on the thermal evolution. A key finding is that the production of the first melt, at the melting point of ammonia dihydrate, ≈176 K, triggers differentiation of rock and ice. The resulting structure comprises a rocky core surrounded by liquids and ice, enclosed within a >100-km thick undifferentiated crust of rock and ice. This structure is especially conducive to the retention of subsurface liquid, and bodies the size of Charon or larger (radii >600 km) are predicted to retain some subsurface liquid to the present day. We discuss the possibility that this liquid can be brought to the surface rapidly via self-propagating cracks. We conclude that cryovolcanism is a viable process expected to affect the surfaces of large KBOs including Charon.  相似文献   

High albedos of low inclination Classical Kuiper belt objects     

M.J. Brucker  W.M. Grundy  J.R. Spencer  E.I. Chiang 《Icarus》2009,201(1):284-496

We present observations of thermal emission from fifteen transneptunian objects (TNOs) made using the Spitzer Space Telescope. Thirteen of the targets are members of the Classical population: six dynamically hot Classicals, five dynamically cold Classicals, and two dynamically cold inner Classical Kuiper belt objects (KBOs). We fit our observations using thermal models to determine the sizes and albedos of our targets finding that the cold Classical KBOs have distinctly higher visual albedos than the hot Classicals and other TNO dynamical classes. The cold Classicals are known to be distinct from other TNOs in terms of their color distribution, size distribution, and binarity fraction. The Classical objects in our sample all have red colors yet they show a diversity of albedos which suggests that there is not a simple relationship between albedo and color. As a consequence of high albedos, the mass estimate of the cold Classical Kuiper belt is reduced from approximately 0.01 M_⊕ to approximately 0.001 M_⊕. Our results also increase significantly the sample of small Classical KBOs with known albedos and sizes from 21 to 32 such objects.  相似文献