共查询到20条相似文献,搜索用时 12 毫秒
1.
J. M. Kirk D. Ward-Thompson R. M. Crutcher 《Monthly notices of the Royal Astronomical Society》2006,369(3):1445-1450
We have mapped linearly polarized dust emission from the pre-stellar cores L1498 and L1517B with the James Clerk Maxwell Telescope (JCMT) using the Submillimetre Common User Bolometer Array (SCUBA) and its polarimeter (SCUBAPOL) at a wavelength of 850 μm. We use these measurements to determine the plane-of-sky magnetic field orientation in the cores. In L1498, we see a magnetic field across the peak of the core that lies at an offset of ∼19°± 12° to the short axis of the core. This is similar to the offsets seen in previous observations of pre-stellar cores. To the south-east of the peak, in the filamentary tail of the core, we see that the magnetic field has rotated to lie almost parallel to the long axis of the filament. We hypothesize that the field in the core may have decoupled from the field in the filament that connects the core to the rest of the cloud. We use the Chandrasekhar–Fermi (CF) method to measure the plane-of-sky field strength in the core of L1498 to be ∼10 ± 7 μG.
In L1517B, we see a more gradual turn in the field direction from the northern part of the core to the south. This appears to follow a twist in the filament in which the core is buried, with the field staying at a roughly constant ∼25°± 6° offset to the short axis of the filament, consistent with previous observations of pre-stellar cores. Hence these two clouds in an apparently similar evolutionary state, that exhibit similar masses, morphologies and densities, have very different magnetic field configurations. We again use the CF method and calculate the magnetic field strength in L1517B to be ∼30 ± 10 μG. Both cores appear to be roughly virialized. Comparison with our previous work on somewhat denser cores shows that, for the denser cores, thermal and non-thermal (including magnetic) support are approximately equal, while for the lower density cores studied here, thermal support dominates. 相似文献
In L1517B, we see a more gradual turn in the field direction from the northern part of the core to the south. This appears to follow a twist in the filament in which the core is buried, with the field staying at a roughly constant ∼25°± 6° offset to the short axis of the filament, consistent with previous observations of pre-stellar cores. Hence these two clouds in an apparently similar evolutionary state, that exhibit similar masses, morphologies and densities, have very different magnetic field configurations. We again use the CF method and calculate the magnetic field strength in L1517B to be ∼30 ± 10 μG. Both cores appear to be roughly virialized. Comparison with our previous work on somewhat denser cores shows that, for the denser cores, thermal and non-thermal (including magnetic) support are approximately equal, while for the lower density cores studied here, thermal support dominates. 相似文献
2.
The origin of Jupiter and the Galilean satellite system is examinedin the light of the new data that has been obtained by
the NASA Galileo Project. In particular, special attention is given to a theory of satellite origin which was put forward
at the start of the Galileo Mission and on the basis of which several predictions have now been proven successful (Prentice,
1996a–c). These predictions concern the chemical composition of Jupiter's atmosphere and the physical structure of the satellites.
According to the proposed theory of satellite origin, each of the Galilean satellites formed by chemical condensation and
gravitational accumulation of solid grains within a concentricfamily of orbiting gas rings. These rings were cast off equatorially
by the rotating proto-Jovian cloud (PJC) which contracted gravitationally to form Jupiter some 4
billion years ago. The PJC formed from the gas and grains left over from the gas ring that had been shed at Jupiter's orbit
by the contracting proto-solar cloud (PSC). Supersonic turbulentconvection provides the means for shedding discrete gas rings.The
temperatures Tn of the system of gas rings shed by the PSCand PJC vary with their respective mean orbital radii Rn (n = 0, 1, 2, Ϊ ) according as Tn ∝ Rn
-0.9. If the planet Mercury condenses at 1640 K, so accounting for the high density ofthat planet via a process of chemical fractionation
between iron and silicates, then Tn at Jupiter's orbit is 158 K. Only 35% of the water vapour condenses out. Thus fractionation between rock and ice, together
with an enhancement in the abundance of solids relative to gas which takes place through gravitational sedimentation of solids
onto the mean orbit of the gas ring, ensures nearly equal proportions of rock and ice in each of Ganymede and Callisto. Io
and Europa condense above the H2O ice point and consist solely of hydrated rock (h-rock). The Ganymedan condensate consists of h-rock and H2O ice. For Callisto, NH3 ice makes up ∼5% of the condensate mass next to h-rock (∼50%) and H2O ice (∼45%).
Detailed thermal and structural models for each of Europa, Ganymedeand Callisto are constructed on the basis of the above
initial bulk chemicalcompositions. For Europa (E), a predicted 2-zone model consisting of a dehydrated rock core of mass 0.912ME and a 150 km thick frozen mantle of salty H2O yields a moment-of-inertiacoefficient which matches the Galileo Orbiter gravity measurement. For Ganymede (G), a 3-zone
model possessing an inner core of solid FeS and mass ∼0.116MG, and an outer H2O ice mantle of mass ∼0.502MG is needed to explain the gravity data.Ganymede's native magnetic field was formed by thermoremanent magnetization of Fe3O4. A new Callisto (C) model is proposed consisting of a core of mass 0.826MC containing a uniform mixture of h-rock (60% by mass) and H2O and NH3 ices, and capped by a mantle of pure ice. This model may have the capacity to yield a thin layer of liquid NH3ċ2H2O at the core boundary, in line with Galileo's discovery of an induced magnetic field
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
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T. A. Schad 《Solar physics》2014,289(5):1477-1498
We study 7530 sunspot umbrae and pores measured by the Hinode Spectropolarimeter (SP) between November 2006 and November 2012. We primarily seek confirmation of the long term secular decrease in the mean magnetic field strength of sunspot umbrae found by Penn and Livingston (IAU Symp. 273, 126, 2011) between 1998 and 2011. The excellent SP photometric properties and full vector magnetic field determinations from full-Stokes Milne–Eddington inversions are used to address the interrelated properties of the magnetic field strength and brightness temperature for all umbral cores. We find non-linear relationships between magnetic field strength and umbral temperature (and continuum contrast), as well as between umbral radius and magnetic field strength. Using disambiguated vector data, we find that the azimuths measured in the umbral cores reflect an organization weakly influenced by Joy’s law. The large selection of umbrae displays a log-normal size spectrum similar to earlier solar cycles. Influenced by the amplitude of the solar cycle and the non-linear relationship between umbral size and core magnetic field strength, the distribution of core magnetic field strengths, fit most effectively with a skew-normal distribution, shows a weak solar cycle dependence. Yet, the mean magnetic field strength does not show a significant long term trend. 相似文献
6.
Using eigenmode expansion of the Mark III and SFI surveys of cosmological radial velocities, a goodness-of-fit analysis is applied on a mode-by-mode basis. This differential analysis complements the Bayesian maximum likelihood analysis that finds the most probable model given the data. Analyzing the surveys with their corresponding most likely models from the CMB-like family of models, as well as with the currently popular LambdaCDM model, reveals a systematic inconsistency of the data with these "best" models. There is a systematic trend of the cumulative chi(2) to increase with the mode number (where the modes are sorted by decreasing order of the eigenvalues). This corresponds to a decrease of the chi(2) with the variance associated with a mode and hence with its effective scale. It follows that the differential analysis finds that on small (large) scales the global analysis of all the modes "puts" less (more) power than actually required by the data. This observed trend might indicate one of the following: (1) the theoretical model (i.e., power spectrum) or the error model (or both) have an excess of power on large scales, (2) velocity bias, or (3) the velocity data suffers from systematic errors that have not yet been corrected. 相似文献
7.
Physical, Chemical, and Mineralogical Properties of Comet 81P/Wild 2 Particles Collected by Stardust
George James Flynn 《Earth, Moon, and Planets》2008,102(1-4):447-459
NASA’s Stardust spacecraft collected dust from the coma of Comet 81P/Wild 2 by impact into aerogel capture cells or into Al-foils.
The first direct, laboratory measurement of the physical, chemical, and mineralogical properties of cometary dust grains ranging
from <10−15 to ∼10−4 g were made on this dust. Deposition of material along the entry tracks in aerogel and the presence of compound craters in
the Al-foils both indicate that many of the Wild 2 particles in the size range sampled by Stardust are weakly bound aggregates
of a diverse range of minerals. Mineralogical characterization of fragments extracted from tracks indicates that most tracks
were dominated by olivine, low-Ca pyroxene, or Fe-sulfides, although one track was dominated by refractory minerals similar
to Ca–Al inclusions in primitive meteorites. Minor mineral phases, including Cu–Fe-sulfide, Fe–Zn-sulfide, carbonate and metal
oxides, were found along some tracks. The high degree of variability of the element/Fe ratios for S, Ca, Ti, Cr, Mn, Ni, Cu,
Zn, and Ga among the 23 tracks from aerogel capture cells analyzed during Stardust Preliminary Examination is consistent with
the mineralogical variability. This indicates Wild 2 particles have widely varying compositions at the largest size analyzed
(>10 μm). Because Stardust collected particles from several jets, sampling material from different regions of the interior
of Wild 2, these particles are expected to be representative of the non-volatile component of the comet over the size range
sampled. Thus, the stream of particles associated with Comet Wild 2 contains individual grains of diverse elemental and mineralogical
compositions, some rich in Fe and S, some in Mg, and others in Ca and Al. The mean refractory element abundance pattern in
the Wild 2 particles that were examined is consistent with the CI meteorite pattern for Mg, Si, Cr, Fe, and Ni to 35%, and
for Ca, Ti and Mn to 60%, but S/Si and Fe/Si both show a statistically significant depletion from the CI values and the moderately
volatile elements Cu, Zn, Ga are enriched relative to CI. This elemental abundance pattern is similar to that in anhydrous,
porous interplanetary dust particles (IDPs), suggesting that, if Wild 2 dust preserves the original composition of the Solar
Nebula, the anhydrous, porous IDPs, not the CI meteorites, may best reflect the Solar Nebula abundances. This might be tested
by elemental composition measurements on cometary meteors. 相似文献
8.
HongZhang Cheng-ZhiZhang 《中国天文和天体物理学报》2001,1(3):275-280
The recent Galileo spacecraft explored Jupiter and its satellite system and provided us with new geodetic data. In order to discuss the dynamical parameters and secular tidal effect of Io, the theory of synchronous satellite is described in detail. Using the new geodetic data of Io, two sets of Io‘s internal structure models are constructed based on the asthenosphere assumption. The liberation parameters α,β,γ and dynamical flattening H are calculated for the models of Io. A comparison of Io with the Moon indicates the they are quite different in many characteristics in spite of the fact that they are approximately equal in mass and size and that they both orbit synchronously. 相似文献
9.
Alan M. Wong Arch M. Reid Stuart A. Hall Virgil L. Sharpton 《Meteoritics & planetary science》2001,36(11):1443-1455
Abstract— In Leon County, Texas, USA, the Marquez Dome, an approximately circular 1.2 km diameter zone of disturbed Cretaceous rocks surrounded by shallow dipping Tertiary sediments, has been interpreted by Gibson and Sharpton (1989) and Sharpton and Gibson (1990) as the surface expression of a buried complex impact crater. New gravity and magnetic anomaly data collected over the Marquez Dome have been combined with well‐log and seismic reflection information to develop a better estimate of the overall geometry of the structure. A three‐dimensional model constructed to a depth of 2000 m from all available information indicates a complex crater 13 km in diameter with an uplift in the center of at least 1120 m. The zone of deformation associated with the cratering event is limited to a depth of <1720 m. No impact breccias were recovered in drilling at two locations, 1.1 and 2 km from the center of the structure, and the central uplift may be the only prominent remnant of this impact into unconsolidated, water‐rich sediments. The magnetic anomaly field shows no correlation with the location and extent of the structure. 相似文献
10.
徐兰平 《中国天文和天体物理学报》1987,(1)
利用暗星云L134的光学观测及分子谱线观测的结果,估算了它的某些基本物理参数,讨论了该星云的稳定性及能量问题。 发现在L134的演化过程中,热压力及转动在制约其自引力坍缩中起一定作用,但不是重要因素,而磁场的支撑作用可能是比较重要的,L134不是在自由下落时间尺度t_(ff)上引力坍缩,而可能是在一个比较长的双极扩散时间尺度t_D上收缩。 本文还计算了L134的冷却率及加热率,表明引力克服热压力作功不是暗星云L134的有效加热源;宇宙线是L134的一个加热源,它能提供所需能量的~20%;磁场通过双极扩散释放能量可能为L134提供了一个重要的加热源。 相似文献
11.
本文简要总结了自1990年8月至1994年1日在紫金山天文台13.7米射电望远镜上进行的水脉泽源的观测,观测结果汇集为表1和图1。在所有435个观测对象中共观测到195个水脉泽源,其中108个为新探测到的 本文对观测仪器和数据处理亦作了简要介绍。 相似文献
12.
Physical and Mechanical Properties of Lunar and Planetary Soils 总被引:3,自引:0,他引:3
Earth, Moon, and Planets - 相似文献
13.
郑兴武 《中国天文和天体物理学报》1995,(3)
利用VLA的观测,我们在猎户座分子云区,发现了11个小质量分子云浓核。它们的平均有效半径为0.03pc,平均质量为3.5M⊙,分布在呈丝状母云南北走向的轴线上。在浓核区3'的范围内,测到有5kms-1pc-1的速度梯度,一个可能的解释是这个核区的慢速转旋。根据我们的NH3(1,1)的观测资料,并与尘埃的毫米波连续辐射和红外辐射比较,我们认为这个区域的大部浓核是还没有星核的年轻分子云核,正处于热动能与自引力的平衡阶段。 相似文献
14.
Hong Zhang 《Celestial Mechanics and Dynamical Astronomy》2003,87(1-2):189-195
Radio Doppler data generated by the Deep Space Network (DSN) from the recent encounters of the Galileo spacecraft with the Galilean satellites have been used to determine the mass (GM) and unnormalized quadruple gravity coefficients in the external gravitational fields of the Galilean satellites. Therefore, a series of internal structure models, which satisfy the given constraints for the mean density and the mean moment of inertia, can be presented by solving Emden equations. And some dynamical parameters can also be calculated based on the density distributions given by internal structure models. 相似文献
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截止到2014年4月21日,已发现了1490多颗系外行星和3705颗Kepler候选体。这从观测角度证明了行星在银河系中是普遍存在的。对系外行星的研究丰富并加深了人们对行星形成与演化的认识。另外,新的观测与发现也不断提出新的科学问题。本论文开展了类地行星的形成演化、内部结构以及大气逃逸的研究。 相似文献
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In this paper,we study five luminous supernovae(LSNe) Ibc(SN 2009ca,ASASSN-15mj,SN 2019omd,SN 2002ued,and SN 2021bmf) whose peak absolute magnitudes Mpeak are ≈-19.5 to-21 mag by fitting their multi-band light curves(LCs) with different energy source models.We find that SN 2009ca might be powered by the 56Ni model since the required 56Ni mass(0.56 M⊙) is comparable to those of energetic SNe Ic,while the rest four SNe cannot be accounted for the 56... 相似文献
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The most direct representation of the measurements provided by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) is a set of Fourier components of the X-ray radiation sampled at discrete points of the spatial-frequency plane, the so called visibilities. Here we review methods for the reconstruction of X-ray and electron maps using RHESSI visibilities and show how the electron maps can be utilized to infer information about the physical properties of the acceleration region during flaring events. 相似文献
19.
利用均匀磁化球模型,对OrionB云中的39个分子云浓核,从它们观测的源半径和分子线线宽,推求它们的数密度和磁场.得到平均磁场110μG,平均密度为8×104/cm3.这些计算值与观测结果一致.对于R>0.2pc的分子云浓核,利用均匀磁化球模型推求磁场和数密度的方法是一种可行的方法. 相似文献