High-speed flows in the vicinity of the Trapezium stars

[OIII] 5007 Å line profiles at high spectral and spatial resolution have been obtained at a single slit position near the Trapezium cluster in the Orion Nebula (M42, NGC1976) using the Manchester Echelle Spectrometer (MES). The very long integration time at this position confirms the earlier tentative identification of a shell on the nearside of the Trapezium cluster with a relative velocity of -100 kms^–1 and a radius of 1 arcminute. No receding counterpart is found. We believe this is the first detection of this feature at optical wavelengths, previous spectroscopic work (O' Dellet al., 1993),(Baldwinet al., 1991),(Castaneda, 1988) having concentrated on the main nebular material at relatively low velocities.     

Gas-drag-assisted capture of Himalia's family

To elucidate the capture of Jupiter's outer moons, we reverse-evolve satellites from their present orbits to their original heliocentric paths in the presence of Jupiter's primordial circumplanetary disk (Lubow et al., 1999, Astrophys. J. 526, 1001-1012; Canup and Ward, 2003, Astron. J. 124, 3404-3423). Our orbital histories use a symplectic integrator that allows dissipation. We assume that the present satellites Himalia, Elara, Lysithea, Leda, and S/2000 J11 are collisional fragments of a single parent. Our simulations show that this “prograde-cluster progenitor” (PCP) could be derived from objects with heliocentric orbits like those of the Hilda asteroid group. We show analytically that this capture is energetically possible. We also compare the spectroscopic characteristics of the prograde cluster members (Grav et al., 2003, Icarus, submitted for publication) with those of the Hildas, and conclude that the surface color of the prograde-cluster progenitor is consistent with an origin within the Hilda group. Accordingly, gas drag in the primordial jovian nebula is found to offer a plausible explanation for the origin of the prograde cluster. A similar capture mechanism is proposed for Saturn's Phoebe.     

Origin of three-dimensional shapes of chondrules: I. Hydrodynamics simulations of rotating droplet exposed to high-velocity rarefied gas flow

The origin of three-dimensional shapes of chondrules is an important information to identify their formation mechanism in the early solar nebula. The measurement of their shapes by using X-ray computed topography suggested that they are usually close to perfect spheres, however, some of them have rugby-ball-like (prolate) shapes [Tsuchiyama, A., Shigeyoshi, R., Kawabata, T., Nakano, T., Uesugi, K., Shirono, S., 2003. Lunar Planet. Sci. 34, 1271-1272]. We considered that the prolate shapes reflect the deformations of chondrule precursor dust particles when they are heated and melted in the high velocity gas flow. In order to reveal the origin of chondrule shapes, we carried out the three-dimensional hydrodynamics simulations of a rotating molten chondrule exposed to the gas flow in the framework of the shock-wave heating model for chondrule formation. We adopted the gas ram pressure acting on the chondrule surface of in a typical shock wave. Considering that the chondrule precursor dust particle has an irregular shape before melting, the ram pressure causes a net torque to rotate the particle. The estimated angular velocity is for the precursor radius of r₀=1 mm, though it has a different value depending on the irregularity of the shape. In addition, the rotation axis is likely to be perpendicular to the direction of the gas flow. Our calculations showed that the rotating molten chondrule elongates along the rotation axis, in contrast, shrinks perpendicularly to it. It is a prolate shape. The reason why the molten chondrule is deformed to a prolate shape was clearly discussed. Our study gives a complementary constraint for chondrule formation mechanisms, comparing with conventional chemical analyses and dynamic crystallization experiments that have mainly constrained the thermal evolutions of chondrules.     

Enrichment in volatiles in the giant planets of the Solar System

We propose an interpretation of the enrichments in volatiles observed in the four giant planets with respect to the solar abundance. It is based on the assumption that volatiles were trapped in the form of solid clathrate hydrates and incorporated in planetesimals embedded in the feeding zones of each of the four giant planets. The mass of trapped volatiles is then held constant with time. The mass of hydrogen and of not trapped gaseous species continuously decreased with time until the formation of the planet was completed, resulting in an increase in the ratio of the mass of trapped volatiles to the mass of hydrogen (Gautier et al., Astrophys. J. 550 (2001) L227). The efficiency of the clathration depends upon the amount of ice available in the early feeding zone. The quasi-uniform enrichment in Ar, Kr, Xe, C, N, and S observed in Jupiter is reproduced because all volatiles were trapped. The non-uniform enrichment observed in C, N and S in Saturn is due to the fact that CH₄, NH₃, and H₂S were trapped but not CO and N₂. The non-uniform enrichment in C, N and S in Uranus and Neptune results from the trapping of CH₄, CO, NH₃ and H₂S, while N₂ was not trapped. Our scenario permits us to interpret the strongly oversolar sulfur abundance inferred by various modelers to be present in Saturn, Uranus and Neptune for reproducing the microwave spectra of the three planets. Abundances of Ar, Kr and Xe in these three are also predicted. Only Xe is expected to be substantially oversolar. The large enrichment in oxygen in Neptune with respect to the solar abundance, calculated by Lodders and Fegley (Icarus 112 (1994) 368) from the detection of CO in the upper troposphere of the planet, is consistent with the trapping of volatiles by clathration. The upper limit of CO in Uranus does not exclude that this process also occurred in Uranus.     

Isotopic analysis of early solar system objects by an ion microprobe: Parametric studies and initial results

Magnesium, potassium and calcium isotope compositions in terrestrial samples and refractory phases from primitive meteorites are determined using an ion microprobe. A thorough investigation of the different instrument parameters is carried out to ensure that conditions necessary for high mass resolution and high precision isotopic studies are adequately satisfied. The instrument can be tuned to achieve mass resolution (M/ΔM) of up to 10,000 (M≤60); it has a very good dynamic stability (ΔB/B≤10 ppm over durations of ≤40 minutes) and the counting system has an effective dead-time of ≤25 nsec and a dynamic background of ≤0·01 c/s. Reproducibility and precision of isotopic measurements are checked by analyzing magnesium and titanium isotopic compositions in terrestrial standards and isotopically doped silicate glasses. A precision of 2‰ (2σ _m ) was achieved during magnesium isotopic analysis in samples with low Mg content (200 ppm). Results from studies of magnesium and potassium isotopic compositions in several Ca−Al-rich refractory inclusions (CAIs) from the primitive meteorites Efremovka and Grosnaja, representing some of the early solar system objects, are presented. The well-behaved Mg−Al isotopic systematics confirm the pristine nature of the Efremovka CAIs inferred earlier from petrographic and trace element studies. The Grosnaja CAIs that have experienced secondary alterations show disturbed magnesium isotopic systematics. Observation of excess²⁶Mg in several of the analyzed CAIs confirms the presence of the now extinct²⁶Al (t _1/2=7×10⁵ years) in the solar nebula at the time of CAI formation. Our data also suggest a relatively uniform distribution of²⁶Al in the solar nebula. Several Efremovka CAIs with excess²⁶Mg also have excess⁴¹K resulting from the decay of⁴¹Ca (t _1/2≃10⁵ years). This observation constrains the time interval between cessation of nucleosynthetic input to the solar nebula and the formation of some of the first solar system solids (CAIs) to less than a million years.     

太阳星云凝聚过程的岩石学模型：（1）球粒陨石的凝聚成因

建立类地行星区太阳星云凝聚过程的岩石学模型，对于合理解释陨石、地球和类地行星的成因关系，探讨地球起源和估算地球的整体成分都有着重要意义。本文中根据天体化学和太阳系演化学说关于太阳星云物理化学条件的基本分析，以及实验凝聚岩石学的研究结果，推断在太阳星云盘的类地行星区中可能有星云的气－固和气－液－固两种凝聚作用发生。通过对球粒陨石中球粒和基质矿物成分及结构构造特征的对比，论证了绝大多数球粒的气－液－固凝聚成因和基质的气－固凝聚成因，并讨论了球粒陨石各化学群的凝聚成因模式。     

Chemical composition of the Orion nebula derived from echelle spectrophotometry

We present echelle spectroscopy in the 3500- to 7060-... range for two positions of the Orion nebula. The data were obtained using the 2.1-m telescope at Observatorio Astronómico Nacional in San Pedro Mártir, Baja California. We have measured the intensities of about 220 emission lines, in particular 81 permitted lines of C⁺, N⁺, N⁺⁺, O⁰, O⁺, Ne⁰, Si⁺, Si⁺⁺ and S⁺, some of them produced by recombination only and others mainly by fluorescence. We have determined electron temperatures, electron densities and ionic abundances using different continuum and line intensity ratios. We derived the He, C and O abundances from recombination lines and find that the C/H and O/H values are very similar to those derived from B stars of the Orion association, and that these nebular values are independent of the temperature structure. We have also derived abundances from collisionally excited lines. These abundances depend on the temperature structure; accurate t ² values have been derived comparing the O II recombination lines with the [O III ] collisionally excited lines. The gaseous abundances of Mg, Si and Fe show significant depletions, implying that a substantial fraction of these atoms is tied up in dust grains. The derived depletions are similar to those found in warm clouds of the Galactic disc, but are not as large as those found in cold clouds. A comparison of the solar and Orion chemical abundances is made.     

Infrared polarimetry and imaging of ultracompact partially ionized optical sources in the orion nebula

Hubble Space Telescope images of the Orion nebula taken with the Wide-Field Camera have revealed subarcsecond structure in several dozen objects which are apparently ionized externally from nearby stars. We have obtained near-IR images and IR polarimetry of the Orion region to search for correlations with the WFC objects. We find that all of the ultracompact WFC objects are associated with IR features of some sort, and that some are associated with strongly polarized IR emission. The object with strongest polarization also shows small IR lobes. In addition, we find some previously unreported sources, showing polarized IR emission, outside the field of the HST images, which we believe may be the same sorts of object. We note that the object with strongest polarization has a double-lobed appearance in the K band image.