Jets And Arcs In The Coma Of Comet Hale-Bopp From August 1996 To April 1997

An observation program for measuring the activity of Comet Hale-Bopp was conducted at the Observatoire de Haute Provence from August 1996 to April 1997. Narrow band images were obtained from the 120 cm telescope. A set of four filters at 513, 516, 527 and 682 nm was used to measure the C₂ emission in the (1,1) and (0,0) band and the intensity of the dust-scattered continuum. The luminosity of the central part of the coma was measured in the continuum and C₂ emissions, as a function of heliocentric distance before perihelion. The activity of the comet considerably increased between October 1996 and February 1997. The images obtained in August, September and October 1996 show the presence of strong jets. In February, March and April 1997, series of roughly concentric arcs were observed in the sunward hemisphere of the coma. The arcs were also detected in the spatial distribution of the I(682 nm)/I(527 nm) red color ratio and the I(513 nm)/I(516 nm) C₂ emission ratio. Using the I(513 nm)/I(516 nm) ratio as a parameter to measure the degree of excitation of C₂ molecules, we can show that the local production of gas, probably resulting from dust fragmentation, should be taken into account. This revised version was published online in July 2006 with corrections to the Cover Date.     

Fundamental Problems in Astrophysics

Progress of modern astrophysics requires the access to the electromagnetic spectrum in the broadest energy range. The Ultraviolet is a fundamental energy domain since it is one of the most powerful tool to study plasmas at temperatures in the 3,000–300,000 K range as well as electronic transitions of the most abundant molecules in the Universe. Moreover, the UV radiation field is a powerful astrochemical and photoionizing agent.The objective of this review is to describe the crucial issues that require access to the UV range. A summary has been added to the end with a more classic view of UV needs by astronomical object type; this approach is followed at length in the rest of the contributions of this issue.     

ORGANISATION DE L'E. G. I. G. Avril 1957

Abstract

The “non-Darcian” behaviour of water in soils is discussed considering the coupling that can arise between the different transport phenomena. In general, it is shown that there is a reason to expect that a difference in osmotic pressure will arise when solutions flow through porous media, and that due to that difference the resulting flow be related in a non-linear fashion to the applied pressure difference. Several numerical examples are presented illustrating this behaviour.     

Stability of Co-Orbital Motion in Exoplanetary Systems

The stability of co-orbital motions is investigated in such exoplanetary systems, where the only known giant planet either moves fully in the habitable zone, or leaves it for some part of its orbit. If the regions around the triangular Lagrangian points are stable, they are possible places for smaller Trojan-like planets. We have determined the nonlinear stability regions around the Lagrangian point L₄ of nine exoplanetary systems in the model of the elliptic restricted three-body problem by using the method of the relative Lyapunov indicators. According to our results, all systems could possess small Trojan-like planets. Several features of the stability regions are also discussed. Finally, the size of the stability region around L₄ in the elliptic restricted three-body problem is determined as a function of the mass parameter and eccentricity.     

The M‐/X‐asteroid menagerie: Results of an NIR spectral survey of 45 main‐belt asteroids

Abstract– Diagnostic mineral absorption features for pyroxene(s), olivine, phyllosilicates, and hydroxides have been detected in the near‐infrared (NIR: approximately 0.75–2.50 μm) spectra for 60% of the Tholen‐classified ( Tholen 1984, 1989 ) M‐/X‐asteroids observed in this study. Nineteen asteroids (42%) exhibit weak Band I (approximately 0.9 μm) ± Band II (approximately 1.9 μm) absorptions, three asteroids (7%) exhibit a weak Band I (approximately 1.05–1.08 μm) olivine absorption, four asteroids (9%) display multiple absorptions suggesting phyllosilicate ± oxide/hydroxide minerals, one (1) asteroid exhibits an S‐asteroid type NIR spectrum, and 18 asteroids (40%) are spectrally featureless in the NIR, but have widely varying slopes. Tholen M‐asteroids are defined as asteroids exhibiting featureless visible‐wavelength (λ) spectra with moderate albedos ( Tholen 1989 ). Tholen X‐asteroids are also defined using the same spectral criterion, but without albedo information. Previous work has suggested spectral and mineralogical diversity in the M‐asteroid population ( Rivkin et al. 1995, 2000 ; Busarev 2002 ; Clark et al. 2004 ; Hardersen et al. 2005 ; Birlan et al. 2007 ; Ockert‐Bell et al. 2008, 2010 ; Shepard et al. 2008, 2010 ; Fornasier et al. 2010 ). The pyroxene‐bearing asteroids are dominated by orthopyroxene with several likely to include higher‐Ca clinopyroxene components. Potential meteorite analogs include mesosiderites, CB/CH chondrites, and silicate‐bearing NiFe meteorites. The Eos family, olivine‐bearing asteroids are most consistent with a CO chondrite analog. The aqueously altered asteroids display multiple, weak absorptions (0.85, 0.92, 0.97, 1.10, 1.40, and 2.30–2.50 μm) indicative of phyllosilicate ± hydroxide minerals. The spectrally featureless asteroids range from metal‐rich to metal‐poor with meteorite analogs including NiFe meteorites, enstatite chondrites, and stony‐iron meteorites.     

Origin and ascent history of unusually crystal-rich alkaline basaltic magmas from the western Pannonian Basin

The last eruptions of the monogenetic Bakony-Balaton Highland Volcanic Field (western Pannonian Basin, Hungary) produced unusually crystal- and xenolith-rich alkaline basalts which are unique among the alkaline basalts of the Carpathian–Pannonian Region. Similar alkaline basalts are only rarely known in other volcanic fields of the world. These special basaltic magmas fed the eruptions of two closely located volcanic centres: the Bondoró-hegy and the Füzes-tó scoria cone. Their uncommon enrichment in diverse crystals produced unique rock textures and modified original magma compositions (13.1–14.2 wt.% MgO, 459–657 ppm Cr, and 455–564 ppm Ni contents). Detailed mineral-scale textural and chemical analyses revealed that the Bondoró-hegy and Füzes-tó alkaline basaltic magmas have a complex ascent history, and that most of their minerals (～30 vol.% of the rocks) represent foreign crystals derived from different levels of the underlying lithosphere. The most abundant xenocrysts, olivine, orthopyroxene, clinopyroxene, and spinel, were incorporated from different regions and rock types of the subcontinental lithospheric mantle. Megacrysts of clinopyroxene and spinel could have originated from pegmatitic veins/sills which probably represent magmas crystallized near the crust–mantle boundary. Green clinopyroxene xenocrysts could have been derived from lower crustal mafic granulites. Minerals that crystallized in situ from the alkaline basaltic melts (olivine with Cr-spinel inclusions, clinopyroxene, plagioclase, and Fe–Ti oxides) are only represented by microphenocrysts and overgrowths on the foreign crystals. The vast amount of peridotitic (most common) and mafic granulitic materials indicates a highly effective interaction between the ascending magmas and wall rocks at lithospheric mantle and lower crustal levels. However, fragments from the middle and upper crust are absent from the studied basalts, suggesting a change in the style (and possibly rate) of magma ascent in the crust. These xenocryst- and xenolith-rich basalts yield divers tools for estimating magma ascent rate that is important for hazard forecasting in monogenetic volcanic fields. According to the estimated ascent rates, the Bondoró-hegy and Füzes-tó alkaline basaltic magmas could have reached the surface within hours to few days, similarly to the estimates for other eruptive centres in the Pannonian Basin which were fed by “normal” (crystal and xenoliths poor) alkaline basalts.