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See detailAbout the origin of extinction in the gravitational lens system MG J0414+0534
Angonin-Willaime, M.-C.; Vanderriest, C.; Courbin, F. et al

in Astronomy and Astrophysics (1999), 347

Photometric measurements of the highly reddened gravitational mirage MG J0414+0534 have been carried out either by PSF subtraction or by applying accurate deconvolution algorithms to optical (R, I) and ... [more ▼]

Photometric measurements of the highly reddened gravitational mirage MG J0414+0534 have been carried out either by PSF subtraction or by applying accurate deconvolution algorithms to optical (R, I) and near-infrared (K) images obtained at CFHT and NOT under sub-arcsecond seeing conditions. It is confirmed that the close pair of images A1-A2 suffers a larger extinction than B and C. While the colours of image A2 obtained from ground-based data seem inconsistent with a simple reddening law, higher resolution images available from HST archives reveal an additional extended component (arc) which introduces significant errors in the photometric decomposition. When the arc component is properly taken into account, the colours of the 4 nucleus images do agree with a classical reddening law, with A2 being by far the most obscured component. Such a differential extinction (maximum difference Delta E(R-I) =~ 0.6) is likely due to the lensing galaxy. This does not mean that {all} the extinction occurs into the lens. Indeed, the fact that the arc is much less red than the images of the nucleus suggests that an important part of the reddening is intrinsic to the source. Finally, no significant variability is observed within this data set, i.e. between 1994 and 1997, while a discrepancy from earlier data is noticed for (A1 + A2). Based on observations collected with the Canada-France-Hawaii Telescope at Mauna Kea (Hawaii, USA), the Nordic Optical Telescope (Canary, Spain) and archive data from the Hubble Space Telescope (NASA/ESA) [less ▲]

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See detailThe orientation within the Galaxy and the Large Magellanic Cloud of nebulae ejected by massive stars
Hutsemekers, Damien ULg

in Astronomy and Astrophysics (1999), 344

The orientation of nebulae ejected by massive stars (Luminous Blue Variables, WR stars, SN1987A) is investigated with respect to the structure of the galaxy to which they belong. In the Galaxy, we find ... [more ▼]

The orientation of nebulae ejected by massive stars (Luminous Blue Variables, WR stars, SN1987A) is investigated with respect to the structure of the galaxy to which they belong. In the Galaxy, we find that the projected long axes of the nebulae most often align with the galactic plane, and then also with the galactic magnetic field. This alignment is statistically significant. In addition, a few nebulae are apparently oriented perpendicular to the galactic plane. In the Large Magellanic Cloud, the nebular axes are found to closely follow the spiral magnetic field. With different inclinations, the Galaxy and the Large Magellanic Cloud probably offer complementary views of the same phenomenon. Although the sample studied thus far is small and the statistics limited, these results suggest that the orientation of massive star ejecta depends on galactic magnetic fields. Since the nebular axes are apparently correlated to the symmetry axes of the stars themselves, and since, in the early evolutionary stages, alignments of accretion disk axes with the interstellar magnetic field have been reported, it is argued that the observed alignment effect results from the star formation process. Based in part on observations collected at the European Southern Observatory (ESO, La Silla) [less ▲]

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See detailAbundance correlations in mildly metal-poor stars
Jehin, Emmanuel ULg; Magain, Pierre ULg; Neuforge, Corinne et al

in Astronomy and Astrophysics (1999), 341

Accurate relative abundances have been obtained for a sample of 21 mildly metal-poor stars from the analysis of high resolution and high signal-to-noise spectra. In order to reach the highest coherence ... [more ▼]

Accurate relative abundances have been obtained for a sample of 21 mildly metal-poor stars from the analysis of high resolution and high signal-to-noise spectra. In order to reach the highest coherence and internal precision, lines with similar dependency on the stellar atmospheric parameters were selected, and the analysis was carried out in a strictly differential way within the sample. With these accurate results, correlations between relative abundances have been searched for, with a special emphasis on the neutron capture elements. This analysis shows that the r elements are closely correlated to the alpha elements, which is in agreement with the generally accepted idea that the r-process takes place during the explosion of massive stars. The situation is more complex as far as the s elements are concerned. Their relation with the alpha elements is not linear. In a first group of stars, the relative abundance of the s elements increases only slightly with the alpha elements overabundance until the latter reaches a maximum value. For the second group, the s elements show a rather large range of enhancement and a constant (and maximum) value of the alpha elements overabundance. This peculiar behaviour leads us to distinguish between two sub-populations of metal-poor stars, namely Pop IIa (first group) and Pop IIb (second group). We suggest a scenario of formation of metal-poor stars based on two distinct phases of chemical enrichment, a first phase essentially consisting in supernova explosions of massive stars, and a second phase where the enrichment is provided by stellar winds from intermediate mass stars. More specifically, we assume that all thick disk and field halo stars were born in globular clusters, from which they escaped, either during an early disruption of the cluster (Pop IIa) or, later, through an evaporation process (Pop IIb). Based on observations obtained at the European Southern Observatory, La Silla, Chile. [less ▲]

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See detailRevised masses of alpha Centauri
Pourbaix, D.; Neuforge-Verheecke; Noels-Grötsch, Arlette ULg

in Astronomy and Astrophysics (1999), 344(1), 172-176

With the precise radial velocities lately published \cite{Murdoch-1993:b}, a simultaneous least-squares adjustment of all visual and spectroscopic observations of the alpha Centauri system is now possible ... [more ▼]

With the precise radial velocities lately published \cite{Murdoch-1993:b}, a simultaneous least-squares adjustment of all visual and spectroscopic observations of the alpha Centauri system is now possible and likely to yield some accurate information. Such an orbit determination leads to a mass ratio in agreement with the astrometric estimate of Kamper & Wesselink \cite*{Kamper-1978} but yields upward revisions of the distance and masses. We examine the effects these revisions on the evolutionary status of the system. [less ▲]

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See detailPolarization properties of a sample of broad absorption line and gravitationally lensed quasars
Hutsemekers, Damien ULg; Lamy, H.; Remy, M.

in Astronomy and Astrophysics (1998), 340

New broad-band linear polarization measurements have been obtained for a sample of 42 optically selected QSOs including 29 broad absorption line (BAL) QSOs. The polarization properties of different sub ... [more ▼]

New broad-band linear polarization measurements have been obtained for a sample of 42 optically selected QSOs including 29 broad absorption line (BAL) QSOs. The polarization properties of different sub-classes have been compared, and possible correlations with various spectral indices searched for. The main results of our study are: (1) Nearly all highly polarized QSOs of our sample belong to the sub-class of BAL QSOs with low-ionization absorption features (LIBAL QSOs). (2) The range of polarization is significantly larger for LIBAL QSOs than for high-ionization (HI) BAL QSOs and non-BAL QSOs. (3) There is some indication that HIBAL QSOs as a class may be more polarized than non-BAL QSOs and therefore intermediate between LIBAL and non-BAL QSOs, but the statistics are not compelling from the sample surveyed thus far. (4) For LIBAL QSOs, the continuum polarization appears significantly correlated with the line profile detachment index, in the sense that LIBAL QSOs with P Cygni-type profiles are more polarized. No correlation was found with the strength of the low- or the high-ionization absorption features, nor with the strength or the width of the emission lines. These results are consistent with a scenario in which LIBAL QSOs constitute a different class of radio-quiet QSOs with more absorbing material and more dust. Higher maximum polarization can therefore be reached, while the actually measured polarization depends on the geometry and orientation of the system as do the line profiles. The observed correlation is interpreted within the framework of recent ``wind-from-disk'' models. Tables~2 and 3 are also available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html}\fnmsep\thanks{Based on observations collected at the European Southern Observatory (ESO, La Silla) [less ▲]

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See detailThree photometric methods tested on ground-based data of Q 2237+0305
Burud, I.; Stabell, R.; Magain, Pierre ULg et al

in Astronomy and Astrophysics (1998), 339

The Einstein Cross, Q 2237+0305, has been photometrically observed in four bands on two successive nights at NOT (La Palma, Spain) in October 1995. Three independent algorithms have been used to analyse ... [more ▼]

The Einstein Cross, Q 2237+0305, has been photometrically observed in four bands on two successive nights at NOT (La Palma, Spain) in October 1995. Three independent algorithms have been used to analyse the data: an automatic image decomposition technique, a CLEAN algorithm and the new MCS deconvolution code. The photometric and astrometric results obtained with the three methods are presented. No photometric variations were found in the four quasar images. Comparison of the photometry from the three techniques shows that both systematic and random errors affect each method. When the seeing is worse than 1farcs0 , the errors from the automatic image decomposition technique and the Clean algorithm tend to be large (0.04-0.1 magnitudes) while the deconvolution code still gives accurate results (1sigma error below 0.04) even for frames with seeing as bad as 1farcs7 . Reddening is observed in the quasar images and is found to be compatible with either extinction from the lensing galaxy or colour dependent microlensing. The photometric accuracy depends on the light distribution used to model the lensing galaxy. In particular, using a numerical galaxy model, as done with the MCS algorithm, makes the method less seeing dependent. Another advantage of using a numerical model is that eventual non-homogeneous structures in the galaxy can be modeled. Finally, we propose an observational strategy for a future photometric monitoring of the Einstein Cross. Based on observations obtained at NOT, La Palma. [less ▲]

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See detailLine profile variability in the spectrum of the O(f) supergiant HD 192639
Rauw, Grégor ULg; Vreux, Jean-Marie ULg

in Astronomy and Astrophysics (1998), 335

We report the results of a medium-resolution spectroscopic investigation of the O(f) supergiant HD 192639. Particular attention is paid to the He {ii}\ lambda 4686 line. This line displays strong profile ... [more ▼]

We report the results of a medium-resolution spectroscopic investigation of the O(f) supergiant HD 192639. Particular attention is paid to the He {ii}\ lambda 4686 line. This line displays strong profile variability on time scales of a few days, changing from a P-Cygni profile with a double-peaked emission component to a pure blue-shifted emission line. It appears that the variability of most of the absorption lines present in our spectra is correlated to the deformation of the He {ii}\ lambda 4686 line and arises probably from a large scale structure in the low-velocity part of the stellar wind rather than from a photospheric phenomenon. We find that the time scale of the variability could be consistent with the estimated rotational period of HD 192639. Based on observations collected at the Observatoire de Haute Provence, France. [less ▲]

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See detailSpectral evolution of Rosetta target comet 46P/Wirtanen
Schulz, R.; Arpigny, Claude ULg; Manfroid, Jean ULg et al

in Astronomy and Astrophysics (1998), 335

The compositional evolution of the coma of Rosetta target comet 46P/Wirtanen was studied along its pre-perihelion orbit. At 2.34 AU the first gaseous species, CN and C_3, were detected. C_2 and NH_2 arose ... [more ▼]

The compositional evolution of the coma of Rosetta target comet 46P/Wirtanen was studied along its pre-perihelion orbit. At 2.34 AU the first gaseous species, CN and C_3, were detected. C_2 and NH_2 arose at 2.0 AU. The production rate curves of gas and dust show a steep increase between 1.8 AU and 1.6 AU indicating a distinct change of the outgassing conditions. The C_2 production increases more rapidly with decreasing solar distance than CN. As one consequence the C_2/CN ratio strongly varies with heliocentric distance. Due to this variation a classification of 46P/Wirtanen by the taxonomy introduced by A'Hearn et al. (\cite{AMSOB}) distinguishing comets with typical abundance ratios from carbon-chain depleted comets, is impossible. It would be classified as depleted beyond about 1.6 AU whereas it would be designated typical at smaller distances. The knowledge of the unexpected changes in the activity curves of 46P/Wirtanen will help to optimize the Rosetta mission scenario in particular with regard to science operations. based on observations obtained at ESO La Silla within ESO programmes No. 57.F-0290 and 58.F-0413 [less ▲]

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See detailThe spatial sodium distribution in the coma of comet Hale-Bopp (C/1995 O1)
Rauer, H.; Arpigny, Claude ULg; Manfroid, Jean ULg et al

in Astronomy and Astrophysics (1998), 334

Long-slit spectra of sodium D-line emission in the coma of comet Hale-Bopp (C/1995 O1) have been obtained on 14 March and 16 April 1997 from the Observatoire de Haute-Provence, France, and the William ... [more ▼]

Long-slit spectra of sodium D-line emission in the coma of comet Hale-Bopp (C/1995 O1) have been obtained on 14 March and 16 April 1997 from the Observatoire de Haute-Provence, France, and the William Herschel Telescope on La Palma, Spain. The spectra show Na spatial profiles within ~ 2.5 10(5) km from the nucleus along and perpendicular to the sun-comet line. In both observing periods secondary maxima are present, indicating spatial structures in the coma sodium distribution. A comparison with the structures seen in the cometary dust continuum is made. A Na production rate of ~ 3 10(24) - 5 10(25) s(-1) could be derived on March 14, 1997, corresponding to less than 0.3% of the normal cosmic abundance ratio Na/O. Based on observations obtained at the William-Herschel-Telescope, La Palma, and the Obs. de Haute-Provence, France [less ▲]

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See detailEvidence for very large-scale coherent orientations of quasar polarization vectors
Hutsemekers, Damien ULg

in Astronomy and Astrophysics (1998), 332

On the basis of a new sample of quasar optical polarization measurements, we have found that, in a region of the sky, the quasar polarization vectors are not randomly oriented as naturally expected, but ... [more ▼]

On the basis of a new sample of quasar optical polarization measurements, we have found that, in a region of the sky, the quasar polarization vectors are not randomly oriented as naturally expected, but appear concentrated around one preferential direction. In order to verify this surprising although preliminary result, we have compiled a large sample of quasar polarization measurements from the literature. With quite severe criteria to eliminate at best the contamination by our Galaxy, a sample of 170 quasars with good quality polarization measurements has been defined. Maps in redshift slices reveal a few regions where the polarization vectors are apparently aligned. To handle the problem more quantitatively, non-parametric 3D statistical tests were designed, as well as a method for visualizing spatially the results. The significance is evaluated through Monte-Carlo simulations. Applied to our sample of 170 polarized quasars, two different statistical tests provide evidence, with significance levels of 0.005 and 0.015 respectively, that the optical polarization vectors of quasars are not randomly distributed over the sky but are coherently oriented on very large spatial scales. This orientation effect appears spatially delimited in the 3D Universe, mainly occuring in a few groups of 10-20 objects. The polarization vectors of objects located along the same line of sight but at different redshifts do not appear accordingly aligned. Essentially for this reason, instrumental bias and contamination by interstellar polarization in our Galaxy are unlikely to be responsible for the observed effect. The very large scale at which this local orientation effect is observed indicates the presence of correlations in objects or fields on spatial scales ~ 1000 h(-1) Mpc at redshifts z =~ 1-2, suggesting an effect of cosmological importance. Several possible and testable interpretations are discussed. Tables~2 and 3 are also available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html [less ▲]

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See detailDetection of the lensing galaxy in HE 1104-1805
Courbin, F.; Lidman, C.; Magain, Pierre ULg

in Astronomy and Astrophysics (1998), 330

We report on deep IR imaging of the double quasar HE 1104-1805. A new image deconvolution technique has been applied to the data in order to optimally combine the numerous frames obtained. The resulting J ... [more ▼]

We report on deep IR imaging of the double quasar HE 1104-1805. A new image deconvolution technique has been applied to the data in order to optimally combine the numerous frames obtained. The resulting J and K' images allow us to detect and study the lensing galaxy between the two lensed QSO images. The near infrared images not only confirm the lensed nature of this double quasar, but also support the previous redshift estimate of z=1.66 for the lensing galaxy. No obvious overdensity of galaxies is detected in the immediate region surrounding the lens, down to limiting magnitudes of J=22 and K=20. The geometry of the system, together with the time delays expected for this lensed quasar, make HE 1104-1805 a remarkable target for future photometric monitoring programs, for the study of microlensing and for the determination of the cosmological parameters in the IR and optical domains. Based on observations obtained at ESO, La Silla, Chile [less ▲]

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See detailField halo stars: the globular cluster connection
Jehin, Emmanuel ULg; Magain, Pierre ULg; Neuforge, Corinne et al

in Astronomy and Astrophysics (1998), 330

High resolution and high signal-to-noise spectra of about 20 metal-poor stars have been analysed. The correlations between the relative abundances of 16 elements have been studied, with a special emphasis ... [more ▼]

High resolution and high signal-to-noise spectra of about 20 metal-poor stars have been analysed. The correlations between the relative abundances of 16 elements have been studied, with a special emphasis on the neutron-capture ones. This analysis reveals the existence of two subpopulations of field halo stars, namely Pop IIa and Pop IIb. They differ by the behaviour of the s-process elements versus the alpha and r-process elements. A scenario for the formation of these stars is presented, which closely relates the origin of field halo stars to the evolution of globular clusters. According to this scenario, the two sub-populations originate from two different stages in the globular cluster's chemical evolution. Based on observations carried out at the European Southern Observatory (La Silla, Chile) [less ▲]

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See detailClose projected QSO-Galaxy associations: are they real?
Claeskens, Jean-François ULg; Surdej, Jean ULg

in Astronomy and Astrophysics (1998), 335

The present status of close projected associations between high redshift quasars and foreground galaxies is reviewed in the framework of gravitational lensing, adopting both an observational and a ... [more ▼]

The present status of close projected associations between high redshift quasars and foreground galaxies is reviewed in the framework of gravitational lensing, adopting both an observational and a theoretical approach. The Non Singular Isothermal Spherical lens model (Hinshaw & Krauss 1987) is used. We confirm that a stronger overdensity q of bright galaxies is expected at small projected angular separations (<= 3arcsec ) from high redshift and bright (thus highly luminous) quasars (HLQs). The effects of a non singular galactic core radius and/or microlensing only slightly enhance this expected overdensity. The predictions reproduce pretty well the scarce observations which are presently available, with the exception of two rather high overdensities previously claimed in the literature (e.g. q=2.9 for theta <= 6arcsec , Webster & Hewett 1990). Uncontrolled morphological selections may affect these controversial results so that additional unbiased observations are badly needed before rejecting gravitational lensing as the mechanism to produce the observed close angular QSO-galaxy associations. We describe three new selected and bias-free samples consisting of a total of 219 different HLQs. Although an apparent overdensity is detected in two of them, these galaxy excesses are found to be statistically not significant. Firm confirmation of the reality of close projected QSO-Galaxy associations is hampered by small number statistics: the expected and observed absolute numbers of such QSO-galaxy associations are still presently found to be very small. Therefore, the corresponding error bars are quite large. We conclude that the best observational strategy to significantly detect (at 3sigma ) an overdensity of galaxies near HLQs is to observe about 1500 HLQs (M_V ~ -29) down to a limiting magnitude Rlim ~ 23. Future automated surveys may provide us with such a large and unbiased database. In conclusions, we find that the lensing-induced correlation between galaxies and HLQs at small angular separations consists of a very interesting but weak effect, much less sensitive to cosmological or astrophysical parameters than the number of multiply imaged sources expected within a sample of HLQs. Based on data collected at the European Southern Observatory, La Silla, Chile [less ▲]

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See detailRedshift estimate of a gravitational lens from the observed reddening of a multiply imaged quasar
Jean, C.; Surdej, Jean ULg

in Astronomy and Astrophysics (1998), 339

Light rays from a multiply imaged quasar usually sample different path lengths across the deflector. Extinction in the lensing galaxy may thus lead to a differential obscuration and reddening between the ... [more ▼]

Light rays from a multiply imaged quasar usually sample different path lengths across the deflector. Extinction in the lensing galaxy may thus lead to a differential obscuration and reddening between the observed macro-lensed QSO images. These effects naturally depend on the precise shape of the extinction law and on the redshift of the lens. By means of numerical Monte-Carlo simulations, using a least-squares fitting method and assuming an extinction law similar to that observed in the Galaxy, we show how accurate photometric observations of multiply imaged quasars obtained in several spectral bands could lead to the estimate of the lens redshift, irrespective of the visibility of the deflector. Observational requirements necessary to apply this method to real cases are thoroughly discussed. If extinction laws turn out to be too different from galaxy to galaxy, we find out that more promising observations should consist in getting high signal-to-noise low resolution spectra of at least three distinct images of a lensed quasar, over a spectral range as wide as possible, from which it should be straightforward to extract the precise shape of the redshifted extinction law. Very high signal-to-noise, low spectral resolution, VLT observations of H 1413+117 and <ASTROBJ>MG 0414+0534</ASTROBJ> should enable one to derive such a redshifted extinction law. [less ▲]

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See detailHST observations of the QSO pair Q1026-0045A,B
Petitjean, Patrick; Surdej, Jean ULg; Smette, Alain et al

in Astronomy and Astrophysics (1998), 334

The spatial distribution of the Lyalpha forest is studied using new HST data for the quasar pair Q 1026-0045 A and B at z_em = 1.438 and 1.520 respectively. The angular separation is 36 arcsec and ... [more ▼]

The spatial distribution of the Lyalpha forest is studied using new HST data for the quasar pair Q 1026-0045 A and B at z_em = 1.438 and 1.520 respectively. The angular separation is 36 arcsec and corresponds to transverse linear separations between lines of sight of ~ 300h(-1}_{50) kpc (q_o = 0.5) over the redshift range 0.833 < z < 1.438. From the observed numbers of coincident and anti-coincident Lyalpha absorption lines, we conclude that, at this redshift, the Lyalpha structures have typical dimensions of ~ 500h(-1}_{50) kpc, larger than the mean separation of the two lines of sight. The velocity difference, Delta V, between coincident lines is surprisingly small (4 and 8 pairs with Delta V < 50 and 200 km s(-1) respectively). Metal line systems are present at z_abs = 1.2651 and 1.2969 in A, z_abs = 0.6320, 0.7090, 1.2651 and 1.4844 in B. In addition we tentatively identify a weak Mg ii system at z_abs = 0.11 in B. It is remarkable that the z_abs = 1.2651 system is common to both lines of sight. The system at z_abs = 1.4844 has strong O vi absorption. There is a metal-poor associated system at z_abs = 1.4420 along the line of sight to A with complex velocity profile. We detect a strong Lyalpha absorption along the line of sight to B redshifted by only 300 km s(-1) relatively to the associated system. It is tempting to interpret this as the presence of a disk of radius larger than 300h(-1}_{50) kpc surrounding quasar A. Based on observations obtained with the NASA/ESA Hubble Space Telescope by the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555 [less ▲]

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See detailScale relativity and quantization of the solar system - Orbit quantization of the planet's satellites
Hermann, Raphaël ULg; Schumacher, G.; Guyard, R.

in Astronomy and Astrophysics (1998), 335(1), 281-286

In a first paper Nottale, Schumacher and Gay have given the bases of the Scale Relativity theory applied to the gravitational field, which leads to the quantization of the solar system. In the present ... [more ▼]

In a first paper Nottale, Schumacher and Gay have given the bases of the Scale Relativity theory applied to the gravitational field, which leads to the quantization of the solar system. In the present paper we show that one more class of objects of the solar system satisfy the rule of quantization, this class including the main satellites and rings of the outer planets. We also give a classification of the satellites by rank, showing that one can predict the existence of certain orbits that are not occupied, or whose objects are not yet discovered. [less ▲]

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See detailProduction and kinematics of sodium atoms in the coma of comet Hale-Bopp
Arpigny, Claude ULg; Rauer, H.; Manfroid, Jean ULg et al

in Astronomy and Astrophysics (1998), 334(Letters), 53-56

High-resolution spectra of sodium D line emission in comet Hale-Bopp (C/1995 O1) were obtained at the Observatoire de Haute-Provence, France, on 25-27 March and on 15-17 April, 1997. The observations have ... [more ▼]

High-resolution spectra of sodium D line emission in comet Hale-Bopp (C/1995 O1) were obtained at the Observatoire de Haute-Provence, France, on 25-27 March and on 15-17 April, 1997. The observations have been used to measure the velocity of sodium atoms in the coma within 2 10(5) km from the nucleus. A comparison between the March and April data provides an illustration of the influence of the heliocentric radial velocity on the strength of the fluorescence (Swings effect), and on the velocity of Na atoms achieved by solar radiation pressure acceleration. Evidence for the presence of a distributed source in the coma is found from the relatively high tailward velocities on the sunward side of the coma, in addition to the sunward extent of sodium emission up to 1.4 10(5) km in April. Based on observations secured at OHP (France) [less ▲]

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See detailSpectroscopic analysis of the Alpha Centauri system
Neuforge-Verheecke, C.; Magain, Pierre ULg

in Astronomy and Astrophysics (1997), 328

A detailed spectroscopic analysis of the two components of the binary system alpha Centauri has been carried out on the basis of extensive high resolution and high signal-to-noise spectra. The ... [more ▼]

A detailed spectroscopic analysis of the two components of the binary system alpha Centauri has been carried out on the basis of extensive high resolution and high signal-to-noise spectra. The temperatures of the stars have been determined from the Fei excitation equilibrium and checked from the H_alpha line wings. The abundances of 17 elements have been obtained and indicate a general overabundance of 0.24 dex relative to the Sun, with no significant difference between the two stars. All elements analysed, from CNO to the iron peak, show basically the same overabundance. Only nickel seems to be significantly more overabundant, while the neutron capture elements appear less enhanced. Based on observations carried out at the European Southern Observatory (La Silla, Chile) [less ▲]

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See detailHST confirmation of the lensed quasar J03.13.
Surdej, Jean ULg; Claeskens, Jean-François ULg; Remy, M. et al

in Astronomy and Astrophysics (1997), 327

In order to definitely prove the claim by Claeskens et al. (1996A&A...305L...9C) that J03.13 is a doubly imaged quasar, WFPC2 direct imaging and FOS spectroscopy of this compact system have been obtained ... [more ▼]

In order to definitely prove the claim by Claeskens et al. (1996A&A...305L...9C) that J03.13 is a doubly imaged quasar, WFPC2 direct imaging and FOS spectroscopy of this compact system have been obtained with HST. These textbook case observations clearly show that J03.13 consists of two point-like images separated by 0.849"+/-0.001" with a magnitude difference of 2.14+/-0.03 in V and I. We see no trace of a lensing galaxy. From the FOS observations, we find that J03.13 A and B have identical spectra within the measurement uncertainties. The data also show that spectroscopic contamination of B by A is negligible. We do confirm the redshift z=2.545 for J03.13 A and B (emission-lines due to Ly-beta lambda1025 and O VI lambdalambda1031, 1037 are also detected), first derived from an unresolved ground-based spectrum of the two components by Claeskens et al. We also find that the absorption line system at z=2.344 (CIV and Ly-alpha) is present in the spectra of A and B. On the contrary, the absorption line system at z=1.085 (MgII) is only present in the spectrum of J03.13 A. If this latter system is associated with the lens, we may expect that image A consists of two radially merging images with a very small angular separation. From the WFPC2 imagery, we can rule out the presence of a third component fainter than A by up to 5.2mag. with an angular separation >=0.13". [less ▲]

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See detailThe LMC transition star R 84 and the core of the LH 39 OB association.
Heydari-Malayeri, M.; Courbin, F.; Rauw, Grégor ULg et al

in Astronomy and Astrophysics (1997), 326

On the basis of sub-arcsecond imaging obtained at the ESO NTT with SUSI and the ESO ADONIS adaptive optics system at the 3.6m telescope, we resolve and study the core components of the LMC OB association ... [more ▼]

On the basis of sub-arcsecond imaging obtained at the ESO NTT with SUSI and the ESO ADONIS adaptive optics system at the 3.6m telescope, we resolve and study the core components of the LMC OB association LH 39. The central star of the association, the rare transition object R 84, is also investigated using CASPEC echelle spectroscopy at the ESO 3.6m telescope. A new, powerful image restoration code that conserves the fluxes allows us to obtain the magnitudes and colors of the components. We bring out some 30 stars in a ~16"x16" area centered on R 84. At a resolution of 0.19"(FWHM), the closest components to R 84 are shown to be stars #21 and #7 lying at 1.1"NW and 1.7"NW respectively of the transition star. The former is possibly a blue star of V=16.7mag and the latter with its V=17.5mag is the reddest star of the field, after R 84. Star #7 turns out to be too faint to correspond to the red M2 supergiant previously reported to contaminate the spectrum of R 84. If the late-type spectrum is due to a line-of-sight supergiant with a luminosity comparable to R 84, it should lie closer than 0.12" to R 84. The transition star shows spectral variability between 1982 and 1991. We also note some slight radial velocity variations of the Of emission lines over timescales of several years. Furthermore, we derive the spectral types of two of the brightest stars of the cluster, using long slit spectra obtained at the NTT telescope equipped with EMMI, and discuss the apparent absence of O type stars in this association. [less ▲]

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