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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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See detailThe onset of photometric variability in red giant stars.
Jorissen, A.; Mowlavi, N.; Sterken, C. et al

in Astronomy and Astrophysics (1997), 324

The onset of variability in red giant stars is studied from the data collected by the `Long-Term Photometry of Variables' project which operates at ESO (La Silla) since 1982. The differential nature of ... [more ▼]

The onset of variability in red giant stars is studied from the data collected by the `Long-Term Photometry of Variables' project which operates at ESO (La Silla) since 1982. The differential nature of the observations combined with a multi-night reduction algorithm enables to study the variability of over 50 red giants with an accuracy of the order of 2 to 3 millimagnitudes on time scales ranging from days to years. All red giants with spectral types late G to early K (i.e. b-y~0.6-0.8) are stable at the level Ï _y_<6millimag (where Ï _y_ is the standard deviation of the Stroemgren y magnitude). Red giants with later spectral types are all variable, and it is found that the minimum variability level Ï _y, min_ increases with increasing b-y, thus defining a minimum-variability boundary. The new data obtained in this paper reveal that the time scale of the associated variability increases with increasing b-y (and thus amplitude), and that the stability of this time scale improves concomitantly. More precisely, irregular variations on time scales of 5 to 10-days (and Ï _y_~7millimag) characterize red giants with b-y~1.0, whereas the variations (with Ï _y_~100millimag) become more regular with periods of the order of 50d for red giants with b-y~1.3. A radial-velocity jitter (of the order of 1.5km/s r.m.s.) is associated with this photometric variability, and suggests that stellar oscillations may be responsible for the observed variations. [less ▲]

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See detailThe geometry of the quadruply imaged quasar PG 1115+080: implications for H_0_.
Courbin, F.; Magain, Pierre ULg; Keeton, C. R. et al

in Astronomy and Astrophysics (1997), 324

Time delay measurements have recently been reported for the lensed quasar PG 1115+080. These measurements can be used to derive H_0_, but only if we can constrain the lensing potential. We have applied a ... [more ▼]

Time delay measurements have recently been reported for the lensed quasar PG 1115+080. These measurements can be used to derive H_0_, but only if we can constrain the lensing potential. We have applied a recently developed deconvolution technique to analyze sub-arcsecond I band images of PG 1115+080, obtained at the Nordic Optical Telescope (NOT) and the Canada France Hawaii Telescope (CFHT). The high performance of the deconvolution code allows us to derive precise positions and magnitudes for the four lensed images of the quasar, as well as for the lensing galaxy. The new measurement of the galaxy position improves its precision by a factor of 3 and thus strengthens the constraints on the lensing potential. With the new data, a range of models incorporating some of the plausible systematic uncertainties yields H_0_=53^+10^_-7_km/s/Mpc. [less ▲]

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See detailThe puzzling Luminous Blue Variable-like object HD 5980 in the Small Magellanic Cloud.
Heydari-Malayeri, M.; Rauw, Grégor ULg; Esslinger, O. et al

in Astronomy and Astrophysics (1997), 322

We have observed the exceptional SMC star HD 5980 during several runs from 1989 to 1995 at ESO La Silla. CASPEC at the 3.6m telescope and EMMI in echelle and long slit modes at NTT were used for ... [more ▼]

We have observed the exceptional SMC star HD 5980 during several runs from 1989 to 1995 at ESO La Silla. CASPEC at the 3.6m telescope and EMMI in echelle and long slit modes at NTT were used for spectroscopy. Sub-arcsecond images were obtained using SUSI at NTT and also an adaptive optics system at the 3.6m telescope. In all our spectra taken before 1994 September HD 5980 shows a spectral type of WN6. The 1994 September spectra were taken shortly after the maximum of the visual light-curve of the LBV-like phenomenon (Bateson & Jones, 1993PVSS...19...50B) and about one month before the observations of Barba et al. (1995ApJ...446L..23B). Near maximum visual brightness, HD 5980 displays a WN11-like spectrum with the HeI lines and the Balmer lines Hdelta and Hgamma showing well-developed PCyg profiles. The sub-arcsecond images (0.17" FWHM), through the near infrared bands J, H, and K, obtained in 1993 and 1996, show no stellar components down to 6.7 mag fainter than HD 5980 in K at a separation of 1.0" and the 3sigma level. For a separation of 0.3" this upper bound is 4.1mag fainter than HD 5980. The observed behavior of this object raises serious problems for our comprehension of the LBV phenomenon in the conventional scenarios of massive star evolution. The present observations cover a crucial period in the evolution of HD 5980 and will therefore be helpful for better understanding this peculiar object especially during its outburst as well as the evolution of W-R stars in general. [less ▲]

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See detailVariations of the AP star HD 208217.
Manfroid, Jean ULg; Mathys, G.

in Astronomy and Astrophysics (1997), 320

We present a new determination of the rotation period of the Ap star HD 208217 based on photometric and magnetic measurements. The variations of this star are remarkable by the strong anharmonicity of its ... [more ▼]

We present a new determination of the rotation period of the Ap star HD 208217 based on photometric and magnetic measurements. The variations of this star are remarkable by the strong anharmonicity of its lightcurves in all colours as well as by the double wave of its mean magnetic field modulus variations. [less ▲]

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See detailSub-arcsecond imaging and spectroscopy of the radio-loud highly polarized quasar PKS 1610-771.
Courbin, F.; Hutsemekers, Damien ULg; Meylan, G. et al

in Astronomy and Astrophysics (1997), 317

We report on imaging and spectroscopic observations of the radio-loud, highly polarized quasar PKS 1610-771 (z=1.71). Our long-slit spectroscopy of the companion 4.55" NW of the quasar confirms the ... [more ▼]

We report on imaging and spectroscopic observations of the radio-loud, highly polarized quasar PKS 1610-771 (z=1.71). Our long-slit spectroscopy of the companion 4.55" NW of the quasar confirms the stellar nature of this object, so ruling out the previously suspected gravitationally lensed nature of this system. PKS 1610-771 looks fuzzy on our sub-arcsecond R and I images and appears located in a rich environment of faint galaxies. Possible magnification, without image splitting of the quasar itself, by some of these maybe foreground galaxies cannot be excluded. The continuum fuzz (made of the closest two objects, viz. A and D) is elongated in a direction orthogonal to the E vector of the optical polarization, as in high-redshift radio-galaxies. The spectrum of PKS 1610-771 appears strongly curved, in a convex way, with a maximum of intensity at ~7,600Å (2,800AÅ rest frame), possibly indicating a strong ultraviolet absorption by dust. [less ▲]

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See detailRadiative Lifetimes for the 4p Excited States of Phosphorus and the Oscillator Strengths of Solar Lines
Berzinsh, U.; Svanberg, S.; Biémont, Emile ULg

in Astronomy and Astrophysics (1997), 326

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See detailExistence of a short period (3.5-4 hours) in the photometric variability of WR 66.
Rauw, Grégor ULg; Gosset, Eric ULg; Manfroid, Jean ULg et al

in Astronomy and Astrophysics (1996), 306

We report here on an independent detection of a short period (~4hours) in the photometric variations of WR66, thus confirming in broad terms the discovery by Antokhin et al.. In addition, we present the ... [more ▼]

We report here on an independent detection of a short period (~4hours) in the photometric variations of WR66, thus confirming in broad terms the discovery by Antokhin et al.. In addition, we present the first spectroscopic variability analysis for this star. A few peculiarities of the spectrum of WR66 are also discussed. Finally, we perform a brief examination of different possible origins of the phenomenon. [less ▲]

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See detailWR22: the most massive Wolf-Rayet star ever weighed.
Rauw, Grégor ULg; Vreux, Jean-Marie ULg; Gosset, Eric ULg et al

in Astronomy and Astrophysics (1996), 306

The results of an extensive spectroscopic campaign on the eclipsing binary WR22 are presented. A new radial velocity curve is deduced for the WN7 component, allowing us to improve the parameters of the ... [more ▼]

The results of an extensive spectroscopic campaign on the eclipsing binary WR22 are presented. A new radial velocity curve is deduced for the WN7 component, allowing us to improve the parameters of the orbit, formerly determined on the basis of photographic spectra. The high signal-to-noise ratio of our data also allows the detection of some weak absorption lines which, for the first time, can definitely be attributed to the companion. A study of their radial velocities gives a mass ratio of m_WR_/m_O_=2.78 leading to a minimum mass of 72M[SUB]sun[/SUB]_ for the WN7 star. The companion can be classified as a "late O" (O6.5-O8.5) star with a luminosity ratio of the system q=L_WR_^y^/L_O_^y^ at 5500A of about 8. The exceptionally high mass of the WN7 star and its high hydrogen mass-fraction suggest that WR22 is at the beginning of its Wolf-Rayet evolution. As a matter of fact, with such a high mass, WR22 most probably is still a hydrogen burning object. Therefore, the WN7 component is much closer to a main sequence O star with a "Wolf-Rayet clothing" than to the other members of the Wolf-Rayet family, which are rather highly evolved He-burning descendants of massive progenitors. [less ▲]

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See detailEmpirical study of departures from the excitation equilibrium of FeI in metal-poor stars.
Magain, Pierre ULg; Zhao, G.

in Astronomy and Astrophysics (1996), 305

A detailed analysis of neutral iron lines in a sample of 13 metal-poor dwarfs and subgiants is carried out on the basis of high resolution spectra obtained with the ESO Coude Echelle Spectrometer. The ... [more ▼]

A detailed analysis of neutral iron lines in a sample of 13 metal-poor dwarfs and subgiants is carried out on the basis of high resolution spectra obtained with the ESO Coude Echelle Spectrometer. The deduced iron abundance is found to depend on the excitation potential of the line used, higher excitation lines generally indicating higher abundances. This could be caused by departures from the local thermodynamic equilibrium (LTE) or by temperature inhomogeneities in the stellar atmospheres. The dependency of this effect on the stellar atmospheric parameters is investigated. From the comparison of iron lines with lines of other elements, it is concluded that the low excitation FeI lines are much more affected than the high excitation lines. The consequences of these effects for the classical abundance analyses are examined. It is found that they may explain, at least in part, some previously reported discrepancies between the results of different authors. [less ▲]

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See detailThe enigmatic flarings of HR2517.
Sterken, C.; Manfroid, Jean ULg

in Astronomy and Astrophysics (1996), 305

We report the discovery of strong flaring of the B3 II-III star HR2517. The evidence is based on Stroemgren differential uvby photometry spanning more than a decade. We discuss the behaviour of HR2517 in ... [more ▼]

We report the discovery of strong flaring of the B3 II-III star HR2517. The evidence is based on Stroemgren differential uvby photometry spanning more than a decade. We discuss the behaviour of HR2517 in terms of two models, viz. a rooted bright spot suddenly appearing and developing with modulation in photospheric temperature producing periodic light and profile variations as the star rotates, or that HR2517 is a Be star in an eccentric (P~33-34d) high-mass X-ray binary (HMXB) with neutron star companion--one of the brightest such systems known. [less ▲]

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