References of "Astronomy and Astrophysics"
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See detailRadiative Lifetime Measurements in Dy II and the Solar Abundance of Dysprosium
Biémont, Emile ULg; Löwe, R. M.

in Astronomy and Astrophysics (1993), 273

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See detailThe evolution of atmospheric extinction at La Silla derived from measurements in the Stromgren photometric system
Sterken, C.; Manfroid, Jean ULg

in Astronomy and Astrophysics (1992), 266

We describe atmospheric extinction data obtained at the La Silla Observatory in the Stromgren photometric system. Our measurements, covering a time interval of almost fifteen years, show three episodes of ... [more ▼]

We describe atmospheric extinction data obtained at the La Silla Observatory in the Stromgren photometric system. Our measurements, covering a time interval of almost fifteen years, show three episodes of increased extinction due to airborne aerosols of volcanic origin. A small-amplitude seasonal variation and a short-timescale pseudo-periodic oscillation are also revealed. Implications on the observing procedures and photometric reduction schemes are discussed. [less ▲]

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See detailPhotometric accuracy - The impact of extrapolation in differing color-transformation schemes
Manfroid, Jean ULg; Sterken, C.; Gosset, Eric ULg

in Astronomy and Astrophysics (1992), 264

We discuss photometric errors that arise from the use of differing color-transformation schemes when extrapolating outside the range of the standard values that define the system. An example based on data ... [more ▼]

We discuss photometric errors that arise from the use of differing color-transformation schemes when extrapolating outside the range of the standard values that define the system. An example based on data from the Long-Term Photometry of Variables project (LTPV) at ESO is analyzed. Practically, the extrapolation errors are most evident as systematic brightness jumps of some program stars, seen when monitored over several observing runs. Conclusions are drawn about the choice of standard stars in the Stromgren system. [less ▲]

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See detailThe Beta Pictoris circumstellar disk. XIII - Survey of the variable CA II lines
Lagrange-Henri, A. M.; Gosset, Eric ULg; Beust, H. et al

in Astronomy and Astrophysics (1992), 264

Over 200 Ca II K and H spectra of Beta Pictoris taken between September 19, 1989 and December 19, 1990 are presented. Many new variable features are observed, at low and high redshifts, together with a ... [more ▼]

Over 200 Ca II K and H spectra of Beta Pictoris taken between September 19, 1989 and December 19, 1990 are presented. Many new variable features are observed, at low and high redshifts, together with a few blueshifted ones. The time scales for the variations are shown to be strongly dependent on the redshift of the variable lines. A rate of events of about 200/yr is derived; it is also shown that in some cases the absorbing cloud covers less than 40 percent of the stellar surface. The observed variations are analyzed and compared to those observed in the UV. These observations make it possible to test the present scenario of evaporating infalling bodies, in terms of velocity of variable lines, occurrence of variable events, small time-scale variations, and size of the absorbing cloud. It is shown that the observed velocity range for the variable components may be explained by the present scenario, as well as the size of the absorbing clouds. [less ▲]

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See detailThe outstanding photometric variations of the Ap star HD 7676 = VV Scl
Renson, Pierre ULg; Manfroid, Jean ULg

in Astronomy and Astrophysics (1992), 263

Contrary to what was formerly believed, VV Scl is not an eclipsing star but an Alpha CVn type variable with period P = 5.0976 +/- 0.0001 d. The range of the luminosity variation is remarkably large for an ... [more ▼]

Contrary to what was formerly believed, VV Scl is not an eclipsing star but an Alpha CVn type variable with period P = 5.0976 +/- 0.0001 d. The range of the luminosity variation is remarkably large for an Ap star, especially in B of Johnson's system, and in b and mostly in v of Stromgren's one. A secondary minimum appears near the maximum in B, b, an v. The main minima are in phase in all colors. [less ▲]

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See detailPhysical constraints on color transformations
Manfroid, Jean ULg

in Astronomy and Astrophysics (1992), 260

Color-transformation procedures in photometry should ideally obey physical principles. A few of these rules, of particular interest in the study of reddened stars, of multiple stars, stellar clusters and ... [more ▼]

Color-transformation procedures in photometry should ideally obey physical principles. A few of these rules, of particular interest in the study of reddened stars, of multiple stars, stellar clusters and galaxies are discussed. Directions to minimize the transformation errors for these objects are given. [less ▲]

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See detailSystematic color transformation effects in Stromgren photometry
Manfroid, Jean ULg; Sterken, C.

in Astronomy and Astrophysics (1992), 258

A major cause of errors in photometry is to be attributed to the color transformation. A thorough analysis of numerous observing runs carried out in the Stromgren system shows how reliable are the ... [more ▼]

A major cause of errors in photometry is to be attributed to the color transformation. A thorough analysis of numerous observing runs carried out in the Stromgren system shows how reliable are the transformations in the different indices. Conclusions are drawn about the reduction methods, the validity of homogenization procedures, and the choice of the standard stars which define the system. [less ▲]

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See detailLight curve of the Ap star TW Columbae
Renson, Pierre ULg; Manfroid, Jean ULg

in Astronomy and Astrophysics (1992), 256

The uvby light curves of the Ap star TW Col are analyzed. The presence of a small-scale dip in the curves appears to be in conflict with a simple spotted-rotator model. TW Col is the second star, after 46 ... [more ▼]

The uvby light curves of the Ap star TW Col are analyzed. The presence of a small-scale dip in the curves appears to be in conflict with a simple spotted-rotator model. TW Col is the second star, after 46 Eri, for which such a phenomenon seems to be confirmed. [less ▲]

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See detailWR 134 line-profile variability revisited
Vreux, Jean-Marie ULg; Gosset, Eric ULg; Bohannan, B. et al

in Astronomy and Astrophysics (1992), 256

Line profile variations of WR 134 are analyzed on more than 200 spectra collected at Haute Provence Observatory and at Kitt Peak National Observatory. No conclusive evidence is found for a periodic ... [more ▼]

Line profile variations of WR 134 are analyzed on more than 200 spectra collected at Haute Provence Observatory and at Kitt Peak National Observatory. No conclusive evidence is found for a periodic behavior. However, for some frequencies, for example, the ones close to nu = (n x 0.5)/d, the data are ill-distributed, and the existence of a related periodic phenomenon cannot be completely ruled out. This does not mean that the variability presents a purely random behavior. Several new characteristics of the observed deformations are extracted from the analysis of the spectra and it is tentatively suggested that these could be the signature of a bipolar flow. [less ▲]

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See detailPhotometric variability of barium stars
Jorissen, A.; Manfroid, Jean ULg; Sterken, C.

in Astronomy and Astrophysics (1992), 253

Results are presented on uvby observations for up to 4 years of 19 southern barium stars. During the observation period, only two of the stars, HD 121447 and HD 46407, showed significant light variations ... [more ▼]

Results are presented on uvby observations for up to 4 years of 19 southern barium stars. During the observation period, only two of the stars, HD 121447 and HD 46407, showed significant light variations. These variables are the stars among the sample with the shortest orbital periods. In the case of HD 46407, evidence for eclipses was also found. It is noted that the evidence of varying phase shifts between the photometric eclipse and the spectroscopic ephemeris based on McClure and Woodsworth's (1990) orbital parameters makes it unlikely that the eclipsed light arises directly from the stellar companion of the barium star. It is also suggested that the flickering displayed by the reddest barium stars is a generic property of all red stars, because it was also observed for the comparison stars HD 204139 and HD 90677. [less ▲]

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See detailQ 1208 + 1011 - The most distant imaged quasar, or a binary?
Magain, Pierre ULg; Surdej, Jean ULg; Vanderriest, C. et al

in Astronomy and Astrophysics (1992), 253(Letters), 13-16

The paper reports the discovery of a new gravitational lens candidate: the high redshift (z = 3.803) and highly luminous (V = 17.5, M[SUB]V[/SUB] = -30.3) quasar Q 1208 + 1011. As derived from the ... [more ▼]

The paper reports the discovery of a new gravitational lens candidate: the high redshift (z = 3.803) and highly luminous (V = 17.5, M[SUB]V[/SUB] = -30.3) quasar Q 1208 + 1011. As derived from the analysis of direct CCD frames taken with the ESO/MPI 2.2 m telescope, this multiple quasar consists of two pointlike images, separated by 0.45 arcsec and characterized by a brightness ratio of 3.5, in red light. Existing spectroscopic data support the gravitational lens interpretation for this system but cannot exclude the hypothesis of a binary quasar. In the former case, the spectrum suggests that, if the metallic absorption line system reported by Steidel (1990) at a redshift z = 2.9157 is associated with the deflector, the mass of the lens should be of the order of M = 7.8 x 10 exp 11 solar masses. Evaluation of a recent HST PC frame obtained for Q 1208 + 1011 within the snapshot survey for gravitational lenses confirms the above results. [less ▲]

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See detailLifetimes in Fe II and the solar abundance of iron
Hannaford, P.; Lowe, R. M.; Grevesse, Nicolas ULg et al

in Astronomy and Astrophysics (1992), 259

New atomic lifetimes have been determined for eight quartet (z 4D, z 4F) levels and ten sextet (z 6D, z 6F, z 6P) levels in Fe II and the results for seven of these levels have been combined with existing ... [more ▼]

New atomic lifetimes have been determined for eight quartet (z 4D, z 4F) levels and ten sextet (z 6D, z 6F, z 6P) levels in Fe II and the results for seven of these levels have been combined with existing experimental branching fractions to obtain a revised set of log gf-values for 15 solar lines in Fe II. The new log gf data are used together with equivalent widths determined from the Liege solar atlas to derive a value for the iron photospheric abundance: 7.48 +/- 0.04, which is consistent with the currently accepted meteoritic result, 7.51 +/- 0.01. The results are compared with the atomic lifetime data, log gf data and solar analyses used in three other recent solar abundance determinations based on Fe II lines. [less ▲]

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See detailElectric Dipole Transitions in Atomic Oxygen and the lifetimes of the 2p3(4S°)3s5S° and 3S° States
Biémont, Emile ULg; Zeippen, C. J.

in Astronomy and Astrophysics (1992), 265

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See detailThe nature of the nebula associated with the luminous blue variable star WRA 751
Hutsemekers, Damien ULg; van Drom, E.

in Astronomy and Astrophysics (1991), 251

Narrow-band filter imagery as well as medium to high resolution spectroscopy of the nebula surrounding the luminous blue variable (LBV) star WRA 751 are presented. The nebula appears as a slowly expanding ... [more ▼]

Narrow-band filter imagery as well as medium to high resolution spectroscopy of the nebula surrounding the luminous blue variable (LBV) star WRA 751 are presented. The nebula appears as a slowly expanding H II region of low excitation characterized by a significant N/O overabundance which may be due to the presence in the nebula of nuclear processed material ejected by the star. With the recent discovery of a nebula around HR Car, all but one known galactic LBVs are now shown to be associated with a nebula. These nebula have rather similar physical characteristics, one of the most important being that all of them apparently contain processed material. [less ▲]

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See detailMG1131 + 0456 - Discovery of the optical Einstein ring with the NTT
Hammer, F.; Angonin, M. C.; Le Fevre, O. et al

in Astronomy and Astrophysics (1991), 250

Deep optical imaging and spectroscopy have been obtained for the well known radio Einstein ring MG1131 + 0456 with the ESO/NTT. The direct CCD frames show a red emission excess at the exact location of ... [more ▼]

Deep optical imaging and spectroscopy have been obtained for the well known radio Einstein ring MG1131 + 0456 with the ESO/NTT. The direct CCD frames show a red emission excess at the exact location of the radio ring. Subsequent subtraction of the lensing galaxy, assumed to be an elliptical galaxy, results in the detection of an optical ring which is strikingly similar to the radio ring. Gravitational lensing models, including source reconstruction, predicts that the source is likely a radio-galaxy for which both the optical and extended radio emissions are sufficiently distorted by the foreground lensing galaxy as to produce a nearly complete ring. The present spectroscopy and broad band photometry suggest that the lensing galaxy has a spectral energy distribution typical of a nonevolved elliptical, tentatively at z of 0.85. The radio-galaxy associated with the radio-ring is likely to be an elliptical at larger redshift, tentatively of 1.13. [less ▲]

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See detailHR Carinae - A luminous blue variable surrounded by an arc-shaped nebula
Hutsemekers, Damien ULg; van Drom, E.

in Astronomy and Astrophysics (1991), 248

New high and medium dispersion spectroscopic observations of the luminous blue variable (LBV) star HR Car are presented, showing that this star has a multiple shell expanding atmosphere and undergoes ... [more ▼]

New high and medium dispersion spectroscopic observations of the luminous blue variable (LBV) star HR Car are presented, showing that this star has a multiple shell expanding atmosphere and undergoes spectral variations apparently correlated with the light variations similarly to the other LBVs. Most striking are the very broad emission wings (FWZI of about 3000 km/s) of the Balmer lines, reminiscent from the broad lines observed in the spectra of Of and WR stars. A kinematic distance of 5.4 kpc and a bolometric magnitude of -9.4 put HR Car among the most luminous stars of the Galaxy. The narrow band filter imagery also reveals an arc-shaped jet like nebula associated with HR Car. Spectroscopic observations of this nebula reveal an emission nature with very low excitation. Like the star itself, the nebula is characterized by a significant N/O overabundance, probably due to the presence of nuclear processed material ejected by the star. [less ▲]

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See detailCalibration of the Alpha Centauri system - Metallicity and age
Noels-Grötsch, Arlette ULg; Grevesse, Nicolas ULg; Magain, Pierre ULg et al

in Astronomy and Astrophysics (1991), 247

The binary system Alpha Cen A and B, with its well known parallax, its high quality orbit and the reasonable hypothesis of a common origin and age for both components, is a useful test of stellar ... [more ▼]

The binary system Alpha Cen A and B, with its well known parallax, its high quality orbit and the reasonable hypothesis of a common origin and age for both components, is a useful test of stellar evolution models. Evolutionary sequences are computed for 1.085 and 0.9 solar masses with different values of Z, Y and alpha (= l/Hp). Adopting luminosities and effective temperatures as derived from the observations, Z = 0.04, Y = 0.32, alpha = 1.6 and t = 5 Gyr. The Z-value derived from evolutionary sequences is very sensitive to the difference in effective temperature between the two components. [less ▲]

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See detailHD160529 - a New Galactic Luminous Blue Variable
Sterken, C.; Gosset, Eric ULg; Juttner, A. et al

in Astronomy and Astrophysics (1991), 247

The galactic early-type A hypergiant HD 160529 has been photometrically monitored in the Strömgren system during the past 18 years. HD 160529 has become fainter in the optical range by 0.5 mag during the ... [more ▼]

The galactic early-type A hypergiant HD 160529 has been photometrically monitored in the Strömgren system during the past 18 years. HD 160529 has become fainter in the optical range by 0.5 mag during the past eight years. In addition, pulsation-like variations with a quasi-period of 57 days and with a peak-to-peak amplitude of about 0.1 mag in b and y were found. The fading was not completely monotonous a fall of at least 0.2 mag in b and y within about 40 days around JD2446500 is particularly remarkable. This jump is accompanied by a change in the temperature-sensitive c[SUB]1[/SUB] index of about -0.14 mag. HD 160529 has become visually fainter and hotter during the past eight years, thus becoming a new case of a galactic Luminous Blue Variable (LBV). Model calculations on the basis of the photometry and previously published as well as new spectroscopic observations have shown HD 160529 to be a close counterpart of R 110, a recently discovered LBV in the LMC. From this comparison an absolute magnitude of M[SUB]V[/SUB] = -8.9 and a distance of 2.5 kpc is derived. The stellar parameters characterising the maximum state are estimated to be T[SUB]eff[/SUB] ≍ 8000 K, log g ≍ 0.55, R ≍ 330 R[SUB]sun[/SUB], from which the mass is estimated to be only M ≍ 13 M[SUB]sun[/SUB]. From the Hα profile a lower limit of the mass-loss rate of HD 160529 was calculated, which is about 10 times higher than that of R 110; this could be a consequence of the higher metallicity of the galactic LBV. The low mass of HD 160529 makes this luminous blue star a good candidate for a post-RSG (red supergiant phase) object, providing some evidence that (as sometimes argued) low metallicity is not a necessary prerequisite for evolution of massive stars backward to higher temperatures. [less ▲]

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See detailAbundances of neutron capture elements in metal-poor dwarfs. I - Yttrium and zirconium
Zhao, G.; Magain, Pierre ULg

in Astronomy and Astrophysics (1991), 244

The yttrium and zirconium abundances are determined in a sample of 20 metal-poor stars on the basis of high resolution, high SNR spectra. Significant differences between the behaviors of these two ... [more ▼]

The yttrium and zirconium abundances are determined in a sample of 20 metal-poor stars on the basis of high resolution, high SNR spectra. Significant differences between the behaviors of these two neighboring elements are found, zirconium being less deficient than yttrium in Population II stars. Moreover, there is a genuine cosmic scatter in the abundances of these two elements relative to iron, of the order of 20 percent. The scatter is lower when these elements are compared to titanium, which might indicate that a significant fraction of the lighter neutron-capture elements are produced in massive stars. [less ▲]

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See detailThe very massive star HDE269676 and its components
Heydari-Malayeri, M.; Hutsemekers, Damien ULg

in Astronomy and Astrophysics (1991), 244

It is shown that one of the most luminous stars of the LMC, HDE269676, is in fact a massive star cluster. On the basis of observations with the ESO New Technology Telescope (NTT) the main components of ... [more ▼]

It is shown that one of the most luminous stars of the LMC, HDE269676, is in fact a massive star cluster. On the basis of observations with the ESO New Technology Telescope (NTT) the main components of the cluster are classified and it is shown that there are several O type stars in HDE269676 instead of the one or two previously known. This is interesting for the initial mass function in the LMC and implies that the number of O type stars in the LMC is generally underestimated. The most massive star of the system being about a factor three less luminous than previously estimated is a super-giant O5f with a zero age main sequence mass of about 80 solar. [less ▲]

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