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Träfflista för sökning "L773:1432 0746 OR L773:0004 6361 srt2:(1995-1999)"

Search: L773:1432 0746 OR L773:0004 6361 > (1995-1999)

  • Result 1-25 of 85
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1.
  • Alberdi, A., et al. (author)
  • The high-frequency compact radio structure of the peculiar quasar 4C 39.25
  • 1997
  • In: Astronomy and Astrophysics. - : Springer-Verlag. - 0004-6361 .- 1432-0746. ; 327:2, s. 513-521
  • Journal article (peer-reviewed)abstract
    • We present new high angular resolution images of the compact non-thermal radio source 4C 39.25 obtained from VLBI observations at λ1.3cm, λ7mm, and λ3mm wavelengths. These maps and Gaussian model-fits show that the milli-arcsecond to sub-milliarcsecond structure of 4C 39.25 consists of a complex bent core-jet structure with embedded moving and stationary VLBI components. Facilitated by the small observing beams and high angular resolutions obtained at mm-wavelengths, we measured the relative positions of the jet components with an accuracy of a few hundred micro-arcseconds. This allows the detailed followup of the ongoing merging process of a westward superluminally moving component (b_) with a stationary component a_, located at ~2.9 mas east of the putative core d_. In contrast to the other components of the structure with steeper spectra, the westernmost component d_ exhibits an inverted spectrum peaking between λ7mm and λ3mm, thus further supporting its identification as the VLBI core, self-absorbed at longer wavelengths. From two VLBI maps obtained nearly simultaneously at λ7mm and λ1.3cm, we made the first spectral index map of 4C 39.25 in this wavelength regime. The main characteristics of the spectral index distribution of the jet are pronounced changes of the spectral index between orientations parallel and transverse to the jet axis. Near the merging components a_ and b_ the spectral index steepens with increasing separation from d_. However, in the bridge of emission c_, which connects d_ with a_ and b_, the spectral index gradient has a direction transverse to the jet axis, suggesting a frequency dependent jet curvature and edge-brightening. A brief discussion of this behaviour within current jet models is presented.
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2.
  • Chu, H.S., et al. (author)
  • VLBI observations of the puzzling BL Lacertae object 0235+164
  • 1996
  • In: Astronomy and Astrophysics. - : Springer. - 0004-6361 .- 1432-0746. ; 307:1, s. 15-20
  • Journal article (peer-reviewed)abstract
    • For over 20 years, many models have been proposed to explain the variability of AO0235+164. Among them, the most favorable has been micro-lensing. We have made a series of VLBI observations on this source in order to better understand its nature. The resultant maps indicate dramatic changes in the position angle of the jet between observations as well as a correlation between the intensity of the VLBI core and flux outbursts which occurred between the observing sessions. These characteristics, in conjunction with other arguments, suggest that the source variability is intrinsic, i.e. microlensing is of minor importance. We suggest two models: 1) a model in which the jet starts at a very small angle to the line of sight and then curves away to become about 6deg at about 1 mas from the core; and 2) a CME (Coronal Mass Ejection) model that also may explain the violent variability in intensity, polarization position angle, and jet direction in AO0235+164.
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3.
  • Erkens, U., et al. (author)
  • Monitoring of active galactic nuclei. VI. The quasar Mrk 876
  • 1995
  • In: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 296:1, s. 90-9898
  • Journal article (peer-reviewed)abstract
    • For pt. V see ibid., vol. 285, no. 3, p. 857-67 (1994). We present the results of a 6 month monitoring campaign of Mrk 876, a low luminosity quasar with M absap-24. High resolution spectroscopy and direct imaging in the Johnson filters B, V, R, I was carried out in the first half of 1990. Line and continuum light curves are presented. Mrk 876 showed a decrease of 17% in the continuum over the whole observing period whereas the line flux of Hbeta and Halpha stayed constant. This implies, that the radius of the broad line region is larger than 6 light months. Coadded spectra of Hbeta and Halpha are presented which confirm an extreme asymmetry of the line profiles to the red. With respect to much earlier spectra the line profiles changed considerably
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7.
  • Matveenko, L.I., et al. (author)
  • The structure of the quasar 3C345 at lambda 49cm and its relation to low-frequency variability
  • 1996
  • In: Astronomy and Astrophysics. - Berlin : Springer-Verlag. - 0004-6361 .- 1432-0746. ; 312, s. 738-744
  • Journal article (peer-reviewed)abstract
    • The structure of the quasar 3C345 has been studied at lambda 49cm with a global VLBI network. The core has a low-freuqncy cut off in its spectrum, and is very weak at this wavelength. The most compact bright knot is the part of the jet nearest to the nucleus, with a size of about 5x4 mas and a brightness temperature of Tb about 0.6 10^12 K. Its flux density and solid angle increased by a factor of about 2 during the period 1983.9 to 1990.8, but the birghtness temperature did not change significantly. The emission at millimeter wavelengths decreased by a factor of about 2 during the same period, while the UV emission from the nucleus also dcreased. We suggest that variations in the low-frequency emissions are caused by changes in the absorption by a cocoon - the thermal plasma surrounding the jet. The electron density in the region is Ne about 10^5 cm^-3and the longitudal field is about 40 microG. The emission measure and the rotation measure vary with r, the distance from the nucleus as r^-3. The data at lamda 49cm indi´cate several components in and near the "hotspot" at the end of the arcsec jet.
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8.
  • Rantakyrö, F.T., et al. (author)
  • 50 μas resolution VLBI images of AGN’s at λ3 mm
  • 1998
  • In: Astronomy and Astrophysics. - Berlin : Springer-Verlag. - 0004-6361 .- 1432-0746. ; 131, s. 451-467
  • Journal article (peer-reviewed)abstract
    • We present 15 images from the global mm-VLBI sessions in 1990 April at 100 GHz and 1993 April at 86 GHz. These observations probe the central engines of the 16 observed AGN's with up to 50 mu as resolution. Among other sources previously observed with lambda 3 mm VLBI we present the first lambda 3 mm maps of 0735+178, 0748+126, 1055+018, 2145+067, and CTA102, in total we have been able to image 13 out of the 16 observed sources. 6 out of the 13 imaged sources observed exhibit curvature and rapid structural changes, although the low dynamic range in two thirds of the maps limits the detection of weak features. Most of the sources have unresolved cores even at this high resolution. There is substantial evidence that the observed sources can be grouped into two general groups: A misaligned population with parsec scale jets in the form of low pitch helices and an aligned population with straight jets with small changes in PA due to intrinsic bends.
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9.
  • Rantakyrö, Fredrik T., et al. (author)
  • Multiband VLBI observations of CTA 102
  • 1996
  • In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 310:1, s. 66-74
  • Journal article (peer-reviewed)abstract
    • The source CTA102, known to exhibit low frequency variability, has been observed at six epochs (three at λ32cm, two at λ18cm, and one at λ1.3cm) with intercontinental VLBI arrays. On the basis of the changes observed in the structure, we believe that the flux density variations at these wavelengths are due to intrinsic processes and not due to interstellar scintillation. This source exhibits behaviour suggestive of being expanding with a very high apparent transverse velocity.
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10.
  • Romeo, Alessandro, 1961 (author)
  • Dynamical effects of softening in N-body simulations of disc galaxies. Method and first applications.
  • 1997
  • In: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 324, s. 523-533
  • Journal article (peer-reviewed)abstract
    • Two questions that naturally arise in N-body simulations of stellar systems are: (o) How can we compare experiments that employ different types of softened gravity? (o) Given a particular type of softened gravity, which choices of the softening length optimize the faithfulness of the experiments to the Newtonian dynamics? We devise a method for exploring the dynamical effects of softening, which provides detailed answers in the case of 2-D simulations of disc galaxies and also solves important aspects of the 3-D problem. In the present paper we focus on two applications that reveal the dynamical differences between the most representative types of softened gravity, including certain anisotropic alternatives. Our method is potentially important not only for testing but also for developing new ideas about softening. Indeed, it opens a direct route to the discovery of optimal types of softened gravity for given dynamical requirements, and thus to the accomplishment of a physically consistent modelling.
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11.
  • Romeo, Alessandro, 1961 (author)
  • Modelling gravity in N-body simulations of disc galaxies. Optimal types of softening for given dynamical requirements
  • 1998
  • In: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 335, s. 922-928
  • Journal article (peer-reviewed)abstract
    • Modelling gravity is a fundamental problem that must be tackled in N-body simulations of stellar systems, and satisfactory solutions require a deep understanding of the dynamical effects of softening. In a previous paper (Romeo 1997), we have devised a method for exploring such effects, and we have focused on two applications that reveal the dynamical differences between the most representative types of softened gravity. In the present paper we show that our method can be applied in another, more fruitful, way: for developing new ideas about softening. Indeed, it opens a direct route to the discovery of optimal types of softened gravity for given dynamical requirements, and thus to the accomplishment of a physically consistent modelling of disc galaxies, even in the presence of a cold interstellar gaseous component and in situations that demand anisotropic resolution.
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13.
  • Alvarez, R, et al. (author)
  • Near-infrared narrow-band photometry of M-giant and Mira stars: models meet observations
  • 1998
  • In: ASTRONOMY AND ASTROPHYSICS. - : SPRINGER VERLAG. - 0004-6361. ; 330:3, s. 1109-1119
  • Journal article (peer-reviewed)abstract
    • From near-infrared, narrow-band photometry of 256 oxygen-rich Mira variables we obtain evidence about the loops that these stars follow in colour-colour diagrams. We also find a phase lag between indices related to molecular band-strength of titanium oxid
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14.
  • Andersen, AC, et al. (author)
  • Optical properties of carbon grains: Influence on dynamical models of AGB stars
  • 1999
  • In: ASTRONOMY AND ASTROPHYSICS. - : SPRINGER VERLAG. - 0004-6361. ; 349:1, s. 243-252
  • Journal article (peer-reviewed)abstract
    • For amorphous carbon several laboratory extinction data are available, which show quite a wide range of differences due to the structural complexity of this material. We have calculated self-consistent dynamic models of circumstellar dust-shells around ca
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15.
  • Ardeberg, Arne, et al. (author)
  • On the history of star formation in the bar of the Large Magellanic Cloud
  • 1997
  • In: ASTRONOMY AND ASTROPHYSICS. - : SPRINGER VERLAG. - 0004-6361. ; 322:2, s. L13-L16
  • Journal article (peer-reviewed)abstract
    • With the HST PC and uvby photometry, we investigate evolutionary signatures in a field in the centre of the LMC Bar. Field stars close to the turn off point in the HR diagram are used for a study of ages and evolution of the stellar popu lations dominatin
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16.
  • Asplund, Martin, et al. (author)
  • A stellar endgame - The born-again Sakurai's object
  • 1997
  • In: ASTRONOMY AND ASTROPHYSICS. - : SPRINGER VERLAG. - 0004-6361. ; 321:2, s. L17-L20
  • Journal article (peer-reviewed)abstract
    • The surface chemical composition of this remarkable star shows that it is hydrogen-deficient, carbon-rich and enriched in the light s-process elements. Spectra taken in May and October 1996 indicate a decrease in the surface hydrogen abundance by 0.7 dex
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17.
  • Asplund, Martin, et al. (author)
  • Abundance similarities between the R CrB star V854 Cen and the born-again Sakurai's object
  • 1998
  • In: ASTRONOMY AND ASTROPHYSICS. - : SPRINGER VERLAG. - 0004-6361. ; 332:2, s. 651-660
  • Journal article (peer-reviewed)abstract
    • The elemental abundances of the mildly hydrogen-deficient R Coronae Borealis (R CrB) star V854 Cen have been estimated. The R CrB stars have been divided into majority and minority classes judging by their abundance patterns. Class assignment has previous
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19.
  • ASPLUND, M (author)
  • SPECTROSCOPY OF RY-SAGITTARII DURING THE 1993 MINIMUM
  • 1995
  • In: ASTRONOMY AND ASTROPHYSICS. - : SPRINGER VERLAG. - 0004-6361. ; 294:3, s. 763-772
  • Journal article (other academic/artistic)abstract
    • The R Coronae Borealis (RCB) star RY Sgr underwent a deep minimum (DELTAV approximately 7.m5) in 1993. Here medium and low resolution observations from approximately 35 days into the decline are presented together with photometry from the early phases unt
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20.
  • Asplund, M (author)
  • The stability of late-type stars close to the Eddington limit
  • 1998
  • In: ASTRONOMY AND ASTROPHYSICS. - : SPRINGER VERLAG. - 0004-6361. ; 330:2, s. 641-650
  • Journal article (other academic/artistic)abstract
    • The opacity-modified Eddington limit has been computed for hydrogen-deficient model atmospheres. The R Coronae Borealis (R CrB) stars are found to be located strikingly close to the limit, which suggests that the unknown trigger mechanism for their visual
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21.
  • Bergvall, Nils, et al. (author)
  • Emission line flux limits in four QSO metal line absorbers
  • 1997
  • In: ASTRONOMY AND ASTROPHYSICS. - : SPRINGER VERLAG. - 0004-6361. ; 321:3, s. 771-775
  • Journal article (peer-reviewed)abstract
    • In an effort to detect the early phases of star formation in massive galaxies, we have searched for Ly alpha and [O II]lambda 3727 emission from four QSO metal absorption line systems and possible associated galaxies, using narrow-band filters. None of th
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22.
  • Bergvall, N, et al. (author)
  • Optical/near-IR surface photometry of blue low surface brightness galaxies
  • 1999
  • In: ASTRONOMY AND ASTROPHYSICS. - : SPRINGER VERLAG. - 0004-6361. ; 341:3, s. 697-708
  • Journal article (peer-reviewed)abstract
    • We present luminosity profiles, colour profiles and disk parameters based on near-infrared JHK surface photometry of a sample of blue low surface brightness galaxies (BLSBGs). The results are discussed along with previously obtained optical data. We find
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23.
  • Berzinsh, U, et al. (author)
  • Radiative lifetimes for the 4p excited states of phosphorus and the oscillator strengths of solar lines
  • 1997
  • In: Astronomy & Astrophysics. - 0004-6361. ; 326:1, s. 412-416
  • Journal article (peer-reviewed)abstract
    • Radiative lifetimes for the levels of the 3s(2)3p(2)4p configuration in neutral phosphorus have been determined experimentally for the first time and also theoretically. Experiments were performed by using two-photon UV laser excitation and subsequent fluorescence detection in a laser-produced plasma. In relativistic Hartree-Fock calculations, the Cowan code extended to 42 configurations was used. Finally, the lifetimes for all the 13 excited states in the 4p configuration were calculated and for 6 of them the values were measured experimentally. Theoretical branching ratios were combined with experimental lifetimes, to deduce oscillator strengths and these were used for the evaluation of the phosphorus abundance in the sun. A value of 5.49 (0.04) was obtained in the usual logarithmic scale.
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  • Result 1-25 of 85

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