W. J. G. Steemers’s research while affiliated with Leiden University and other places

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Publications (5)


The 64W part of the WRST Survey (Oort+, 1988)
  • Article

August 2001

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6 Reads

M. J. A. Oort

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W. J. G. Steemers

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R. A. Windhorst

We present a deep 92cm survey down to 4.5mJy (5{sigma}) carried out with the Westerbork Synthesis Radio Telescope (WSRT) of a field in Lynx, previously observed at three other radio frequencies: 6, 21, and 49cm. The observations, totalising 4*12h were made in the redundancy mode, which makes it possible to achieve a high dynamic range. In total, 384 sources above a 5{sigma} peak flux limit were detected, of which 337 form a complete sample. (1 data file).


A deep 92 CM survey of the Lynx area

March 1988

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12 Reads

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2 Citations

Astronomy and Astrophysics Supplement Series

The paper presents a deep 92 cm survey down to 4.5 mJy (5 sigma) carried out with the Westerbork Synthesis Radio Telescope (WSRT) of a field in Lynx, previously observed at three other radio frequencies: 6, 21, and 49 cm. The observations were made in the redundancy mode, which makes it possible to achieve a high dynamic range. In total, 384 sources above a 5 sigma peak flux limit were detected, of which 337 form a complete sample. For the 92-cm complete sample, the spectral index distribution was determined as a function of 92 cm and 21 cm flux density. The fainter sources have significantly flatter spectra between 92 cm and 21 cm than the brighter sources, with a median spectral index that changes from about 0.7 above 25 mJy to about 0.55 below 25 mJy (at 21 cm). A similar decrease is seen as a function of 92 cm flux density. A much smaller effect is shown by the median spectral index between 49 and 21 cm. There is, therefore, clear evidence for a spectral turnover between 92 and 49 cm in a large fraction of the radio sources. From optical identifications it appears that the spectral turnover occurs primarily in the blue radio galaxies. The 327 MHz source counts above 4.75 mJy are presented.


A high precision photometric investigation of the micro-variations of Wolf-Rayet stars

September 1987

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5 Reads

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14 Citations

Astronomy and Astrophysics

VBLUW photometric observations of seven WR stars, obtained with standard deviation less than about 0.002 mag using the 90-cm Dutch telescope at ESO during March-April 1986, are reported. The observation and data-reduction techniques are discussed, and the results are presented in extensive tables and graphs and characterized in detail. Some of the light and color variations are tentatively attributed to temperature-induced changes in the continuum emission (e.g., nonuniform illumination of the pseudophotosphere due to dynamical inhomogeneities such as temporarily hotter eddies or blobs). Particular attention is given to the contrast between the strong W-band (323.6-nm) excess presently observed in WR 6 (EZ CMa) and the FUV deficiency observed by van der Hucht et al. in 1974-1976; an explanation based on increased outer-envelope transparency is proposed.


VBLUW photometry of two hypergiants HD 80077 (B2Ia+) and HD 74180 (F2Ia+) and of the open cluster PISMIS 11

December 1985

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16 Reads

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3 Citations

Astronomy and Astrophysics

The authors present and discuss VBLUW-photometry of HD 80077 and HD 74180, two galactic hypergiants, and in addition also of the young open cluster Pismis 11, of which HD 80077 presumably is a member. Both hypergiants were observed for several months during two consecutive years, and appear to show small, irregular variations in brightness and colours. The results of the observations concerning distances and reddening do not contradict other studies of these stars.


An investigation of the micro-variations of highly luminous OBA type stars. III - The S DOR type star R 71 = HDE 169006 in the LMC

November 1985

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18 Reads

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1 Citation

Astronomy and Astrophysics

A. M. van Genderen

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W. J. G. Steemers

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P. T. M. Feldbrugge

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[...]

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A. K. van den Boogaart

VBLUW photometry of the LMC S Dor type star R 71 = HDE 269006 is presented and discussed. It appears that the star similar to other high luminous OB type stars shows optical micro variations. The light amplitude is ≡0m.1 and the typical time scale ≡20d. The star fits well into the PLC relation for variable supergiants, suggesting that some type of pulsation is the cause. Contrary to a few other normal B type supergiants, the light curves of individual cycles are smooth and the V-B and B-L colours vary in anti-phase with each other. The largest colour variations usually occur in the B-U viz. ≡0m.02 (the U band contains the Balmer jump).