Search Publications
Effective temperature of B-type stars from the SI II lines of the UV multiplet 13.04 at 130.5-130.9 nm.
Castelli, F.; Singh, J.
An analysis was performed for the 129.3-132.0 nm region of ten normal and two helium-weak peculiar B-type stars of spectral types ranging from B2 to B9 and luminosity classes IV-V. Computations of spectra made it possible to determine the Si abundance from the Si III lines and to estimate the effective temperature from the Si III profiles. The rot…
The HIPPARCOS Input Catalogue. II. Astrometric Data.
Le Poole, R. S.; Jahreiß, H.; Argue, A. N. +12 more
A positional accuracy of 1.5 arcsec rms at the epoch of observation was required for the stars proposed for observation by the Hipparcos satellite. How this was achieved by collecting and investigating already available positions and proper motions in a special compilation catalog, carrying out new ground-based measurements, and a detailed analysi…
A different view of plasma flow inside P/Halley
Balsiger, H.; Neubauer, F. M.; Ip, W. -H. +4 more
The Giotto spacecraft carried two different instruments - the JPA and the IMS - for the observation of hot ions in the coma of P/Halley. Although there are many similarities in the time and distance profiles of the plasma flow parameters (bulk velocity, number density, and temperature) computed from the two data sets, there are also some significa…
X-ray variability in a sample of Seyfert 1 type AGN.
Walter, R.; Courvoisier, T. J. -L.
X-ray observations of several Seyfert 1 type AGN observed many times with EXOSAT are presented and analysed. The spectral parameters are extracted independently in the medium and soft X-ray spectral ranges. The variations observed in the two spectral bands and their relation are discussed and compared to the spectral variations observed in other w…
Solar system objects observed by HIPPARCOS
Bec-Borsenberger, A.
While the vast majority of objects contained in the Hipparcos observing program are stars, 48 minor planets and two satellites of the major planets are also observable and included within the observing program. These observations have very particular scientific values, and the inclusion of the objects in the Input Catalogue have posed very specifi…
HIPPARCOS photometry - NDAC reductions
van Leeuwen, Floor; Hog, E.; Penston, Margaret J. +2 more
The Hipparcos mission will provide astrometry for about 120,000 stars. It will also give very accurate photometry for these stars. The resulting data base will by far surpass any existing photometric catalog, both in accuracy and number of stars measured. This paper describes the calibrations and initial reductions of the photometry carried out by…
A link between X-ray variability and ultraviolet emission in Seyfert 1 type AGN.
Walter, R.; Courvoisier, T. J. -L.
We analyse the behaviour of X-ray spectral variations in Seyfert 1 type active galactic nuclei observed many times in the 2-10 keV energy band. The selected sources are compatible with a separation into two groups distinguished by different kinds of X-ray variability and by different ratios of ultraviolet to X-ray fluxes. In the first group of obj…
Comparison of the first results from the HIPPARCOS star mappers with the HIPPARCOS Input Catalogue
Evans, D. W.; Arenou, F.; van Leeuwen, F. +1 more
Preliminary positions and magnitudes derived from the analysis of 12 weeks of observations from the Hipparcos star mappers are systematically compared with the various sources of ground-based data used in the Hipparcos Input Catalogue. These comparisons allow us to cross-check the accuracies claimed by the various sources of ground-based data and …
X-ray spectrum of NGC 3783 : detection of soft excess and an emission line.
Ghosh, K. K.; Soundararajaperumal, S.; Sivarani, T. +1 more
Results of the X-ray (0.1-10 keV) spectral analysis of the Seyfert I galaxy, NGC 3783, which was observed with EXOSAT on three epochs between 1983 and 1985, are presented here. No low energy absorption, intrinsic to the source, was found in this galaxy and the spectral index of the power law fit did not vary during the EXOSAT observations. Soft ex…
AI II as an electron density diagnostic for the upper chromosphere/lower transition region in late-type stars.
Doyle, J. G.; Keenan, F. P.; Harra, L. K. +2 more
The ratio of the Al II resonance line 3s2 1S-3sp 1P1 at 1670 A to the inter-system line 3s2 1S-3s3p 3P1 at 2669 A is a useful electron density diagnostic for N(e) greater than 10 exp 10/cu cm. However, it is also sensitive to the adopted electron temperature, and, in fact, should be a useful electron temperature diagnostic when Ne is not greater t…