Search Publications
Trace hydrogen in DB white dwarfs : GD 408 - the lowest H abundance case.
Koester, D.; Weidemann, V.
Improved IUE observations revealed the presence of trace hydrogen in the helium atmosphere of GD408, a DB star at Teff of 13,500 K. The hydrogen abundance determined by spectroscopic analysis, log H/He = -6, is considerably smaller than the value determined for other DB stars, which are classified DBA due to the presence of visible hydrogen lines.…
On the magnetic energy density in the X-ray emission of active galactic nuclei.
Ghisellini, G.; Fabian, A. C.; Celotti, A.
We estimate upper limits on the magnetic energy density in the compact X-ray emission region of the Seyfert galaxies NGC 4051 and NGC 6814 and the quasars 3C273, 3C279 and H1821 + 643 using synchrotron self-Compton theory. The size of the emission region is obtained from the X-ray variability time-scale, and the electron density from the assumptio…
Quasi-linear pitch angle and energy diffusion of pickup ions near comet Halley
Coates, A. J.; Neubauer, F. M.; Johnstone, A. D. +1 more
The process of ion pickup in the environment of Halley's comet is studied in order to see if velocity diffusion driven by the observed level of turbulence can explain the observed development of the implanted ion distribution. The theoretical description used is based on a quasi-linear approach and considers the implantation and transport of comet…
The Ultraviolet Variations of K Scuti
Monier, R.
This paper reports on low resolution observations of the pulsating star Delta Scuti (F2IIIp) obtained with the International Ultraviolet Explorer (IUE) at different phases of the 4 hr and 40 min pulsation cycle. The ultraviolet flux varies in phase with the optical flux measured with the Fine Error Sensor (FES). The amplitude of the ultraviolet va…
IUE Observations of NGC 1068: The Extended High-Ionization Emission Region
Bruhweiler, F. C.; Truong, K. Q.
UV emission morphology of the inner disk of NGC 1068 reveals extremely strong emission of the highly ionized species, N V λ1240, C IV λ 1550, and He II λ1640 in regions of distances up to 25"(2.5 kpc) from the nucleus. The strong emission from these species cannot be due to photoionization from OB stars and requires either photoionization from the…
Search for fast ultraviolet variability in NGC 5548 and non-linearities in IUE's SWP camera.
Walter, R.; Courvoisier, T. J. -L.
, Large observation campaigns of Seyfert galaxies give the opportunity to check the linearity of IUE short wavelength camera at low flux number and moderate background. In particular, many pairs of quasi simultaneous observations of NGC 5548 have been obtained with IUE. We selected pairs of spectra which include one well exposed and one poorly exp…
The variable broad emission line spectrum of 4C 37.43.
Gondhalekar, P. M.; O'Brien, P. T.
The results of a short campaign to monitor the quasar 4C37.43 using the International Ultraviolet Explorer (IUE) satellite are presented. Combining these data with archive data we find clear evidence for ultraviolet continuum and Ly α emission line variability. Most of the Ly α variability occurs in the wings of the line. The C IV λ1549 line may v…
Giotto extended mission
Schwehm, Gerhard
The navigation of the ESA spacecraft Giotto to its encounter with comet P/Halley on 14 March 1986 required just 10% of the fuel available. Although the spacecraft was damaged by dust impacts during its close flyby at the nucleus of P/Halley it was retargeted to return close to Earth to maintain the option to extend the mission to encounter another…
The nearby interstellar material in the general direction towards M 22.
Bates, B.; Gilheany, S.; Wood, K. D.
High-resolution optical spectra of interstellar absorption lines are used to examine reddening variations across the globular cluster M22 which are produced by the nearby low-velocity gas. Fifteen cluster stars together with three stars within 1.3 deg of the cluster were observed in the Na D lines and in some case also the K I (7699 A) line. Optic…
Carbon and nitrogen abundances in the BN supergiant HD 93840, and their implications for normal galactic supergiants.
Massa, D.; Lamers, H. J. G. L. M.; Altner, B. +1 more
It is demonstrated how carbon and nitrogen abundances of luminous B stars can be determined by a combined analysis of their UV photospheric and wind lines. It is shown that HD 93840 has nearly the same temperature and surface gravity as the normal B1 Ib star Zeta Per. These two stars are compared on the basis of their UV photospheric silicon spect…