A survey of nova remnants

Exnovae and nova shells are generally faint and difficult to observe. Only a few have been studied, and even less of them in sufficient detail. We give a progress report on our survey of exnovae and nova shells, especially on the results of a spectral survey of novae in the southern Milky Way. The three-dimensional structure in the light of H derived for the nebula of GK Per is displayed.Based on observations obtained at the European Southern Observatory, La Silla, Chile, and at the Centro Astronomico Hispano-Aleman Calar Alto, Spain, operated by the Max-Planck-Institut für Astronomie, Heidelberg.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986.     

Atomic data relevant to line formation in strongly magnetized white dwarf stars

The discovery of magnetic field strengths in the range of 500 million Gauss in the objects Grw+70°8247 and PG1031+224 has given enormous impetus to investigations of magnetic white dwarf stars. We have determined intensities of Balmer transitions for B>10⁸ Gauss as a function of field and find strong oscillations of the transitions strengths, which are of importance to the quantitative analysis of the observed spectra and the interpretation of polarimetric measurements in these objects.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986.     

Electron impact polarization of resonance lines from Lithium-like ions

The Oppenheimer-Penney theory, as developed by Percival and Seaton (1958), is applied to calculate the polarization of resonance lines from Li-like ions. Two laws for the pitch-angle distribution of electrons around the magnetic field are accounted. The degrees of polarization are averaged over the energy of non-thermal electrons generated during the initial phase of solar flares. It is found that for the full space pitch-angle distribution, as adopted by Chandra and Joshi (1984), the degrees of polarization are nearly independent of the atomic number of ion. Whereas for the forward-come distribution used by Haug (1981), they depend on the choice of the free parameterE ₀. The polarization of the resonance lines from Li-like ions is two times larger than that of the L radiations from H-like ions. Hence, under favourable conditions, it may be detected during solar flares.     

The interaction of nova shells with the interstellar medium

Decelerations of the shells around the novae V603 Aq1, V476 Cyg, DQ Her and GK Per were determined from photographs and recent CCD observations. The deceleration is larger for higher expansion velocities, the mean half-lifetime, after which the expansion velocity has dropped to half its initial value, is 75 years.Based on observations obtained at the European Southern Observatory, La Silla, Chile, and at the Centro Astronomico Hispano-Aleman Calar Alto, operated by the Max-Planck-Institut für Astronomie, HeidelbergPaper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986.     

Group Doppler effect in anisotropic plasmas

In anisotropic plasmas, the radiative power emitted and the power observed per unit solid angle should be calculated along the direction of the group velocityv _g . The two power functions referred differ by a product of two factors: one is the group Doppler factor and the other is the squeezing effect of the radiative energy due to the dependence ofv _g on direction. In this paper, the group Doppler factor is derived using two different methods, and the relevant physical concepts are analyzed in details. A number of numerical examples pertaining to astrophysical situations are presented, to illustrate the significance of the group Doppler effect with respect to the wave Doppler effect which is valid in isotropic media.     

Astrometric investigation with the Tautenburg Schmidt telescope

The observations and the plate reduction technique for the determination of positions and absolute proper motions which is used in Potsdam are described. Recent results have shown that an accuracy of about 0 _. 1 for positions and 0 _. ⁷ cent _. ^–1 for proper motions can be achieved both for bright (8^m–12^m) and faint (16^m–18^m) stars. Three astrometric programmes using the Tautenburg plates are presented.     

Realisation of a celestial system based on VLBI geodynamics programs

The relative orientations of various VLBI celestial reference frames are evaluated on the basis of coordinate differences of common sources. It is shown that an accuracy better than 0.001 can be achieved. Possible regional deformations in the different catalogues are investigated; they are found to reach a few 0.001 in some restricted zones. The application of these studies to the realisation of a combined celestial reference frame consistent with the BIH Terrestrial System is outlined.     

Unstable chromospheric dark features and type-III bursts association

The temporal association between the kinematic parameters of chromospheric dark features (DF) and the production of radio type-III bursts is investigated during a period of five months. The Doppler shifts inside six different DF are measured by means of the Meudon Multichannel Subtracting Double Pass Spectrograph (MSDP) during periods of some minutes around 24 type-III bursts. The position of the radio bursts has been checked to be associated with the same active region observed by MSDP, by using the Nançay Radioheliograph. It appears that 23 out of 24 bursts take place when the DF is totally or predominantly blue-shifted. In 18 out of 21 cases, a maximum of the outward velocity is observed in the optical image closest in time to the radio burst. The following peculiarities are also shown by the analyzed DF:

1. All of them present a lengthened shape, in most cases pointing toward a sunspot: a bright region coinciding with a parasitic polarity is observed in between.
2. Horizontal velocities along the DF major axis are often observed, always in a direction opposite to the sunspots.

This association is interpreted by assuming that a shock wave, generated by the moving chromospheric material, is able to accelerate coronal electrons (according to a mechanism proposed by Leroy and Mangeney, 1984) thus producing type-III bursts.     

A possible observational constraint on the production of lithium by galactic novae

Recent computations (D'Antona and Mazzitelli, 1982), together with the general scheme of evolution of cataclysmic binaries (CBs), lead to conclude that the secondaries in those CBs having periods shorter than 4.5 hr have a large³He content in the envelope, ranging from 10^–4 (P=4.5 hr) to >2×10^–3 (P=3 hr) in mass fraction, if the nova systems have an age of some billion years. The consequence on the frequency of novae outbursts is shortly examined. If lithium is produced by galactic novae, the⁷Li content of old disk stars should be very close to the Population II content.Presented at the 5th European Workshop on White Dwarfs, Kiel, 1984.