Structural investigations in the central part of the coma cluster on the base of equidensity diagrams. II

On the base of Tautenburg UBV-Schmidt plates equidensitometric diameters of galaxies of ROOD and BAUM 's list are given and the ellipticities in the three colours are determined. It was found that there is no difference in the ellipticities depending on the colours, on the average. The percentage of spherical objects (ε = 0.00) was found to 21.5 per cent. The surface brightness for 213 objects in the system V is given. The mean surface brightness is 22.75 mag/□′ with a dispersion of 2.5 mag/□′. It shows only a very slight dependence on the integrated magnitude.     

Struktur und Entwicklung kompakter Radioquellen

A model is suggested which accounts for (i) the observed shape and angular variation of compact radio sources (especially the apparent superrelativistic velocities and the absence of contracting sources), (ii) the flux variation associated with the angular variation, and (iii) all the known cases of apparent occurence of surface brightness exceeding the theoretical upper limit provided by the inverse COMPTON effect, preserving the usual premises: cosmological origin of the redshift and incoherent synchrotron radiation of electrons. The model consists of a plasma ring expanding with moderate relativistic velocity. It provides two possibilities for estimating the distance of the sources by radio data: from the time dependence of the angular expansion and from the angular diameter and the shape of flux variation. In the context of cosmology this distances are „angular distances”︁ and therefore, if the redshift is also known, the HUBBLE constant or the acceleration parameter q₀ may be obtained. The second method is applied to BL Lac yielding approximately 6 Mpc. So the underlying galaxy would be a dwarf system of M ≈︂–13. The active nucleus of M ≈︂ – 16 is rather below the normal quasars. This seems very satisfactory in view of the short time scale of variations in BL Lac compared to the quasars.     

Suspected OB stars near the andromeda nebula

An investigation of suspected OB stars on plates of the Schmidt telescope of Karl Schwarzschild Observatory Tautenburg gave the following results: I. There exists a sharp boundary of the M 31 disk. For distances larger than R = 30 kpc from the centre no OB stars of the Andromeda galaxy could be found. 2. The isolated OB stars in the outer regions of M 31 fit into the spiral structure. 3. The suspected OB stars outside M 31 (R>30 kpc) in all probability belong to our own galaxy. It is to be expected that they partly are blue horizontal branch stars of extreme population II, partly white dwarfs rather far left of the black body line in the two colour diagram.     

Ob-Stars on the utmost Borders of M 31

In a field of 4 square degrees, including the southwestern part of M 31 18 blue starlike objects are found by a photometric statistics in U,B,V of all starlike objects in this field. With great probability 13 of them are OB stars of M 31. Their utmost distances from the nucleus of M 31 are 35…50 kpc. Three further objects are probably quasistellar objects. Two others are striking by their extraordinary position in the two-colour diagram and cannot be classified in a distinct category of objects.     

A new stellar standard sequence in the coma cluster of galaxies. (preliminary report)

Photographic magnitudes in the UBV system of 39 stars in the Coma cluster of galaxies are given.     

Photometry of Galaxies in the Field of the Galaxy Cluster aBELL 1781

A method for determing the brightness and diameter of galaxies in a large field on Schmidt plates by automatic scanning with a microphotometer is described and applied to the ABELL cluster A 1781. The accuracy of this method is tested. The overall errors (r.m.s.) of brightness and diameter are 0.16 mag and 0″.44. The cluster A 1781 has been prooved to be a very poor cluster of about 10 members up to m_B ≈ 19^m.5.     

Kukarkin-Parenago relationship,period gap,and cause of dwarf nova outbursts

It is shown that it can be useful to incorporate the orbital period P as an additional parameter into the Kukarkin-Parenago relationship. Then it turns out that the relationship splits into two, one holding for objects above and the other for objects below the well-known and much-discussed period gap between 2.2 and 2.8 hours A = -2.03 + 2.26 log C + 8.80 log P for P > 2.2 h A = 2.07 + 2.46 log C - 3.68 log P for P > 2.8 h (A amplitude (mag), C cycle length (d), P orbital period (h)). Future theories of the dwarf nova phenomenon and the origin of the period gap will have to deal with this.     

The neighbourhoods of the nearest hickson groups

The comparison of the spatial distribution of galaxies in the surroundings of the nearest compact and loose groups of galaxies confirms that compact groups are very likely to be part of looser groups and shows that loose groups associated to compact groups tend to have higher spatial density than normal ones.