Distance estimation of the LMC,SMC, and M33 from the apparent radius distribution ofHii-regions

The size distribution ofHii-regions in the LMC, SMC, and M33 is used to estimate their distances. The values deduced are: 42.66±2.51, 89.73±5.03, and 755.11±47.14 kpc, respectively.     

30 Doradus as the active centre of the Large Magellanic Cloud

Evidence is presented for the hypothesis that the supergiantHii-complex 30 Doradus (NGC 2070) is the mildly active galactic nucleus of the Large Magellanic Cloud. For this purpose the general properties of galactic nuclei and the characteristics of active nuclei are reviewed (Section 2). Examination of 30 Doradus shows that it plasy the same exceptional role among allHii-regions of the LMC as Sgr A among those of our Galaxy, and has all the properties of a galactic nucleus (luminosity, emission spectrum, IR source, semistellar central object R 136, symmetry centre of an starting point of spiral structure). Evidence for the activity is given by the peculiar filamentary structure (Figure 1), the young spiral filaments superposed on old, broad and smooth near-circular arms (Figure 2), the splitting of the [Oiii] 5007 profile in two components corresponding to an expansion velocity of 50 km s^–1, and the strong non-thermal component (Section 3). The mass loss of 30 Dor is estimated at 0.05M /a. It is speculated that the nucleus of a galaxy may be wandering due to explosive events.     

Ring nebulae associated with of stars: Statistics,classification, origin

The ring nebulae associated with galactic Of stars is considered on the grounds of the list of Of nebulae proposed by lozinskaya and Lomovsky (1982). Taking into account the selection effects, about 80% of Of stars are shown to be associated withHii regions and about 30–50% of these regions have shell structures. Four types of nebulae associated with Of stars are resolved: amorphousHii regions, ring-likeHii regions, wind-blown bubbles, and stellar ejectas. These types appear to be identical to the morphology of nebulae around WR stars proposed by Chu (1981). Observational data are presented and the nature of a number of Of ring nebulae of different types is discussed.     

Radio emission of galactic clusters at 1.4 GHz and 2.7 GHz

41 galactic clusters containing stars with spectral types from O5 to B9 have been observed at frequencies of 1.4 GHz and 2.7 GHz. Only clusters with spectral types earlier than B1 show thermal radio emission. Emission measure, mean electron density, and mass ofHii gas have been computed from the observed data, and a comparison between the radio flux and H- and H-flux densities has been made. The ratio of hydrogen mass and total stellar mass of the clusters and the gas to dust ratio are given.     

Hii region statistics in spirals

From high-resolution maps of the spiral galaxies NGC 3992 and NGC 4321, obtained with the TAURUS camera at the 4.2 m William Herschel Telescope, we have inferred: (a) The total number of separately identifiableHii regions in their discs classified according to their membership of a spiral arm or the interarm disc, (b) the radial distribution ofHii regions, (c) the frequency distribution of diameters ofHii regions, (d) the luminosity function; each parameter rpesented separately for theHii regions in the arms and in the interarm region, showing the differences between them. From these observations we have also derived the radial dependence of the product of the filling factor, the electron density, and the ionized hydrogen density, which does not appear to change between arm and interarm disc.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

The injection of energy into the interstellar medium by stars (invited review)

Stars inject energy into the interstellar medium (ISM) by radiation, stellar winds, and supernova explosions. This energy injection causes the ISM to be inhomogeneous, which in turn alters the manner in which the energy is transferred through the ISM. A significant fraction of the energy is injected by massive stars, which formHii regions in the ISM. The structure and evolution ofHii regions in a cloudy medium deffers significantly from that in a homogeneous one. The strong stellar winds produced by massive stars form bubbles in the ISM, and the structure of these bubbles is often dominated by the structure of theHii region in which they are embedded. Finally, when the star explodes as a supernova, the evolution and appearance of the resulting remnant is determined by the structure of the bubble andHii region formed by the star during its lifetime.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.     

Neutral hydrogen in the vicinity of galactic radio sources

In the preceding paper by Vengeret al. (1984) the results of observations of neutral gas in the vicinity of some galacticHii regions were considered; and it was demonstrated that 17 of the regions observed are surrounded by expandingHi envelopes. This paper describes a model of interaction between theHii regions and the surrounding interstellar medium constructed on the basis of the said results. It is assumed that the main dynamic factor in the envelope formation mechanism is the total stellar wind from the stars which ionze theHii regions. The employment of the observations of the line at =21 cm, of the radio continuum and IR dust continuum made it possible to determine the differential mass spectra of the stars exciting theHii zones and calculate some characteristics of the stellar population. The mass spectrum index of the objects considered turned out to be much higher than that for the mass spectra of background stars and scattered clusters of stars.     

On the effect of collisional ionization during the time evolution of Hii regions

The set of equations describing the time evolution ofHii regions, accounting for collisional ionization, are presented. Differential forms of these equations are deduced, and it is shown that it is not necessary within this context to consider changes in the potential energy due to ionization of the gas.     

Study of WATCH GRB error boxes

We have studied the first WATCH GRB Catalogue of-ray Bursts in order to find correlations between WATCH GRB error boxes and a great variety of celestial objects present in 33 different catalogues. No particular class of objects has been found to be significantly correlated with the WATCH GRBs.     

Global photometric observations and physical properties ofHii regions in the small magellanic cloud

Integrated photoelectric measurements of the equivalent widthW _H, the [Oiii]/H ratio and the H emission line flux were obtained for 30Hii regions in the SMC. Physical properties of theHii regions and their ionizing stellar associations were derived. Some aspects of the recent star formation in the SMC and the evolution ofHii regions are discussed.     

An electron temperature distribution derived from optical and radio measurements of HII regions

Radio measurements of the electron temperature ofHii regions are obtained from the ratio of the brightness temperature of a hydrogen recombination line to that of the adjacent continuum, while optical measurements are obtained from the ratio of [Oiii] forbidden-line intensities. The radio and optical measurements made under the assumption of an isothermalHii region,T _R andT _opt respectively, are combined to derive a temperature distribution for an entire nebula. A sphericalHii region in local thermodynamic equilibrium with constant density which is optically thin in both the line and the continuum is used as a model. Assuming linear temperature gradients withT _R=6000K andT _opt=10000K, it is found thatT=12000K (1–0.74r/R), wherer is the distance from the center andR is the radius of the nebula.     

BN objects — A class of very young and massive stars

The Becklin-Neugebauer objects are identified by means of a wide range of observable features as a separate class of very young and massive stars surrounded by optically thick dust shells. We show that they evolutionarily connect real protostars to compactHii regions. We give criteria which should be appropriate to segregate the BN objects from compactHii regions. Finally, we describe the structure of a typical BN object.Paper presented at a Workshop on The Role of Dust in Dense Regions of Interstellar Matter, held at Georgenthal, G.D.R., in March, 1986.     

The origin of cosmic gamma-ray bursts

The positions of 46 γ-ray burst sources on the sky are used to show that the majority of these objects either originate in very distant regions with redshifts ?30000 km s^?1 or within ?0.5β of the Sun; where β is the scale-height of the parent population perpendicular to the galactic disc. An origin of the majority of γ-ray bursts in the more distant parts of the galactic disc, the galactic nuclear bulge, the Virgo supercluster, in galaxies withm _pg<18 and rich in Abell clusters of distance classes 0 to 4 is excluded by the data.     

Das geschwindigkeitsellipsoid derHII-regionen in M33

The peculiar radial velocities ofHii regions in M33 varies with position angle in a manner that can be explained by an ellipsoidal distribution with minor axis in the direction of rotation. The amplitude of the variations, however, is too great as compared with theory or experience in the galaxy.

---

---

Mitteilungen Serie A, Nr. 36.     

The colours of the interstellar medium

On the basis ofI-I plots, we find that the ISM radiates preferentially at two pairs of far-infrared frequencies which correspond to (scattered) black-body temperatures of (23 ± 1, 187 ± 5) K and (39 ± 1, 104 ± 5) K. The first pair is emitted by the cold matrix, the second pair byHii regions and supernova shells.     

Simulation of the apparent anomalies of the galactic structure

In the maps of the galactic structure based on the kinematical method, several systematic heliocentric anomalies are found: in the northern galactic hemisphere the spiral arms are more tightly wound and the extent of neutral hydrogen is smaller than in the southern hemisphere; with separate rotation curves for the north and the south the arms become anomalously circular with a consequent discrepancy to the stellar distribution; there are straight portions in the arms pointing towards the Sun, as well as systematic strong curvatures and knee-like features; the inner arms affect the structure of the outer arms; with the northern rotation model, Hii-regions and Hi avoid the southern tangential circle; in the rear of the Galaxy, at symmetric longitudes, enhanced Hi-densities are found; the Perseus arm is displaced atl=180°. All of these anomalies can be explained with a simple model involving a non-velocity redshift field within the Galaxy, with an enhancement within the spiral arms. This is demonstrated by numerical simulations of the structural anomalies. Reducing the redshift effect from the kinematic data, the Galaxy's structure and kinematics appear symmetric. The significance of the result for the redshift problem is discussed.     

Energy spectra of gamma-ray bursts detected by the APEX experiment on phobos mission

Molecular clouds are clumpy on length scales down to the limits of observational resolution. At least some ultracompactHii regions (UCHiiR) may result from the interaction of a young early type star and this type of cloud. The clumps can act as reservoirs of ionized gas distributed within theHii region. These models reproduce the relatively long lifetimes implied by the population statistics of UCHiiR. We present line profile and emission measure plots based on the simplest case where the flow remains supersonic through to a recombination front. The morphology agrees with the shell-like UCHiiR as classified by Churchwell. The predicted line profiles are broad and double peaked with a separation of about 50 km s^–1 for the example given.     

Recombination line observations in southernHii regions

SeveralHii regions were observed in the Instituto Argentina de Radioastronomía (IAR) in the H166 line and 1.4 GHz radio continuum. Resulting physical parameters derived from low-frequency (H166 line) and high-frequency (H110, H109, H76 line, made by other authors) observations ofHii regions are compared. Low-frequency derived parameters describe better the low-density ionized gas conditions, whereas high-frequency observations are more sensitive to high density ionized gas located in the central parts of the nebulae.     

Neutral hydrogen observations onHii regions

Nine of the cataloguedHii regions in the southern hemisphere have been selected for correlation with neutral hydrogen observed at the 21 cm line.The radiotelescope used for theHi line observations was the 30 m Carnegie telescope of the Instituto Argentino de Radioastronomía and the 56 channel, 10 KHz bandwidth receiver.The observational results are analyzed for eachHii region. They are compared with previous optical and radio results.For three of the nine observed regions it has been possible to find neutral hydrogen in absorption with similar velocities. In three cases absorption has been found but no component at theHii region velocity is seen. Finally in three cases, it has not been possible to find any absorption at all.Member of the Carrera del Investigador Científico del Cosejo Nacional de Investigaciones Científicas y Técnicas.     

Giant infrared bubble in Cepheus

A ring-shaped infrared emission region is recognizable on the IRAS Sky Flux images of a Cepheus region which happens to include the association Cepheus OB 2. The ring is easily visible both at 60 and 100 microns. The approximate galactic coordinates of its centre arel=102°.8 andb=+6°.7, with an outer diameter of 7 deg. IC 1396 and several otherHii regions, such as S 129, S 133, S 134, and S 140 are apparently parts of the ring. If it is assumed that these Hii regions are physically connected to the ring its distance must be about 900 pc and its diameter 120 pc. The existence of several arc-shaped H filaments along the ring, the proper motion of the nearby runaway star Cephei, and the possible presence of the [Fex] 6375 interstellar line in the spectra of two stars of Cep OB 2 combine to suggest that the infrared ring might well be a result of a supernova explosion which occurred in this region about 2–3 million years ago.