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.     

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.     

The effects of stark broadening in the radio recombination line temperatures

Observations of Orion A, M 17, and W 3 made at the National Radio Astronomy Observatory show Stark broadening in the , , and hydrogen recombination lines at 22 cm. Stark effect has such a large effect on the line profiles that fitted Gaussian profiles cannot be used to measure the area of the line. Theoretical line profiles have been derived from modelHii regions. Voigt profiles fitted to the theoretical line profiles indicate that the areas of the lines derived from the fitted profiles may be systematically too small. Using Voigt profiles fitted by least squares to the observations, we found that the line intensities at 22 cm agree with the intensity ratios predicted by LTE theory.     

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.     

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.     

A high galactic latitude HI 21 cm-line absorption survey using the GMRT: I. Observations and spectra

We have used the Giant Meterwave Radio Telescope (GMRT) to measure the Galactic HI 21-cm line absorption towards 102 extragalactic radio continuum sources, located at high (|b| > 15°) Galactic latitudes. The Declination coverage of the present survey is δ}> - 45°. With a mean rms optical depth of ∼ 0.003, this is the most sensitive Galactic HI 21-cm line absorption survey to date. To supplement the absorption data, we have extracted the HI 21-cm line emission profiles towards these 102 lines of sight from the Leiden Dwingeloo Survey of Galactic neutral hydrogen. We have carried out a Gaussian fitting analysis to identify the discrete absorption and emission components in these profiles. In this paper, we present the spectra and the components. A subsequent paper will discuss the interpretation of these results.     

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.     

Observations of high velocities inHii regions with a two-etalon Fabry-Pérot spectrometer

High internal motions of the ionized material in theHii regions M 8, M16, M 17 and the Orion Nebula were searched for with a two-etalon Fabry-Perot monochromotor. The profiles of the [Oiii], 5007 Å and in one case the 4959 Å line were obtained at many positions from these nebulae. Non-gaussian wings of up to –60 km/s were found on the profiles from M 17 and M 16 over regions several minutes across. Line doubling of up to 20 km/s was definitely found in M17 and M 8. Small components with radial velocities of up to –55 km/s with respect to the means were suspected in M 8 and the Orion Nebula.     

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.     

Surface photometry of simpleHii regions

We have carried out surface photometry of six simpleHii regions which are characterized by the existence of an exciting star earlier than B0, and a nearly round shape. On the basis of on balibrated image data and adopting a spherical model, we derive the distributions of gas and dust densities in theHii regions.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.On leave from the Bosscha Observatory, Institute of Technology Bandung, Indonesia.     

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.     

The equation of state of a cosmological selfgravitating fluid influenced by thermal or turbulent motion and the primeval background radiation

A variety of new optical observations have been made over the bright core of the supermassiveHii region 30 Doradus and the ionized filamentary material surrounding this object. In addition, a more detailed analysis of previously published interstellar absorption andHi emission profiles has been undertaken.The velocity and density structure of this complex region is discussed and the new results analysed here shown to be compatible to the model of 30 Doradus presented in Cantóet al. (1980) and Meaburn (1980).     

Surface photometry of theHii regions S254–S257

The surface photometry of S254–S257 has been carried out by means of a wide range image processing technique in the reduction system. The photographic plates in the H+[NII] andV-bands are taken with the Schmidt telescope. Especially, we have obtained the calibrated map of theHii region, superposing two or more plates with different exposure times, and removing the star images. Three kinds of calibrated maps of theHii regions are drawn: (1)E-map in the (H+[NII]+continuum) (2)V-map in the continuum atV-band, (3)(E-V)-map in the (H+[NII]) line emission. The intensity profiles across the nebular centers were also obtained. Based on calibrated maps, the morphological structure and mass distribution of S255 and S257 are discussed. The location of observed nebulae on the (m _H–m _v) diagram, wherem _H andm _v denote the surface brightness, expressed in the magnitude per square arcmin, is shown together with that of some other nebulae. Some arguements on the age sequence of observedHii regions are presented.     

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.     

Observations of ammonia in galacticHii regions

The ammonia emission associated with southern galacticHii regions have been studied. Some physical parameters, computed for each individual sources, were derived and a brief discussion follows.     

Chemical abundances of the WR-ring nebulae NGC 2359 and RCW 78

We report on extensive spectroscopic observations of the WR-ring nebulae NGC 2359 and RCW 78, respectively, excited by the WN5 stars HD 56925 and WN8 HD 117688. For the first object we have determined abundances for O/H, Ne/H, N/H, and He/H in many different positions, including the ionized gas in the bubble, inside the optical shell structure, and the outermost zones associated with the S 298 Hii region. We do not find any significant difference in the N/H and O/H abundances over the entire nebula. The O/H and N/H abundances expected are close to those for a normal Hii region located at similar distance. In the case of He/H we find indication of local enhancements which sum to the abundance of metal rich galactic Hii regions like M17. RCW 78 appears to show slight overabundances of He/H and N/H in the two observed positions.The ionising temperature for the WN central star (HD 56925) of NGC 2359 is determined from the observed Hii region spectrum giving a value of 50000 K, appropriate to its spectral type.     

High-chromosphere and low-transition-region network: A different organization in an equatorial coronal hole?

In order to investigate the high chromosphere and the low transition region in a coronal hole, we have analysed Ca ii, Mg ii and hydrogen resonance lines, recorded by the OSO-8 spectrometer in 1975. We present the comparison between average profiles observed in and out of the equatorial coronal hole which was at the center of the solar disk between 27 and 29 November, 1975. We separate internetwork and quiet-Sun (network+internetwork) profiles: for the internetwork, we observe that the hydrogen and Mg ii profiles recorded in the hole are stronger than the profiles recorded out of the hole; a similar result, but with a much lower contrast, is found for the quiet Sun. We discuss this surprising result.     

A photometric study of the double ring structure of NGC 4736

In this study we present photometric results for the galaxy NGC 4736: infrared and visible profiles. After a careful correction for the extinction within the galaxy based on measured neutral gas surface densities, we interpret the profiles in the individual bands and in colour indices, in terms of the radial distribution of stellar populations. We pick out the behaviour of the two rings, an inner ring some 40–50 arc sec from the nucleus, and an outer ring some 300 arc sec away. We show how the photometry allows us to make tentative physical inferences about the nature of these two structures, showing that the inner ring is connected with an outflow of gas observed via itsHii regions, and is probably the result of an axisymmetric starburst, while the outer ring is a site of star formation which appears to be further from the centre than the typical resonant structures associated with a density wave.     

The continuous emission fromHii regions

The contribution to the continuous emission by electron scattering in anHii region (Pottasch's suggestion) is studied. The phenomenon is evaluated for different cases, and compared with the contribution due to atomic processes. It is proved, taking into account all the possible variables, that the contribution is neglected for typical Hii regions in our Galaxy, and in other galaxies.     

Absorption in the 21-cm Line in Primordial Matter Density Fluctuations at the Stage of Their Nonlinear Compression

The mechanisms of absorption formation in the cosmic microwave background (CMB) spectrum at the frequency of the 21-cm line of the transition between the ground-state hyperfine sublevels of the hydrogen atom are analyzed. We show that a strong nonlinearity at the compression stage of primordial matter density fluctuations can give rise to a significant (in depth) absorption even before the explosions of the first stars. In this case, the main effect is due to the heating of matter in a certain narrow range of temperatures under cloud compression. We consider a steady-state radiative transfer in the 21-cm line in a medium that represents a contracting primordial matter density fluctuation at a given redshift z modeled by a homogeneous spherically symmetric cloud in the state of collapse with an adiabatic change in the gas temperature. For a sequence of cloud states with different degrees of compression we have calculated the frequency profiles of the line in the flux of radiation emerging from the cloud. In the initial state we specify the cloud radius r₀, while the gas density is assumed to be equal to the mean cosmological density for a given redshift. We show that for a separate cloud at z = 20, r₀ = 1 kpc, and a degree of radius compression of 1.9 the absorption depth in the line center can reach 0.9 K. When averaged over an ensemble of clouds, the central frequency of the line and its width are determined by the details of the fluctuation evolution dynamics.