The physical parameters and exciting stars of six Hii regions

The radio spectra of particular radio sources (mainly Hii regions) are plotted on the same graph for comparison. Various physical parameters of these objects are obtained from the radio data through a model. Predictions about the number and the nature of the exciting stars of six Hii regions are made. A comparison with the so far identified exciting stars is also presented.     

A review of recent observations of dust in Hii regions

The upper limit for the absorption cross section σ _H ^ext , of dust in Hii regions in the wave-length range 912–504 Å derived by Mezgeret al. (1974), is compatible with that expected for large dust grains, and a gas-to-dust ratio equal to that in the general interstellar medium. The albedo of the small grains must be high for λ>504 Å. This restriction is lifted if the visual extinction cross section of the grains in Hii regions is less than that for grains in the general interstellar medium. New observations of the Orion Nebula indicate that the visual extinction cross section is within a factor 2 of the value in the general interstellar medium.     

Radio observations of IRAS-selected Southern hemisphere classical Be stars

We present the first radio observations of a sample of 13 optically and IR-bright Southern hemisphere classical Be stars made from the Australian Telescope Compact Array at 3.5 and 6.3 cm simultaneously. One star, δ Cen, was detected at 3.5 cm, and a second, μ Cen, was also thought to have been detected; further observations of this source are required to confirm this detection. No sources were detected at 6.3 cm, although δ Cen was previously detected at this wavelength by other observers at a higher flux than our detection limit. The radio observations show that the spectral energy distribution undergoes a turnover between the far-IR and radio wavelengths, as was seen in previous studies. Likewise we find no simple correlation between far-IR and radio flux. Lower limits to the outer disc radius were found to be of the order of a few hundred solar radii i.e. of the order of those found previously by Taylor et al.     

Sub-mm and far-infrared continuum observations of herbig-haro exciting stars and other yso's

We show initial results from a survey of the mm to far-IR continuum spectra of 30 YSO's known to be exciting Herbig-Haro objects. The data are also compared with line intensities of C¹⁸O and H₂CO. We include in this analysis results from other sub-mm continuum surveys of compact HII regions, T-Tauri stars and class 0 YSO's. The results provide a statistical sample of the long-wavelength dust spectra of 60 Young Stellar Objects. Data are displayed on mm to FIR colour-colour diagrams, with the aim of trying to discriminate between different stages of star formation through general spectral characteristics, rather than detailed model fits to individual sources.     

Meter-decameter observations of dMe flare stars with the Clark lake Radio telescope

The now-closed Clark Radio Observatory was used in 1984 and 1985 to search for flaring emission from a number of dMe flare stars in the 30.9 to 110.6 MHz frequency range. No emission was found to greatly exceed detection limits which range from about 1 Jy for 1 hr averaging, to about 50 Jy for 1 s averaging, even though flares were often seen to tens of mJy at 20 cm using the VLA for those times when VLA-CLRO observations were coordinated. There are marginal detections of flaring from AD Leo over two periods on December 15, 1985 which mark the beginning and the end of along-lasting, narrow-band flare at 1415 MHz.     

The structure of the dense cores of Hii regions

The latest observational and theoretical evidence which suggests the existence of a large number of partially ionized globules (PIG's) in the cores of Hii regions is presented. The consequences of this proposition to the structure of the Orion Nebula are then considered. It is suggested that the dense ionized core of M42 is caused by two major ionization fronts eating into a large mass of neutral material which contains many dense neutral globules. The ‘Hourglass’ in M8 is shown to be very similar.     

Tides and Arp's hypothesis about distribution of Hii regions

We discuss Arp's hypothesis that the HII regions are more numerous and more conspicuous on the side of a galaxy facing its companion. Arp's hypothesis seems not to be true if we add to Hodge's sets of galaxies only the most probably tidally-interacting cases.     

Sampling effects in the emission-line spectra of Hii regions

We show that sampling effects in the initial mass function are very important in the low-mass cluster case. To this aim, we compute photoionization models ionized by realistic clusters made up of various combinations of individual stars and clusters built with a synthesis model. We discuss the differences in the position on diagnostic diagrams, their implications, and future and ongoing applications of the present work.     

The forbidden line spectrum in the edges of Hii regions

The influence of a relative motion between a hot star and the surrounding medium on the forbidden lines emission spectrum of the latter is described in an example.     

Grains and the galactic electron temperature gradient of Hii regions

The effect of dust on the electron temperature gradient of galactic Hii regions is investigated taking into account the photoelectric heating mechanism and using a recent galactic rotation curve. It is shown that the grains are unable to contribute substantially to the observed gradient of electron temperature.     

A classification of the available astrophysical data of particular Hii regions

A detailed classification of the available data on W3 is presented. Radio, optical and infrared maps of the object are shown in order of increasing angular resolution. The physical parameters and energy spectra of the various components of W3 and their sources are presented. A summary of the current theories on W3 is also included.     

A classification of the available astrophysical data of particular Hii regions

The physical parameters of the object compiled from various authors are classified in a separate table. Line ratios are presented for their significance in determining the density and temperature of the nebula. Data for the electron temperature of the object are also presented. A correlation of several interesting features of the core of the nebular complex is also shown.     

A classification of the available astrophysical data of particular Hii regions

Various contour maps of the object are presented. The physical parameters of the nebula are also classified and presented either in separate tables or in contour map form.     

W28-A possible association of supernova remnants and Hii regions

The nature of various components of the W28 complex region is investigated. The radio spectra of W28-A1 (G 6.4-0.2), M20 (G 7.0-0.2), W28-A2 (G 5.9-0.4), W28-A4 (G 5.3-1.1), KE59 (G 6.6-0.3) and G 6.4-0.5 are established over a wide range of frequencies. The W28-A1 (G 6.4-0.2) source is a SNR (sp. index –0.41), the M20, W28-A2 and KE59 seem to be thermal sources (sp. indexes –0.06, –0.15 and –0.04 respectively) whereas the W28-A4 and G 6.4-0.5 are possibly mixed sources containing thermal and non-thermal features. Certain physical parameters of the thermal components are derived by adopting a model. The physical properties of the W28-A1 SNR are investigated. The possibility of a SNR-Hii regions association in the W28 region is also discussed.     

Hipparcos observations of pre-main-sequence stars

We present first results of Hipparcos observations of nearby low-mass pre-main-sequence (PMS) stars (T Tauri and Herbig Ae/Be stars). The data obtained by Hipparcos allow us to derive weighted mean parallaxes for three major nearby star-forming regions (SFRs), Lupus, Chamaeleon I and Taurus–Auriga. Furthermore, results on the isolated objects AB Dor and TW Hya are presented. Finally, we discuss the evolutionary status of Herbig Ae/Be (HAEBE) stars on the basis of Hipparcos results.     

Infrared observations of symbiotic stars

Near infrared measurements in the J, H and K bands have been carried out for a number of symbiotic stars with the 1.5 m telescope at the Rothney Astrophysical Observatory (RAO). A comparison with the earlier observations shows that the S-type symbiotic stars do not have any significant variation in the infrared flux over the past five years. However a small variation ∼ 0.3 magnitude in the infrared flux has been detected for CH Cygni. The observations of HM Sagittae show large decreases in the infrared flux compared to the previous measurements. The variability in the infrared fluxes of both these objects could be attributed to a variation in the temperature due to the cooling of the dust shell. The variability observed for V1016 Cygni is found consistent with the previous measurements.     

Supernova remnants and content in high mass stars of the giant Hii region NGC 604

An optical survey of NGC 604, the brightestHII complex in M33, has lead to the discovery of a single supernova remnant, in agreement with the predominantly thermal radio spectrum of the region. For the same NGC 604, we have derived from the H flux and the evolutionary tracks of the stars, the population and the birth-rates of Main Sequence, high-mass stars. The computation is in agreement with the observations of the upper part of the stellar luminosity function. From the stellar population one predicts in the region a number of remnants significantly larger than 1. To explain this discrepancy, it is suggested the presence in NGC 604 of at least one overluminous, hot star (M _B9), which contributes to a large fraction of the ionizations.On leave from the Asiago Observatory.On leave from Istituto di Fisica Generale, Università di Torino.     

Iras observations of solar-like stars

IRAS has detected 70% of the 66 F, G, K nearby dwarf stars investigated here. The sample included chromospherically active as well as non-active dwarfs. The detected stars show emission at 12 and 25 m. Their 12 m luminosity is in the range 1–13×10³⁰ erg s^–1 and it is strongly correlated to the star's total luminosity (L _bol).There are indications that some of the stars possess IR emission in excess of that expected from a stellar photosphere.Paper presented at the 11th European Regional Astronomical Meeting of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

Spectrophotometric observations of variable stars

We present the results of our visual and near-infrared spectrophotometric observations for 77 variable stars obtained during 1971–1991 in Chile, Armenia, and Bolivia. The quasi-monochromatic, extraatmospheric fluxes from the stars are given in absolute energy units (W m^?2 m^?1) at all wavelengths of the spectral range at 2.5-nm intervals.     

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.