Differential efficiency of massive star formation in NGC 628

The efficiency ratio of massive star formation in the arms of the spiral NGC 628 to that in the interarm region has been determined from the sizes and positions of itsHii regions following the method described in Cepa and Beckman (1989a, b). The variation of this relative star formation efficiency with radius shows a very similar behaviour for each of the three arms of this galaxy with a minimum at 100 arc sec from the centre, which can be identified with the corotation radius distances at which the arms show bifurcations or sudden changes in their pitch angles. This result, together with the higher star formation efficiency in the arms than that in the interarm zones, is evidence supporting the hypothesis that the spiral arms of NGC 628 are driven by a density wave which triggers the star formation according to the spiral density wave theory.     

The stellar content of a spiral arm of NGC 4321

The effects of the passage of a spiral arm through the disc of the giant Virgo Sc galaxy NGC 4321 are investigated with Hubble Space Telescope WFPC2 images in two colours. Concentrating on a portion of the southern spiral arm of NGC 4321, we have applied a new program to solve for the star formation histories in the arm and interarm regions separately. The observational uncertainties and the variable crowding across the spiral arm are taken into account using the results of artificial star tests. In the interarm regions the data are consistent with a constant star formation rate for the last 50 Myr while the stars in the arm region show a star formation rate four times larger than in the interarm regions in the last 5 Myr.     

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 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.     

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.     

Evidence for high velocity gas in giantHii regions

Spectroscopy of selected areas of the two GiantHii Regions (GHR) NGC 5471 and NGC 604 shows convincing evidence for the existence of ionised gas at high velocities (1000 km s^–1) from the analysis of the profile of the Balmer (in NGC 604) and the [Oiii] (in NGC 5471) lines. The association of these effects with the presence of WR clusters and possible SNR inside the complex are analysed and the results are compared with previous indications in the literature for 30 Dor (in front of R 136) as well as with recent evidence from high-dispersion spectroscopy for some GHRs in M101.     

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.     

The structure of a typical double-ring planetary nebula NGC 2392

Monochromatic photographs in H, [Nii] 6584 and [Oiii] 5007 Å show many different details in the morphological structure of the Eskimo Nebula (NGC 2392). H and [Nii] images show, in a first approximation, similar structure (elliptical inner ring and broken outer ring) whereas both rings in [Oiii] are quite circular and regular in brightness. A photometric study using the method described by Louise (1974) gives various geometrical parameters of both rings which are practically the same for the observed lines. In other words, the classical stratification structure in planetary nebulae is not clearly observed in NGC 2392, in good agreement with previous observations (Wilson, 1950). This fact is probably a consequence of the peculiar structure in the geometry of the nebula. A model consisting of an inner toroid surrounded by a spherical shell is proposed to account for both photometric and spectroscopic observations.     

Radial variations of line ratios across NGC 5236

The ionized gas in NGC 5236 was studied by spectroscopic means. The radial behaviour of the line ratios [Nii]/[Sii], H/[Sii] suggest a real nitrogen overabundance in the central regions of that galaxy.     

NGC 1144, the fastest rotating galaxy?

The photometry and kinematics of the interacting galaxy system Arp 118 are presented. Its eastern component, NGC 1144, has a Seyfert 2 nucleus and is dominated by a ring-like structure of giantHii regions, the radial velocities of which vary by more than 1000 km s^–1. Kinematical models including a close encounter with the elliptical companion NGC 1143 lead to extremely high values for the rotational velocity, the mass of NGC 1144 and itsM/L ratio.     

Physical conditions in NGC 6164-5

From spectroscopic observations of the emission lines H, 4959 and 5007[Oiii], H, 6584[Nii], 6717 and 6731[Sii] and some interferometric data in H and [Nii] it was established that NGC 6164 and NGC 6165 have characteristics of normal Hii regions for Ne and Te. They are embedded in a very tenuous medium nearly coincident with the Strömgren sphere of the central star HD 148937, corresponding to the outer peripheral structure described by Westerlund. Variations of the relative abundances of N, O, and S are discussed.     

An improved evolutionary model for stellar population applied to NGC 2903

We outline the revised physical assumption which have been incorporated into our previously published evolutionary model for stellar population synthesis in galaxies. Applying the present version of the model to well-defined zones on the nucleus and along a spiral arm of NGC 2903 we show that the star formation rate has increased during the lifetime of the galaxy, steadily but only slightly in the arm, and spectacularly in the nucleus, where there is current starburst activity. We also infer that considerable gaseous inflow has occurred in the arm during the lifetime of the disc.Paper presented at the 11 th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

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.     

Efficiency of massive star formation in grand design spirals

The present study estimates the efficiency ratio for massive star formation between the arms and the interarm discs of three grand design spirals. The estimate is based on H mapping observations of theHii regions in the galaxies. We find that this efficiency ratio is 10 in the zones between the Lindbalad resonances and the radius which we infer to be co-rotation, dropping to values close to unity at these three resonance raddii. these results point to a dominant influence of resonance structure in stimulating star formation in grand design spirals.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.     

Nebular complex in the region of Cas OB2 association; ring nebula Sh157

This paper presents the results of monochromatic [Oiii], [Nii], and [Sii] observations of ring nebula Sh157 around the star (WR + B0III) HD 219460 belonging to the Ba 3 cluster. A stratification of radiation typical for photoionization excitation has been revealed. The observations suggest that the Sh157 ring structure may arise as a result of the HD 219460 stellar wind blowing the surroundingHii region, and the bubble age is found to bet(2–5)×10⁵ yr. Three outer envelopes have been distinguished: a weaker extended emission shell apparently blown out by the wind from B-stars of the Ba 3 cluster, and two dust shells which are likely to be associated with NGC 7510 and Cas OB2. The paper is also concerned with the discussion of young star aggregates Ba 3, NGC 7510, Cas OB1, OB2, OB4, OB5, OB7, and Cep OBI and the associatedHii regions, shells and supershells of gas and dust, molecular clouds, and supernova remnants which may be probable members of a single giant stellar complex where the star formation process is in progress.     

Narrow band CCD imagery of southern galaxies in strong lines and continua: NGC 613

CCD narrow band imagery of the southern barred spiral NGC 613 has been obtained in the light of H, H, [Oiii] 5007 Å, [Oii] (3726+29) Å and continua at 3765, 5230, and 6500 Å. Absolute fluxes have been obtained for the lines H and [Oiii] 5007 Å and continua at 5230 and 6500 Å. Distribution of ages, monochromatic colour vs absolute brightness and [Oiii]/H are discussed.     

Monochromatic photographs of giantHii regions in the Magellanic Clouds

Narrow band-pass direct photographs have been secured of the central region of 30 Doradus and of several other nebulosities in the Magellanic Clouds. We have utilized an image tube camera attached to the Yale one-meter telescope at the Cerro Tololo Interamerican Observatory. Interference filters whose band passes were selected to allow for the radial velocities of the Magellanic Clouds have been utilized to record monochromatic images of [Nii] 6584, H, Hei 5876, and [Oiii] 5007. In 30 Doradus the main filamentary structures appear to be very similar in H, Hei, [Nii], and [Oiii] at least in the inner regions where most spectroscopic measurements have been made. The features appear to be material bounded rather than radiation bounded and the excitation differences are very small. Likewise, in NGC 346 in the Small Magellanic Cloud the [Oiii] and H images are very similar, but in N44 in the Large Magellanic Cloud excitation differences do appear. The implications of these observations for chemical compositions of nebulosities in the Magellanic Clouds would appear to be that spectroscopic and scanner studies should be supplemented by monochromatic photographs. Some nebulosities can be handled by simple models while for others excitation differences must be taken into account.Guest investigators at Cerro Tololo Interamerican Observatory operated by the Association of Universities for Research in Astronomy under contract with the National Science Foundation     

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.     

The electron temperature distributions and internal kinematics of seven diffuse nebulae

A Fabry-Pérot spectrophotometer is used to derive values of the intensity ratio H/[Nii] at 98 points in the seven bright diffuse nebulae M8, M20, M16, M17, NGC7000, M42, IC434. The fraction of nitrogen in the singly ionized state is estimated in the different objects, and is found to be sufficiently constant within any one nebula so that the above intensity ratio may be used to derive accurate electron temperature distributions. The position of the peak of the nebular line, its excess non-thermal width, its shape and relative intensity are used to derive kinematical models of these objects.It is found that values of H/[Nii]1 are representative of the bright central cores of these nebulae. Temperatures between 7000K and 12000K were derived in the different objects. Although some of this apparent variation is due to the different conditions of excitation in the various nebulae, it is shown that a convincing progression of temperature in M8, M16, M17 is supported by radio recombination line results. The temperature variation within any one object was generally significantly less than 1500K.No evidence was found for velocities of mass motion at more than twice the speed of sound. Relative radial velocities of generally less than 15 km sec^–1 characterized the velocity fields of M8, M20, M16, M42. The velocities in M17 were measured as about 20 km sec^–1. Motions in NGC 7000 and IC 434 were much lower (5 km sec^–1) although here the number of points taken was too small to construct meaningful kinematical models.It is concluded that the internal motions of radiatively ionizedHii regions of Pop. I will not significantly affect the results of existing surveys for determining the rotation of the galaxy with radial velocities deduced from nebular emission lines.     

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.