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Journal articles on the topic 'HII regions'

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Published: 4 June 2021

Last updated: 8 June 2024

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1

Chu, You Hua, and Robert C. Kennicutt. "Internal motions of HII regions and giant HII regions." Astrophysics and Space Science 216, no. 1-2 (June 1994): 253–62. http://dx.doi.org/10.1007/bf00982502.

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2

Kurtz, Stan. "Hypercompact HII regions." Proceedings of the International Astronomical Union 1, S227 (May 2005): 111–19. http://dx.doi.org/10.1017/s1743921305004424.

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3

Shapiro, Paul R., Ira Wasserman, and Mark L. Giroux. "Cosmological HII Regions." Symposium - International Astronomical Union 117 (1987): 366. http://dx.doi.org/10.1017/s0074180900150533.

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We have generalized the classical description of ionization front propagation to the case of a point source in a uniform, cosmologically expanding gas. We present illustrative curves for the comoving radius and peculiar velocity for several turn-on redshifts, z0N, for Ωtot = 1, Ωb = 0.1, h = 1. The quantity RS is the generalized Strömgren radius [RS = RSi (1 + z0N)/(1 + z), RSi = (3Nu/4πnH,i2 α2)1/3, Nu = photoionizing number flux per source, α2 = recombination rate to n = 2, nH,i = nHo (1 + z0N)3]. The quantity T0N = 2 (1 + z0N)−3/2/(3H0). We also plot ζ, the value of (2nQoNph, Q/3HonHo) needed to ionize the IGM with overlapping QSO HII regions by redshift z0V for QSO turn-on at various z0N, where Nph, Q = ionizing photon luminosity per QSO, nQo = QSO number density (present co-moving value), nH = H density of IGM, and nH/nHo = nQ/nQo = (1 + z)3. From a recent preprint by Koo (1985), we estimate ζ ≲ 1 (for Ωb = 0.1, h = 1) for QSO's with L ∼ 1045 erg s−1. In this case, the observed QSO's cannot be the sole source of the IGM ionization that is implied by the null detection of the Gunn-Peterson effect for QSO's with z > 2.

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4

Hughes, V. A. "Variable HII Regions." International Astronomical Union Colloquium 120 (1989): 104–10. http://dx.doi.org/10.1017/s0252921100023526.

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Initial CO observations of the Cepheus OB cloud by Sargent (1977), identified the condensation known as Cepheus A. Other observations have shown that it has the normal empirical indications of a star forming region. In the radio continuum, using the WSRT, it consists of two thermal components (Hughes and Wou-terloot 1982). The West component is associated with the Herbig-Haro object GGD-37, and will not be considered further here. The East component when observed further with the VLA, is seen to consist of two strings of about 14 compact HII regions, and is the subject of the present paper.

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5

O'dell, C. R. "Turbulence in HII Regions." Publications of the Astronomical Society of the Pacific 98 (November 1986): 1106. http://dx.doi.org/10.1086/131943.

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6

Fuentes-Masip, O., H. O. Castañeda, and C. Muñoz-Tuñón. "New Methods to Measure HII Regions Diameters." International Astronomical Union Colloquium 149 (1995): 143–47. http://dx.doi.org/10.1017/s025292110002282x.

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AbstractObservations of the giant irregular galaxy NGC 4449 are being used to study the correlations between the diameter or luminosity and the velocity dispersion of its giant HII regions (GHRs), understanding as GHRs those HII regions with supersonic velocity dispersions. In the central part of this galaxy the HII regions overlap, and also there is a strong, morphologically diffuse emission that permeates the main body of NGC 4449, both effects making difficult the detection, identification and separation of HII regions. We have developed new techniques to solve these problems, obtaining HII regions parameters equivalent to the ones that would be got if the HII regions did not overlap and if the diffuse emission did not exist.

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7

Chu, You-Hua, and Robert C. Kennicutt. "Supernova Remnants in Giant HII Regions." International Astronomical Union Colloquium 101 (1988): 201–4. http://dx.doi.org/10.1017/s0252921100102374.

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Giant HII regions contain large numbers of massive stars, and hence are expected to contain large numbers of SNRs. Until recently, however, only a few SNRs have been identified in extragalactic giant HII regions. Moreover, most of these SNRs are located at the outskirts of HII regions, instead of the core where most of the stars are located. The low detection rate and the outlying locations of the SNRs may be due to: 1) observational difficulties - the background HII regions are much more luminous than the SNRs in both optical line emission and radio continuum; 2) intrinsic invisibility of SNRs - stellar wind and SNRs may have created a supershell (Mac Low and McCray 1987), and the core of a giant HII region is filled with hot tenuous coronal gas; or 3) a genuine deficiency of supernovae and SNRs in the HII regions (Sramek and Weedman 1986).

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8

Kaufman, Michele, R. C. Kennicutt, and F. N. Bash. "Giant H II Regions in M81." Symposium - International Astronomical Union 116 (1986): 503–4. http://dx.doi.org/10.1017/s0074180900149459.

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Giant HII regions are important tracers of recent star formation in distant galaxies. For a selection of HII regions in our galaxy where the exciting stars can be identified, Rumstay (1985) finds that the measured Hα and radio continuum luminosities of an HII region correlate with the stellar ionizing flux derived from model atmospheres and the known exciting stars. Therefore, we use flux measurements of giant HII regions as an index of the distribution of O stars in M81.

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9

Stéphan, G., P. Schilke, J. Le Bourlot, A. Schmiedeke, R. Choudhury, B. Godard, and Á. Sánchez-Monge. "Chemical modeling of internal photon-dominated regions surrounding deeply embedded HC/UCHII regions." Astronomy & Astrophysics 617 (September 2018): A60. http://dx.doi.org/10.1051/0004-6361/201730639.

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Aims. We aim to investigate the chemistry of internal photon-dominated regions (PDRs) surrounding deeply embedded hypercompact (HC) and ultracompact (UC) HII regions. We search for specific tracers of this evolutionary stage of massive star formation that can be detected with current astronomical facilities. Methods. We modeled hot cores with embedded HC/UCHII regions (called HII region models in the article despite the fact that we do not model the HII region itself), by coupling the astrochemical code Saptarsy to a radiative transfer framework obtaining the spatio-temporal evolution of abundances as well as time-dependent synthetic spectra. In these models where we focused on the internal PDR surrounding the HII region, the gas temperature is set to the dust temperature and we do not include dynamics thus the density structure is fixed. We compared this to hot molecular core (HMC) models and studied the effect on the chemistry of the radiation field which is included in the HII region models only during the computation of abundances. In addition, we investigated the chemical evolution of the gas surrounding HII regions with models of different densities at the ionization front, different sizes of the ionized cavity and different initial abundances. Results. We obtain the time evolution of synthetic spectra for a dozen of selected species as well as ratios of their integrated intensities. We find that some molecules such as C, N2H+, CN, and HCO do not trace the inner core and so are not good tracers to distinguish the HII/PDR regions to the HMCs phase. On the contrary, C+ and O trace the internal PDRs, in the two models starting with different initial abundances, but are unfortunately currently unobservable with the current achievable spatial resolution because of the very thin internal PDR (Δ rPDR < 100 AU). The emission of these two tracers is very dependent on the size of the HII region and on the density in the PDR. In addition, we find that the abundance profiles are highly affected by the choice of the initial abundances, hence the importance to properly define them.

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10

O'Dell, C. R. "Comparison of turbulence in HII regions and molecular clouds." Symposium - International Astronomical Union 147 (1991): 476–79. http://dx.doi.org/10.1017/s0074180900240072.

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Both the HII Regions and the Molecular Clouds show broadening of their emission lines beyond that expected from thermal motion and this is ascribed to turbulence. Turbulence in molecular clouds generally agrees with a model where the velocity of motion is determined by the Alfv én velocity.Turbulence in Galactic HII Regions and Giant Extragalactic HII Regions can also be studied by the width of the emission lines. The magnitude of the turbulent velocities in these regions are characteristically about 10 km/s. There is a general increase in turbulent velocity with the size of the HII Region, and this relation is close to but different from the one third power dependence expected from the most naive application of Kolmogorov theory. When a detailed study is conducted of each Galactic HII Region by means of the structure function, one finds that there is not agreement with Kolmogorov theory.The Size-Turbulent versus Velocity relation for Galactic HII Regions differs slightly from the better defined velocity relation for Giant Extragalactic HII Regions. This difference is probably due to the fact that the larger extragalactic objects are probably complexes of multiple individual HII Regions. There is no evidence that broadening of extragalactic HII Regions is due to motion about a common center of mass.

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11

O'Dell, C. R. "Comparison of turbulence in HII regions and molecular clouds." Symposium - International Astronomical Union 147 (1991): 476–79. http://dx.doi.org/10.1017/s0074180900199450.

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Both the HII Regions and the Molecular Clouds show broadening of their emission lines beyond that expected from thermal motion and this is ascribed to turbulence. Turbulence in molecular clouds generally agrees with a model where the velocity of motion is determined by the Alfv én velocity.Turbulence in Galactic HII Regions and Giant Extragalactic HII Regions can also be studied by the width of the emission lines. The magnitude of the turbulent velocities in these regions are characteristically about 10 km/s. There is a general increase in turbulent velocity with the size of the HII Region, and this relation is close to but different from the one third power dependence expected from the most naive application of Kolmogorov theory. When a detailed study is conducted of each Galactic HII Region by means of the structure function, one finds that there is not agreement with Kolmogorov theory.The Size-Turbulent versus Velocity relation for Galactic HII Regions differs slightly from the better defined velocity relation for Giant Extragalactic HII Regions. This difference is probably due to the fact that the larger extragalactic objects are probably complexes of multiple individual HII Regions. There is no evidence that broadening of extragalactic HII Regions is due to motion about a common center of mass.

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12

Klaassen, P. D., K. G. Johnston, J. S. Urquhart, J. C. Mottram, T. Peters, R. Kuiper, H. Beuther, F. F. S. van der Tak, and C. Goddi. "The evolution of young HII regions." Astronomy & Astrophysics 611 (March 2018): A99. http://dx.doi.org/10.1051/0004-6361/201731727.

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Context. High-mass stars form in much richer environments than those associated with isolated low-mass stars, and once they reach a certain mass, produce ionised (HII) regions. The formation of these pockets of ionised gas are unique to the formation of high-mass stars (M > 8 M⊙), and present an excellent opportunity to study the final stages of accretion, which could include accretion through the HII region itself. Aim. This study of the dynamics of the gas on both sides of these ionisation boundaries in very young HII regions aims to quantify the relationship between the HII regions and their immediate environments. Methods. We present high-resolution (~0.5″) ALMA observations of nine HII regions selected from the red MSX source survey with compact radio emission and bolometric luminosities greater than 104 L⊙. We focus on the initial presentation of the data, including initial results from the radio recombination line H29α, some complementary molecules, and the 256 GHz continuum emission. Results. Of the six (out of nine) regions with H29α detections, two appear to have cometary morphologies with velocity gradients across them, and two appear more spherical with velocity gradients suggestive of infalling ionised gas. The remaining two were either observed at low resolution or had signals that were too weak to draw robust conclusions. We also present a description of the interactions between the ionised and molecular gas (as traced by CS (J = 5 − 4)), often (but not always) finding the HII region had cleared its immediate vicinity of molecules. Conclusions. Of our sample of nine, the observations of the two clusters expected to have the youngest HII regions (from previous radio observations) are suggestive of having infalling motions in the H29α emission, which could be indicative of late stage accretion onto the stars despite the presence of an HII region.

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13

Stephens, Ian W., Leslie W. Looney, Remy Indebetouw, You-Hua Chu, Robert A. Gruendl, C. H. Rosie Chen, Jonathan P. Seale, and Jessica Marie Evans. "Evolution of HII Regions around Massive YSOs." Proceedings of the International Astronomical Union 8, S292 (August 2012): 56. http://dx.doi.org/10.1017/s1743921313000379.

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AbstractWe survey HII free-free emission around ∼60 spectroscopically confirmed young stellar objects (YSOs) in the Large Magellanic Cloud using the Australia Telescope Compact Array (ATCA) at 3.3 and 5.5 cm. From each YSOs' infrared spectrum, we: a) quantify how embedded/evolved the YSO is through principle component analysis (PCA) of the silicate absorption (Seale et al. 2009); and b) estimate the mass from SED models (Robitaille et al. 2007). We have four main results: (1) Based on mass estimates from SED models and ATCA detection limits, we find that most massive YSOs are in HII regions regardless of age; (2) Older massive YSOs (as indicated by silicate PCA index) are much more likely to be resolved than younger YSOs, indicating evolving HII regions; (3) Resolved (typically older) sources usually have lower densities. Thus, in our survey we see a transition from ultra-compact HII to HII regions; and (4) We find that accretion about the massive YSO is likely non-spherical, resulting in HII regions in the shape of prolate spheroids.

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14

Rodríguez, M. "Iron abundance in HII regions." Astronomy & Astrophysics 389, no. 2 (June 27, 2002): 556–67. http://dx.doi.org/10.1051/0004-6361:20011823.

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15

Dale, J. E. "The effects of HII regions." EAS Publications Series 75-76 (2015): 353–58. http://dx.doi.org/10.1051/eas/1575070.

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16

Tenorio-Tagle, G., C. Muñoz-Tuñón, E. Pérez, S. Silich, and E. Telles. "On the structure of giant HII regions and HII galaxies." Proceedings of the International Astronomical Union 2, S237 (August 2006): 238–41. http://dx.doi.org/10.1017/s1743921307001536.

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AbstractWe review the structural properties of giant extragalactic HII regions and HII galaxies based on two dimensional hydrodynamic calculations, and propose an evolutionary sequence that accounts for their observed detailed structure. The model assumes a massive and young stellar cluster surrounded by a large collection of clouds. These are thus exposed to the most important star-formation feedback mechanisms: photoionization and the cluster wind. The models show how the two feedback mechanisms compete with each other in the disruption of clouds and lead to two different hydrodynamic solutions: The storage of clouds into a long lasting ragged shell that inhibits the expansion of the thermalized wind, and the steady filtering of the shocked wind gas through channels carved within the cloud stratum that results into the creation of large-scale superbubbles. Both solutions are here claimed to be concurrently at work in giant HII regions and HII galaxies, causing their detailed inner structure.

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17

Danehkar, A. "Radiatively Cooling Superwinds in Ultracompact Hii Regions." Proceedings of the International Astronomical Union 17, S373 (August 2021): 25–27. http://dx.doi.org/10.1017/s1743921322003994.

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AbstractUltracompact Hii regions (UC-HII) are the young, very dense cores of massive star-forming regions in dwarf galaxies, where newly formed massive OB stars are surrounded by natal molecular clouds. Thermal energy deposited by mechanical feedback from a cluster of massive OB stars can form a superwind, which may lead to a wind-blown bubble as well as radiative cooling. We investigate the formation of radiatively cooling superwinds in UC-HII using a radiative cooling module in the hydrodynamics program. We built a grid of hydrodynamic simulations to determine the dependence of radiative cooling on the cluster radius, mass-deposition rate, wind velocity, and ambient medium in UC-HII. Our findings could help to better understand star formation in massive star-forming regions, where cool superwinds could trigger the formation of molecular clumpy regions.

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18

Omar, A., J. N. Chengalur, and D. A. Roshi. "Multifrequency GMRT observations of HII regions." Symposium - International Astronomical Union 199 (2002): 313–14. http://dx.doi.org/10.1017/s0074180900169207.

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The electron temperatures of the compact cores of the galactic HII regions S206 and S209 have been determined by radio continuum observations near 235, 327 and 610 MHz using the Giant Meterwave Radio Telescope (GMRT). The resolution of our maps are 11″ and 6″ at 327 and 610 MHz respectively. These are the highest resolution low frequency maps of these HII regions.

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19

Xiao, Lin, J. J. Eldridge, Elizabeth Stanway, and L. Galbany. "Emission-line Diagnostics of Nearby HII Regions Including Supernova Hosts." Proceedings of the International Astronomical Union 12, S329 (November 2016): 49–53. http://dx.doi.org/10.1017/s1743921317002915.

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AbstractWe present a new model of the optical nebular emission from HII regions by combining the results of the Binary Population and Spectral Synthesis (bpass) code with the photoionization code cloudy (Ferland et al. 1998). We explore a variety of emission-line diagnostics of these star-forming HII regions and examine the effects of metallicity and interacting binary evolution on the nebula emission-line production. We compare the line emission properties of HII regions with model stellar populations, and provide new constraints on their stellar populations and supernova progenitors. We find that models including massive binary stars can successfully match all the observational constraints and provide reasonable age and mass estimation of the HII regions and supernova progenitors.

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20

Wofford, Aida. "Superbubble Hii regions: how self-enriched should they be?" Proceedings of the International Astronomical Union 5, S265 (August 2009): 386–87. http://dx.doi.org/10.1017/s1743921310001055.

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AbstractI modeled the pollution of low metallicity (Z=0.001) superbubble Hii regions with the ejecta from single stellar populations of 104−106M⊙ in mass. I found that the He, C, N, and O abundance enhancements in the Hii regions, due to pollution with the enriched winds from Wolf-Rayet stars, are insignificant at 5Myr. The few localized metal enhancements observed so far in resolved extragalactic Hii regions are not associated with superbubbles and remain to be modeled in detail.

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21

Melnick, Jorge. "Superassociations in the Arms of Normal and Active Galaxies." Symposium - International Astronomical Union 121 (1987): 545–65. http://dx.doi.org/10.1017/s0074180900155603.

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The properties of violent star formation regions in the arms of spiral galaxies are reviewed with the aim of providing the foundations on which models of nuclear starbursts can be built. It is argued that the classical examples of extranuclear starbursts, giant HII regions and Superassociations, are closely related but fundamentally different classes of objects; their properties are reviewed and discussed in detail.It is shown that giant HII regions are a homogeneous class of gravitationally bound objects ionized by starburst clusters and that the initial mass functions of these clusters change according to their chemical composition being flatter for metal poor systems. Superassociations are loose associations of associations which generally contain one or more giant HII regions. Star formation in superassociations is self-sustaining and therefore these structures may last much longer than individual giant HII regions.Isolated superassociations or HII galaxies are shown to have the same global properties as giant HII regions in late type galaxies. In particular, the correlations between Hβ luminosity and emission line velocity width are similar for both classes. Since HII galaxies can be observed out to large distances this correlation provides a potentially powerful method to calibrate the extragalactic distance scale. A preliminary calibration gives HO = 95 ± 9 km/sec/Mpc.The relations between the properties of giant HII regions and Superassociations with those of their parent galaxies are briefly discussed.

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22

Abe, R., A. Hara, T. Hayakawa, S. Kato, A. Kawamura, A. Mizuno, N. Mizuno, et al. "Molecular Clouds in the LMC Observed with NANTEN: III. Comparison with HII Regions." Symposium - International Astronomical Union 190 (1999): 130–31. http://dx.doi.org/10.1017/s0074180900117620.

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We have made 12CO(J=1-0) observations in the LMC with NANTEN, and compared the detected giant molecular clouds (GMCs) with HII regions and stellar clusters. It is found that ~ 80% of the GMCs are associated with HII regions. The results of comparisons of the GMCs with the HII regions and the stellar clusters are presented.

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23

Godbout, Steve, Gilles Joncas, and Laurent Drissen. "Kinematical Study of Old HII Regions and Optical Counterpart to the DRAO Canadian Galactic Plane Survey." Publications of the Astronomical Society of Australia 15, no. 1 (1998): 60–63. http://dx.doi.org/10.1071/as98060.

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AbstractThis project aims to tackle a few unresolved problems related to the interstellar medium (ISM) by acting as an optical counterpart to the Canadian Galactic Plane Survey (CGPS). We have three main objectives: (1) the study of large/old HII regions; (2) observations of targets-of-opportunity that may be found by the CGPS; (3) comparison of the kinematics of extragalactic (M33) and galactic HII regions. (1) Old HII regions having large spatial extents will be observed to establish their kinematical structure when almost no molecular material is left to produce photodissociated flows. Does turbulence play a role in these objects as for younger nebulae? (2) Ionised nebulae observed by the CGPS that are peculiar, either morphologically or by special association with neutral hydrogen, will also be observed. (3) The kinematical behaviour of old HII regions in M33 will be compared with that of galactic HII regions. With both data sets in hand we will check if HII regions, like supernovae, dump energy into the neutral ISM. The instrument used as well as some very preliminary data are presented.

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24

Sabbatini, L., F. Cavaliere, G. Dall'Oglio, R. D. Davies, L. Martinis, A. Miriametro, R. Paladini, L. Pizzo, P. A. Russo, and L. Valenziano. "Millimetric observations of southern HII regions." Astronomy & Astrophysics 439, no. 2 (July 29, 2005): 595–600. http://dx.doi.org/10.1051/0004-6361:20041615.

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25

Russeil, D., C. Adami, and Y. M. Georgelin. "Revised distances of Northern HII regions." Astronomy & Astrophysics 470, no. 1 (May 10, 2007): 161–71. http://dx.doi.org/10.1051/0004-6361:20066051.

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26

Aksaker, N., M. Sahan, I. Yegingil, and N. Emrahoglu. "DEFPOS Hα observations of Hii regions." New Astronomy 16, no. 8 (December 2011): 485–91. http://dx.doi.org/10.1016/j.newast.2011.04.005.

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27

Rodríguez, Luis F. "Radio observations of ultracompact HII regions." Proceedings of the International Astronomical Union 1, S227 (May 2005): 120–27. http://dx.doi.org/10.1017/s1743921305004436.

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28

Copetti, M. V. F. "Integrated photometry of galactic HII regions." Astronomy and Astrophysics Supplement Series 147, no. 1 (November 2000): 93–97. http://dx.doi.org/10.1051/aas:2000291.

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29

Della Bruna, Lorenza, Angela Adamo, Arjan Bik, Michele Fumagalli, Rene Walterbos, Göran Östlin, Gustavo Bruzual, et al. "Studying the ISM at ∼10 pc scale in NGC 7793 with MUSE." Astronomy & Astrophysics 635 (March 2020): A134. http://dx.doi.org/10.1051/0004-6361/201937173.

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Context. Studies of nearby galaxies reveal that around 50% of the total Hα luminosity in late-type spirals originates from diffuse ionised gas (DIG), which is a warm, diffuse component of the interstellar medium that can be associated with various mechanisms, the most important ones being “leaking” HII regions, evolved field stars, and shocks. Aims. Using MUSE Wide Field Mode adaptive optics-assisted data, we study the condition of the ionised medium in the nearby (D = 3.4 Mpc) flocculent spiral galaxy NGC 7793 at a spatial resolution of ∼10 pc. We construct a sample of HII regions and investigate the properties and origin of the DIG component. Methods. We obtained stellar and gas kinematics by modelling the stellar continuum and fitting the Hα emission line. We identified the boundaries of resolved HII regions based on their Hα surface brightness. As a way of comparison, we also selected regions according to the Hα/[SII] line ratio; this results in more conservative boundaries. Using characteristic line ratios and the gas velocity dispersion, we excluded potential contaminants, such as supernova remnants (SNRs) and planetary nebulae (PNe). The continuum subtracted HeII map was used to spectroscopically identify Wolf Rayet stars (WR) in our field of view. Finally, we computed electron densities and temperatures using the line ratio [SII]6716/6731 and [SIII]6312/9069, respectively. We studied the properties of the ionised gas through “BPT” emission line diagrams combined with velocity dispersion of the gas. Results. We spectroscopically confirm two previously detected WR and SNR candidates and report the discovery of the other seven WR candidates, one SNR, and two PNe within our field of view. The resulting DIG fraction is between ∼27 and 42% depending on the method used to define the boundaries of the HII regions (flux brightness cut in Hα = 6.7 × 10−18 erg s−1 cm−2 or Hα/[SII] = 2.1, respectively). In agreement with previous studies, we find that the DIG exhibits enhanced [SII]/Hα and [NII]/Hα ratios and a median temperature that is ∼3000 K higher than in HII regions. We also observe an apparent inverse correlation between temperature and Hα surface brightness. In the majority of our field of view, the observed [SII]6716/6731 ratio is consistent within 1σ with ne < 30 cm−3, with an almost identical distribution for the DIG and HII regions. The velocity dispersion of the ionised gas indicates that the DIG has a higher degree of turbulence than the HII regions. Comparison with photoionisation and shock models reveals that, overall, the diffuse component can only partially be explained via shocks and that it is most likely consistent with photons leaking from density bounded HII regions or with radiation from evolved field stars. Further investigation will be conducted in a follow-up paper.

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30

Zhi-yao, Yu. "Star Clusters in Giant Extragalactic HII Regions." Symposium - International Astronomical Union 207 (2002): 439–44. http://dx.doi.org/10.1017/s0074180900224157.

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In this paper we study the relationship between the star formation efficiency and luminosity of Hα emission, Lyman continuum radiation, and Hβ emission on 35 giant extragalactic HII regions in seven galaxies. Using the observational results we obtain the relationship, and find that the star formation efficiency is correlation with Halpha luminocity, and Lyman continuum luminosity, and Hβ lumonosity, respectively. Key words: external galaxy—giant HII region—star cluster

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31

Cioni, M. R., S. Cortiglioni, M. Orsini, E. Palazzi, G. G. C. Palumbo, R. L. Sorochenko, and A. P. Tsivilev. "New Measurements of the 4He Abundance in Galactic HII Regions." International Astronomical Union Colloquium 166 (1997): 429–32. http://dx.doi.org/10.1017/s0252921100071384.

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AbstractPreliminary results of the 4He relative abundance determination from Radio Recombination Lines observations at different frequencies in the Orion, Rosette and W3 HII regions are presented. The Orion HII region has been mapped at both 22 GHz and 36 GHz with the same beamwidth (2 arcmin) using the Medicina and Puschino radio telescopes respectively. The RRLs parameters, together with their variation with frequency and with distance were determined by centering the map on the star θ1OriC. Three positions were observed in the Rosette HII region at 8.3 GHz leading to the first detection in this region of the transition He92α. The derived 4He relative abundance is considerably greater than the ones obtained from previous measurements. The W3 HII region was observed at 36 GHz and the 4He/H value derived was compared with previous measurements performed with higher spatial resolution.

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32

Churchwell, Ed. "Massive Star Formation: New Results." Highlights of Astronomy 12 (2002): 159–60. http://dx.doi.org/10.1017/s1539299600013137.

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In the past year several new observations with important implications for massive star formation (MSF) have been obtained. Among these were the detection of UC HII region precursor candidates at 350µm and the discovery of many hard X-ray point sources in the Orion and W3 MSF regions. These observations are summarized below.Hunter et al. (1998, 2000) imaged 25 MSF regions at 350µm to search for candidate precursors of UC HII regions (i.e. luminous submm/FIR emission, maser sources, and no HII emission). Of the 28 sources detected, 10 appear to be UC HII precursor candidates.

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33

Filipovic, M. D., P. A. Jones, and G. L. Vhite. "The relation between radio flux density and ionizing ultra-violet flux for HII regions and supernova remnants in the Large Magellanic cloud." Serbian Astronomical Journal, no. 166 (2003): 31–37. http://dx.doi.org/10.2298/saj0366031f.

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We present a comparison between the Parkes radio surveys (Filipovic et al 1995) and Vacuum Ultra-Violet (VUV) surveys (Smith et al. 1987) of the Large Magellanic Clouds (LMC). We have found 72 sources in common in the LMC which are known HII regions (52) and supernova remnants (SNRs) (19). Some of these radio sources are associated with two or more UV stellar associations. A comparison of the radio flux densities and ionizing UV flux for HII regions shows a very good correlation, as expected from theory. Many of the Magellanic Clouds (MCs) SNRs are embedded in HII regions, so there is also a relation between radio and UV which we attribute to the surrounding HII regions.

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34

Turner, Jean L. "The Youngest Star-Forming Regions in Galaxies: Giant Compact HII Regions and Protoglobular Clusters?" Symposium - International Astronomical Union 205 (2001): 224–27. http://dx.doi.org/10.1017/s0074180900221049.

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Subarcsecond radio and infrared observations reveal a class of luminous, obscured, optically thick HII regions associated with extremely large young clusters in nearby starburst galaxies. VLA images show bright radio nebulae with ne ∼ 104 cm−3, densities characteristic of young Galactic compact HII regions. Excitation of the nebulae requires the presence of several thousand O stars within regions of 1-10 pc extent, corresponding to clusters containing 105–106 stars. The compact nebulae are also bright in the mid-infrared, and can for significant fractions of not only the total IR luminosity, but also the total bolometric luminosity, of the parent galaxies. The prototype for these “supernebulae” is the large, obscured cluster in the dwarf galaxy NGC 5253.

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35

Balser, Dana S., Trey V. Wenger, T. M. Bania, and L. D. Anderson. "Metallicity Structure across the Galactic Disk: Radio Observations of H ii Regions." Proceedings of the International Astronomical Union 13, S334 (July 2017): 275–76. http://dx.doi.org/10.1017/s1743921317007530.

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AbstractHii regions are the sites of massive star formation and are the archetypal tracers of spiral arms. Because of their short lifetimes (<10 Myr) their abundances provide a measure of the nuclear processing of many stellar generations. Here we review our ongoing efforts to explore the metallicity structure of the Galactic disk by observing radio recombination line (RRL) and thermal radio continuum emission toward Hii regions. The RRL-to-continuum ratio provides an accurate measure of the electron temperature which is used as a proxy for metallicity. Since collisionally excited lines from metals (e.g., O, C) are the main coolant in Hii regions, the thermal electron temperature is well correlated with metallicity (e.g., [O/H]). We determine Hii region distances from maser parallax measurements when possible; otherwise we use kinematic distances. Such radio diagnostics of Hii regions yield an extinction free tracer to map the metallicity distribution across the entire Galactic disk.

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36

Kravchuk, S. G. "Observational appearance of circumstellar ionized regions and ultracompact HII regions." Astronomische Nachrichten: A Journal on all Fields of Astronomy 310, no. 4 (1989): 337–39. http://dx.doi.org/10.1002/asna.2113100424.

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37

Zaragoza-Cardiel, J., J. E. Beckman, J. Font, A. Camps-Fariña, B. García-Lorenzo, and S. Erroz-Ferrer. "Star formation enhancement characteristics in interacting galaxies." Proceedings of the International Astronomical Union 10, S309 (July 2014): 354. http://dx.doi.org/10.1017/s174392131401045x.

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AbstractWe have observed 12 interacting galaxies using the Fabry-Perot interferometer GHαFaS (Galaxy Hα Fabry-Perot system) on the 4.2m William Herschel Telescope (La Palma). We have extracted the physical properties (sizes, Hα luminosity and velocity dispersion) of 236 HII regions for the full sample of interacting galaxies. We have derived the physical properties of 664 HII regions for a sample of 28 isolated galaxies observed with the same instrument in order to compare both populations of HII regions, finding that there are brighter and denser star forming regions in the interacting galaxies compared with the isolated galaxies sample.

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38

Crocker, Deborah A., Pamela D. Baugus, and R. Buta. "HII Regions in Resonance-Ring Spiral Galaxies." International Astronomical Union Colloquium 157 (1996): 80–82. http://dx.doi.org/10.1017/s0252921100049459.

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Resonance ring galaxies are barred (and sometimes nonbarred) spiral galaxies which display an inner, outer, or nuclear ring in the light distribution. This study is of the HII region properties in 32 probable resonance ring galaxies selected mainly from the Catalog of Southern Ringed Galaxies (CSRG; Buta 1995). The objective of the study is to illustrate the distributions of HII regions in classic examples of ringed galaxies, and to relate the observed properties to resonance theory. Here we present a brief summary of a more extensive paper on the results of this study (Crocker, Baugus, & Buta 1996).

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39

Ye, Taisheng. "Diameter distribution of HII regions in galaxies." Monthly Notices of the Royal Astronomical Society 255, no. 1 (March 1992): 32–40. http://dx.doi.org/10.1093/mnras/255.1.32.

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40

Damineli, A., R. D. Blum, E. Figuerêdo, and P. S. Conti. "NIR studies of galactic giant HII regions." Proceedings of the International Astronomical Union 1, S227 (May 2005): 407–12. http://dx.doi.org/10.1017/s1743921305004813.

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41

Cox, Pierre. "Studies of Compact HII Regions with ISO." Highlights of Astronomy 12 (2002): 164–66. http://dx.doi.org/10.1017/s1539299600013150.

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42

Henning, Th, M. Feldt, B. Stecklum, and R. Klein. "High-resolution imaging of ultracompact HII regions." Astronomy & Astrophysics 370, no. 1 (April 2001): 100–111. http://dx.doi.org/10.1051/0004-6361:20010227.

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43

Casta�eda, H�ctor O. "Supersonic turbulence in giant extragalactic HII regions." Astrophysics and Space Science 216, no. 1-2 (June 1994): 285–89. http://dx.doi.org/10.1007/bf00982506.

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44

Breitschwerdt, D., and F. D. Kahn. "Turbulent mixing in wind-blown HII regions." Astrophysics and Space Science 216, no. 1-2 (June 1994): 297–301. http://dx.doi.org/10.1007/bf00982508.

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45

Lumsden, Stuart, and Melvin Hoare. "Infrared velocity mapping of compact HII regions." Astrophysics and Space Science 224, no. 1-2 (February 1995): 505–6. http://dx.doi.org/10.1007/bf00667927.

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46

Wilcots, Eric M., and Paul Hodge. "Faint HII regions in the Magellanic Clouds." Symposium - International Astronomical Union 148 (1991): 226–27. http://dx.doi.org/10.1017/s0074180900200478.

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We present CCD observations of several faint regions of active star formation in the Magellanic Clouds. For the five regions for which we were able to obtain photometric data we have extended the luminosity function (LF) two orders of magnitude below the previous faint limit of 1037 erg s−1 (1). Our preliminary results indicate that the LF turns over at 1036 erg s−1 for the LMC and 1036.5 erg s−1 for the SMC. It is also apparent that much of the Ha emission fainter than 1036 erg s−1 is diffuse or filamentary in nature.

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47

Courtès, G. "Turbulent and Ordered Motions in HII Regions." International Astronomical Union Colloquium 120 (1989): 80–95. http://dx.doi.org/10.1017/s0252921100023502.

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SummaryThe Fill regions morphology have suggested, from the beginning of this century, the evidence of internal motions, very few gas radial velocities (RV) on the brightest “nebulae” had been obtained, because of obvious technical difficulties. The modern Fabry-Perot interferometers and the new integral Field Spectrograph provide abundant RV fields at various scales in the interstellar gas. New ways in imagery, owing to these new instruments, lead to a better analyse of both, the velocities field and their precise morphology free of stellar, dusty, and non thermal continua. The richness and the guallty of RV data, a better understanding of the expansion phenomena authorize again a rebirth of the turbulence observations of the ionized interstellar gas.

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48

Franco, José, Guillermo Tenorio-Tagle, and Peter Bodenheimer. "Expansion of HII Regions in Density Gradients." International Astronomical Union Colloquium 120 (1989): 96–103. http://dx.doi.org/10.1017/s0252921100023514.

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AbstractThe main features of HII regions expanding in spherical and disk-like clouds with density gradients are reviewed. The spherical cases assume power-law density stratifications, r~w, and the disk-like cases include exponential, gaussian, and sech2 distributions. For power-law profiles, there is a critical exponent, wcrit = 3/2, above which the ionization front cannot be “trapped” and the cloud becomes fully ionized. For clouds with w < 3/2, the radius of the ionized region grows as t4/(7-2w) and drives a shock front into the ambient neutral medium. For w = wcrit = 3/2 the shock wave cannot detach from the ionization front and the two move together with a constant speed equal to about 2ci, where ci is the sound speed in the ionized gas. For w > 3/2 the expansion corresponds to the “champagne phase”, and two regimes, fast and slow, are apparent: between 3/2 < w ≤ 3, the slow regime, the inner region drives a weak shock moving with almost constant velocity through the cloud, and for w > 3, the fast regime, the shock becomes strong and accelerates with time.For the case of disk-like clouds, which are assumed cylindrically symmetric, the dimensions of the initial HII regions along each azimuthal angle, θ, are described in terms of the Strömgren radius for the midplane density, Ro, and the disk scale height, H. For yo = Rosin(θ)/H ≤ α (where α is a constant dependent on the assumed density distribution) the whole HII region is contained within the disk, and for yo > α a conical section of the disk becomes totally ionized. The critical azimuthal angle above which the HII region becomes unbounded is defined by θcrit =sin-1(αH/Ro). The expansion of initially unbounded HII regions (i.e. with yo > α) proceeds along the z-axis and, if the disk column density remains constant during the evolution, the ionization front eventually recedes from infinity to become trapped within the expanding disk. For clouds threaded by a B-field oriented parallel to the symmetry axis, as expected in magnetically dominated clouds, this effect can be very prominent. The expanding gas overtaken by the receding ionization front maintains its linear momentum after recombination and is transformed into a high-velocity neutral outflow. In the absence of magnetic fields, the trapping has only a short duration.

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Breitschwerdt, D., and F. D. Kahn. "Turbulent Mixing in Wind-Blown HII Regions." International Astronomical Union Colloquium 120 (1989): 117–21. http://dx.doi.org/10.1017/s025292110002354x.

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AbstractTurbulent mixing between an ionization bounded HII region and a hot shocked stellar wind (HSSW), which keeps it under pressure, is examined. Recently we have shown that acoustic disturbances can grow there to finite amplitude in a time scale which is comparable to the sound crossing time in the HII layer. The resulting turbulence will then stretch fluid elements and the frozen-in magnetic field. A condition under which turbulence can decay down to the viscous scale, where mixing is very efficient, is derived. For a uniform and plane parallel magnetic field Bo and a constant density ρo of the ambient medium, we find that efficient mixing takes place near the polar regions. Subsequently the rate of mass addition to the hot bubble is calculated and it is shown that catastrophic cooling is likely to occur. In the case of NGC 6334(A) it seems that this has just happened and we predict an upper limit for Bo of 4 x 10-5 gauss there. This model may also explain the existence of highly ionized species (e.g. OVI), soft X-rays and high velocity flows of the order of 100 km/s.

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Masegosa, J., and M. Moles. "Age Effects in Giant Extragalactic HII Regions." International Astronomical Union Colloquium 120 (1989): 369. http://dx.doi.org/10.1017/s0252921100024118.

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Spatial inhomogeneities for a sample of Giant Extragalactic HII Regions (GEHR) have been analyzed. The results indicate that, whitin the observational errors, the O/H and He/H abundance ratios are constant for a given GEHR. The Ne/O ratio on the other hand shows a trend with the evolution of the ionizing stellar clusters.

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