Ii Regions (ii + regions)

Distribution by Scientific Domains

Kinds of Ii Regions

  • h ii regions


  • Selected Abstracts


    Mitochondrial DNA variability in Poles and Russians

    ANNALS OF HUMAN GENETICS, Issue 4 2002
    B. A. MALYARCHUK
    Mitochondrial DNA (mtDNA) sequence variation was examined in Poles (from the Pomerania-Kujawy region; n = 436) and Russians (from three different regions of the European part of Russia; n = 201), for which the two hypervariable segments (HVS I and HVS II) and haplogroup-specific coding region sites were analyzed. The use of mtDNA coding region RFLP analysis made it possible to distinguish parallel mutations that occurred at particular sites in the HVS I and II regions during mtDNA evolution. In total, parallel mutations were identified at 73 nucleotide sites in HVS I (17.8%) and 31 sites in HVS II (7.73%). The classification of mitochondrial haplotypes revealed the presence of all major European haplogroups, which were characterized by similar patterns of distribution in Poles and Russians. An analysis of the distribution of the control region haplotypes did not reveal any specific combinations of unique mtDNA haplotypes and their subclusters that clearly distinguish both Poles and Russians from the neighbouring European populations. The only exception is a novel subcluster U4a within subhaplogroup U4, defined by a diagnostic mutation at nucleotide position 310 in HVS II. This subcluster was found in common predominantly between Poles and Russians (at a frequency of 2.3% and 2.0%, respectively) and may therefore have a central-eastern European origin. [source]


    Sixty four nights of UBV photometry of early-type stars at La Sila,

    ASTRONOMISCHE NACHRICHTEN, Issue 3 2010
    P. Mayer
    Abstract UBV measurements of early-type stars, mostly eclipsing binaries, obtained at La Silla in the years 1990 to 1994 with the ESO 50 cm telescope are presented. Most of these data were already used in our individual studies of several binaries. Now all photometric measurements were reduced again with an advanced technique and are made available electronically. Our data for MY Ser have not yet been published; new light curve is given and solved. The result is that MY Ser is a contact binary, with very large fill-out parameter. Also a light curve and its solution for V1051 Cen are provided, and the problem of the period of V871 Cen is pointed out. Besides binaries (and the comparison and check stars) data for several stars in southern H II regions are included. Extinction and transformation coefficients are given (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


    Spatially Resolved Very Large Array 74 MHz Observations Toward the Galactic Center

    ASTRONOMISCHE NACHRICHTEN, Issue S1 2003
    C. L. Brogan
    Abstract We present the highest resolution and sensitivity low frequency image (<300 MHz) of the Galactic center to date using the Very Large Array at 74 MHz in its A, B, C, & D configurations. The resulting images have a resolution of 2.1, × 1.2, and a dynamic range of ,400 From this data we have been able to identify a region of enhanced 74 MHz emission about 5° in extent that is coincident with the high density molecular gas surrounding the Galactic center known as the Central Molecular Zone. In addition to giving an unprecedented view of the extended nonthermal emission surrounding the Galactic center, the 74 MHz image shows deep free-free absorption across the Galactic center itself, as well as, part of the Galactic center radio lobe, and a number of H II regions in the field. This absorption is due to ionized thermal gas in front of, or in some cases embedded in, the nonthermal Galactic center (GC) emission. Such absorption allows us to unambiguously place some of the H II regions in the direction of the GC along the line of sight for the first time. The morphology, nature, and relationship to the Galactic center of the 74 MHz absorption and emission is discussed. [source]


    Purification, crystallization and preliminary X-ray analysis of the GTP-binding protein Rab9 implicated in endosome-to-TGN vesicle trafficking

    ACTA CRYSTALLOGRAPHICA SECTION D, Issue 3 2004
    Julia G. Wittmann
    Rab GTP-binding proteins are involved in the regulation of distinct vesicular-transport events involving membrane targeting and fusion. They differ from other small GTPases by the presence of specific loop regions that serve as effector-binding sites in addition to the classical switch I and switch II regions. While the structures of many small GTP-binding proteins of the Ras superfamily are available in both GDP- and GTP-bound forms, Rab proteins are less well characterized than Ras proteins at the structural level. The crystallization of Rab9, a key regulatory component in the recycling of mannose-6-­phosphate receptors from endosomes to the trans-Golgi network, is described here. [source]


    ROS-inhibitory activity of YopE is required for full virulence of Yersinia in mice

    CELLULAR MICROBIOLOGY, Issue 7 2010
    Warangkhana Songsungthong
    Summary YopE, a type III secreted effector of Yersinia, is a GTPase Activating Protein for Rac1 and RhoA whose catalytic activity is critical for virulence. We found that YopE also inhibited reactive oxygen species (ROS) production and inactivated Rac2. How YopE distinguishes among its targets and which specific targets are critical for Yersinia survival in different tissues are unknown. A screen identifying YopE mutants in Yersinia pseudotuberculosis that interact with different Rho GTPases showed that YopE residues at positions 102, 106, 109 and 156 discern among switch I and II regions of Rac1, Rac2 and RhoA. Two mutants, which expressed YopE alleles with different antiphagocytic, ROS-inhibitory and cell-rounding activities, YptbL109A and YptbESptP, were studied in animal infections. Inhibition of both phagocytosis and ROS production were required for splenic colonization, whereas fewer YopE activities were required for Peyer's patch colonization. This study shows that Y. pseudotuberculosis encounters multiple host defences in different tissues and uses distinct YopE activities to disable them. [source]


    A study of the massive star-forming region M8 using images from the Spitzer Infrared Array Camera

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2010
    Dewangan Lokesh Kumar
    ABSTRACT We present photometry and images (3.6, 4.5, 5.8 and 8.0 ,m) from the Spitzer Infrared Array Camera (IRAC) of the star-forming region Messier 8 (M8). The IRAC photometry reveals ongoing star formation in the M8 complex, with 64 class 0/I and 168 class II sources identified in several locations in the vicinity of submm gas cores/clumps. Nearly 60 per cent of these young stellar objects (YSOs) occur in about seven small clusters. The spatial surface density of the clustered YSOs is determined to be about 10,20 YSOs pc,2. Fresh star formation by the process of ,collect and collapse' might have been triggered by the expanding H ii regions and winds from massive stars. IRAC ratio images are generated and studied in order to identify possible diagnostic emission regions in M8. The image of 4.5/8.0 ,m reveals a Br, counterpart of the optical Hourglass H ii region, while the ratio 8.0/4.5 ,m indicates PAH emission in a cavity-like structure to the east of the Hourglass. The ratio maps of 3.6/4.5, 5.8/4.5 and 8.0/4.5 ,m seem to identify PAH emission regions in the sharp ridges and filamentary structures seen east to west and north-east to south-west in the M8 complex. [source]


    Diffuse continuum transfer in H ii regions

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2009
    R. J. R. Williams
    ABSTRACT We compare the accuracy of various methods for determining the transfer of the diffuse Lyman continuum in H ii regions by comparing them with a high-resolution discrete-ordinate integration. We use these results to suggest how, in multidimensional dynamical simulations, the diffuse field may be treated with acceptable accuracy without requiring detailed transport solutions. The angular distribution of the diffuse field derived from the numerical integration provides insight into the likely effects of the diffuse field for various material distributions. [source]


    Modification of the 21-cm power spectrum by X-rays during the epoch of reionization

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2009
    L. Warszawski
    ABSTRACT We incorporate a contribution to reionization from X-rays within analytic and seminumerical simulations of the 21-cm signal arising from neutral hydrogen during the epoch of reionization. The relatively long X-ray mean free path (MFP) means that ionizations due to X-rays are not subject to the same density bias as UV ionizations, resulting in a substantive modification to the statistics of the 21-cm signal. We explore the impact that X-ray ionizations have on the power spectrum (PS) of 21-cm fluctuations by varying both the average X-ray MFP and the fractional contribution of X-rays to reionization. In general, prior to the epoch when the intergalactic medium (IGM) is dominated by ionized regions (H ii regions), X-ray-induced ionization enhances fluctuations on spatial scales smaller than the X-ray MFP, provided that X-ray heating does not strongly suppress galaxy formation. Conversely, at later times when H ii regions dominate, small-scale fluctuations in the 21-cm signal are suppressed by X-ray ionization. Our modelling also shows that the modification of the 21-cm signal due to the presence of X-rays is sensitive to the relative scales of the X-ray MFP and the characteristic size of H ii regions. We therefore find that X-rays imprint an epoch and scale-dependent signature on the 21-cm PS, whose prominence depends on fractional X-ray contribution. The degree of X-ray heating of the IGM also determines the extent to which these features can be discerned. We further show that the presence of X-rays smoothes out the shoulder-like signature of H ii regions in the 21-cm PS. For example, a 10 per cent contribution to reionization from X-rays translates to a 20,30 per cent modulation in the 21-cm PS across the scale of H ii regions. We show that the Murchison Widefield Array will have sufficient sensitivity to detect this modification of the PS, so long as the X-ray photon MFP falls within the range of scales over which the array is most sensitive (,0.1 Mpc,1). In cases in which this MFP takes a much smaller value, an array with larger collecting area would be required. As a result, an X-ray contribution to reionization has the potential to substantially complicate analysis of the 21-cm PS. On the other hand, a combination of precision measurements and modelling of the 21-cm PS promises to provide an avenue for investigating the role and contribution of X-rays during reionization. [source]


    Reionization bias in high-redshift quasar near-zones

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2008
    J. Stuart B. Wyithe
    ABSTRACT Absorption spectra of high-redshift quasars exhibit an increasingly thick Ly, forest, suggesting that the fraction of neutral hydrogen in the intergalactic medium (IGM) is increasing towards z, 6. However, the interpretation of these spectra is complicated by the fact that the Ly, optical depth is already large for neutral hydrogen fractions in excess of 10,4, and also because quasars are expected to reside in dense regions of the IGM. We present a model for the evolution of the ionization state of the IGM which is applicable to the dense, biased regions around high-redshift quasars as well as more typical regions in the IGM. We employ a cold dark matter based model in which the ionizing photons for reionization are produced by star formation in dark matter haloes spanning a wide range of masses, combined with numerical radiative transfer simulations which model the resulting opacity distribution in quasar absorption spectra. With an appropriate choice for the parameter which controls the star formation efficiency, our model is able to simultaneously reproduce the observed Ly, forest opacity at 4 < z < 6, the ionizing photon mean-free-path at z, 4 and the rapid evolution of highly ionized near-zone sizes around high-redshift quasars at 5.8 < z < 6.4. In our model, reionization extends over a wide redshift range, starting at z, 10 and completing as H ii regions overlap at z, 6,7. We find that within 5 physical Mpc of a high-redshift quasar, the evolution of the ionization state of the IGM precedes that in more typical regions by around 0.3 redshift units. More importantly, when combined with the rapid increase in the ionizing photon mean-free-path expected shortly after overlap, this offset results in an ionizing background near the quasar which exceeds the value in the rest of the IGM by a factor of ,2,3. We further find that in the post-overlap phase of reionization the size of the observed quasar near-zones is not directly sensitive to the neutral hydrogen fraction of the IGM. Instead, these sizes probe the level of the background ionization rate and the temperature of the surrounding IGM. The observed rapid evolution of the quasar near-zone sizes at 5.8 < z < 6.4 can thus be explained by the rapid evolution of the ionizing background, which in our model is caused by the completion of overlap at the end of reionization by 6 ,z, 7. [source]


    Australia Telescope Compact Array 1.2-cm observations of the massive star-forming region G305.2+0.2

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
    Andrew J. Walsh
    ABSTRACT We report on Australia Telescope Compact Array observations of the massive star-forming region G305.2+0.2 at 1.2 cm. We detected emission in five molecules towards G305A, confirming its hot core nature. We determined a rotational temperature of 26 K for methanol. A non-local thermodynamic equilibrium excitation calculation suggests a kinematic temperature of the order of 200 K. A time-dependent chemical model is also used to model the gas-phase chemistry of the hot core associated with G305A. A comparison with the observations suggest an age of between 2 × 104 and 1.5 × 105 yr. We also report on a feature to the south-east of G305A which may show weak Class I methanol maser emission in the line at 24.933 GHz. The more evolved source G305B does not show emission in any of the line tracers, but strong Class I methanol maser emission at 24.933 GHz is found 3 arcsec to the east. Radio continuum emission at 18.496 GHz is detected towards two H ii regions. The implications of the non-detection of radio continuum emission towards G305A and G305B are also discussed. [source]


    A census of the Carina Nebula , II.

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
    Energy budget, global properties of the nebulosity
    ABSTRACT The first paper in this series took a direct census of energy input from the known OB stars in the Carina Nebula, and in this paper we study the global properties of the surrounding nebulosity. This detailed comparison may prove useful for interpreting observations of extragalactic giant H ii regions and ultraluminous infrared (IR) galaxies. We find that the total IR luminosity of Carina is about 1.2 × 107 L,, accounting for only about 50,60 per cent of the known stellar luminosity from Paper I. Similarly, the ionizing photon luminosity derived from the integrated radio continuum is about 7 × 1050 s,1, accounting for ,75 per cent of the expected Lyman continuum from known OB stars. The total kinetic energy of the nebula is about 8 × 1051 erg, or ,30 per cent of the mechanical energy from stellar winds over the lifetime of the nebula, so there is no need to invoke a supernova (SN) explosion based on energetics. Warm dust grains residing in the H ii region interior dominate emission at 10,30 ,m, but cooler grains at 30,40 K dominate the IR luminosity and indicate a likely gas mass of ,106 M,. We find an excellent correlation between the radio continuum and 20,25 ,m emission, consistent with the idea that the ,80-K grain population is heated by trapped Ly, photons. Similarly, we find a near perfect correlation between the far-IR optical depth map of cool grains and 8.6-,m hydrocarbon emission, indicating that most of the nebular mass resides as atomic gas in photodissociation regions and not in dense molecular clouds. Synchronized star formation around the periphery of Carina provides a strong case that star formation here was indeed triggered by stellar winds and ultraviolet radiation. This second generation appears to involve a cascade toward preferentially intermediate- and low-mass stars, but this may soon change when , Carinae and its siblings explode. If the current reservoir of atomic and molecular gas can be tapped at that time, massive star formation may be rejuvenated around the periphery of Carina much as if it were a young version of Gould's Belt. Furthermore, when these multiple SNe occur, the triggered second generation will be pelted repeatedly with SN ejecta bearing short-lived radioactive nuclides. Carina may therefore represent the most observable analogue to the cradle of our own Solar system. [source]


    Spitzer observations of M83 and the hot star, H ii region connection

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
    Robert H. Rubin
    ABSTRACT We have undertaken a programme to observe emission lines of [S iv] 10.51, [Ne ii] 12.81, [Ne iii] 15.56, and [S iii] 18.71 ,m in a number of extragalactic H ii regions with the Spitzer Space Telescope. Here we report our results for the nearly face-on spiral galaxy M83. A subsequent paper will present our data and analysis for another substantially face-on spiral galaxy M33. The nebulae selected cover a wide range of galactocentric radii (RG). The observations were made with the infrared spectrograph in the short wavelength, high dispersion configuration. The above set of four lines is observed cospatially, thus permitting a reliable comparison of the fluxes. From the measured fluxes, we determine the ionic abundance ratios including Ne++/Ne+, S3+/S++ and S++/Ne+ and find that there is a correlation of increasingly higher ionization with larger RG. By sampling the dominant ionization states of Ne and S for H ii regions, we can approximate the Ne/S ratio by (Ne++ Ne++)/(S+++ S3+). Our findings of ratios that significantly exceed the benchmark Orion Nebula value, as well as a decrease in this ratio with increasing RG, are more likely due to other effects than a true gradient in Ne/S. Two effects that will tend to lower these high estimates and to flatten the gradient are first, the method does not account for the presence of S+ and second, S but not Ne is incorporated into grains. Both Ne and S are primary elements produced in ,-chain reactions, following C and O burning in stars, making their yields depend very little on the stellar metallicity. Thus, it is expected that Ne/S remains relatively constant throughout a galaxy. We stress that this type of observation and method of analysis does have the potential for accurate measurements of Ne/S, particularly for H ii regions that have lower metallicity and higher ionization than those here, such as those in M33. Our observations may also be used to test the predicted ionizing spectral energy distribution (SED) of various stellar atmosphere models. We compare the ratio of fractional ionizations ,Ne++,/,S++, and ,Ne++,/,S3+, versus ,S3+,/,S++, with predictions made from our photoionization models using several of the state-of-the-art stellar atmosphere model grids. The overall best fit appears to be the nebular models using the supergiant stellar atmosphere models of Pauldrach, Hoffmann & Lennon and Sternberg, Hoffmann & Pauldrach. This result is not sensitive to the electron density and temperature range expected for these M83 nebulae. Considerable computational effort has gone into the comparison between data and models, although not all parameter studies have yet been performed on an ultimate level (e.g. in the present paper the stellar atmosphere model abundances have been fixed to solar values). A future paper, with the benefit of more observational data, will continue these studies to further discriminate how the ionic ratios depend on the SED and the other nebular parameters. [source]


    On the relation between electron temperatures in the O+ and O++ zones in high-metallicity H ii regions

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006
    Leonid S. Pilyugin
    ABSTRACT We suggest a new way to establish the relation between the electron temperature t3 within the [O iii] zone and the electron temperature t2 within the [O ii] zone in high-metallicity (12 + log(O/H) > 8.25) H ii regions. The t2,t3 diagram is constructed by applying our method to a sample of 372 H ii regions. We find that the correlation between t2 and t3 is tight and can be approximated by a linear expression. The new t2,t3 relation can be used to determine t2 and accurate abundances in high-metallicity H ii regions with a measured t3. It can also be used in conjunction with the ff relation for the determination of t3 and t2 and oxygen abundances in high-metallicity H ii regions, where the [O iii],4363 auroral line is not detected. The derived t2,t3 relation is independent of photoionization models of H ii regions. [source]


    An ATCA radio-continuum study of the Small Magellanic Cloud , IV.

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006
    A multifrequency analysis of the N 66 region
    ABSTRACT Traditional identification of supernova remnants (SNRs) include the use of radio spectral index, optical spectral studies (including strong [S ii], [N ii], [O i], [O ii] and [O iii] lines) and X-ray co-identifications. Each of these can have significant limitations within the context of a particular SNR candidate and new identification methods are continually sought. In this paper, we explore subtraction techniques by Ye, Turtle and Kennicutt to remove thermal emission estimated from H, flux from radio-continuum images. The remaining non-thermal emission allows the identification of SNRs embedded within these H ii regions. Subtraction images of the N 66 region in the Small Magellanic Cloud (SMC) using H, wide-field optical CCD images from the Curtis Schmidt Telescope and the recent Australia Telescope Compact Array (ATCA)/Parkes radio-continuum (1420, 2370, 4800 and 8640 MHz) data are presented as an example. These show three SNRs (B0057 , 724, B0056 , 724 and B0056 , 725) separated from their surrounding H ii radio emission. 2.3-m dual-beam spectrograph long-slit spectra from selected regions within N 66 suggest the presence of an additional SNR with no radio or X-ray emission. Radio spectral index, [S ii]/H, ratio and archived Chandra images of N 66 combine to give a more coherent picture of this region, confirming B0057 , 724 as an SNR. The N 66 nebula complex is divided into 10 components, composed separately of these SNRs and H ii regions. [source]


    Reionization history from coupled cosmic microwave background/21-cm line data

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2005
    R. Salvaterra
    ABSTRACT We study cosmic microwave background (CMB) secondary anisotropies produced by inhomogeneous reionization by means of cosmological simulations coupled with the radiative transfer code crash. The reionization history is consistent with the Wilkinson Microwave Anisotropy Probe Thomson optical depth determination. We find that the signal arising from this process dominates over the primary CMB component for l, 4000 and reaches a maximum amplitude of l(l+ 1)Cl/2,, 1.6 × 10,13 on arcmin scales (i.e. l as large as several thousands). We then cross-correlate secondary CMB anisotropy maps with neutral hydrogen 21-cm line emission fluctuations obtained from the same simulations. The two signals are highly anticorrelated on angular scales corresponding to the typical size of H ii regions (including overlapping) at the 21-cm map redshift. We show how the CMB/21-cm cross-correlation can be used: (i) to study the nature of the reionization sources; (ii) to reconstruct the cosmic reionization history; (iii) to infer the mean cosmic ionization level at any redshift. We discuss the feasibility of the proposed experiment with forthcoming facilities. [source]


    MSX mid-infrared imaging of massive star birth environments , II.

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2004
    Giant H ii regions
    ABSTRACT We conduct a Galactic census of giant H ii (GH ii) regions, based on the all-sky 6-cm data set of Kuchar & Clark, plus the kinematic distances obtained by Russeil. From an inspection of mid-infrared (MIR) Mid-course Space Experiment (MSX) and far-IR IRAS Sky Survey Atlas images, we identify a total of 56 GH ii regions in the Milky Way, of which 15 per cent (65 per cent) can be seen at optical (near-IR) wavelengths. The mid to far-IR fluxes from each GH ii region are measured, and sample the thermal emission from the ubiquitous dust present within the exciting clusters of OB stars, arising from the integrated luminosity of the hot stars heating the cluster dust, for which we obtain log L(IR) = 5.5,7.3 L,. The MIR 21-,m spatial morphology is presented for each GH ii region, and often indicates multiple emission sources, suggesting complicated cluster formation. IR colour,colour diagrams are presented, providing information concerning the temperature distribution and the optical depth of the dust. For the clusters of our study, the dust is not optically thick to all stellar radiation, thus the measured infrared luminosity is lower than Lbol. As the dust environment of a cluster begins to dissipate, the thermal emission and its optical depth ought to decrease even before the stars evolve appreciably. We see evidence of this in our empirical relationship between the integrated IR and Lyman continuum luminosities. [source]


    Australia Telescope Compact Array H i observations of the NGC 6845 galaxy group

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2003
    Scott Gordon
    ABSTRACT We present the results of Australia Telescope Compact Array (ATCA) H i line and 20-cm radio continuum observations of the galaxy quartet NGC 6845. The H i emission extends over all four galaxies but can only be associated clearly with the two spiral galaxies, NGC 6845A and B, which show signs of strong tidal interaction. We derive a total H i mass of at least 1.8 × 1010 M,, most of which is associated with NGC 6845A, the largest galaxy of the group. We investigate the tidal interaction between NGC 6845A and B by studying the kinematics of distinct H i components and their relation to the known H ii regions. No H i emission is detected from the two lenticular galaxies, NGC 6845C and D. A previously uncatalogued dwarf galaxy, ATCA J2001,4659, was detected 4.4 arcmin NE from NGC 6845B and has an H i mass of ,5 × 108 M,. No H i bridge is visible between the group and its newly detected companion. Extended 20-cm radio continuum emission is detected in NGC 6845A and B as well as in the tidal bridge between the two galaxies. We derive star formation rates of 15,40 M, yr,1. [source]


    Stationary models for fast and slow logarithmic spiral patterns in disc galaxies

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2002
    Yu-Qing Lou
    A recent wavelet analysis on multiwavelength image data of the nearby spiral galaxy NGC 6946 revealed a multi-arm spiral structure that persists well into the outer differentially rotating disc region. The extended spiral arms in polarized radio-continuum emission and in red light appear interlaced with each other, while the spiral arms in emissions of total radio continuum, of H, from H ii regions, and of neutral hydrogen all trace the red-light spiral arms, although to a somewhat lesser extent. The key issue now becomes how to sustain extended slow magnetohydrodynamic (MHD) density wave features in a thin magnetized disc with a flat rotation curve. We describe here a theoretical model to examine stationary non-axisymmetric logarithmic spiral configurations constructed from a background equilibrium of a magnetized singular isothermal disc (MSID) with a flat rotation curve and with a non-force-free azimuthal magnetic field. It is found analytically that two types of stationary spiral MSID configurations may exist, physically corresponding to the two possibilities of fast and slow spiral MHD density waves. Such stationary MHD density waves are possible only at proper MSID rotation speeds. For the fast MSID configuration, logarithmic spiral enhancements of magnetic field and gas density are either in phase in the tight-winding regime or shifted with a spatial phase difference ,,/2 for open spiral structures. For the slow MSID configuration, logarithmic spiral enhancements of magnetic field and gas density are either out of phase in the tight-winding regime or shifted with a spatial phase difference for open spiral structures and persist in a flat rotation curve. For NGC 6946, several pertinent aspects of the slow MSID scenario with stationary logarithmic spiral arms are discussed. The two exact solutions can be also utilized to test relevant numerical MHD codes. [source]


    Narrow-band CCD photometry of giant H ii regions

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2002
    Guillermo Bosch
    We have obtained accurate CCD narrow-band H, and H, photometry of giant H ii regions (GEHRs) in M33, NGC 6822 and M101. Comparison with previous determinations of emission-line fluxes shows large discrepancies; their probable origins are discussed. Combining our new photometric data with global velocity dispersion (,) derived from emission linewidths, we review the relation. A re-analysis of the properties of the GEHRs included in our sample shows that age spread and the superposition of components in multiple regions introduce a considerable spread in the regression. Combining the information available in the literature regarding ages of the associated clusters, evolutionary footprints on the interstellar medium, and kinematical properties of the knots that build up the multiple GEHRs, we find that a subsample , which we refer to as young and single GEHRs , do follow a tight relation in the plane. [source]


    A comprehensive study of reported high-metallicity giant H ii regions , I. Detailed abundance analysis

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2002
    Marcelo Castellanos
    We present long-slit observations in the optical and near-infrared of 14 H ii regions in the spiral galaxies NGC 628, 925, 1232 and 1637, all of them reported to have solar or oversolar abundances according to empirical calibrations. For seven of the observed regions, ion-weighted temperatures from optical forbidden auroral to nebular line ratios are obtained and, for six of them, the oxygen abundances derived by standard methods turn out to be significantly lower than solar. The other one, named CDT1 in NGC 1232, shows an oxygen abundance of , and constitutes, to the best of our knowledge, the first high-metallicity H ii region for which accurate line temperatures, and hence elemental abundances, have been derived. For the rest of the regions no line temperature measurements could be made, and the metallicity has been determined by means of both detailed photoionization modelling and the sulphur abundance parameter S23. Only one of these regions shows values of O23 and S23 implying a solar or oversolar metallicity. According to our analysis, only two of the observed regions can therefore be considered as of high metallicity. These two fit the trends previously found in other high-metallicity H ii regions, i.e., N/O and S/O abundance ratios seem to be higher and lower than solar respectively. [source]