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Dynamical Formation of Dark Molecular Hydrogen Clouds around Diffuse H II Regions
We examine the process that triggers molecular cloud formation arounddiffuse H II regions. We calculate the time evolution of the shell, aswell as the H II region, in a two-phase neutral medium, solving the UVand FUV radiative transfer and the thermal and chemical processes with atime-dependent hydrodynamics code. In the cold neutral medium, theambient gas is swept up in a cold (T~30-40 K) and dense(n~103 cm-3) shell around the H II region. In theshell, H2 molecules are formed from the swept-up H I gas, buthardly any CO is formed. This is a result of the different efficienciesof self-shielding effects between H2 and CO molecules. Thephysical and chemical properties of gas in the shell are justintermediate between those of the neutral medium and molecular cloudsobserved in CO emission. We suggest that the predicted cold ``dark'' HI/H2 gas should be detectable as an H I self-absorption(HISA) feature. We have sought such features in recent observationaldata and found shell-like HISA features around the giant H II regions W4and W5. These features shows good spatial correlation with dustemission, but poor correlation with CO emission. Our quantitativeanalysis shows that the HISA cloud can be as cold as a few tens ofkelvins. In the warm neutral medium, on the other hand, an expandingdiffuse H II region is much simpler, owing to a small pressure excess.The UV photons only ionize the neutral medium and produce a warm ionizedmedium.
| Globulettes as Seeds of Brown Dwarfs and Free-Floating Planetary-Mass Objects
Some H II regions surrounding young stellar clusters contain tiny dustyclouds, which on photos look like dark spots or teardrops against abackground of nebular emission. From our collection of Hα imagesof 10 H II regions gathered at the Nordic Optical Telescope, we found173 such clouds, which we call ``globulettes,'' since they are muchsmaller than normal globules and form a distinct class of objects. Manyglobulettes are quite isolated and located far from the molecular shellsand elephant trunks associated with the regions. Others are attached tothe trunks (or shells), suggesting that globulettes may form as aconsequence of erosion of these larger structures. None of our objectsappear to contain stellar objects. The globulettes were measured forposition, dimension, and orientation, and we find that most objects aresmaller than 10 kAU. The Rosette Nebula and IC 1805 are particularlyrich in globulettes, for which the size distributions peak at mean radiiof ~2.5 kAU, similar to what was found by Reipurth and coworkers and DeMarco and coworkers for similar objects in other regions. We estimatetotal mass and density distributions for each object from extinctionmeasures and conclude that a majority contain <13 MJ,corresponding to planetary-mass objects. We then estimate the internalthermal and potential energies and find, when also including the effectsfrom the outer pressure, that a large fraction of the globulettes couldbe unstable and would contract on short timescales, <10 6yr. In addition, the radiation pressure and ram pressure exerted on theside facing the clusters would stimulate contraction. Since theglobulettes are not screened from stellar light by dust clouds fartherin, one would expect photoevaporation to dissolve the objects. However,surprisingly few objects show bright rims or teardrop forms. Wecalculate the expected lifetimes against photoevaporation. Theselifetimes scatter around 4×106 yr, much longer thanestimated in previous studies and also much longer than the free-falltime. We conclude that a large number of our globulettes have time toform central low-mass objects long before the ionization front, drivenby the impinging Lyman photons, has penetrated far into the globulette.Hence, the globulettes may be one source in the formation of browndwarfs and free-floating planetary-mass objects in the galaxy.Based on observations made with the Nordic Optical Telescope, operatedon the island of La Palma jointly by Denmark, Finland, Iceland, Norway,and Sweden in the Spanish Observatorio del Roque de los Muchachos of theInstituto de Astrofísica de Canarias.
| The Fragmenting Superbubble Associated with the H II Region W4
New observations at high latitudes above the H II region W4 show thatthe structure formerly identified as a chimney candidate, an opening tothe Galactic halo, is instead a superbubble in the process offragmenting and possibly evolving into a chimney. Data at high Galacticlatitudes (b>5deg) above the W3/W4 star-forming region at1420 and 408 MHz Stokes I (total power) and 1420 MHz Stokes Q and U(linear polarization) reveal an egg-shaped structure with morphologicalcorrelations between our data and the Hα data of Dennison,Topasna, and Simonetti. Polarized intensity images show depolarizationextending from W4 up the walls of the superbubble, providing strongevidence that the radio continuum is generated by thermal emissioncoincident with the Hα emission regions. We conclude that theparts of the H II region hitherto known as W4 and the newly revealedthermal emission are all ionized by the open cluster OCl 352. At anassumed distance of 2.35 kpc, the ovoid structure is 164 pc wide andextends 246 pc above the midplane of the Galaxy. The shell's emissiondecreases in total intensity and polarized intensity in variouslocations, appearing to have a break at its top and another on one side.Using a geometric analysis of the depolarization in the shell's walls,we estimate that a magnetic field line-of-sight component of 3-5 μGexists in the shell. We explore the connection between W4 and theGalactic halo, considering whether sufficient radiation can escape fromthe fragmenting superbubble to ionize the kiloparsec-scale Hα loopdiscovered by Reynolds, Sterling and Haffner.
| X-Ray Study of Triggered Star Formation and Protostars in IC 1396N
The IC 1396N cometary globule (CG) within the large nearby H II regionIC 1396 has been observed with the ACIS detector on board the ChandraX-Ray Observatory. We detect 117 X-ray sources, of which ~50-60 arelikely members of the young open cluster Trumpler 37 dispersedthroughout the H II region, and 25 are associated with young starsformed within the globule. Infrared photometry (2MASS and Spitzer) showsthat the X-ray population is very young: 3 older Class III stars, 16classical T Tauri stars, and 6 protostars including a Class 0/I system.We infer a total T Tauri population of ~30 stars in the globule,including the undetected population, with a star formation efficiency of1%-4%. An elongated source spatial distribution with an age gradientoriented toward the exciting star is discovered in the X-ray populationof IC 1396N, supporting similar findings in other cometary globules. Thegeometric and age distribution is consistent with the radiation-drivenimplosion (RDI) model for triggered star formation in CGs by H II regionshocks. The inferred velocity of the shock front propagating into theglobule is ~0.6 km s-1. The large number of X-ray-luminousprotostars in the globule suggests either an unusually high ratio ofClass I/0 to Class II/III stars or a nonstandard initial mass functionfavoring higher mass stars by the triggering process. We find that theChandra source associated with the luminous Class 0/I protostar IRAS21391+5802 is one of the youngest stars ever detected in the X-ray band.We also establish for the first time that the X-ray absorption inprotostars arises from the local infalling envelopes rather than fromambient molecular cloud material.
| Giant Molecular Clouds in M31. I. Molecular Cloud Properties
We present Berkeley-Illinois-Maryland Association (BIMA) millimeterinterferometer observations of giant molecular clouds (GMCs) along aspiral arm in M31. The observations consist of a survey using thecompact configuration of the interferometer and follow-up, higherresolution observations on a subset of the detections in the survey. Thedata are processed using an analysis algorithm designed to extract GMCsand correct their derived properties for observational biases, therebyfacilitating comparison with Milky Way data. The algorithm identifies 67GMCs, of which 19 have a sufficient signal-to-noise ratio to accuratelymeasure their properties. The GMCs in this portion of M31 areindistinguishable from those found in the Milky Way, having a similarsize-line width relationship and distribution of virial parameters,confirming the results of previous, smaller studies. The velocitygradients and angular momenta of the GMCs are comparable to the valuesmeasured in M33 and the Milky Way, and in all cases are below expectedvalues based on the local galactic shear. The studied region of M31 hasan interstellar radiation field, metallicity, Toomre Q parameter, andmidplane volume density similar to those of the inner Milky Way, so thesimilarity of GMC populations between the two systems is not surprising.
| Young Stars in the Camelopardalis Dust and Molecular Clouds. I. The Cam OB1 Association
The distribution of dust and molecular clouds in the direction ofGalactic longitudes 132--158° and latitudes ± 12\degr\ isinvestigated. The maps of dust distribution in the area were plottedfrom the following surveys: the star counts in the DSS I database byDobashi et al. (2005), the survey of the average infrared color excessesby Froebrich et al. (2007) and the thermal dust emission survey at 100μ m by Schlegel et al. (1998). The distribution of molecular cloudswas taken from the whole sky CO survey by Dame et al. (2001). All thesesurveys show very similar cloud patterns in the area. Using the radialvelocities of CO, the distances to separate clouds are estimated. Arevised list of the Cam OB1 association members contains 43 stars andthe open cluster NGC 1502. 18 young irregular variable and Hαemission stars are identified in the area. All this proves that the starforming process in the Camelopardalis clouds is still in progress.
| Sequential Formation of Low-Mass Stars in the BRC 14 Region
We carried out a deep near-infrared survey of a bright-rimmed molecularcloud, BRC 14 (IC 1848A). The 10σ limiting magnitude of the surveyis 17.7mag at the K-band. Seventy-four sources are classified as youngstellar object (YSO) candidates based on a near-infrared color-colordiagram. The faintest YSO candidates may have masses on the order oftenths of the solar mass, assuming an age of 1Myr. We examined threevalues as indicators of star formation: fraction of the YSO candidates,extinctions of all sources, and near-infrared excesses of the YSOcandidates. All indicators increase from outside of the rim to thecenter of the molecular cloud, which suggests that the formation of thelow-mass stars in the BRC 14 region proceeds from outside to the centerof the cloud.
| Kinematics of the Open Cluster System in the Galaxy
Absolute proper motions and radial velocities of 202 open clusters inthe solar neighborhood, which can be used as tracers of the Galacticdisk, are used to investigate the kinematics of the Galaxy in the solarvicinity, including the mean heliocentric velocity components(u1,u2,u3) of the open cluster system,the characteristic velocity dispersions(σ1,σ2,σ3), Oortconstants (A,B) and the large-scale radial motion parameters (C,D) ofthe Galaxy. The results derived from the observational data of propermotions and radial velocities of a subgroup of 117 thin disk young openclusters by means of a maximum likelihood algorithm are:(u1,u2,u3) =(-16.1+/-1.0,-7.9+/-1.4,-10.4+/-1.5) km s-1,(σ1,σ2,σ3) =(17.0+/-0.7,12.2+/-0.9,8.0+/-1.3) km s-1,(A,B) =(14.8+/-1.0,-13.0+/-2.7) km s-1 kpc-1, and (C,D) =(1.5+/-0.7,-1.2+/-1.5) km s-1 k pc-1. A discussionon the results and comparisons with what was obtained by other authorsis given.
| Hierarchical Star Formation in the Spiral Galaxy NGC 628
The distributions of size and luminosity for star-forming regions in thenearby spiral galaxy NGC 628 are studied over a wide range of scalesusing progressively blurred versions of an image from the Hubble SpaceTelescope Advanced Camera for Surveys. Four optical filters areconsidered for the central region, including Hα. Two filters areused for an outer region. The features in each blurred image are countedand measured using SExtractor. The cumulative size distribution is foundto be a power law in all passbands, with a slope of approximately -1.5over 1.8 orders of magnitude. The luminosity distribution isapproximately a power law as well, with a slope of approximately -1 forlogarithmic intervals of luminosity. The results suggest a scale-freenature for stellar aggregates in a galaxy disk. Fractal models of thindisks reproduce the projected size distribution and suggest a projectedmass distribution slope of approximately -0.5 for these extendedregions. This mass slope converts to the observed luminosity slope if weaccount for luminosity evolution and longer lifetimes in larger regions.
| Modeling the Neutral Hydrogen Interstellar Medium: A Better Kinematic Distance Tool
An advanced approach to the kinematic distance method is developed. Themethod is applicable to second- and third-quadrant Galactic objects withknown velocities. It is based on fitting a model of the density andvelocity features in an isothermal H I disk to observed H I data. Thevelocity field of the gas is modeled with a power law for basic circularrotation, underlying noncircular motions from a two-armed density wavepattern. With a reasonable number of adjustable parameters andconstraints the model reproduces observations toward many Galacticobjects, and accurate distances are found from the modeled velocityfield. High-resolution H I spectral line data from the Canadian GalacticPlane Survey (CGPS; Taylor et al.) are used to discriminate clouds fromthe intercloud medium (the ``stratum'') for which the model is intended.The ability of the model to reproduce these data is demonstrated in one-[Tb(v)] and two- [Tb(l, v)] dimensional fits.Distances to 22 H II regions and SNRs calculated by the fitted velocityfield compare extremely well with other kinematically independentdistances.
| Bias-free Measurement of Giant Molecular Cloud Properties
We review methods for measuring the sizes, line widths, and luminositiesof giant molecular clouds (GMCs) in molecular-line data cubes with lowresolution and sensitivity. We find that moment methods are robust andsensitive, making full use of both position and intensity information,and we recommend a standard method to measure the position angle, majorand minor axis sizes, line width, and luminosity using moment methods.Without corrections for the effects of beam convolution and sensitivityto GMC properties, the resulting properties may be severely biased. Thisis particularly true for extragalactic observations, where resolutionand sensitivity effects often bias measured values by 40% or more. Wecorrect for finite spatial and spectral resolutions with a simpledeconvolution, and we correct for sensitivity biases by extrapolatingproperties of a GMC to those we would expect to measure with perfectsensitivity (i.e., the 0 K isosurface). The resulting method recoversthe properties of a GMC to within 10% over a large range of resolutionsand sensitivities, provided the clouds are marginally resolved with apeak signal-to-noise ratio greater than 10. We note that interferometerssystematically underestimate cloud properties, particularly the fluxfrom a cloud. The degree of bias depends on the sensitivity of theobservations and the (u,v) coverage of the observations. In an Appendixto the paper we present a conservative, new decomposition algorithm foridentifying GMCs in molecular-line observations. This algorithm treatsthe data in physical rather than observational units (i.e., parsecsrather than beams or arcseconds), does not produce spurious clouds inthe presence of noise, and is sensitive to a range of morphologies. As aresult, the output of this decomposition should be directly comparableamong disparate data sets.
| Star formation associated with H II regions
Star formation associated with H II regions is briefly reviewed. Specialemphasis is laid on our series of observational studies on bright-rimmedclouds (BRCs), in which we found a phenomenon called "small-scalesequential star formation." In addition a new hypothesis is advocated onthe two modes of star formation associated with H II regions, i.e., thecluster and dispersed modes. The former gives birth to a rich clusterand in the associated H II region BRCs are formed only at a later stageof its evolution in the peripheries. In the latter mode no clusters oronly loose ones are formed, but BRCs can appear at earlier stages ininner part of the H II region. Presumably these modes depend on theinitial density distribution of the natal molecular cloud.
| The Cosmological Significance of High-Velocity Cloud Complex H
We have used new and archival infrared and radio observations to searchfor a dwarf galaxy associated with the high-velocity cloud (HVC) knownas `complex H.' Complex H is a large (Ω>~400 deg2)and probably nearby (d=27 kpc) HVC whose location in the Galactic planehas hampered previous investigations of its stellar content. The H Imass of the cloud is 2.0×107(d/27 kpc)2Msolar, making complex H one of the most massive HVCs if itsdistance is more than ~20 kpc. Virtually all similar H I clouds in othergalaxy groups are associated with low surface brightness dwarf galaxies.We selected mid-infrared sources observed by the MSX satellite in thedirection of complex H that appeared likely to be star-forming regionsand observed them at the wavelength of the CO J=1-->0 rotationaltransition in order to determine their velocities. Of the 60 observedsources, 59 show emission at Milky Way velocities, and we detected noemission at velocities consistent with that of complex H. We use theseobservations to set an upper limit on the ongoing star formation rate inthe HVC of <~5×10-4 Msolaryr-1. We also searched the 2MASS database for evidence of anydwarf-galaxy-like stellar population in the direction of the HVC andfound no trace of a distant red giant population, with an upper limit onthe stellar mass of ~106 Msolar. Given the lack ofevidence for either current star formation or an evolved population, weconclude that complex H cannot be a dwarf galaxy with properties similarto those of known dwarfs. Complex H is therefore one of the most massiveknown H I clouds that does not contain any stars. If complex H isself-gravitating, then this object is one of the few known dark galaxycandidates. These findings may offer observational support for the ideathat the cold dark matter substructure problem is related to thedifficulty of forming stars in low-mass dark matter halos;alternatively, complex H could be an example of a cold accretion flowonto the Milky Way.
| Binary and Multiple O-Type Stars in the Cassiopeia OB6 Association
We present the results of time-resolved spectroscopy of 13 O-type starsin the Cas OB6 stellar association. We conducted a survey for radialvelocity variability in search of binary systems, which are expected tobe plentiful in young OB associations. Here we report the discovery oftwo new single-lined binaries, and we present new orbital elements forthree double-lined binaries (including one in the multiple-star systemHD 17505). One of the double-lined systems is the eclipsing binarysystem DN Cas, and we present a preliminary light-curve analysis thatyields the system inclination, masses, and radii. We compare the spectraof the single stars and the individual components of the binary starswith model synthetic spectra to estimate the stellar effectivetemperatures, gravities, and projected rotational velocities. We alsomake fits of the spectral energy distributions to derive E(B-V),R=AV/E(B-V), and angular diameter. A distance of 1.9 kpcyields radii that are consistent with evolutionary models. We find that7 of 14 systems with spectroscopic data are probable binaries,consistent with the high binary frequency found for other massive starsin clusters and associations.
| Precession of the Orbital Plane of Binary Pulsars and Significant Variabilities
There are two ways of expressing the precession of orbital plane of abinary pulsar system, given by Barker & O'Connell, Apostolatos etal. and Kidder, respectively. We point out that these two ways actuallycome from the same Lagrangian under different degrees of freedom. Damour& Schäfer and Wex & Kopeikin applied Barker &O'Connell's orbital precession velocity in pulsar timing measurement.This paper applies Apostolatos et al.'s and Kidder's orbital precessionvelocity. We show that Damour & Schäfer's treatment correspondsto negligible Spin-Orbit induced precession of periastron, while Wex& Kopeikin and this paper both found significant (but notequivalent) effects. The observational data of two typical binarypulsars, PSR J2051 0827 and PSR J1713+0747, apparently support asignificant Spin-Orbit coupling effect. Specific binary pulsars withorbital plane nearly edge on could discriminate between Wex &Kopeikin and this paper: if the orbital period derivative of thedouble-pulsar system PSRs J0737 3039 A and B, with orbital inclinationangle i = 87.7-29+17 deg, is much larger than thatof the gravitational radiation induced one, then the expression in thispaper is supported, otherwise Wex & Kopeikin's is supported.
| The Spiral Arms and Interarm Separation of the Milky Way: An Updated Statistical Study
Measurements of the spiral arms (their pitch angle, number, shape, andthe interarm separation) in the Milky Way have been published in theperiod from 1980 to early 2005, using different methods that yieldedslightly or widely differing values, depending on hidden or unknownbiases. A meta-analysis would be useful for searching for trends amongthese individual studies. First, we find a convergence for threeparameters: the pitch angle, the number of arms, and the shape of thearms. Second, we find a change for the interarm separation near the Sun,a decrease of about 25%; it is our determination of these parametersthat evolves, not the actual physical arms. Third, we recalibrate arecent arm model to take account of that gradual change and compare itwith the spread in each arm parameter.
| Astrophysical parameters of Galactic open clusters
We present a catalogue of astrophysical data for 520 Galactic openclusters. These are the clusters for which at least three most probablemembers (18 on average) could be identified in the ASCC-2.5, a catalogueof stars based on the Tycho-2 observations from the Hipparcos mission.We applied homogeneous methods and algorithms to determine angular sizesof cluster cores and coronae, heliocentric distances, mean propermotions, mean radial velocities, and ages. For the first time we derivedistances for 200 clusters, radial velocities for 94 clusters, and agesof 196 clusters. This homogeneous new parameter set is compared withearlier determinations, where we find, in particular, that the angularsizes were systematically underestimated in the literature.
| Interaction of the Supernova Remnant HB3 with the Ambient Interstellar Gas
The well-known shell supernova remnant (SNR) HB3 is part of afeature-rich star-forming region together with the nebulae W3, W4, andW5. We study the HI structure around this SNR using five RATAN-600 driftcurves obtained at a wavelength of 21 cm with an angular resolution of2′ in one coordinate over the radial-velocity range -183 to +60 kms-1 in a wider region of the sky and with a higher sensitivity than inprevious works by other authors. The spatial-kinematic distribution ofHI features around the SNR clearly shows two concentric expanding shellsof gas that surround the SNR and coincide with it in all threecoordinates (α, δ, and V). The outer shell has a radius of133 pc, a thickness of 24 pc, and an expansion velocity of 48 km s-1.The mass of the gas in it is ≈2.3 × 105 M ȯ. For the innershell, these parameters are 78 pc, 36 pc, 24 km s- 1, and 0.9 ×105 M ȯ, respectively. The inner shell is immediately adjacent tothe SNR. Assuming that the outer shell was produced by the stellar windand the inner shell arose from the shock wave of the SNR proper, weestimated the age of the outer shell, ≈1.7 × 106 yr, and themechanical luminosity of the stellar wind, 1.5 × 1038 erg s-1. Theinner shell has an age of ≈106 yr and corresponds to a totalsupernova explosion energy of ≈1052 erg.
| Analysis of the Radio Spectrum of HB3
The supernova remnant HB3 was re-observed at 408MHz and 1420MHz with theDominion Radio Astrophysical Observatory Synthesis Telescope as part ofthe Canadian Galactic Plane Survey. These observations supercedeprevious observations, as they have higher sensitivity and significantlyimproved calibration and dynamic range.HB3 is adjacent to the bright HII regions W3, W4 and W5, so that theimproved imaging significantly improves the contamination by the HIIregions' emission. The current analysis includes identification andremoval of background point sources to improve the measurement of theradio flux due to the supernova remnant. Spectral index is measured forseveral region in HB3 using integrated flux. We also apply the T-T plotmethod, which is sensitive to the spectral index of spatially variableemission. The results are compared with previously published values. Theanalysis shows that removal of background point sources and thecontribution from extended emission from W3 are both important. Theaverage spectral index of HB3 is 0.3-0.4, flatter than previouslyreported. The difference is consistent with being due the spectrum ofthe background sources. The filamentary emission from HB3 is found tohave spatially variable spectral index. The possible mechanisms for thisare discussed.This work supported by the Natural Sciences and Engineering ResearchCouncil of Canada.
| A radio and mid-infrared survey of northern bright-rimmed clouds
We have carried out an archival radio, optical and infrared wavelengthimaging survey of 44 Bright-Rimmed Clouds (BRCs) using the NRAO/VLA SkySurvey (NVSS) archive, images from the Digitised Sky Survey (DSS) andthe Midcourse Space eXperiment (MSX). The data characterise the physicalproperties of the Ionised Boundary Layer (IBL) of the BRCs. We haveclassified the radio detections as: that associated with the ionisedcloud rims; that associated with possible embedded Young Stellar Objects(YSOs); and that unlikely to be associated with the clouds at all. Thestars responsible for ionising each cloud are identified and acomparison of the expected ionising flux to that measured at the cloudrims is presented. A total of 25 clouds display 20 cm radio continuumemission that is associated with their bright optical rims. The ionisingphoton flux illuminating these clouds, the ionised gas pressure and theelectron density of the IBL are determined. We derive internal molecularpressures for 9 clouds using molecular line data from the literature andcompare these pressures to the IBL pressures to determine the pressurebalance of the clouds. We find three clouds in which the pressureexerted by their IBLs is much greater than that measured in the internalmolecular material. A comparison of external pressures around theremaining clouds to a global mean internal pressure shows that themajority of clouds can be expected to be in pressure equilibrium withtheir IBLs and hence are likely to be currently shocked byphotoionisation shocks. We identify one source which shows 20 cmemission consistent with that of an embedded high-mass YSO and confirmits association with a known infrared stellar cluster. This embeddedcluster is shown to contain early-type B stars, implying that at leastsome BRCs are intimately involved in intermediate to high mass starformation.Figure \ref{fig:images} and Table \ref{tbl:istars1} are only availablein electronic form at http://www.edpsciences.org
| Cosmic magnetic fields - as observed in the Universe, in galactic dynamos, and in the Milky Way
Cosmic magnetism has that exotic ``Je ne sais quoi''! Magnetism has beenobserved in various objects, located near the edge of the Universe andall the way down to the Milky Way's center. The observed magnetic fieldcan take the cell-type shape in randomly-oriented large blobs found inintracluster gas or outside of clusters of galaxies, the helix shape insynchrotron jets, the longitudinal shape in ram-pressured shocks inradio lobes near elliptical galaxies, the spiral shape of logarithmicarms in spiral galaxies, or the egg shape of an enlarged interstellarbubble. In strength, the magnetic field varies from 0.1 nG(cosmological), to 20 μG (galaxies, jets, superbubbles), and to 1 mGin the Milky Way filaments.Magnetism plays a small physical role in the formation of largestructures. It acts as a tracer of the dynamical histories ofcosmological and intracluster events, it guides the motion of theinterstellar ionised gas, and it aligns the charged dust particles.Batteries and dynamos are often employed in models to create and amplifyseed magnetic fields. Starting soon after the Big Bang (redshiftz>2000), this review covers the cosmological background surface(z~1100, distance ~4.3 Gpc), the epoch of first stars (z~20 distance~4.1 Gpc), the currently observable Universe (z~10, distance ~3.9 Gpc),superclusters of galaxies (size ~50 Mpc), intracluster gas (size ~10Mpc), galaxies (~30 kpc), spiral arms (~10 kpc), interstellarsuperbubbles (~100 pc), synchrotron filaments (~10 pc), and the MilkyWay's center.
| Comparison of the Luminosity Functions of Open Clusters Based on USNO-A1 Data
The luminosity and mass functions of a group of Galactic open clustersare constructed by applying a statistical method to photometric datafrom the USNO-A1 catalog. Despite some limitations, this catalog can beused for statistical analyses in Galactic astronomy. Pairwisecomparisons of the derived cluster luminosity functions are performedfor five age intervals. The differences between the luminosity functionsof the open clusters are not statistically significant in most cases. Itis concluded that the luminosity functions are approximately universalthroughout a large volume in the solar neighborhood. Combined luminosityand mass functions are constructed for six age intervals. The slope ofthe mass spectrum may vary somewhat from cluster to cluster, and themean slope may be somewhat higher than the Salpetervalue.
| Interstellar Turbulence I: Observations and Processes
Turbulence affects the structure and motions of nearly all temperatureand density regimes in the interstellar gas. This two-part reviewsummarizes the observations, theory, and simulations of interstellarturbulence and their implications for many fields of astrophysics. Thefirst part begins with diagnostics for turbulence that have been appliedto the cool interstellar medium and highlights their main results. Theenergy sources for interstellar turbulence are then summarized alongwith numerical estimates for their power input. Supernovae andsuperbubbles dominate the total power, but many other sources spanning alarge range of scales, from swing-amplified gravitational instabilitiesto cosmic ray streaming, all contribute in some way. Turbulence theoryis considered in detail, including the basic fluid equations, solenoidaland compressible modes, global inviscid quadratic invariants, scalingarguments for the power spectrum, phenomenological models for thescaling of higher-order structure functions, the direction and localityof energy transfer and cascade, velocity probability distributions, andturbulent pressure. We emphasize expected differences betweenincompressible and compressible turbulence. Theories of magneticturbulence on scales smaller than the collision mean free path areincluded, as are theories of magnetohydrodynamic turbulence and theirvarious proposals for power spectra. Numerical simulations ofinterstellar turbulence are reviewed. Models have reproduced the basicfeatures of the observed scaling relations, predicted fast decay ratesfor supersonic MHD turbulence, and derived probability distributionfunctions for density. Thermal instabilities and thermal phases have anew interpretation in a supersonically turbulent medium. Large-scalemodels with various combinations of self-gravity, magnetic fields,supernovae, and star formation are beginning to resemble the observedinterstellar medium in morphology and statistical properties. The roleof self-gravity in turbulent gas evolution is clarified, leading to newparadigms for the formation of star clusters, the stellar mass function,the origin of stellar rotation and binary stars, and the effects ofmagnetic fields. The review ends with a reflection on the progress thathas been made in our understanding of the interstellar medium and offersa list of outstanding problems.
| Emission of CO, C I, and C II in W3 Main
We used the KOSMA 3m telescope to map the core 7' × 5' of theGalactic massive star forming region W3 Main in the two fine structurelines of atomic carbon and four mid-J transitions of CO and13CO. The maps are centered on the luminous infrared sourceIRS 5 for which we obtained ISO/LWS data comprising four high-J COtransitions, [C II], and [O I] at 63 and 145 μm. In combination witha KAO map of integrated line intensities of [C II] \citep{howe1991},this data set allows us to study the physical structure of the molecularcloud interface regions where the occurence of carbon is believed tochange from C+ to C0, and to CO. The molecular gasin W3 Main is warmed by the far ultraviolet (FUV) field created by morethan a dozen OB stars. Detailed modelling shows that most of theobserved line intensity ratios and absolute intensities are consistentwith a clumpy photon dominated region (PDR) of a few hundred unresolvedclumps per 0.84 pc beam, filling between 3 and 9% of the volume, with atypical clump radius of 0.025 pc (2.2''), and typical mass of 0.44Mȯ. The high-excitation lines of CO stem from a 100-200K layer, as do the [C I] lines. The bulk of the gas mass is however atlower temperatures.
| Far-infrared loops in the 2nd Galactic Quadrant
We present the results of an investigation of the large-scale structureof the diffuse interstellar medium in the 2nd Galactic Quadrant(90°≤l≤180°). 145 loops were identified on IRAS-basedfar-infrared maps. Our catalogue lists their basic physical properties.The distribution clearly suggests that there is an efficient processthat can generate loop-like features at high Galactic latitudes.Distances are provided for 30 loops. We also give an observationalestimate of the volume filling factor of the hot gas in the Local Arm,4.6%≤f2nd<6.4%.Appendices A-C are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/131
| A study of the Galactic star forming region IRAS 02593+6016/S 201 in infrared and radio wavelengths
We present infrared and radio continuum observations of the S 201 starforming region. A massive star cluster is seen, which contains differentclasses of young stellar objects. The near-infrared colour-colour andcolour-magnitude diagrams are studied to determine the nature of thesesources. We have discovered knots of molecular hydrogen emission at2.122 μm in the central region of S 201. These knots are clearly seenalong the diffuse emission to the north-west and are probably obscuredHerbig-Haro objects. High sensitivity and high resolution radiocontinuum images from GMRT observations at 610 and 1280 MHz show anarc-shaped structure due to the interaction between the HII region andthe adjacent molecular cloud. The ionization front at the interfacebetween the HII region and the molecular cloud is clearly seen comparingthe radio, molecular hydrogen and Brγ images. The emission fromthe carriers of Unidentified Infrared Bands in the mid-infrared 6-9μm (possibly due to PAHs) as extracted from the Midcourse SpaceExperiment survey (at 8, 12, 14 and 21 μm) is compared with the radioemission. The HIRES processed IRAS maps at 12, 25, 60 and 100 μm havealso been used for comparison. The spatial distribution of thetemperature and the optical depth of the warm dust component around theS 201 region has been generated from the mid-infrared images.This paper is based on observations made with the Italian TelescopioNazionale Galileo (TNG) operated on the island of La Palma by the CentroGalileo Galilei of the CNAA (Consorzio Nazionale per l'Astronomia el'Astrofisica) at the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofisica de Canarias.This publication makes use of data products from the Two Micron All SkySurvey, which is a joint project of the University of Massachusetts andthe Infrared Processing and Analysis Center/California Institute ofTechnology, funded by the National Aeronautics and Space Administrationand the National Science Foundation. IPAC is thanked for providing HIRESprocessed IRAS data.
| Searching for signs of triggered star formation toward IC 1848
We have carried out an in-depth study of three bright-rimmed cloudsSFO 11, SFO 11NE and SFO 11Eassociated with the HII region IC 1848, using observations carried outat the James Clerk Maxwell Telescope (JCMT) and the Nordic OpticalTelescope (NOT), plus archival data from IRAS, 2MASS and the NVSS. Weshow that the overall morphology of the clouds is reasonably consistentwith that of radiative-driven implosion (RDI) models developed topredict the evolution of cometary globules. There is evidence for aphotoevaporated flow from the surface of each cloud and, based upon themorphology and pressure balance of the clouds, it is possible thatD-critical ionisation fronts are propagating into the molecular gas. Theprimary O star responsible for ionising the surfaces of the clouds isthe 06V star HD 17505. Each cloud is associated witheither recent or ongoing star formation: we have detected 8 sub-mm coreswhich possess the hallmarks of protostellar cores and identify YSOcandidates from 2MASS data. We infer the past and future evolution ofthe clouds and demonstrate via a simple pressure-based argument that theUV illumination may have induced the collapse of the dense molecularcores found at the head of SFO 11 and SFO11E.
| Morphological Analysis of H I Features. I. Metric Space Technique
This is the first of two papers on the morphological analysis of H Ifeatures. In this first paper, we use the so-called metric spacetechnique, developed by F. C. Adams and J. Wiseman. The metric spacetechnique is an image analysis, mathematical formalism used toquantitatively compare astrophysical maps according to complexity.Instead of comparing maps on a pixel-by-pixel basis, we compare themaps' one-dimensional ``output functions,'' which characterize specificmorphological/physical aspects of the maps. The tool is used to analyze28 H I features of known origin taken from the Canadian Galactic PlaneSurvey (CGPS), where the maps are scaled at 18" per pixel (resolution of1cosδ arcmin). Technical and mathematical improvements to theformalism are presented. After classifying the 28 maps according tocomplexity, we searched for correlations between this complexity rankingand other quantifiable aspects of the H I features such as age, area, HI area, distance, flux from the ionizing star(s), fractal dimension, H Imass, and |z| (the absolute value of the height of the objects, above orbelow the Galactic plane). The most interesting correlations are (1) thehigher the flux of UV photons, the more complex is the photodissociatedH I feature, and (2) the older the supernova remnant, the more complexthe H I associated with it. There is no correlation between the fractaldimension of the maps and their complexity or their physicalcharacteristics, thus showing that the metric space technique could beused as a solution to the degeneracy of the fractal dimension.
| W4 revisited: a chimney candidate in the Milky Way galaxy explored using radio continuum and polarization observations. (thesis abstract).
Not Available
| A New Distance Technique for Galactic Plane Objects
We present a new method based on H I column densities for determinationof distances within the disk of the Galaxy. The technique is useful forall Galactic plane objects, including H II regions and supernovaremnants (SNRs), provided a line-of-sight velocity can be assigned tothe object. Our method uses 21 cm spectral-line data to find the atomichydrogen column density to an object, and beyond it to the Galacticedge. A model of the smooth large-scale Galactic distribution of H Imaterial seen in emission (which principally traces the smooth structureof the Galaxy) is constructed. Our model accounts for scale-heightflaring with increasing Galactocentric radius and includes the Galacticwarp, which is prominent in the first and second quadrants of theGalaxy. The model's ability to trace the observed distribution of H I isdemonstrated on lines of sight toward SNR DA 530 (l=93.3d,b=7deg) and H II region Sh 121 (l=90.2d, b=1.7d). We thenapply the new technique to 29 Sharpless H II regions with knownphotometric distances across the second quadrant. We measureline-of-sight velocities for the H II regions from associated12CO emission, using 1' resolution 12CO (J=1-0)data from the Canadian Galactic Plane Survey. Our distance method yieldsdistances to these objects that are consistent with their photometricdistances and which are markedly smaller than the kinematic distancesfound from a flat Galactic rotation curve.
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