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Hierarchical Triggering of Star Formation by Superbubbles in W3/W4
It is generally believed that expanding superbubbles and mechanicalfeedback from massive stars trigger star formation, because there arenumerous examples of superbubbles showing secondary star formation attheir edges. However, although these systems show an age sequence, theydo not provide strong evidence of a causal relationship. The W3/W4Galactic star-forming complex suggests a three-generation hierarchy: thesupergiant shell structures correspond to the oldest generation; thesetriggered the formation of IC 1795 in W3, the progenitor of a molecularsuperbubble that in turn triggered the current star-forming episodes inthe embedded regions W3-North, W3-Main, and W3-OH. We present UBVphotometry and spectroscopic classifications for IC 1795, which show anage of 3-5 Myr. This age is intermediate between the reported 6-20 Myrage of the supergiant shell system and the extremely young ages(104-105 yr) for the embedded knots of theultracompact H II regions, W3-North, W3-Main, and W3-OH. Thus, an agesequence is indeed confirmed for the entire W3/W4 hierarchical system.This therefore provides some of the first convincing evidence thatsuperbubble action and mechanical feedback are indeed a triggeringmechanism for star formation.

New infrared star clusters in the Northern and Equatorial Milky Way with 2MASS
We carried out a survey of infrared star clusters and stellar groups onthe 2MASS J, H and Ks all-sky release Atlas in the Northernand Equatorial Milky Way (350deg < l < 360deg, 0deg < l < 230 deg). Thesearch in this zone complements that in the Southern Milky Way (Dutra etal. \cite{Dutra03}a). The method concentrates efforts on the directionsof known optical and radio nebulae. The present study provides 167 newinfrared clusters, stellar groups and candidates. Combining the twostudies for the whole Milky Way, 346 infrared clusters, stellar groupsand candidates were discovered, whereas 315 objects were previouslyknown. They constitute an important new sample for future detailedstudies.

Probing the Interstellar Medium Using H I Absorption and Emission toward the W3 H II Region
H I spectra toward the W3 H II complex are presented and used to probethe Galactic structure and interstellar medium conditions between us andthis region. The overall shape of the spectra is consistent with thepredictions of the Two-Arm Spiral Shock model wherein the gas found inthe -40 to -50 km s^-1 range has been accelerated by some 20 km s^-1from its rotation curve velocity. Spin temperatures of ~100 K arederived for the Local Arm gas, lower than found in a previous, similarstudy toward DR 7. For the interarm region, values on the order of 300 Kare found, implying a negligible filling factor for the cold neutralmedium (<<1%). Some of the absorbing gas at velocities near -40 kms^-1 is confirmed to be associated with the H II regions.

Dust Scattering of Emission Lines in H II Regions. I. Plane-parallel Models and Application to the Orion Nebula (M42)
I present a comprehensive plane-parallel radiative transfer model forthe scattering by dust of emission lines in a blister H II region on thesurface of a molecular cloud. The effects of dust in the molecularcloud, within the H II region, and in a layer of overlying neutralmaterial are all included. The spectral intensity and polarizationprofiles of the scattered lines are calculated, as well as the effectivealbedo of the scattering. Analytical, semianalytical and Monte Carlonumerical techniques are used, as appropriate. The model is applied indetail to observations of the [O III] lambda 5007 A line in the innerOrion Nebula. It is shown that the observations are consistent with abackscattering origin for the broad redshifted component that is seen inthis line, as well as in lines of other ionized species. High-resolutionspectropolarimetry of the emission line would provide a conclusive testof the scattering model. Model fits suggest that the optical depth ofdust between the ionization front and the [O III] emitting region is inthe range 0.5-1.0 in the visible. Foreground scattering in the neutrallid that overlies the nebula is shown to have little effect on the lineprofile, although it could account for some of the unexplainedbroadening seen in all optical lines.

VLA Zeeman observations of W3
The 100-microG field strengths and field reversals observed by Trolandet al. (1989) in the -38 km/s feature of the W3 core are confirmed. Theweak positive field observed by Van der Werf and Goss (1990) in the -46km/s feature is confirmed as well. The statistical summing technique isused to determine the magnetic field throughout much of the W3 sample. Amodel of an initially uniform magnetic field which has been pinched bythe collapse of the surrounding molecular cloud is proposed to explainthe particular morphology of the field in the -38 km/s velocitycomponent toward W3 A and W3 B. Measurements of the line-of-sightmagnetic field south of W3 A and W3 B suggest that the magnetic field inthis region has been distorted into a convex structure, perhaps by theexpansion of the H II region NG 896.

Radio recombination line observations of the galactic H II complex W3
This paper presents the results of multifrequency high resolutionobservations of the giant H II complex in the hydrogen, helium, andcarbon 76-alpha, 110-alpha, 166-alpha, and 272-alpha radio recombinationlines. Locally strong helium lines are observed toward W3A and B inwhich the He to H line intensity ratio is found to be greater than 20percent or more. Possible explanations for these high line ratios arelocal He enrichment of the ISM due to an evolved stellar object or thepresence of a very hard radiation field leading to underionization ofhydrogen. Carbon emission is detected toward the core region of W3. Nocarbon emission is observed at locations away from the strong continuumsources, which must be due to the C II cloud being located in front ofthese sources. Stimulated emission is the main emission mechanism forthe C166-alpha and C110-alpha lines observed toward W3A.

High resolution H I observations of H II regions. II - H I and the magnetic field near W3
H I absorption observations of the W3 complex, conducted using theWesterbork Synthesis Radio Telescope (angular resolution 13 arcsec,velocity resolution 1 km/s), are presented. The observed velocitystructure is explained by the influence of the shock front associatedwith the H II region W4 on the W3 molecular cloud. The model provides anexplanation for the observed velocities of ionized, neutral, andpartially ionized gas in the various components of the W3 complex. Theline-of-sight magnetic field in the absorbing gas is determined from theH I Zeeman effect. Significant small-scale structure is found. Themagnetic field exhibits a centrally peaked morphology of theline-of-sight magnetic field toward W3A in the -45.8 km/s H I component.It is suggested that the magnetic field is a vestige of recent formationof massive stars in that area. The estimated time scale for ambipolardiffusion is consistent with this hypothesis.

Carbon radio recombination line observations of W3
The H II complex W3 has been observed with a 13.4 x 14.2-arcsec angularresolution and 2 km/s velocity resolution in the C166-alpha radiorecombination line. These are the first interferometric C166-alphaobservations. The line is detected only from a region of 40-arcsecdiameter centered on component A, the more evolved component in the coreof the complex. The distribution of the emission and the velocitystructure seem to support the suggestion that at frequencies as low as21 cm, only emission from a cold cloud in front of the H II region isobserved. In agreement with earlier studies, the main contributingemission mechanism is most likely a stimulated emission.

The W3 molecular cloud
Extensive J = 1 to 0 (C-12)(O-16) and (C-13)(O-16) observations of theW3 molecular cloud and the surrounding region are presented anddiscussed. The velocity structure in the region is strongly suggestiveof a model of large-scale, externally induced star formation. It isshown that star formation occurred in W3 and the nearby star-formingregion W3(OH) after the gas within which they lie was swept up by theexpanding W4 ionization front. Two condensations dominate the massstructure of the core of W3, one associated with IRS 4 and the otherwith IRS 5 and 1. A velocity difference between the two condensations isinterpreted as indicating the two sources actually are discrete knots.

Internal reddening of H II-regions and HH-objects, with special reference to NGC 896 (W3)
Simple analytical models are used in a study of the effect of scatteringby dust internal to H II regions, for assumed universal opticalparameters of the dust. A procedure for determining the externalinterstellar extinction and the internal gas-to-dust abundance ratio ofthe H II regions has been developed and applied as a test to the OrionNebula. New observations of the H II region NGC 896 have been analyzedto identify the effects of internal dust on its spectrum, and toestablish its relationship to the background radio source W3.

A comparison of high resolution optical and radio observations of W3
High resolution maps of the W3 complex for radio continuum and selectedoptical emission lines are used to derive the visual extinction A(V),the excitation, and the evolutionary state of the component H IIregions. A large diffuse H II region in the east, which is the mosthighly evolved of this type, is found, together with a southern complexof H II regions and a northern complex of young and compact H II regionsstill embedded in the W3 molecular core. A comparison of the observeddust distribution with ionized and atomic hydrogen and several othermolecules in W3 shows the functional relationship between the C-130column density and A(V) to be in accord with Strohacker's (1978) resultfor dark clouds.

Comparative study of radio and optical photometry of several H II regions.
Abstract image available at:http://adsabs.harvard.edu/abs/1971AJ.....76..571S

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Observation and Astrometry data

Right ascension:02h25m48.00s
Apparent magnitude:99.9

Catalogs and designations:
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NGC 2000.0NGC 896

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