A NEAR-INFRARED STUDY OF THE HIGHLY-OBSCURED ACTIVE STAR-FORMING REGION W51B

By comparing radial velocities of radio bright compact Hii regions with their Hi absorption profiles, we discovered expanding shells of neutral hydrogen around them. These shells are revealed by absorption of the radio continuum emission from the Hii regions at velocities indicating greater distances than the observed radial velocity. We believe that these shells are shock zones at the outer edge of the expanding ionized region. Additionally we found evidence for a velocity inversion inside the Perseus arm caused by a spiral shock, which results in a deep absorption line in the spectra of compact Hii regions behind it. In this paper we present a novel technique for the detec- tion and study of expanding Hi structures around compact Hii regions that uses 21-cm Hi absorption spectra. The technique allows Hi surrounding Hii regions to be studied even if the Hii region is not resolved. In a number of cases we detect absorp- tion features that are associated with shocked expanding lay- ers of photodissociated gas. We focus on compact Hii regions within the observed area of the Canadian Galactic Plane Survey

We present a multi-wavelength analysis of the infrared dust bubble S24, and its environs, with the aim of investigating the characteristics of the molecular gas and the interstellar dust linked to them, and analyzing the evolutionary status of the young stellar objects (YSOs) identified there. Using APEX data, we mapped the molecular emission in the CO(2-1), $^{13}$CO(2-1), C$^{18}$O(2-1), and $^{13}$CO(3-2) lines in a region of about 5'x 5' in size around the bubble. The cold dust distribution was analyzed using ATLASGAL and Herschel images. Complementary IR and radio data were also used.The molecular gas linked to the S24 bubble, G341.220-0.213, and G341.217-0.237 has velocities between -48.0 km sec$^{-1}$ and -40.0 km sec$^{-1}$. The gas distribution reveals a shell-like molecular structure of $\sim$0.8 pc in radius bordering the bubble. A cold dust counterpart of the shell is detected in the LABOCA and Herschel images.The presence of extended emission at 24 $\mu$m and radio continuum emission inside the bubble indicates that the bubble is a compact HII region. Part of the molecular gas bordering S24 coincides with the extended infrared dust cloud SDC341.194-0.221. A cold molecular clump is present at the interface between S24 and G341.217-0.237. As regards G341.220-0.213, the presence of an arc-like molecular structure at the northern and eastern sections of this IR source indicates that G341.220-0.213 is interacting with the molecular gas. Several YSO candidates are found to be linked to the IR extended sources, thus confirming their nature as active star-forming regions. The total gas mass in the region and the H$_2$ ambient density amount to 10300 M$_{\odot}$ and 5900 cm$^{-3}$, indicating that G341.220-0.213, G341.217-0.237, and the S24 HII region are evolving in a high density medium. A triggering star formation scenario is also investigated.

Data on thermal radio emission and absorption in and near the directions towards supernova remnants are used to estimate the distribution of ionized gas surrounding remnants of type II supernovae. The amount of absorption and emission toward the supernova remnants are determined by two types of HII regions. The first are extended HII regions around the supernova remnants (Stromgren spheres), while the second are more compact and bright HII regions surrounding early-type stars. In the early stages of evolution of the supernova remnants (1000–3000 yrs), the amount of thermal absorption and emission is minimum, apparently indicating that only the supernova Stromgren zones contribute in these stages, while there is an absence of absorption or emission from the compact HII regions. Possible mechanisms for this scenario are discussed.

13 CO (J=1-0) emission of massive star forming region including 15 ultracompact and 4 compact HII regions in Galactic plane was mapped with the 13.7 m millimeter wave telescope of Purple Mountain Observatory. The present observations provide the first complete structure of the clouds in 13 CO with a higher spatial resolution and a wide-field coverage of 28′×45′. Combined with the images of far-infrared emission and dust color temperature obtained from ISSA, various possible dynamical connections between the compact HII regions and associated clouds were found. We presente some reasons to explain the formation of new dense cold core and molecular emission cavity in the massive star formation and early evolution. The luminosities of excitation stars for all HII regions and the main parameters of associated clouds are also derived. The results show that the newborn stars' luminosities are correlated with the 13 CO column densities, masses (in 55″beam) and 13 CO velocity widths obviously.

We present high spatial resolution well sampled observations of the NGC 6334 cloud complex in the 12CO, 13CO and C18O isotopic species, and 1.0-mm continuum emission from cool dust, covering the far infrared emission Peaks I-V, a new Peak IV, and the cold (T=20K) sub-mm Peak I(North) cloud (Cheung et al. 1978, Gezari 1982). The 12CO and 13CO emission was mapped previously by Scoville and Wannier (1976) with 2 – 4 arcmin sampling. These 1.0-mm continuum maps were obtained at the CTIO 4-m telescope with 65 arcsec FWHM resolution in June 1981, and the CO line observations were made at the NRAO 36-foot telescope with 1 arcmin FWHM resolution in April 1982. Several new observational results are presented here, including the large-scale detailed structure of the 1.0-mm continuum and 13CO line emission a ~0.7 pc scale (Figure la-c), and half beam width (0.5 arcmin) sampled 12CO, 13CO and C18O maps of the dense Peak I(North) region (Figure 2). A comparison of the I.0-mm continuum (Figure la) and CO isotopes (Figure 2) shows that the highest TA *(12CO) = 16K occurs at the position of sub-mm Peak I and compact HII region “E” (Rodriguez, Canto and Moran 1982), consistent with the distribution expected of optically thick gas heated by warm dust associated with the compact HII region. In contrast, the map of integrated TA *(13CO) shows two distinct peaks similar to the 400 µm continuum structure mapped by Gezari (1982). The highest TA *(13CO) = 20K at Peak I(North), and the 13CO/12CO ratio is greater there than at Peak I indicating that the cold Peak I(North) source can not be optically thin in 13CO and is massive, in spite of relativly weak 12CO emission. The optically thin C18O source (Figure 3) is very compact and also brightest at Peak I(North), the position of the highest 1.0-mm dust column density (NH2 = 1 × 1024 cm−2). The C18O distribution confirms that Peak I(North) apparently has among the highest derived visual extinction (mv > 100 mag). The 13CO lines at I(North) are narrow (6 km s−1 FWHM) and may arise from the envelope of a protostellar cloud still in the collapse phase. The four color IRAS obvservations (Figure 3), while considerably lower in spatial resolution, show that the warmest (12 – 25 µm) dust emission is concentrated about 5 arcmin SW of the strongest 1.0-mm and CO emission, and is associated with the prominent, extended visible HII region NGC 6334. This work was partially supported by NSF grant ASF-86-18763, and by NASA RTOP 188-41-55. Open image in new window Figure 1 a) 1.0-mm continuum map of the NGC 6334 complex with 65 arcsec (FWHM) beam. The region was sampled on a 30 aresec grid near the strong peaks, and 1 arcmin elswhere. The contour interval in 1.0-mm brightness is 15 Janskys/beam (peak = 135); since the dust is optically thin this is equivalent to an interval of 1 × 1023 cm−2 in derived molecular hydrogen column density. (Note: Peaks labled I-IV by Cheung et al. (1978) are actually I/North, I, II and III in current nomenclature). b) Peak TA *(13CO), contour interval is 2K (peak = 20K), full beam width sampled (1 arcmin) over the region and half beam sampled near Peak I. c) Integrated TA *(13CO) line intensity, contour interval is 6 Jy km s−1arcmin−2 (peak = 131), same sampling as in Figure lb. Open image in new window Figure 2 12CO, 13CO and C18O peak TA * and integrated TA * intensity maps near far infrared NGC 6334 peak I, half beam width sampled (30 arcsec). The cross (+) marks the cold sum-mm peak I(North) object, an asterisk (*) marks compact HII region “E”. Peak TA * (12CO) = 16K,peak integrated TA * (12CO) line intensity = 166 K km s−larcmin−2; peak TA * (13CO) = 20K, peak integrated TA * (13CO) = 131 K km s−larcmin−2; peak TA *(C18O) = 4.5K, peak integrated TA *(C18O) line = 20 K km s−larcmin−2. Open image in new window Figure 3 IRAS maps of the NGC 6334 cloud complex at 12 25, 60 and 100 µm.

The luminosity of the central star of the compact HII regions of NGC 7538 was estimated from the solid angle of the IR sources subtended relative to the central star, and was found to be 5 ∼ 10 times as intense as that of IR sources. Under the single central star approximation, the luminosity gives a stellar UV photon rate NU(∗)(s −1 )o f∼3.0×10 48 , ∼1.5×10 49 , ∼5.1×10 49 , and ∼1.7 × 10 47 for the compact HII regions of NGC 7538-IRS1(A/2), B, IRS2, and IRS3, respectively. NU(∗) and the observed electron density, ne, provide the dust opacity of the ionizing photon, τSd, for the optical path out to the Stromgren sphere radius rS, assuming a gas with standard dust content. Ionizing photon opacity over the same optical path but with the actual dust content τSda is also derived from ri/rS ,w hereri is the radius of the ionized sphere, which is estimated from NU(∗ )a nd the observed volume emission measure n 2(4πr 3 /3) (Spitzer 1978). An observational trend of γ{NU(∗)/4πr 2 } 1/2 ∼ constant, where γ = τSda/τSd, was obtained for the 4 compact HII regions of the NGC 7538(N). Fourteen selected compact HII regions from data catalogued by VLA observations were examined for this trend, and a similar result was obtained. A limit of γ as 15 ≥ γ ≥ 0.1 was given for the 14 selected sources. The size of the dust-depleted cavity of the NGC 7538(N) suggested by Chini et al. (1986) coincides with that of the ionized sphere of the IRS2 of the region.

Context. High-excitation compact Hii regions of the Magellanic Clouds are sites of recent massive star formation in low metallicity environments. Aims. Detailed study of these regions and their environments using high-spatial resolution observations is necessary to better understand massive star formation, which is still an unsolved problem. We aim at a detailed study of the Small Magellanic Cloud compact Hii region N26, which is only 4 00 in diameter. Methods. This study is based on high spatial resolution imaging ( 0: 00 1‐0: 00 3) in JHKs and L 0 bands, using the VLT equipped with the NAOS adaptive optics system. A larger region ( 50 pc 76 pc) was also imaged at medium spatial resolution, using the ESO 2.2 m telescope in optical wavelengths. We also used the JHKs archival data from the IRSF survey and the Spitzer Space Telescope SAGESMC survey. Results. Our high-resolution JHKs data of the compact high-excitation Hii region N26 reveal a new, bright component (C) between the two already known optical components A and B. Components A and C are resolved into several stars. Component A is the main ionization source of N26 and coincides with the radio continuum source B0046-7333. A new compact Hii region with very faint [Oiii] 5007 emission has been discovered. In the mid-infrared, our field resembles a shell formed by filaments and dust clumps, coinciding with the molecular cloud SMCB2. Region N22, located in the center of the shell, is the most excited Hii region of the complex and seems to have created a cavity in SMCB2. We derive nebular parameters from spectra, and using color‐magnitude and color‐color diagrams, we identify stellar sources that show significant near-infrared excess emission in order to identify the best YSO candidates.

We investigate the diffuse absolute calibration of the InfraRed Array Camera on the Spitzer Space Telescope at 8.0microns using a sample of 43 HII regions with a wide range of morphologies near GLON=312deg. For each region we carefully measure sky-subtracted,point-source- subtracted, areally-integrated IRAC 8.0-micron fluxes and compare these with Midcourse Space eXperiment (MSX) 8.3-micron images at two different spatial resolutions, and with radio continuum maps. We determine an accurate median ratio of IRAC 8.0-micron/MSX\8.3-micron fluxes, of 1.55+/-0.15. From robust spectral energy distributions of these regions we conclude that the present 8.0-micron diffuse calibration of the SST is 36% too high compared with the MSX validated calibration, perhaps due to scattered light inside the camera. This is an independent confirmation of the result derived for the diffuse calibration of IRAC by the Spitzer Science Center (SSC). From regression analyses we find that 843-MHz radio fluxes of HII regions and mid-infrared (MIR) fluxes are linearly related for MSX at 8.3-microns and Spitzer at 8.0 microns, confirming the earlier MSX result by Cohen & Green. The median ratio of MIR/843-MHz diffuse continuum fluxes is 600 times smaller in nonthermal than thermal regions, making it a sharp discriminant. The ratios are largely independent of morphology up to a size of ~24 arcsec. We provide homogeneous radio and MIR morphologies for all sources. MIR morphology is not uniquely related to radio structure. Compact regions may have MIR filaments and/or diffuse haloes, perhaps infrared counter- parts to weakly ionized radio haloes found around compact HII regions. We offer two IRAC colour-colour plots as quantitative diagnostics of diffuse HII regions.

In an effort to understand the factors that govern the transition from low- to high-mass star formation, we identify for the first time a sample of intermediate-mass star-forming regions (IM SFRs) where stars up to - but not exceeding - 8 solar masses are being produced. We use IRAS colors and Spitzer Space Telescope mid-IR images, in conjunction with millimeter continuum and CO maps, to compile a sample of 50 IM SFRs in the inner Galaxy. These are likely to be precursors to Herbig AeBe stars and their associated clusters of low-mass stars. IM SFRs constitute embedded clusters at an early evolutionary stage akin to compact HII regions, but they lack the massive ionizing central star(s). The photodissociation regions that demarcate IM SFRs have typical diameters of ~1 pc and luminosities of ~10^4 solar luminosities, making them an order of magnitude less luminous than (ultra)compact HII regions. IM SFRs coincide with molecular clumps of mass ~10^3 solar masses which, in turn, lie within larger molecular clouds spanning the lower end of the giant molecular cloud mass range, 10^4-10^5 solar masses. The IR luminosity and associated molecular mass of IM SFRs are correlated, consistent with the known luminosity-mass relationship of compact HII regions. Peak mass column densities within IM SFRs are ~0.1-0.5 g/cm^2, a factor of several lower than ultra-compact HII regions, supporting the proposition that there is a threshold for massive star formation at ~1 g/cm^2.

We investigated the star formation activities in the AFGL 333 region, which is in the vicinity of the W 4 expanding bubble, by conducting NH3 (1,1), (2,2), and (3,3) mapping observations with the 45 m Nobeyama Radio Telescope an angular resolution of 75″. The morphology of the NH3 (1,1) map shows a bow-shaped structure with the size of 2.0 × 0.6 pc as seen in the dust continuum. At the interface between the W 4 bubble and the dense NH3 cloud, the compact H ii region G134.2+0.8, associated with IRAS 02245+6115, is located. Interestingly, just at the north and south of G134.2+0.8 we found NH3 emission exhibiting large velocity widths of ∼2.8 km s−1, compared to 1.8 km s−1 at the other positions. As the possibility of mechanical energy injection through the activity of young stellar objects (YSOs) is low, we considered the origin of the large turbulent gas motion as an indication of interaction between the compact H ii region and the periphery of the dense molecular cloud. We also found expanding motion of the CO emission associated with G134.2+0.8. The overall structure of the AFGL 333-Ridge might have been formed by the expanding bubble of W 4. However, the small velocity widths observed to the west of IRAS 02245+6115, around the center of the dense molecular cloud, suggest that interaction with the compact H ii region is limited. Therefore the YSOs (dominantly Class 0/I) in the core of the AFGL 333-Ridge dense molecular cloud most likely formed in quiescent mode. As previously suggested for the large-scale star formation in the W 3 giant molecular cloud, our results show an apparent coexistence of induced and quiescent star formations in this region. It appears that star formation in the AFGL 333 region has proceeded without significant external triggers, but accompanying stellar feedback environment.

We have obtained deep spectra of 26 planetary nebulae (PNe) and 9 compact HII regions in the nearby spiral galaxy NGC 300, and analyzed them together with those of the giant HII regions previously observed. We have determined the physical properties of all these objects and their He, N, O, Ne, S and Ar abundances in a consistent way. We find that, globally, compact HII regions have abundance ratios similar to those of giant HII regions, while PNe have systematically larger N/O ratios and similar Ne/O and Ar/O ratios. We demonstrate that the nitrogen enhancement in PNe cannot be only due to second dredge-up in the progenitor stars, since their initial masses are around 2--2.5\,\msun. An extra mixing process is required, perhaps driven by stellar rotation. Concerning the radial abundance distribution, PNe behave differently from HII regions: in the central part of the galaxy their average O/H abundance ratio is 0.15 dex smaller. Their abundance dispersion at any galactocentric radius is significantly larger than that shown by HII regions and many of them have O/H values higher than HII regions at the same galactocentric distance. This suggests that not only nitrogen, but also oxygen is affected by nucleosynthesis in the PN progenitors, by an amount which depends at least on the stellar rotation velocity and possibly other parameters. The formal O/H, Ne/H and Ar/He abundance gradients from PNe are significantly shallower that from HII regions. We argue that this indicates a steepening of the metallicity gradient in NGC 300 during the last Gyr, rather than an effect of radial stellar motions, although the large observed dispersion makes this conclusion only tentative.

Given the rarity of young O star candidates, compact HII regions embedded in dense molecular cores continue to serve as potential sites to peer into the details of high-mass star formation. To uncover the ionizing sources of the most luminous and compact HII regions embedded in the RCW106 and RCW122 giant molecular clouds, known to be relatively nearby (2-4 kpc) and isolated, thus providing an opportunity to examine spatial scales of a few hundred to a thousand AU in size. High spatial resolution (0.3"), mid-infrared spectra (R=350), including the fine structure lines [ArIII] and [NeII], were obtained for four luminous compact HII regions, embedded inside the dense cores within the RCW106 and RCW122 molecular cloud complexes. At this resolution, these targets reveal point-like sources surrounded by nebulosity of different morphologies, uncovering details at spatial dimensions of <1000AU. The point-like sources display [ArIII] and [NeII] lines - the ratios of which are used to estimate the temperature of the embedded sources. The derived temperatures are indicative of mid-late O type objects for all the sources with [ArIII] emission. Previously known characteristics of these targets from the literature, including evidence of disk or accretion suggest that the identified sources may grow more to become early-type O stars by the end of the star formation process.

By matching infrared-selected, massive young stellar objects (MYSOs) and compact HII regions in the RMS survey to massive clumps found in the submillimetre ATLASGAL survey, we have identified ~1000 embedded young massive stars between 280\degr < $\ell$ < 350\degr and 10degr < $\ell$ < 60\degr with |b|<1.5degr. Combined with an existing sample of radio-selected methanol masers and compact HII regions, the result is a catalogue of ~1700 massive stars embedded within ~1300 clumps located across the inner Galaxy, containing three observationally distinct subsamples, methanol-maser, MYSO and HII-region associations, covering the most important tracers of massive star formation, thought to represent key stages of evolution. We find that massive star formation is strongly correlated with the regions of highest column density in spherical, centrally condensed clumps. We find no significant differences between the three samples in clump structure or the relative location of the embedded stars, which suggests that the structure of a clump is set before the onset of star formation, and changes little as the embedded object evolves towards the main sequence. There is a strong linear correlation between clump mass and bolometric luminosity, with the most massive stars forming in the most massive clumps. We find that the MYSO and HII-region subsamples are likely to cover a similar range of evolutionary stages and that the majority are near the end of their main accretion phase. We find few infrared-bright MYSOs associated with the most massive clumps, probably due to very short pre-main sequence lifetimes in the most luminous sources.

We compile measurements made with high angular resolution of the H76α line and radio continuum emission from over thirty compact HII regions. The aim was to investigate global trends exhibited by the observed and derived parameters and to provide explanations for the tendencies in terms of the physical conditions of the ionized gas. The line widths of compact HII regions are considerably broader than those of diffuse regions. We conclude that the large widths are due to turbulent motions produced by the progressive ionization of a magnetized and inhomogeneous medium around a newly formed star. The line to continuum ratios are spread over a much wider range than that span by extended regions. We show that the large ratios are due to line enhancement by stimulated emission, while the small ones can be attributed to line suppression in regions with large continuum opacities.KeywordsMolecular CloudDiffuse RegionHigh Angular ResolutionDoppler WidthPeak LineThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Supersonic gas velocities and large density gradients are observed in compact HII regions. Possible causes of such flows are examined analytically. It is shown that density gradients in either the HII region or the exterior molecular cloud can naturally give supersonic gas speeds. Cases discussed include the swelling of uniform HII bubbles into clouds with decreasing densities, isothermal shock propagation within the ionized region, and homologous expansion of non-uniform HII bubbles. Supersonic flows are readily generated under reasonable conditions. Thus the observed supersonic motions and density gradients in compact HII regions may be intimately related.