Context. This study deals with infrared bubbles, the H ii regions they enclose, and triggered massive-star formation on their borders. Aims: We attempt to determine the nature of the bubbles observed by Spitzer in the Galactic plane, mainly to establish if possible their association with massive stars. We take advantage of the very simple morphology of these objects to search for star formation triggered by H ii regions, and to estimate the importance of this mode of star formation. Methods: We consider a sample of 102 bubbles detected by Spitzer-GLIMPSE, and catalogued by Churchwell et al. (2006; hereafter CH06). We use mid-infrared and radio-continuum public data (respectively the Spitzer-GLIMPSE and -MIPSGAL surveys and the MAGPIS and VGPS surveys) to discuss their nature. We use the ATLASGAL survey at 870 μm to search for dense neutral material collected on their borders. The 870 μm data traces the distribution of cold dust, thus of the dense neutral material where stars may form. Results: We find that 86% of the bubbles contain ionized gas detected by means of its radio-continuum emission at 20-cm. Thus, most of the bubbles observed at 8.0 μm enclose H ii regions ionized by O-B2 stars. This finding differs from the earlier CH06 results (~25% of the bubbles enclosing H ii regions). Ninety-eight percent of the bubbles exhibit 24 μm emission in their central regions. The ionized regions at the center of the 8.0 μm bubbles seem to be devoid of PAHs but contain hot dust. PAH emission at 8.0 μm is observed in the direction of the photodissociation regions surrounding the ionized gas. Among the 65 regions for which the angular resolution of the observations is high enough to resolve the spatial distribution of cold dust at 870 μm, we find that 40% are surrounded by cold dust, and that another 28% contain interacting condensations. The former are good candidates for the collect and collapse process, as they display an accumulation of dense material at their borders. The latter are good candidates for the compression of pre-existing condensations by the ionized gas. Thirteen bubbles exhibit associated ultracompact H ii regions in the direction of dust condensations adjacent to their ionization fronts. Another five show methanol masers in similar condensations. Conclusions: Our results suggest that more than a quarter of the bubbles may have triggered the formation of massive objects. Therefore, star formation triggered by H ii regions may be an important process, especially for massive-star formation. Appendices are only available in electronic form at http://www.aanda.org

/pdf/a-gallery-of-bubbles-the-nature-of-the-bubbles-observed-by-4p6anfen4h.pdf

A gallery of bubbles. The nature of the bubbles observed by Spitzer and what ATLASGAL tells us about the surrounding neutral material

The relative importance of primordial molecular cloud fragmentation versus large-scale accretion still remains to be assessed in the context of massive core/star formation. Studying the kinematics of the dense gas surrounding massive-star progenitors can tell us the extent to which large-scale flow of material impacts the growth in mass of star-forming cores. Here we present a comprehensive dataset of the 5500(±800) M⊙ infrared dark cloud SDC335.579-0.272 (hereafter SDC335), which exhibits a network of cold, dense, parsec-long filaments. Atacama Large Millimeter Array (ALMA) Cycle 0 observations reveal two massive star-forming cores, MM1 and MM2, sitting at the centre of SDC335 where the filaments intersect. With a gas mass of 545(-385+770) M⊙ contained within a source diameter of 0.05 pc, MM1 is one of the most massive, compact protostellar cores ever observed in the Galaxy. As a whole, SDC335 could potentially form an OB cluster similar to the Trapezium cluster in Orion. ALMA and Mopra single-dish observations of the SDC335 dense gas furthermore reveal that the kinematics of this hub-filament system are consistent with a global collapse of the cloud. These molecular-line data point towards an infall velocity Vinf = 0.7( ± 0.2) km s-1, and a total mass infall rate Ṁinf ≃ 2.5(±1.0) × 10-3 M⊙ yr-1 towards the central pc-size region of SDC335. This infall rate brings 750(±300) M⊙ of gas to the centre of the cloud per free-fall time (tff = 3 × 105 yr). This is enough to double the mass already present in the central pc-size region in 3.5-1.0+2.2 × tff. These values suggest that the global collapse of SDC335 over the past million year resulted in the formation of an early O-type star progenitor at the centre of the cloud’s gravitational potential well.

https://www.aanda.org/articles/aa/pdf/2013/07/aa21318-13.pdf

Global collapse of molecular clouds as a formation mechanism for the most massive stars

We present the results of a high angular resolution Very Large Array (VLA) Class I 44 GHz and Class II 6.7 GHz CH{sub 3}OH maser survey of a sample of {approx}20 massive young stellar object (MYSO) outflow candidates selected on the basis of extended 4.5 {mu}m emission in Spitzer Galactic Legacy Infrared Mid-Plane Survey Extraordinaire images. These 4.5 {mu}m selected candidates are referred to as extended green objects (EGOs), for the common coding of this band as green in three-color Infrared Array Camera images. The detection rate of 6.7 GHz Class II CH{sub 3}OH masers, which are associated exclusively with massive YSOs, toward EGOs is {approx}>64%-nearly double the detection rate of surveys using other MYSO selection criteria. The detection rate of Class I 44 GHz CH{sub 3}OH masers, which trace molecular outflows, is {approx}89% toward EGOs associated with 6.7 GHz CH{sub 3}OH masers. The two types of CH{sub 3}OH masers exhibit different spatial distributions: 6.7 GHz masers are centrally concentrated and usually coincide with 24 {mu}m emission, while 44 GHz masers are widely distributed and generally trace diffuse 4.5 {mu}m features. We also present results of a complementary James Clerk Maxwell Telescope (JCMT) single-pointing molecular line survey of EGOs inmore » the outflow tracers HCO{sup +}(3-2) and SiO(5-4). The HCO{sup +} line profiles and high SiO detection rate (90%) are indicative of the presence of active outflows. No 44 GHz continuum emission is detected at the 5 mJy beam{sup -1} (5{sigma}) level toward 95% of EGOs surveyed, excluding bright ultracompact H II regions as powering sources for the 4.5 {mu}m outflows. The results of our surveys constitute strong evidence that EGOs are young, massive YSOs, with active outflows, presumably powered by ongoing accretion.« less

http://iopscience.iop.org/article/10.1088/0004-637X/702/2/1615/pdf

A Class I and Class II CH3OH Maser Survey of EGOs from the GLIMPSE Survey

We present a highly reliable flux-limited census of 18,949 point sources in the Galactic midplane that have intrinsically red mid-infrared colors. These sources were selected from the Spitzer Space Telescope Galactic Legacy Infrared Midplane Survey Extraordinaire (GLIMPSE) I and II surveys of 274 deg2 of the Galactic midplane, and consist mostly of high- and intermediate-mass young stellar objects (YSOs) and asymptotic giant branch (AGB) stars. The selection criteria were carefully chosen to minimize the effects of position-dependent sensitivity, saturation, and confusion. The distribution of sources on the sky and their location in the Infrared Array Camera and the Multiband Image Photometer for Spitzer 24 μm color-magnitude and color-color space are presented. Using this large sample, we find that YSOs and AGB stars can be mostly separated by simple color-magnitude selection criteria into approximately 50%-70% of YSOs and 30%-50% of AGB stars. Planetary nebulae and background galaxies together represent at most 2%-3% of all the red sources. 1004 red sources in the GLIMPSE II region, mostly AGB stars with high mass-loss rates, show significant (≥0.3 mag) variability at 4.5 and/or 8.0 μm. With over 11,000 likely YSOs and over 7000 likely AGB stars, this is to date the largest uniform census of AGB stars and high- and intermediate-mass YSOs in the Milky Way Galaxy.

/pdf/intrinsically-red-sources-observed-by-spitzer-in-the-55nb2ob6sh.pdf

Intrinsically Red Sources Observed by Spitzer in the Galactic Midplane

Context. The APEX Telescope Large Area Survey of the GALaxy (ATLASGAL) is the first systematic survey of the inner Galactic plane in the sub-millimetre. The observations were carried out with the Large APEX Bolometer Camera (LABOCA), an array of 295 bolometers observing at 870 μ m (345 GHz). Aims. Here we present a first version of the compact source catalogue extracted from this survey. This catalogue provides an unbiased database of dusty clumps in the inner Galaxy. Methods. The construction of this catalogue was made using the source extraction routine SExtractor. We have cross-associated the obtained sources with the IRAS and MSX catalogues, in order to constrain their nature.Results. We have detected 6639 compact sources in the range from 330 ≤ l  ≤ 21 degrees and  |b |  ≤ 1.5 degrees. The catalogue has a 99% completeness for sources with a peak flux above 6σ , which corresponds to a flux density of  ~0.4 Jy beam-1 . The parameters extracted for sources with peak fluxes below the 6σ completeness threshold should be used with caution. Tests on simulated data find the uncertainty in the flux measurement to be  ~12%, however, in more complex regions the flux values can be overestimated by a factor of 2 due to the additional background emission. Using a search radius of 30′′ we found that 40% of ATLASGAL compact sources are associated with an IRAS or MSX point source, but,  ~50% are found to be associated with MSX 21 μ m fluxes above the local background level, which is probably a lower limit to the actual number of sources associated with star formation. Conclusions. Although infrared emission is found towards the majority of the clumps detected, this catalogue is still likely to include a significant number of clumps that are devoid of star formation activity and therefore excellent candidates for objects in the coldest, earliest stages of (high-mass) star formation.

/pdf/atlasgal-compact-source-catalogue-330-l-21-q5j52tzvfb.pdf

ATLASGAL – compact source catalogue: 330° < ℓ < 21°

Using images from the Spitzer Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE), we have identified more than 300 extended 4.5 μm sources (Extended Green Objects (EGOs), for the common coding of the [4.5] band as green in three-color composite InfraRed Array Camera images). We present a catalog of these EGOs, including integrated flux density measurements at 3.6, 4.5, 5.8, 8.0, and 24 μm from GLIMPSE and the Multiband Imaging Photometer for Spitzer Galactic Plane Survey. The average angular separation between a source in our sample and the nearest IRAS point source is greater than 1'. The majority of EGOs are associated with infrared dark clouds (IRDCs), and where high-resolution 6.7 GHz CH3OH maser surveys overlap the GLIMPSE coverage, EGOs and 6.7 GHz CH3OH masers are strongly correlated. Extended 4.5 μm emission is thought to trace shocked molecular gas in protostellar outflows; the association of EGOs with IRDCs and 6.7 GHz CH3OH masers suggests that the extended 4.5 μm emission may pinpoint outflows specifically from massive protostars. The mid-IR colors of EGOs lie in regions of color-color space occupied by young protostars still embedded in infalling envelopes.

https://iopscience.iop.org/article/10.1088/0004-6256/136/6/2391/pdf

A Catalog of Extended Green Objects in the GLIMPSE Survey: A New Sample of Massive Young Stellar Object Outflow Candidates

Using images from the Spitzer GLIMPSE Legacy survey, we have identified more than 300 extended 4.5 micron sources (abbreviated EGO, Extended Green Object, for the common coding of the [4.5] band as green in 3-color composite IRAC images). We present a catalog of these EGOs, including integrated flux density measurements at 3.6, 4.5, 5.8, 8.0, and 24 microns from the GLIMPSE and MIPSGAL surveys. The average angular separation between a source in our sample and the nearest IRAS point source is >1 arcminute. The majority of EGOs are associated with infrared dark clouds (IRDCs), and where high-resolution 6.7 GHz methanol maser surveys overlap the GLIMPSE coverage, EGOs and 6.7 GHz methanol masers are strongly correlated. Extended 4.5 micron emission is thought to trace shocked molecular gas in protostellar outflows; the association of EGOs with IRDCs and 6.7 GHz methanol masers suggests that extended 4.5 micron emission may pinpoint outflows specifically from massive protostars. The mid-infrared colors of EGOs lie in regions of color-color space occupied by young protostars still embedded in infalling envelopes.

E. Holden

Papers

A Catalog of Extended Green Objects in the GLIMPSE Survey: A New Sample of Massive Young Stellar Object Outflow Candidates

A Catalog of Extended Green Objects (EGOs) in the GLIMPSE Survey: A new sample of massive young stellar object outflow candidates