Yellowballs are a collection of approximately 900 compact, infrared sources identified and named by volunteers participating in the Milky Way Project (MWP), a citizen-science project that uses GLIMPSE/MIPSGAL images from Spitzer to explore topics rel
ated to Galactic star formation. In this paper, through a combination of catalog cross-matching and infrared color analysis, we show that yellowballs are a mix of compact star-forming regions, including ultra-compact and compact HII regions, as well as analogous regions for less massive B-type stars. The resulting MWP yellowball catalog provides a useful complement to the Red MSX Source (RMS) survey. It similarly highlights regions of massive star formation, but the selection of objects purely on the basis of their infrared morphology and color in Spitzer images identifies a signature of compact star-forming regions shared across a broad range of luminosities, and by inference, masses. We discuss the origin of their striking mid-infrared appearance, and suggest that future studies of the yellowball sample will improve our understanding of how massive and intermediate-mass star-forming regions transition from compact to more extended bubble-like structures.
We present the first detailed study of the large, ~30 pc diameter, inner-Galaxy HII region W 39. Radio recombination line observations combined with HI absorption spectra and Galactic rotation models show that the region lies at V(LSR) = +65.4+/-0.5
km/s corresponding to a near kinematic distance of 4.5+/-0.2 kpc. Analysis of radio continuum emission shows that the HII region is being powered by a cluster of OB stars with a combined hydrogen-ionizing luminosity of log(Q) >=50, and that there are three compact HII regions located on the periphery of W 39, each with log(Q)~48.5 (single O7 - O9 V star equivalent). In the infrared, W 39 has a hierarchical bubble morphology, and is a likely site of sequential star formation involving massive stars. Kinematic models of the expansion of W 39 yield timescales of order Myr consistent with a scenario where the formation of the smaller HII regions has been triggered by the expansion of W 39. Using Spitzer GLIMPSE and MIPSGAL data we show that star-formation activity is not distributed uniformly around the periphery of W 39 but is concentrated in two areas that include the compact HII regions as well as a number of intermediate-mass Class I and Class II YSOs.
We use 2MASS and MSX infrared observations, along with new molecular line (CO) observations, to examine the distribution of young stellar objects (YSOs) in the molecular cloud surrounding the halo HII region KR 140 in order to determine if the ongoin
g star-formation activity in this region is dominated by sequential star formation within the photodissociation region (PDR) surrounding the HII region. We find that KR 140 has an extensive population of YSOs that have spontaneously formed due to processes not related to the expansion of the HII region. Much of the YSO population in the molecular cloud is concentrated along a dense filamentary molecular structure, traced by C18O, that has not been erased by the formation of the exciting O star. Some of the previously observed submillimetre clumps surrounding the HII region are shown to be sites of recent intermediate and low-mass star formation while other massive starless clumps clearly associated with the PDR may be the next sites of sequential star formation.
A new catalogue of extended radio sources has been prepared based on arcminute-resolution 1420 MHz images from the Canadian Galactic Plane Survey (CGPS). The new catalogue provides both 1420 MHz and 408 MHz flux density measurements on sources found
near the Galactic plane in the second quadrant of our Galaxy. In addition cross-identifications are made with other major radio catalogues and information is provided to facilitate the recovery of CGPS image data associated with each catalogued source. Numerous new radio sources are identified and the catalogue provides a comprehensive summary of both newly discovered and previously known HII regions and supernova remnants in the outer Galaxy. The catalogue should be of use both for synoptic studies of Galactic structure and for placing higher resolution observations, at radio and other wavelengths, in context.
Arcminute-resolution radio continuum images at 408 and 1420 MHz from the Canadian Galactic Plane Survey (CGPS) have been used to reexamine radio sources listed in the Kallas and Reich (1980) catalogue. This catalogue is of particular interest to Gala
ctic studies as it lists both extended and compact radio sources found in the second Galactic quadrant. We have determined the nature (extended vs. compact, Galactic vs. extragalactic) of all of these bright radio sources. A number of large HII regions with no optical counterparts are highlighted along with a sample of large radio galaxies. Many sources previously thought to be extended Galactic objects are shown to be point sources. A sample of point sources with flat or rising spectra between 408 and 1420 MHz has been compiled, and within this sample likely Gigahertz Peaked Spectrum sources have been identified.
