In a previous paper we have investigated the molecular environment towards the eastern border of the SNR G18.8+0.3. Continuing with the study of the surroundings of this SNR, in this work we focus on its southern border, which in the radio continuum
emission shows a very peculiar morphology with a corrugated corner and a very flattened southern flank. We observed two regions towards the south of SNR G18.8+0.3 using the Atacama Submillimeter Telescope Experiment (ASTE) in the 12CO J=3-2. One of these regions was also surveyed in 13CO and C18O J=3-2. The angular and spectral resolution of these observations were 22, and 0.11 km/s. We compared the CO emission to 20 cm radio continuum maps obtain as part of the Multi-Array Galactic Plane Imaging Survey (MAGPIS) and 870 um dust emission extracted from the APEX Telescope Large Area Survey of the Galaxy. We discovered a molecular feature with a good morphological correspondence with the SNRs southernmost corner. In particular, there are indentations in the radio continuum map that are complemented by protrusions in the molecular CO image, strongly suggesting that the SNR shock is interacting with a molecular cloud. Towards this region we found that the 12CO peak is not correlated with the observed 13CO peaks, which are likely related to a nearby hii~region. Regarding the most flattened border of SNR G18.8+0.3, where an interaction of the SNR with dense material was previously suggested, our 12CO J=3-2 map show no obvious indication that this is occurring.
HII regions are particularly interesting because they can generate dense layers of gas and dust, elongated columns or pillars of gas pointing towards the ionizing sources, and cometary globules of dense gas, where triggered star formation can occur.
Understanding the interplay between the ionizing radiation and the dense surrounding gas is very important to explain the origin of these peculiar structures, and hence to characterize triggered star formation. G46.5-0.2 (G46), a poorly studied galactic HII region located at about 4 kpc, is an excellent target to perform this kind of studies. Using public molecular data extracted from the Galactic Ring Survey (13CO J=1-0) and from the James Clerk Maxwell Telescope data archive (12CO, 13CO, C18O J=3-2, HCO+ and HCN J=4-3), and infrared data from the GLIMPSE and MIPSGAL surveys, we perform a complete study of G46, its molecular environment and the young stellar objects placed around it. We found that G46, probably excited by an O7V star, is located close to the edge of the GRSMC G046.34-00.21 molecular cloud. It presents a horse-shoe morphology opening in direction of the cloud. We observed a filamentary structure in the molecular gas likely related to G46 and not considerable molecular emission towards its open border. We found that about 10 towards the southwest of G46 there are some pillar-like features, shining at 8 um and pointing towards the HII region open border. We propose that the pillar-like features were carved and sculpted by the ionizing flux from G46. We found several young stellar objects likely embedded in the molecular cloud grouped in two main concentrations: one, closer to the G46 open border consisting of Class II type sources, and other one mostly composed by Class I type YSOs located just ahead the pillars-like features, strongly suggesting an age gradient in the YSOs distribution.
We perform a multiwavelength study toward the SNR G18.1-0.1 and nearby several HII regions (infrared dust bubbles N21 and N22, and the HII regions G018.149-00.283 and G18.197-00.181). Our goal is to provide observational evidence supporting that mass
ive stars usually born in clusters from the same molecular cloud, which then produce, along their evolution, different neighboring objects such as HII regions, interstellar bubbles and supernova remnants. We suggest that the objects analysed in this work belong to a same complex located at the distance of about 4 kpc. Using molecular data we inspected the interstellar medium toward this complex and from optical and X-ray observations we looked for OB-type stars in the region. Analysing public 13CO J=1--0 data we found several molecular structures very likely related to the HII region/SNR complex. We suggest that the molecular gas is very likely being swept and shaped by the expansion of the HII regions. From spectroscopic optical observations obtained with the 2.15 m telescope at CASLEO, Argentina, we discovered three O-type stars very likely exciting the bubbles N21 and N22, and an uncatalogued HII region northward bubble N22, respectively. Also we found four B0-5 stars, one toward the bubble N22 and the others within the HII region G18.149-0.283. By inspecting the Chandra Source Catalog we found two point X-ray sources and we suggest that one of them is an early O-type star. Finally we inspected the large scale interstellar medium around this region. We discovered a big molecular shell of about 70 pc x 28 pc in which the analysed complex appears to be located in its southern border.
