Aims. The main goal of this work is to a have a new neutral hydrogen (H i) supershell candidate catalog to analyze their spatial distribution in the Galaxy and to carry out a statistical study of their main properties. Methods. This catalog was carried out making use of the Leiden-Argentine-Bonn (LAB) survey. The supershell candidates were identified using a combination of two techniques: a visual inspection plus an automatic searching algorithm. Our automatic algorithm is able to detect both closed and open structures. Results. A total of 566 supershell candidates were identified. Most of them (347) are located in the second Galactic quadrant, while 219 were found in the third one. About 98% of a subset of 190 structures (used to derive the statistical properties of the supershell candidates) are elliptical with a mean weighted eccentricity of 0.8 ± 0.1, and ∼70% have their major axes parallel to the Galactic plane. The weighted mean value of the effective radius of the structures is ∼160 pc. Owing to the ability of our automatic algorithm to detect open structures, we have also identified some "galactic chimney" candidates. We find an asymmetry between the second and third Galactic quadrants in the sense that in the second one we detect structures as far as 32 kpc, while for the 3rd one the farthest structure is detected at 17 kpc. The supershell surface density in the solar neighborhood is ∼8 kpc −2 , and decreases as we move farther away form the Galactic center. We have also compared our catalog with those by other authors.

Context. The Milky Way, when viewed in the neutral hydrogen line emission, presents large structures called Galactic supershells (GSs). The origin of these structures is still a subject of debate. The most common scenario invoked is the combined action of strong winds from massive stars and their subsequent explosion as supernova.Aims. The aim of this work is to determine the origin of 490 GSs that belong to the Catalog of H i supershell candidates in the outer part of the Galaxy. Methods. To know the physical processes that took place to create these expanding structures, it is necessary to determine their kinetic energies. To obtain all the GS masses, we developed and used an automatic algorithm, which was tested on 95 GSs whose masses were also estimated by hand.Results. The estimated kinetic energies of the GSs vary from 1 × 10 47 to 3.4 × 10 51 erg. Considering an efficiency of 20% for the conversion of mechanical stellar wind energy into the kinetic energy of the GSs, the estimated values of the GS energies could be reached by stellar OB associations. For the GSs located at high Galactic latitudes, the possible mechanism for their creation could be attributed to collision with high velocity clouds (HVC). We have also analysed the distribution of GSs in the Galaxy, showing that at low Galactic latitudes, |b| < 2 • , most of the structures in the third Galactic quadrant seem to be projected onto the Perseus Arm. The detection of GSs at very high distances from the Galactic centre may be attributed to diffuse gas associated with the circumgalactic medium of M31 and to intra-group gas in the Local Group filament.

We present a study of a new molecular shell, G 126.1-0.8-14, using available multiwavelength Galactic plane surveys and optical Gemini observations. A well-defined shell-like structure is observed in the CO(1-0) line emission at (l,b) = (126. • 1, -0. • 8), in the velocity range -10.5 to -15.5 km s −1 . The H I emission shows a region of low emissivity inside G 126.1-0.8-14, while radio continuum observations reveal faint non-thermal emission possibly related to this shell. Optical spectra obtained with Gemini South show the existence of B-type stars likely to be associated with G 126.1-0.8-14. An estimate of the stellar wind energy injected by these stars shows that they alone cannot be able to create such a structure. On the other hand, one supernova explosion would provide enough energy to generate the shell. Using the MSX, IRAS and WISE point source catalogues we have found about 30 young stellar object candidates, whose birth could have been triggered by the expansion of G 126.1-0.8-14. In this context, Sh2-187 could be a consequence of the action on its surroundings of the most massive (and thus most evolve) of the stars formed by the expanding molecular shell.

Context. Massive stars have a profound effect on the surrounding interstellar medium. They ionize and heat the neutral gas, and with their strong winds they sweep up the gas, forming large H i shells. In this way, they generate a dense shell that provides the physical conditions for the formation of new stars. Aims. The aim of this study is to analyze the origin and evolution of the large H i shell GS 100-02-41 and its role in triggering star-forming processes. Methods. To characterize the shell and its environs, we carried out a multi-wavelength study. We analyzed the H i 21 cm line, the radio continuum, and infrared emission distributions.Results. The analysis of the H i data shows an expanding shell structure centered at (l, b) = (100.• 6, -2.• 04) in the velocity range from -29 to -51.7 km s −1 . Taking into account noncircular motions, we infer for GS 100-02-41 a kinematical distance of 2.8 ± 0.6 kpc. Several massive stars belonging to Cep OB1 are located in projection within the large H i shell boundaries. The analysis of the radio continuum and infrared data reveals that there is no continuum counterpart of the H i shell. On the other hand, three slightly extended radio continuum sources are observed in projection onto the dense H i shell. From their flux density determinations we infer that they are thermal in nature. An analysis of the H i emission distribution in the environs of these sources shows a region of low emissivity for each of them, which correlates well morphologically with the ionized gas in a velocity range similar to the one where GS 100-02-41 is detected. Conclusions. Based on an energy analysis, we conclude that the origin of GS 100-02-41 could have been mainly caused by the action of the Cep OB1 massive stars located inside the H i shell. The obtained age difference between the H i shell and the H ii regions, together with their relative location, lead us to conclude that the ionizing stars could have been created as a consequence of the shell evolution.

Aims. With the aim of studying the physical properties of Galactic IR bubbles and to explore their impact in massive star formation, we present a study of the IR bubble S169, associated with the massive star forming region IRAS 12326-6245. Methods. We used CO (2–1),13CO (2–1), C18O (2–1), HCN (3–2), and HCO+ (3–2) line data obtained with the APEX telescope using the on-the-fly full sampling technique to study the properties of the molecular gas in the nebula and the IRAS source. To analyze the properties and distribution of the dust, we made use of images obtained from the IRAC-GLIMPSE, Herschel, and ATLASGAL archives. The properties of the ionized gas in the nebula were studied using radio continuum and Hα images obtained from the SUMSS survey and SuperCOSMOS database, respectively. In our search for stellar and protostellar objects in the region, we used point source calalogs obtained from the MSX, WISE, GLIMPSE, 2MASS, AAVSO, ASCC-2.5V3, and GAIA databases. Results. The new APEX observations allowed us to identify three molecular components, each one associated with different regions of the nebula, namely: at −39 km s−1 (component A), −25 km s−1 (component B), and −17 km s−1 (component C). Component A is shown to be the most dense and clumpy. Six molecular condensations (MC1 to MC6) were identified in this component, with MC3 (the densest and more massive one) being the molecular counterpart of IRAS 12326-6245. For this source, we estimated an H2 column density up to 8 × 1023 cm−2. An LTE analysis of the high density tracer lines HCO+ (3–2) and HCN (3–2) on this source, assuming 50 and 150 K, respectively, indicates column densities of N(HCO+) = (5.2 ± 0.1) × 1013 cm−2 and N(HCN) = (1.9 ± 0.5) × 1014 cm−2. To explain the morphology and velocity of components A, B, and C, we propose a simple model consisting of a partially complete semisphere-like structure expanding at ~12 km s−1. The introduction of this model has led to a discussion about the distance to both S169 and IRAS 12326-6245, which was estimated to be ~2 kpc. Several candidate YSOs were identified, projected mostly onto the molecular condensations MC3, MC4, and MC5, which indicates that the star-formation process is very active at the borders of the nebula. A comparison between observable and modeled parameters was not enough to discern whether the collect-and-collapse mechanism is acting at the edge of S169. However, other processes such as radiative-driven implosion or even a combination of both mechanisms, namely, collect-and-collapse and radiative-driven implosion, could be acting simultaneously in the region.