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Cloud environment is thought to play a critical role in determining the mechanism of formation of massive stars. In this contribution we review the physical characteristics of the environment around recently formed massive stars. Particular emphasis is given to recent high angular resolution observations which have improved our knowledge of the physical conditions and kinematics of compact regions of ionized gas and of dense and hot molecular cores associated with luminous O and B stars. We will show that this large body of data, gathered during the last decade, has allowed significant progress in the understanding of the physical processes that take place during the formation and early evolution of massive stars.

Globulettes are small (radii <10 kAU) dark dust clouds, seen against the background of bright nebulae. A majority of the objects have planetary mass. These objects may be a source of brown dwarfs and free floating planetary mass objects in the galaxy. In this paper we investigate how many globulettes could have formed in the Milky Way and how they could contribute to the total population of free floating planets. In order to do that we examine H-alpha images of 27 H II regions. In these images, we find 778 globulettes.We find that a conservative value of the number of globulettes formed is 5.7×1010. If 10% of the globulettes form free floating planets then they have contributed with 5.7×109 free floating planets in the Milky Way. A less conservative number of globulettes would mean that the globulettes could contribute 2.0×1010 free floating planets. Thus the globulettes could represent a non-negligible source of free floating planets in the Milky Way.

H ii regions are ionized nebulae surrounding massive stars. They exhibit a wealth of emission lines that form the basis for estimation of chemical composition. Heavy elements regulate the cooling of interstellar gas, and are essential to the understanding of several phenomena such as nucleosynthesis, star formation and chemical evolution 1 , 2 . For over 80 years 3 , however, a discrepancy exists of a factor of around two between heavy-element abundances derived from collisionally excited lines and those from the weaker recombination lines, which has thrown our absolute abundance determinations into doubt 4 , 5 . Here we report observational evidence that there are temperature inhomogeneities within the gas, quantified by t 2  (ref. 6 ). These inhomogeneities affect only highly ionized gas and cause the abundance discrepancy problem. Metallicity determinations based on collisionally excited lines must be revised because these may be severely underestimated, especially in regions of lower metallicity such as those recently observed with the James Webb Space Telescope in high- z galaxies 7 – 9 . We present new empirical relations for estimation of temperature and metallicity, critical for a robust interpretation of the chemical composition of the Universe over cosmic time. The authors report observational evidence that, within interstellar gas, there are temperature inhomogeneities affecting only highly ionized gas and causing the abundance discrepancy problem, and provide new empirical relations for estimation of temperature and metallicity.

Unbound young stellar systems, the loose ensembles of physically related young bright stars, trace the typical regions of recent star formation in galaxies. Their morphologies vary from small few pc-size associations of newly formed stars to enormous few kpc-size complexes composed of stars few 100 Myr old. These stellar conglomerations are located within the disks and along the spiral arms and rings of star-forming disk galaxies, and they are the active star-forming centers of dwarf and starburst galaxies. Being associated with star-forming regions of various sizes, these stellar structures trace the regions where stars form at various length- and timescales, from compact clusters to whole galactic disks. Stellar associations, the prototypical unbound young systems, and their larger counterparts, stellar aggregates, and stellar complexes, have been the focus of several studies for quite a few decades, with special interest on their demographics, classification, and structural morphology. The compiled surveys of these loose young stellar systems demonstrate that the clear distinction of these systems into well-defined classes is not as straightforward as for stellar clusters, due to their low densities, asymmetric shapes and variety in structural parameters. These surveys also illustrate that unbound stellar structures follow a clear hierarchical pattern in the clustering of their stars across various scales. Stellar associations are characterized by significant sub-structure with bound stellar clusters being their most compact parts, while associations themselves are the brighter denser parts of larger stellar aggregates and stellar complexes, which are members of larger super-structures up to the scale of a whole star-forming galaxy. This structural pattern, which is usually characterized as self-similar or fractal, appears to be identical to that of star-forming giant molecular clouds and interstellar gas, driven mainly by turbulence cascade. In this short review, I make a concise compilation of our understanding of unbound young stellar systems across various environments in the local universe, as it is developed during the last 60 years. I present a factual assessment of the clustering behavior of star formation, as revealed from the assembling pattern of stars across loose stellar structures and its relation to the interstellar medium and the environmental conditions. I also provide a consistent account of the processes that possibly play important role in the formation of unbound stellar systems, compiled from both theoretical and observational investigations on the field.

FEEDBACK is a SOFIA (Stratospheric Observatory for Infrared Astronomy) legacy program dedicated to study the interaction of massive stars with their environment. It performs a survey of 11 galactic high mass star-forming regions in the 158 m (1.9 THz) line of [C ii] and the 63 m (4.7 THz) line of [O i]. We employ the 14 pixel Low Frequency Array and 7 pixel High Frequency Array upGREAT heterodyne instrument to spectrally resolve (0.24 MHz) these far-infrared fine structure lines. With a total observing time of 96h, we will cover ∼6700 arcmin2 at 14 1) angular resolution for the [C ii] line and 6 3 for the [O i] line. The observations started in spring 2019 (Cycle 7). Our aim is to understand the dynamics in regions dominated by different feedback processes from massive stars such as stellar winds, thermal expansion, and radiation pressure, and to quantify the mechanical energy injection and radiative heating efficiency. This is an important science topic because feedback of massive stars on their environment regulates the physical conditions and sets the emission characteristics in the interstellar medium (ISM), influences the star formation activity through molecular cloud dissolution and compression processes, and drives the evolution of the ISM in galaxies. The [C ii] line provides the kinematics of the gas and is one of the dominant cooling lines of gas for low to moderate densities and UV fields. The [O i] line traces warm and high-density gas, excited in photodissociations regions with a strong UV field or by shocks. The source sample spans a broad range in stellar characteristics from single OB stars, to small groups of O stars, to rich young stellar clusters, to ministarburst complexes. It contains well-known targets such as Aquila, the Cygnus X region, M16, M17, NGC7538, NGC6334, Vela, and W43 as well as a selection of H ii region bubbles, namely RCW49, RCW79, and RCW120. These [C ii] maps, together with the less explored [O i] 63 m line, provide an outstanding database for the community. They will be made publically available and will trigger further studies and follow-up observations.

Hi-GAL, the Herschel infrared Galactic Plane Survey, is an Open Time Key Project of theHerschel Space Observatory. It will make an unbiased photometric survey of the inner Galactic plane by mapping a2° 2 ° wide strip in the longitude range∣l∣ < 60° ∣ l ∣ < 60 ° in five wavebands between 70 μm and 500 μm. The aim of Hi-GAL is to detect the earliest phases of the formation of molecular clouds and high-mass stars and to use the optimum combination ofHerschelwavelength coverage, sensitivity, mapping strategy, and speed to deliver a homogeneous census of star-forming regions and cold structures in the interstellar medium. The resulting representative samples will yield the variation of source temperature, luminosity, mass and age in a wide range of Galactic environments at all scales from massive YSOs in protoclusters to entire spiral arms, providing an evolutionary sequence for the formation of intermediate and high-mass stars. This information is essential to the formulation of a predictive global model of the role of environment and feedback in regulating the star-formation process. Such a model is vital to understanding star formation on galactic scales and in the early universe. Hi-GAL will also provide a science legacy for decades to come with incalculable potential for systematic and serendipitous science in a wide range of astronomical fields, enabling the optimum use of future major facilities such asJWSTand ALMA.

In this work, we propose a proper plasma analysis practice (PPAP), an updated procedure of plasma diagnostics in the era of spatially resolved spectroscopy. In particular, we emphasize the importance of performing both of the extinction correction and the direct method of plasma diagnostics simultaneously as an integrated process. This approach is motivated by the reciprocal dependence between critical parameters in these analyses, which can be resolved by iteratively seeking a converged solution. The use of PPAP allows us to eliminate unnecessary assumptions that prevent us from obtaining an exact solution at each element of the spectral imaging data. Using a suite of Hubble Space Telescope/WFC3 narrowband images of the planetary nebula, NGC 6720, we validate PPAP by (1) simultaneously and self-consistently deriving the extinction, c (H β ), and electron density/temperature distribution, ( n e ([S ii ]), T e ([N ii ])), maps that are consistent with each other, and (2) obtaining identical metal abundance distribution maps, ( n (N + )/ n (H + ), n (S + )/ n (H + )), from multiple emission line maps at different wavelengths/transition energies. We also determine that the derived c (H β ) consists both of the interstellar medium and circumsource components and that the ionized gas-to-dust mass ratio in the main ring is at least 437 and as high as about 1600. We find that, unless we deliberately seek self-consistency, uncertainties at tens of % can easily arise in outcomes, making it impossible to discern actual spatial variations that occurs at the same level, defeating the purpose of conducting spatially resolved spectroscopic observations.

A revised catalogue of 310 Galactic supernova remnants (SNRs) and some statistics on their properties are presented. 21 SNRs have been added to the catalogue since the previously published version from 2019, and 5 entries have been removed, as they have been identified as H ii regions. Also discussed are some basic statistics of the remnants in the catalogue, the selection effects that apply to the identification of Galactic SNRs and their consequences.

The NGC 346 young stellar system and associated N66 giant H ii region in the Small Magellanic Cloud are the nearest example of a massive star-forming event in a low metallicity ( Z ≈ 0.2 Z ⊙ ) galaxy. With an age of ≲3 Myr this system provides a unique opportunity to study relationships between massive stars and their associated H ii region. Using archival data, we derive a total H α luminosity of L (H α ) = 4.1 × 10 38 erg s −1 corresponding to an H-photoionization rate of 3 × 10 50 s −1 . A comparison with a predicted stellar ionization rate derived from the more than 50 known O-stars in NGC 346, including massive stars recently classified from Hubble Space Telescope far-ultraviolet (FUV) spectra, indicates an approximate ionization balance. Spectra obtained with SALT suggest the ionization structure of N66 could be consistent with some leakage of ionizing photons. Due to the low metallicity, the FUV luminosity from NGC 346 is not confined to the interstellar cloud associated with N66. Ionization extends through much of the spatial extent of the N66 cloud complex, and most of the cloud mass is not ionized. The stellar mass estimated from nebular L (H α ) appears to be lower than masses derived from the census of resolved stars which may indicate a disconnect between the formation of high and low mass stars in this region. We briefly discuss implications of the properties of N66 for studies of star formation and stellar feedback in low metallicity environments.

In this paper, we pioneer a new machine-learning method to search for H ii regions in spectra from The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). H ii regions are emission nebulae created when young and massive stars ionize nearby gas clouds with high-energy ultraviolet radiation. Having more H ii region samples will help us understand the formation and evolution of stars. Machine-learning methods are often applied to search for special celestial bodies such as H ii regions. LAMOST has conducted spectral surveys and provided a wealth of valuable spectra for the research of special and rare celestial bodies. To overcome the problem of sparse positive samples and diversification of negative samples, a novel method called the self-calibrated convolution network is introduced and implemented for spectral processing. A deep network classifier with a structure called a self-calibrated block provides a high precision rate, and the recall rate is improved by adding the strategy of positive-unlabeled bagging. Experimental results show that this method can achieve better performance than other current methods. Eighty-nine spectra are identified as Galactic H ii regions after cross-matching with the WISE Catalog of Galactic H ii Regions, confirming the effectiveness of the method proposed in this paper.