Maser effects occur in recombination lines when the plasma departs from local thermodynamic equilibrium (LTE). Its consequence is not as dramatic as that found in molecular masers, and therefore it is more difficult to recognize. Besides, it occurs in compact high density regions, and its lines fall at millimeter and submillimeter wavelengths, only recently available with good angular resolution. This review will focus on the theoretical aspects of maser recombination lines and on the recent detection of these masers in different astronomical objects.

We report the results of a 7-year monitoring program using the Australia Telescope Compact Array (ATCA) for the 9.9 GHz class I methanol maser in G331.13-0.24 where a periodic class II methanol maser is present. The great deal of the project was to control systematics at an unprecedented level. Although no periodic flux variation was found, the maser shows a very stable decline of 166±7 μJy/day. The radial velocity of the maser is stable down to 1 m/s level. We also report a marginal periodic signal in radial velocity (comparable to the level of systematics) of about 20±7 cm/s with the period of 475±22 days, close to that of the 6.7-GHz maser in the source. No hyperfine split was detected which suggests preferential excitation of a single hyperfine transition.

The Australia Telescope Compact Array (ATCA) has been used for an interferometric follow up observation at 104.3 GHz of the targets where either this or the 9.9-GHz maser was previously detected. We confirm the significant difference (by more than 1.5 orders of magnitude) from source to source of the flux density ratio for these two maser transitions. Based on the morphology and location of continuum sources, the most likely explanation of this discrepancy is the difference in the flux density of the seed radiation at the two frequencies. We also report absolute positions (with arcsec accuracy) for all detected 104.3 GHz masers.

The Alicante Survey of MAssive Stars in Hii Regions (A-SMASHeR) is aimed at finding the ratio of massive stars that are born in isolation. We present LIRIS/WHT images and EMIR/GTC spectra of the massive stellar content in A-SMASHeR regions. Our preliminary analysis yields ∼20% of regions hosting relatively (or truly) isolated massive stars.

In this paper, we present a glimpse of our observations of two Wolf-Rayet (WR) nebulae, NGC2359 and NGC6888 obtained with the SITELLE imaging Fourier transform spectrograph. The data are of unprecedented spatial coverage and cover a broad wavelength range.

We study the He II λ4686 emission south of multiple-star system HD 5980, which hosts two WN-type stars. We use optical VLT FORS1 long-slit spectra. The observations are close to the eclipse phase. Broad He II emission with a SNR >5 is observed as far as 7.6 pc from HD 5980. The He II emission that is closest to HD 5980 is 1.2 pc directly south and has a redshifted component with FWHM 1450 km s-1 originating in the eclipsing WN5-6 star, and a blueshifted component with FWHM 600 km s-1. The second component cannot be explained by either the nearly- eclipsed WN star or the nearby supernova remnant, SNR B0057-724. We suggest that the additional He II emission comes from the colliding winds of the two WN stars.

Many simulations have been performed to elucidate the formation process of first stars. In first star formation, radiative feedback is a key process in determining stellar masses. However, previous simulations which follow the feedback process don’t resolve the small scale ( 10 AU) to realize long-term calculation, and the structure near massive protostars is still unknown. To clarify how the radiation from the protostar works, we need to resolve small scale and calculate the interaction between the radiation and the dense gas in such a region. As a first step towards understanding the phenomenon in this region, we perform the high-resolution simulation around the massive protostar without radiative transfer. We find that dense gas covers the protostar even in the polar direction and the HII region cannot expand. Solving the radiative transfer for getting accurate results is our future work. We are currently developing the new radiation hydrodynamics code for that.

Considering as many as 70% of massive stars interact with a binary companion (Sana et al.2012, 2014), we created a new model of the optical nebular emission of HII regions by combining the results of the Binary Population and Spectral Synthesis (BPASS, Eldridge, Stanway et al.2017) code with the photoionization code (CLOUDY). This is discussed more in detail in Xiao et al.2018a. Then we use this model to explore a variety of emission-line diagnostics of CCSN host HII regions from the PMAS/PPAK Integral-field Supernova hosts COmpilation (PISCO, Galbany et al.2018). We determine the age, metallicity and gas parameters for H II regions associated with CCSNe, contrasting the above variables to distribution type II and type Ibc SNe. We find their nebular emission and CCSN progenitor types are largely determined by past and ongoing binary interactions, for example mass loss, mass gain and stellar mergers. However we note these two types SNe have little preference in their host environment metallicity measured by oxygen abundance or in progenitor initial mass, except that at lower metallicities supernovae are more likely to be of type II. The BPASS and nebular emission models are available from bpass.auckland.ac.nz and warwick.ac.uk/bpass.

We present radio continuum and water maser observations toward the high-mass star-forming region IRAS 23151+5912 from the VLA and VLBA archive, respectively. We detected a continuum source, which seems to be a hypercompact HII region. In addition, a water maser group about 4″ south from the continuum source was detected. We present preliminary results of the analysis of three observations epochs of the water masers, which are tracing an arc-like structure. However, its kinematics is quite complex, since while one section of the structure seems to be moving away from one center, another section seems to be approaching.

The detection of hydrogen radio-recombination maser lines (RRLs) toward MWC349A in the year 1989 opened the chance to place constraints on the kinematics of an ionized circumstellar disk around a massive star. Since then, a significant number of observations have allowed improving our understanding of this source to the point that we have established that its ionized wind launching occurs at a distance of ~24 au as claimed by disk wind models. On the other hand, this field of study has undergone considerable development over the last six years with the detection of new RRL maser sources. Here, we present a brief summary of all these recent advances and the promising future prospects.

We present a new model of the optical nebular emission from HII regions by combining the results of the Binary Population and Spectral Synthesis (bpass) code with the photoionization code cloudy (Ferland et al. 1998). We explore a variety of emission-line diagnostics of these star-forming HII regions and examine the effects of metallicity and interacting binary evolution on the nebula emission-line production. We compare the line emission properties of HII regions with model stellar populations, and provide new constraints on their stellar populations and supernova progenitors. We find that models including massive binary stars can successfully match all the observational constraints and provide reasonable age and mass estimation of the HII regions and supernova progenitors.

We use the stellar census of OB associations in the Large Magellanic Cloud (LMC) to predict the Hα luminosities of the host HII regions, based on results from stellar atmosphere models. These values are compared to the observed HII region luminosities, yielding an estimate for the mean fraction of H-ionising photons that escape the local nebulae in this sample. We formally estimate that, overall, 0–51% of the ionising radiation escapes the local HII regions and is available to ionise the warm, ionised medium in the LMC. We find both nebulae that appear to be density-bounded and ones that appear to be radiation-bounded.

We suggest and give some evidence that as an HII region expands and the O stars evolve into B giants, remains of the molecular clouds first appear as bright-rimmed clouds, then as cometary globules and finally as small clouds which are visible by the reflected light from the B giants. We propose to call the last of these ‘reflection clouds’ and all three categories collectively ‘remnant clouds’. A list is presented of about 80 objects of these remnant clouds in the Ori OB 1 association. In the Belt region there is a beautiful spatial sequence from bright-rimmed clouds through cometary globules to reflection clouds. We suspect that retarded star formation in remnant clouds can explain the presence of so-called dispersed T Tau stars in the peripheries of OB associations.

This project is aimed at understanding the dependence of star formation on the environment by analysing young stellar populations in two very different positions in disk galaxies: circumnuclear and outer disk giant regions. Integral field spectroscopy (IFS) provide an ideal means to achieve these goals providing simultaneous spatial and spectral resolution. Here we present the characterization of the work sample, composed by 671 outer regions and 725 inner regions from 263 isolated spirals galaxies observed by the CALIFA survey. The wide number of regions in both samples allows us to obtain statistically relevant results about the influence of metallicity, density and environment on star formation, and how it disseminates over the galaxy, to obtain evolutionary stories for the star-forming regions and to compare our results with models of massive star formation and galactic chemical evolution.

SITELLE is the new imaging Fourier transform spectrograph of the Canada-France-Hawaii Telescope. It produces an impressive 4 million spectra in a single datacube in selected bandpasses from 350 to 900 nm. Its large FOV (11′x11′) and its high spatial sampling (0.32′′/pixel, seeing limited) allow us to study extended objects with an unprecedented view (Drissen et al. 2014). SITELLE’s first observations of nearby galaxies revealed its capabilities to conduct detailed studies of emission line regions.

We found that star-forming regions in extended ultraviolet (XUV) disks are generally consistent with the molecular-hydrogen Kennicutt-Schmidt law that applies within the inner, optical disk. This is true for star formation rates based on Hα + 24 μm data or FUV + 24 μm data. We estimated that the star-forming regions have ages of 1 − 7 Myr and propose that the presence or absence of molecular gas provides an additional “clock” that may help distinguish between aging and stochasticity as the explanation for the low Hα-to-FUV flux ratios in XUV disks. This contribution is a summary of the work originally presented in Watson et al. (2016).

We have made near-infrared (JHKs) imaging polarimetry toward 24 bright-rimmed clouds in the southern hemisphere in order to reveal their magnetic field structures. The obtained polarization vector maps show that the magnetic field directions inside the bright rim are different from its ambient magnetic field direction, implying that magnetic field structures just inside the ionized front of the clouds are due to the gas compression by UV radiation from nearby massive star. Our investigation into the relation between the magnetic field configuration and the shape of the cloud suggests that the magnetic field configuration affects the evolution of the cloud shape.

We have combined observations of the Antennae galaxies from the radio interferometer ALMA and from the optical interferometer GHαFaS (Galaxy Hα Fabry-Perot System). The two sets of observations have comparable angular and spectral resolutions, enabling us to identify 142 giant molecular clouds (GMCs) and 303 Hii regions. We have measured, and compared, their basic physical properties (radius, velocity dispersion, luminosity). We study the mass functions of Hii regions and GMCs, where a double population of Hii regions due to a double population of GMCs is found. Using the velocity dispersion measurements, we claim that the difference between the regimes is the nature of the dominant binding force. For the regions in the lower mass range, the dominant force is the external pressure, while in the higher mass range it is the internal gravity of the clouds. We have applied the same method to similar observations (GHαFaS & ALMA) of Arp 236 and NGC 1614 finding similar results.