Ultracompact HII Regions Research Articles

Revisiting the massive star-forming complex RCW 122: New millimeter and submillimeter study

In this paper, we present a new multifrequency study of the giant star-forming complex RCW 122. We used molecular data obtained with the ASTE 10 m and the APEX 12 m telescopes, along with infrared observations spanning from 3.6 µm to 870 µm, obtained from available databases. We also incorporated a range of public datasets, including the radio continuum at 3 GHz, narrowband Ha images, and deep JHK photometry. Our analysis focuses mostly on cataloged ATLASGAL sources, showcasing a spectrum of evolutionary stages from infrared dark cloud (IRDC)/high-mass protostellar object (HMPO) to ultra-compact HII region (UCHII), as inferred from preliminary inspections of the public dataset. Based on ASTE HCO+(4−3) and CO(3−2) data, we identified five molecular clumps, designated A, B, C, D, and E, as molecular counterparts of the ATLASGAL sources. These clumps have radial velocities ranging from ~−15 km s−1 to −10 km s−1, confirming their association with RCW 122. In addition, we report the detection of 20 transitions from 11 distinct molecules in the APEX spectra in the frequency ranges from 258.38 GHz to 262.38 GHz, 228.538 GHz to 232.538 GHz, and 218.3 GHz to 222.3 GHz, unveiling a diverse chemical complexity among the clumps. Utilizing CO(2−1) and C18O(2−1) data taken from the observations with the APEX telescope, we estimated the total LTE molecular mass, ranging from 200 M⊙ (clump A) to 4400 M⊙ (clump B). Our mid- to far-infrared (MIR-FIR) flux density analysis yielded minimum dust temperatures of 23.7 K (clump A) to maximum temperatures of 33.9 K (clump B), indicating varying degrees of internal heating among the clumps. The bolometric luminosities span 1.7×103 L⊙ (clump A) to 2.4×105 L⊙ (clump B), while the total (dust+gas) mass ranges from 350 M⊙ (clump A) to 3800 M⊙ (clump B). Our analysis of the molecular line richness, L/M ratios, and CH3CCH and dust temperatures reveals an evolutionary sequence of A/E→C→D/B, consistent with preliminary inferences of the ATLASGAL sources. In this context, clumps A and E exhibit early stages of collapse, with clump A likely in an early HMPO phase, which is supported by identifying a candidate molecular outflow. Clump E appears to be in an intermediate stage between IRDC and HMPO. Clumps D and B show evidence of being in the UCHII phase, with clump B likely more advanced. Clump C likely represents an intermediate stage between HMPO and HMC. Our findings suggest clump B is undergoing ionization and heating by multiple stellar and protostellar members of the stellar cluster DBS 119. Meanwhile, other cluster members may be responsible for ionizing other regions of RCW 122 that have evolved into fully developed HII regions, beyond the molecular dissociation stage.

Core to ultracompact HII region evolution in the W49A massive protocluster

Aims. We aim to identify and characterize cores in the high-mass protocluster W49A, determine their evolutionary stages, and measure the associated lifetimes. Methods. We built a catalog of 129 cores extracted from an ALMA 1.3 mm continuum image at 0.26″ (2900 au) angular resolution. The association between cores and hypercompact or ultracompact HII (H/UC HII) regions was established from the analysis of VLA 3.3 cm continuum and H30α line observations. We also looked for emission of hot molecular cores (HMCs) using the methyl formate doublet at 218.29 GHz. Results. We identified 40 cores associated with an H/UC HII region and 19 HMCs over the ALMA mosaic. The 52 cores with an H/UC HII region and/or an HMC are assumed to be high-mass protostellar cores, while the rest of the core population likely consists of prestellar cores and low-mass protostellar cores. We found a good agreement between the two tracers of ionized gas, with 23 common detections and only four cores detected at 3.3 cm and not in H30α. The spectral indexes from 3.3 cm to 1.3 mm range from 1, for the youngest cores with partially optically thick free-free emission, to about −0.1, which is for the optically thin free-free emission obtained for cores that are likely more evolved. Conclusions. Using the H/UC HII regions as a reference, we found the statistical lifetimes of the HMC and massive protostellar phases in W49N to be about 6 × 104 yr and 1.4 × 105 yr, respectively. We also showed that HMCs can coexist with H/UC HII regions during a short fraction of the core lifetime, about 2 × 104 yr. This indicates a rapid dispersal of the inner molecule envelope once the HC HII is formed.

Searching for stellar population in the molecular cloud GRSMC 045.49+00.04

Abstract Understanding the characteristics of young stellar populations is essential for deriving insights into star formation processes within parent molecular clouds and the influence of massive stars on these processes. This study primarily aims to investigate the young stellar objects (YSOs) within the molecular cloud G 045.49+00.04, including three ultra-compact HII (UC HII) regions: G 45.48+0.13 (IRAS 19117+1107), G 45.45+0.06 (IRAS 19120+1103), and G 45.47+0.05. We used near-, mid-, and far-infrared photometric data along with radiation transfer models and the modified blackbody fitting to identify and study the YSOs and the interstellar medium (ISM). In total, we identified 1482 YSOs in a 12 arcmin radius covering GRSMC 045.49+00.04, with a mass range from 1.5 M ${}_{\odot}$ to 22 M ${}_{\odot}$ . Of these, 315 objects form relatively dense clusters in the UC HII regions, close to the IRAS 19120+1103 and 19117+1107 sources. In each UC HII region, several high-mass stars have been identified, which in all likelihood are responsible for the ionization. The YSOs with 21.8 M ${}_{\odot}$ and 13.7 ± 0.4 M ${}_{\odot}$ are associated with IRAS 19120+1103 and 19117+1107, respectively. The non-cluster YSOs (1168) are uniformly distributed on the field. The distribution of YSOs from both samples on the colour-magnitude diagram and by the evolutionary ages is different. About 75% of objects in the IRAS clusters are concentrated around the Zero Age Main Sequence and have a well-defined peak at an age of Log(Age[years]) $\approx$ 6.75, with a narrow spread. The non-cluster objects have two concentrations located to the right and left of the 0.1 Myr isochrone and two well-defined peaks at Log(Age) $\approx$ 6.25 and 5.25. The fraction of the near-infrared excess stars, as well as the mass function confirm that the evolutionary age of the cluster is about 1 Myr. The K luminosity functions’ α slopes for the IRAS clusters and non-cluster objects are 0.32 ± 0.04 and 0.72 ± 0.13, respectively. The steeper α slope is suggesting that the non-cluster objects are less evolved, which is well consistent with the evolutionary age. Similar results – including evolutionary age, narrow age spread, and the less evolved nature of non-cluster objects – were also observed for the YSOs in the neighbouring G 45.14+00.14. The both regions (G 045.49+00.04 and G 45.14+00.14) are located and distinguished by their brightness and density at the edge of the bubble around the highly variable X-ray binary GRS 1915+105, which includes a black hole and a K-giant companion. Based on the above, we can assume that the process of star formation in the young IRAS clusters was triggered by the GRS 1915+105-initiated shock front inside the ISM massive condensation, through the process of ‘collecting and collapse’. Most non-cluster objects probably belong to a later generation. Their formation could be triggered by the recurrent activity of GRS 1915+105 and/or through the edge collapse scenario and mass accumulation through the gas flows along the ISM filaments.

The evolution of sulphur-bearing molecules in high-mass star-forming cores

Context. To understand the chemistry of sulphur (S) in the interstellar medium, models need to be tested by observations of S-bearing molecules in different physical conditions. Aims. We aim to derive the column densities and abundances of S-bearing molecules in high-mass dense cores in different evolutionary stages and with different physical properties. Methods. We analysed observations obtained with the Institut de RadioAstronomie Millimétrique (IRAM) 30 m telescope towards 15 well-known cores classified in the three main evolutionary stages of the high-mass star formation process: high-mass starless cores, high-mass protostellar objects, and ultracompact HII regions. Results. We detected rotational lines of SO, SO+, NS, C34S, 13CS, SO2, CCS, H2S, HCS+, OCS, H2CS, and CCCS. We also analysed the lines of the NO molecule for the first time to complement the analysis. From a local thermodynamic equilibrium approach, we derived the column densities of each species and excitation temperatures for those that are detected in multiple lines with different excitation. Based on a statistical analysis of the line widths and the excitation temperatures, we find that NS, C34S, 13CS, CCS, and HCS+ trace cold, quiescent, and likely extended material; OCS, and SO2 trace warmer, more turbulent, and likely denser and more compact material; SO and perhaps SO+ trace both quiescent and turbulent material, depending on the target. The nature of the emission of H2S, H2CS, and CCCS is less clear. The molecular abundances of SO, SO2, and H2S show the strongest positive correlations with the kinetic temperature, which is thought to be an indicator for evolution. Moreover, the sum of all molecular abundances shows an enhancement of gaseous S from the less evolved to the more evolved stages. These trends could be due to the increasing amount of S that is sputtered from dust grains owing to the increasing protostellar activity with evolution. The average abundances in each evolutionary group increase, especially in the oxygen-bearing molecules, perhaps due to the increasing abundance of atomic oxygen with evolution owing to photodissociation of water in the gas phase. Conclusions. Our observational work represents a test-bed for theoretical studies aimed at modelling the chemistry of sulphur during the evolution of high-mass star-forming cores.

Physical and chemical complexity in high-mass star-forming regions with ALMA

Context. High-mass star formation is a hierarchical process from cloud (>1 pc), to clump (0.1−1 pc), to core scales (<0.1 pc). Modern interferometers that achieve high angular resolutions at millimeter wavelengths allow us to probe the physical and chemical properties of the gas and dust of protostellar cores in the earliest evolutionary formation phases. Aims. In this study we investigate how physical properties, such as the density and temperature profiles, evolve on core scales through the evolutionary sequence during high-mass star formation ranging from protostars in cold infrared-dark clouds to evolved ultracompact HII (UCHII) regions. Methods. We observed 11 high-mass star-forming regions with the Atacama Large Millimeter/submillimeter Array (ALMA) at 3 mm wavelengths. Based on the 3 mm continuum morphology and H(40)α recombination line emission - which trace locations with free-free (ff) emission - the fragmented cores analyzed in this study are classified as either “dust” or “dust+ff” cores. In addition, we resolved three cometary UCHII regions with extended 3 mm emission that is dominated by free-free emission. The temperature structure and radial profiles (T ~ r−q) were determined by modeling the molecular emission of CH3CN and CH313CN with XCLASS and by using the HCN-to-HNC intensity ratio as a probe for the gas kinetic temperature. The density profiles (n ~ r−p) were estimated from the 3 mm continuum visibility profiles. The masses (M) and H2 column densities (N(H2)) were then calculated from the 3 mm dust continuum emission. Results. We find a large spread in mass and peak H2 column density in the detected sources, ranging from 0.1 to 150 M⊙ and 1023 to 1026 cm−2, respectively. Including the results of the CORE and CORE-extension studies to increase the sample size, we find evolutionary trends on core scales for the temperature power-law index (q) increasing from 0.1 to 0.7 from infrared-dark clouds to UCHII regions, while for the density power-law index (p) on core scales, we do not find strong evidence for an evolutionary trend. However, we find that on the larger clump scales the density profile flattens from p ≈ 2.2 to p ≈ 1.2 during these evolutionary phases. Conclusions. By characterizing a large statistical sample of individual fragmented cores, we find that the physical properties, such as the temperature on core scales and the density profile on clump scales, evolve even during the earliest evolutionary phases in high-mass star-forming regions. These findings provide observational constraints for theoretical models that describe the formation of massive stars. In follow-up studies we aim to further characterize the chemical properties of the regions by analyzing the large amount of molecular lines detected with ALMA in order to investigate how the chemical properties of the molecular gas evolve during the formation of massive stars.

Importance of source structure on complex organics emission

Context. The hot molecular core phase of massive star formation shows emission from complex organic molecules. However, these species are only detected toward a fraction of high-mass protostars. In particular, there is a spread of ~2 orders of magnitude in methanol emission intensity from high-mass protostars. Aims. The goal of this work is to answer the question of whether high-mass disks can explain the lack of methanol emission from some massive protostellar systems. Methods. We considered an envelope-only and an envelope-plus-disk model and used the code RADMC-3D to calculate the methanol emission. High and low millimeter (mm) opacity dust (representing large and small dust distributions) were considered for both models separately, and the methanol abundance was parameterized. Viscous heating was included due to the high accretion rates of these objects in the disk. Results. In contrast with low-mass protostars, the presence of a disk does not significantly affect the temperature structure and methanol emission. The shadowing effect of the disk is not as important for high-mass objects, and the disk midplane is hot because of viscous heating, which is effective due to the high accretion rates. The methanol emission is lower for models with high mm opacity dust because the dust attenuation blocks the emission in the envelope and hides it in the disk through continuum oversubtraction, but the disk needs to be large for this to become effective. A minimum disk size of ~2000–2500 au is needed (at L = 104 L⊙) with high mm opacity dust for drop of a factor of about one order of magnitude in the methanol emission compared with the envelope-only models with low mm opacity dust. Consistent with observations of infrared absorption lines toward high-mass protostars, we find a vertical temperature inversion, that is, higher temperatures in the disk midplane than the disk surface, at radii ≲50 au for models with L = 104 L⊙ and high mm opacity dust as long as the envelope mass is ≳550 M⊙ (Ṁ = 3.6 × 10−3 M⊙ yr−1). Conclusions. The large observed scatter in methanol emission from massive protostars can be mostly explained toward lower-luminosity objects (~103 L⊙) with the envelope-plus-disk models including low and high mm opacity dust. The methanol emission variation toward sources with high luminosities (≳104 L⊙) cannot be explained by models with or without a disk with a relatively high gas-phase abundance of methanol. However, the luminosity-to-mass ratios of these objects suggest that they might be associated with hypercompact or ultracompact HII regions. Therefore, the low methanol emission toward the high-luminosity sources can be explained by them hosting an HII region in which methanol is absent.

Global kinematics study of OH masers in W49N

Context. Star formation is underway in the W49N molecular cloud (MC) at a high level of efficiency, with almost twenty ultra-compact (UC) HII regions observed thus far, indicating a recent formation of massive stars. Previous works have suggested that this cloud is undergoing a global contraction. Aims. We analyse the data on OH masers in the molecular cloud W49N, observed with the VLBA at the 1612, 1665, and 1667 MHz transitions in left circular polarization (LCP) and right circular polarization (RCP) with an aim to study the global kinematics of the masers. Methods. We carried out our study based on the locations and observed velocities of the maser spots, Vobs. We found the location (α, δ)m of the maximum correlation between V = Vobs−Vsys (with Vsys the systemic velocity) and distance to it. The velocities were fitted to the straight line of Vobs−Vsys versus d(α,δ)m, resulting in Vftd. The difference between the fitted values and those obtained from observations is ∆ V = (Vobs−Vsys)-Vftd. The Vobs−Vsys velocity shows a gradient as a function of the distance to (α, δ)m, where the closer spots have the largest velocities. Spots with similar velocities are located in different sectors, with respect to (α, δ)m. Then, we assumed that the spots are moving towards a contraction centre (CCOH), which is at the apex of a CONUS. We also assumed that the distance of each spot to CCOH is dcc = √2 d(α,δ)m and that they fall with a velocity Vcc = √2Vftd, with the total velocity being VTot = Vcc + √2 Δ V. Using this velocity, we estimated the free-fall velocity. Results. The coordinates of (α, δ)m are effectively (α2000 = 19:10:13.1253, δ2000 = 9:6:13.570). The observed dispersion with respect to the global trend against dcc, shows a maximum at 0.12 pc, with a decay from 0.12 to 0.19 pc, which is faster than that taking place between 0.19 and 0.42 pc. Based on VTot, an inner mass of Minn = 2500 M⊙ was estimated. In addition, the estimated accretion rate is Ṁ = 1.4×10−3 M⊙yr−1, which requires a time of tinn = 1.8×106 yr to accumulate Minn. The free-fall time, assuming n = 1×10−4 cm−3, is tff = 3.4×105 yr. Performing the same procedure with published data that are of lower spatial resolution (than the VLBA data) produces similar results. For example, based on the available data, we find that (α, δ)m = (19:10:13.1392, 9:6:13.4387) J2000, which is at ≲ 0.3 asec from what has been calculated with the VLBA data, with an estimated inner mass of 2700 M⊙. A sub-collapse appears to be taking place in the region traced by the OH maser spots. Based on methanol maser cloudlets data, which lie in a smaller region, another possible centre of contraction is identified, which could be due to a sub-collapse towards a 75 M⊙ inner mass. Conclusions. The velocities of the OH spots at W49N, along with their positions with respect to (α, δ)m, make it possible to trace a global kinematics that is apparently due to a sub-collapse in the W49N MC.

The stellar content of UCHII regions: the molecular cloud GRSMC045.49+00.05

Ultra-compact Hii (UCHII) regions are an important phase in the formation and early evolution of massive stars. The main objectives of this work are to study the stellar content associated with the G045.49+00.04 and G045.14+00.14 star-forming regions located in the GRSMC45.46+0.05 molecular cloud at a distance of about 8kpc. Both regions contain a number of UCHII regions. The main objective is to characterize the embedded young stellar objects (YSOs), such as their mass, evolutionary age and age spread, spatial distribution, luminosity function. We used near-, mid-, and far-infrared photometric data to identify and classify the YSOs. Their main parameters were determined by the spectral energy distribution (SED) fitting tool using radiation transfer models. Totally, we identified 2864 YSOs. We also constructed a colour-magnitude diagram to compare the parameters of stellar objects with the results of the radiative transfer models. The density distribution of the identified YSOs showed the presence of dense clusters in the UCHII regions. The parameters of YSOs in the IRAS clusters and non-cluster objects surrounding them show some differences. In general, the YSOs in these clusters have an evolutionary age larger than 106 years with an age spread of a few Myr. The clusters include several high- and intermediate-mass zero-age main sequence stellar objects. The small age spread suggests that the clusters may originate from a single triggering event.

Search and study of young infrared stellar clusters

The “bricks” of the Galactic disc are giant molecular clouds, which are birthplaces of stellar population. Therefore, there is a genetic connection between young stellar objects (YSOs) and their surrounding Interstellar Medium (ISM). The thesis is devoted to a search for young stellar clusters in the vicinity of IRAS sources and a detailed study of three selected star-forming regions: IRAS 05137+3919, 05168+3634, and 19110+1045, which includes determining the parameters of the ISM based on far-infrared data, the identification and classification of YSOs using color-color and color-magnitude diagrams, the modeling of stellar parameters, and the construction and interpretation of the luminosity functions. It is likely that the three young stellar clusters were formed under different scenarios. The age spread of the IRAS05137+3919 and IRAS05168+3634 regions is much larger, and, therefore, we concluded that the stellar population is formed as a result of independent condensations. The age spread of the IRAS clusters’ members in the third region, which is a pair of ultra-compact HII regions (UCHIIs), namely G45.12+0.13 and G45.07+0.13, is small. The small age spread suggests that the clusters may originate from a single triggering event. Moreover, high-mass YSOs were obtained only in the G45.07+0.13 and G45.12+0.13 UCHII regions where the ISM initial density was higher and the star formation proceeded relatively quickly.

The physical and chemical structure of Sagittarius B2

Context. The giant molecular cloud Sagittarius B2 (hereafter SgrB2) is the most massive region with ongoing high-mass star formation in the Galaxy. Two ultra-compact Hii (UCHii ) regions were identified in SgrB2’s central hot cores, SgrB2(M) and SgrB2(N). Aims. Our aim is to characterize the properties of the Hii regions in the entire SgrB2 cloud. Comparing the Hii regions and the dust cores, we aim to depict the evolutionary stages of different parts of SgrB2. Methods. We use the Very Large Array in its A, CnB, and D configurations, and in the frequency band C (~6GHz) to observe the whole SgrB2 complex. Using ancillary VLA data at 22.4 GHz and ALMA data at 96 GHz, we calculated the physical parameters of the UCHii regions and their dense gas environment. Results. We identify 54 UCHii regions in the 6 GHz image, 39 of which are also detected at 22.4 GHz. Eight of the 54 UCHii regions are newly discovered. The UCHii regions have radii between 0.006 pc and 0.04 pc, and have emission measure between 106 pc cm6 and 109 pc cm6. The UCHii regions are ionized by stars of types from B0.5 to O6. We found a typical gas density of ~106–109 cm3 around the UCHii regions. The pressure of the UCHii regions and the dense gas surrounding them are comparable. The expansion timescale of these UCHii regions is determined to be ~104–105 yr. The percentage of the dust cores that are associated with Hii regions are 33%, 73%, 4%, and 1% for SgrB2(N), SgrB2(M), SgrB2(S), and SgrB2(DS), respectively. Two-thirds of the dust cores in SgrB2(DS) are associated with outflows. Conclusions. The electron densities of the UCHii regions we identified are in agreement with that of typical UCHii regions, while the radii are smaller than those of the typical UCHii regions. The dust cores in SgrB2(M) are more evolved than in SgrB2(N). The dust cores in SgrB2(DS) are younger than in SgrB2(M) or SgrB2(N).

Properties of interstellar matter and stellar population in two star-forming regions

This study aims to compare the properties of interstellar medium in two star-forming regions with different stellar content, with and without high-mass YSOs. The first region is an extended molecular cloud surrounding five IRAS sources: 05168+3634, 05184+3635, 05177+3636, 05162+3639, and IRAS 05156+3643. The second one is a physically connected pair of ultra compact HII regions, G45.07+0.13 & G45.12+0.13, associated with IRAS 19110+1045 and IRAS 19111+1048 sources, respectively. Using these two star formation regions as an example, one can see the relationship between the initial parameters of the parent molecular cloud (hydrogen column density, dust temperature), the process of star formation itself (external triggering shock or independent condensations), and the parameters of the stellar content. High-mass YSOs were obtained only in the G45.07+0.13 & G45.12+0.13 regions, in which, apparently, the initial density of the parent molecular cloud was higher and the star formation process was initiated by an external triggering shock. In addition, in the IRAS 05168+3634 region, there is a relationship between the density of the interstellar medium and the activity of the star formation process. In those subregions, where the mass and density of the initial, parent molecular cloud is greater, the process of star formation is likely to be more active and have a longer duration. In addition, in these sub-groups, on average, the mass of stars is larger.

The Cygnus Allscale Survey of Chemistry and Dynamical Environments: CASCADE

Context. While star formation on large molecular cloud scales and on small core and disk scales has been investigated intensely over the past decades, the connection of the large-scale interstellar material with the densest small-scale cores has been a largely neglected field. Aims. We wish to understand how the gas is fed from clouds down to cores. This covers dynamical accretion flows as well as the physical and chemical gas properties over a broad range of spatial scales. Methods. Using the IRAM facilities NOEMA and the IRAM 30 m telescope, we mapped large areas (640 arcmin2) of the archetypical star formation complex Cygnus X at 3.6 mm wavelengths in line and continuum emission. The data were combined and imaged together to cover all accessible spatial scales. Results. The scope and outline of The Cygnus Allscale Survey of Chemistry and Dynamical Environments (CASCADE) as part of the Max Planck IRAM Observatory Program (MIOP) is presented. We then focus on the first observed subregion in Cygnus X, namely the DR20 star formation site, which comprises sources in a range of evolutionary stages from cold pristine gas clumps to more evolved ultracompact Hii regions. The data covering cloud to cores scales at a linear spatial resolution of <5000 au reveal several kinematic cloud components that may be part of several large-scale flows around the central cores. The temperature structure of the region is investigated by means of the HCN/HNC intensity ratio and compared to dust-derived temperatures. We find that the deuterated DCO+ emission is almost exclusively located toward regions at low temperatures below 20 K. Investigating the slopes of spatial power spectra of dense gas tracer intensity distributions (HCO+, H13CO+, and N2H+), we find comparatively flat slopes between −2.9 and −2.6, consistent with high Mach numbers and/or active star formation in DR20. Conclusions. This MIOP large program on star formation in Cygnus X provides unique new data connecting cloud with core scales. The analysis of the DR20 data presented here highlights the potential of this program to investigate in detail the different physical and chemical aspects and their interrelations from the scale of the natal molecular cloud down to the scale of accretion onto the individual protostellar cores.

A multiwavelength study of the W33 Main ultracompact HII region

Aims. The dynamics of ionized gas around the W33 Main ultracompact HII region is studied using observations of hydrogen radio recombination lines and a detailed multiwavelength characterization of the massive star-forming region W33 Main is performed. Methods. We used the Giant Meterwave Radio Telescope (GMRT) to observe the H167α recombination line at 1.4 GHz at an angular resolution of 10″, and Karl. G. Jansky Very Large Array (VLA) data acquired in the GLOSTAR survey that stacks six recombination lines from 4–8 GHz at 25″ resolution to study the dynamics of ionized gas. We also observed the radio continuum at 1.4 GHz and 610 MHz with the GMRT and used GLOSTAR 4-8 GHz continuum data to characterize the nature of the radio emission. In addition, archival data from submillimeter to near-infrared wavelengths were used to study the dust emission and identify young stellar objects in the W33 Main star-forming region. Results. The radio recombination lines were detected at good signal to noise in the GLOSTAR data, while the H167α radio recombination line was marginally detected with the GMRT. The spectral index of radio emission in the region determined from GMRT and GLOSTAR shows the emission to be thermal in the entire region. Along with W33 Main, an arc-shaped diffuse continuum source, G12.81–0.22, was detected with the GMRT data. The GLOSTAR recombination line data reveal a velocity gradient across W33 Main and G12.81–0.22. The electron temperature is found to be 6343 K and 4843 K in W33 Main and G12.81–0.22, respectively. The physical properties of the W33 Main molecular clump were derived by modeling the dust emission using data from the ATLASGAL and Hi-GAL surveys and they are consistent with the region being a relatively evolved site of massive star formation. The gas dynamics and physical properties of G12.81–0.22 are consistent with the HII region being in an evolved phase and its expansion on account of the pressure difference is slowing down.

Kinematics and structure of Ionized gas in the UCHII regions of W33 main

Abstract High mass proto-stars create Ultra-Compact H ii regions (UCHII) at the stage of evolution when most of the accretion is finished but the star is still heavily embedded in molecular material. The morphologies of UCHII regions reflect the interactions of stellar winds, stellar motions, and density structure in the molecular cloud; they are complex and it has been very difficult to interpret them. We here present data obtained with TEXES on the NASA IRTF of the [Nei,ii] emission line in the proto-cluster of young OB stars in W33 Main. The data cube has a spatial resolution of ∼1.4 arcsec and true velocity resolution ∼5 km s-1; with Aλ ∼ 0.02AV it is relatively unaffected by extinction. We have run 3D hydrodynamic and line profile simulations, using PLUTO and RADMC-3D, of the gas structures created by multiple windy stars moving relative to each other through the ambient cloud. By iterative comparison of the data cube and the simulations, we arrive at models that reproduce the different morphology and kinematic structure of each UCHII region in W33 Main. The results indicate that each sub-source probably holds multiple exciting stars, permitting an improved view of the stellar population, and finds the stellar trajectories, which may determine the dynamical development of the proto-cluster.

Evolutions of CH3CN abundance in molecular clumps

To investigate the effects of massive star evolution on surrounding molecules, we select nine massive clumps previously observed with the Atacama Pathfinder Experiment (APEX) telescope and the Submillimeter Array (SMA) telescope. Based on the observations of APEX, we obtain luminosity to mass ratios L clump/M clump that range from 10 to 154 L ⊙/M ⊙, where some of them embedded ultra compact (UC) HII region. Using the SMA, CH3CN (12K–11K) transitions were observed toward nine massive star-forming regions. We derive the CH3CN rotational temperature and column density using the XCLASS program, and calculate its fractional abundance. We find that CH3CN temperature seems to increase with the increase of L clump/M clump when the ratio is between 10 to 40 L ⊙/M ⊙, then decrease when L clump/M clump ≥ 40 L ⊙/M ⊙. Assuming that the CH3CN gas is heated by radiation from the central star, the effective distance of CH3CN relative to the central star is estimated. The distance ranges from ∼ 0.003 to ∼ 0.083 pc, which accounts for ∼ 1/100 to ∼ 1/1000 of clump size. The effective distance increases slightly as L clump/M clump increases (R eff ∼ (L clump/M clump)0.5±0.2). Overall, the CH3CN abundance is found to decrease as the clumps evolve, e.g., X CH3CN ∼ (L clump/M clump)−1.0 ± 0.7. The steady decline of CH3CN abundance as the clumps evolution can be interpreted as a result of photodissociation.

Radio Stars of the SKA

Radio emission from stars can be used, for example, to study ionized winds or stellar flares. The radio emission is faint and studies have been limited to few objects. The Square Kilometer Array (SKA) brings a survey ability to the topic of radio stars. In this paper we investigate what the SKA can detect, and what sensitivity will be required for deep surveys of the stellar Milky Way. We focus on the radio emission from OB stars, Be stars, flares from M dwarfs, and Ultra Compact HII regions. The stellar distribution in the Milky Way is simulated using the Besançon model, and various relations are used to predict their radio flux. We find that the full SKA will easily detect all UltraCompact HII regions. At the limit of 10 nJy at 5 GHz, the SKA can detect 1500 Be stars and 50 OB stars per square degree, out to several kpc. It can also detect flares from 4500 M dwarfs per square degree. At 100 nJy, the numbers become about 8 times smaller. SKA surveys of the Galactic plane should be designed for high sensitivity. Deep imaging should consider the significant number of faint flares in the field, even outside the plane of the Milky Way.

EXPLORING THE NATURE OF COMPACT RADIO SOURCES ASSOCIATED TO UCHII REGIONS

We present Very Large Array 7 mm continuum observations of four ultracompact (UC) HII regions, observed previously at 1.3 cm, in order to investigate the nature of the compact radio sources associated with these regions. We detect a total of seven compact radio sources, four of them with thermal emission, and two compact radio sources with clear non- thermal emission. The thermal emission is consistent with the presence of an ionized envelope, either static (i.e., trapped in the gravitational radius of an associated massive star) or flowing away (i.e., a photo-evaporative flow). The nature of the non-thermal sources remains unclear and several possibilities are proposed. The possibility that most of these compact radio sources are photo-evaporating objects, and the remaining ones more evolved objects, is consistent with previous studies on UCHII regions.

14 years of 6.7 GHz periodic methanol maser observations towards G188.95+0.89

AbstractWe report the results of 14 years of monitoring of G188.95+0.89 periodic 6.7 GHz methanol masers using the Hartebeesthoek 26-m radio telescope. G188.95+0.89 (S252, AFGL5180) is a radio-quiet methanol maser site that is often interpreted as precursors of ultra-compact HII regions or massive protostar sites. At least five bright spectral components were identified. The maser feature at 11.36 km s-1 was found to experience an exponential decay during the monitoring period. The millimetre continuum reveals two cores associated with the source.

Simultaneous 22 GHz Water and 44 GHz Methanol Maser Survey of Ultracompact H ii Regions

We have carried out a multi-epoch simultaneous survey of 22 GHz H$_2$O and 44 GHz Class I CH$_3$OH masers toward 103 ultracompact HII regions (UCHIIs) between 2010 and 2017. We detected H$_2$O and CH$_3$OH maser emission in 74 (72%) and 55 (53%) UCHIIs, respectively. Among them, 3 H$_2$O and 27 CH$_3$OH maser sources are new detections. These high detection rates suggest that the occurrence periods of both maser species are significantly overlapped with the UCHII phase of massive star formation. The CH$_3$OH maser lines always have small (< 10 km s$^{-1}$) relative velocities with respect to the natal molecular cores, while H$_2$O maser lines often show larger relative velocities. Twenty four H$_2$O maser-detected sources have maser lines at relative velocities > 30 km s$^{-1}$, and thirteen of them show extremely high-velocity (> 50 km s$^{-1}$) components. The appearance and disappearance of H$_2$O maser lines are quite frequent over six-month or one-year time intervals. In contrast, CH$_3$OH maser lines usually do not exhibit significant variations in the line intensity, velocity, or shape for the same periods. The isotropic luminosities of both masers appear to correlate with the bolometric luminosities of the central stars. This correlation becomes much stronger in the case that data points in the low- and intermediate-mass regimes are added. The maser luminosities also tend to increase with the radio continuum luminosities of UCHIIs and the submillimeter continuum luminosities of the associated dense cores.

Beyond the Solar Circle − Trends in Massive Star Formation Between the Inner and Outer Galaxy

We have compiled the most complete compact and ultracompact HII region catalogue to date via multi-wavelength inspection of survey data. We utilise data from the recently available SASSy 850$\mu$m survey to identify massive star forming clumps in the outer Galaxy ($R_{\rm{GC}}>8.5$ kpc) and cross-match with infrared and radio data of known UC HII regions from the RMS database. For the inner Galaxy sample ($R_{\rm{GC}}<8.5$ kpc), we adopt the compact HII regions from previous works that used similar methods to cross match ATLASGAL with either CORNISH or RMS, depending on the location within the Galactic plane. We present a new UC HII region catalogue that more than doubles the original sample size of previous work, totaling 536 embedded HII regions and 445 host clumps. We examine the distance independent values of N$_{\rm{Ly}}/$M and L$_{\rm{bol}}/$M as proxies for massive star formation efficiency and overall star formation efficiency, respectively. We find a significant trend showing that L$_{\rm{bol}}/$M decreases with increasing $R_{\rm{GC}}$, suggesting that the overall star formation per unit mass is less in the outer Galaxy.