We report ALMA observations with resolution ≈ 0.5 at 3 mm of the extended Sgr B2 cloud in the Central Molecular Zone (CMZ). We detect 271 compact sources, most of which are smaller than 5000 AU. By ruling out alternative possibilities, we conclude that these sources consist of a mix of hypercompact H ii regions and young stellar objects (YSOs). Most of the newly-detected sources are YSOs with gas envelopes which, based on their luminosities, must contain objects with stellar masses M * 8 M . Their spatial distribution spread over a ∼ 12 × 3 pc region demonstrates that Sgr B2 is experiencing an extended star formation event, not just an isolated 'starburst' within the protocluster regions. Using this new sample, we examine star formation thresholds and surface density relations in Sgr B2. While all of the YSOs reside in regions of high column density (N (H 2 ) 2 × 10 23 cm −2 ), not all regions of high column density contain YSOs. The observed column density threshold for star formation is substantially higher than that in solar vicinity clouds, implying either that high-mass star formation requires a higher column density or that any star formation threshold in the CMZ must be higher than in nearby clouds. The relation between the surface density of gas and stars is incompatible with extrapolations from local clouds, and instead stellar densities in Sgr B2

A survey toward 674 Planck cold clumps of the Early Cold Core Catalogue (ECC) in the J=1-0 transitions of 12 CO, 13 CO and C 18 O has been carried out using the PMO 13.7 m telescope. 673 clumps were detected with the 12 CO and 13 CO, and 68% of the samples have C 18 O emission. Additional velocity components were also identified. A close consistency of the three line peak velocities was revealed for the first time. Kinematic distances are given out for all the velocity components and half of the clumps are located within 0.5 and 1.5 kpc. Excitation temperatures range from 4 to 27 K, slightly larger than those of T d . Line width analysis shows that the majority of ECC clumps are low mass clumps. Column densities N H 2 span from 10 20 to 4.5×10 22 cm −2 with an average value of (4.4±3.6)×10 21 cm −2 . N H 2 cumulative fraction distribution deviates from the lognormal distribution, which is attributed to optical depth. The average abundance ratio of the 13 CO to C 18 O in these clumps is 7.0±3.8, higher than the terrestrial value. Dust and gas are well coupled in 95% of the clumps. Blue profile, red profile and line asymmetry in total was found in less than 10% of the clumps, generally indicating star formation is not developed yet. Ten clumps were mapped. Twelve velocity components and 22 cores were obtained. Their morphologies include extended diffuse, dense isolated, cometary and filament, of which the last is the majority. 20 cores are starless. Only 7 cores seem to be in gravitationally bound state. Planck cold clumps are the most quiescent among the samples of weak-red IRAS, infrared dark clouds, UC Hii region candidates, EGOs and methanol maser sources, suggesting that Planck cold clumps have expanded the horizon of cold Astronomy.

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Context. Sagittarius B2 (north) is a chemically rich, high-mass star-forming region located within the giant molecular cloud complex Sgr B2 in the central molecular zone of our Galaxy. Dust continuum emission at 242 GHz, obtained at high angular resolution with the Atacama Large Millimeter Array (ALMA), reveals that it has a filamentary structure on scales of 0.1 pc. Aims. We aim to characterize the filamentary structure of Sgr B2(N) and its kinematic properties using multiple molecular dense gas tracers. Methods. We have used an unbiased, spectral line-survey that covers the frequency range from 211 to 275 GHz and obtained with ALMA (angular resolution of 0.′′4, or 3300 au) to study the small-scale structure of the dense gas in Sgr B2(N). In order to derive the kinematic properties of the gas in a chemically line-rich source like Sgr B2(N), we have developed a python-based tool that stacks all the detected line transitions of any molecular species. This allows us to increase the signal-to-noise ratio (S/N) of our observations and average out line blending effects, which are common in line-rich regions. Results. A filamentary network is visible in Sgr B2(N) in the emission maps of the molecular species CH3OCHO, CH3OCH3, CH3OH and H2CS. In total, eight filaments are found that converge to the central hub (with a mass of 2000 M⊙, assuming a temperature of 250 K) and extending for about 0.1 pc (up to 0.5 pc). The spatial structure, together with the presence of the massive central region, suggest that these filaments may be associated with accretion processes, transporting material from the outer regions to the central dense hub. We derive velocity gradients along the filaments of about 20–100 km s−1 pc−1, which are 10–100 times larger than those typically found on larger scales (~1 pc) in other star-forming regions. The mass accretion rates of individual filaments are ≾0.05 M⊙ yr−1, which result in a total accretion rate of 0.16 M⊙ yr−1. Some filaments harbor dense cores that are likely forming stars and stellar clusters. We determine an empirical relation between the luminosity and stellar mass of the clusters. The stellar content of these dense cores is on the order of 50% of the total mass. The timescales required for the dense cores to collapse and form stars, exhausting their gas content, are compared with the timescale of their accretion process onto the central hub. We conclude that the cores may merge in the center when already forming stellar clusters but still containing a significant amount of gas, resulting in a “damp” merger. Conclusions. The high density and mass of the central region, combined with the presence of converging filaments with high mass, high accretion rates and embedded dense cores already forming stars, suggest that Sgr B2(N) may have the potential to evolve into a super stellar cluster.

Context. Radio observations at metre-centimetre wavelengths shed light on the nature of the emission of Hii regions. Usually this category of objects is dominated by thermal radiation produced by ionised hydrogen, namely protons and electrons. However, a number of observational studies have revealed the existence of Hii regions with a mixture of thermal and non-thermal radiation. The latter represents a clue as to the presence of relativistic electrons. However, neither the interstellar cosmic-ray electron flux nor the flux of secondary electrons, produced by primary cosmic rays through ionisation processes, is high enough to explain the observed flux densities. Aims. We investigate the possibility of accelerating local thermal electrons up to relativistic energies in Hii region shocks. Methods. We assumed that relativistic electrons can be accelerated through the first-order Fermi acceleration mechanism and we estimated the emerging electron fluxes, the corresponding flux densities, and the spectral indexes. Results. We find flux densities of the same order of magnitude of those observed. In particular, we applied our model to the 'deep south' (DS) region of Sagittarius B2 and we succeeded in reproducing the observed flux densities with an accuracy of less than 20% as well as the spectral indexes. The model also gives constraints on magnetic field strength (0.3 − 4 mG), density (1 − 9 × 10 4 cm −3 ), and flow velocity in the shock reference frame (33 − 50 km s −1 ) expected in DS. Conclusions. We suggest a mechanism able to accelerate thermal electrons inside Hii regions through the first-order Fermi acceleration. The existence of a local source of relativistic electrons can explain the origin of both the observed non-thermal emission and the corresponding spectral indexes.

The low dust temperatures (<14 K) of Planck Galactic cold clumps (PGCCs) make them ideal targets to probe the initial conditions and very early phase of star formation. "TOP-SCOPE" is a joint survey program targeting ∼2000 PGCCs in J=1-0 transitions of CO isotopologues and ∼1000 PGCCs in 850 μm continuum emission. The objective of the "TOP-SCOPE" survey and the joint surveys (SMT 10 m, KVN 21 m, and NRO 45 m) is to statistically study the initial conditions occurring during star formation and the evolution of molecular clouds, across a wide range of environments. The observations, data analysis, and example science cases for these surveys are introduced with an exemplar source, PGCC G26.53+0.17 (G26), which is a filamentary infrared dark cloud (IRDC). The total mass, length, and mean line mass (M/L) of the G26 filament are ∼6200 M ☉ , ∼12 pc, and ∼500 M ☉ pc −1 , respectively. Ten massive clumps, including eight starless ones, are found along the filament. The most massive clump as a whole may still be in global collapse, while its denser part seems to be undergoing expansion owing to outflow feedback. The fragmentation in the G26 filament from cloud scale to clump scale is in agreement with gravitational fragmentation of an isothermal, nonmagnetized, and turbulent supported cylinder. A bimodal behavior in dust emissivity spectral index (β) distribution is found in G26, suggesting grain growth along the filament. The G26 filament may be formed owing to large-scale compression flows evidenced by the temperature and velocity gradients across its natal cloud.

Context. The giant molecular cloud Sagittarius B2 (hereafter Sgr B2) is the most massive region with ongoing high-mass star formation in the Galaxy. In the southern region of the 40-pc large envelope of Sgr B2, we encounter the Sgr B2(DS) region, which hosts more than 60 high-mass protostellar cores distributed in an arc shape around an extended H II region. Hints of non-thermal emission have been found in the H II region associated with Sgr B2(DS). Aims. We seek to characterize the spatial structure and the spectral energy distribution of the radio continuum emission in Sgr B2(DS). We aim to disentangle the contribution from the thermal and non-thermal radiation, as well as to study the origin of the non-thermal radiation. Methods. We used the Very Large Array in its CnB and D configurations, and in the frequency bands C (4–8 GHz) and X (8–12 GHz) to observe the whole Sgr B2 complex. Continuum and radio recombination line maps are obtained. Results. We detect radio continuum emission in Sgr B2(DS) in a bubble-shaped structure. From 4 to 12 GHz, we derive a spectral index between − 1.2 and − 0.4, indicating the presence of non-thermal emission. We decomposed the contribution from thermal and non-thermal emission, and find that the thermal component is clumpy and more concentrated, while the non-thermal component is more extended and diffuse. The radio recombination lines in the region are found to be not in local thermodynamic equilibrium but stimulated by the non-thermal emission. Conclusions. Sgr B2(DS) shows a mixture of thermal and non-thermal emission at radio wavelengths. The thermal free–free emission is likely tracing an H II region ionized by an O 7 star, while the non-thermal emission can be generated by relativistic electrons created through first-order Fermi acceleration. We have developed a simple model of the Sgr B2(DS) region and found that first-order Fermi acceleration can reproduce the observed flux density and spectral index.