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Context. The Milky Way’s central molecular zone (CMZ) has been measured to form stars ten times less efficiently than in the Galactic disk, based on emission from high-mass stars. However, the CMZ’s low-mass (⩽2M_⊙) protostellar population, which accounts for most of the initial stellar mass budget and star formation rate (SFR), is poorly constrained observationally due to limited sensitivity and resolution. Aims. We aim to perform a cloud-wide census of the protostellar population in three massive CMZ clouds. Methods. We present the Dual-band Unified Exploration of three CMZ Clouds (DUET) survey, targeting the 20 km s⁻¹cloud, Sgr C, and the dust ridge cloud “e” using the Atacama Large Millimeter/submillimeter Array (ALMA) at 1.3 and 3 mm. The mosaicked observations achieve a comparable resolution of 0.^′′2–0.^′′3 (∼2000 au) and a sky coverage of 8.3–10.4 arcmin², respectively. Results. We report 563 continuum sources at 1.3 mm and 330 at 3 mm, respectively, and a dual-band catalog with 450 continuum sources. These sources are marginally resolved at a resolution of 2000 au. We find a universal deviation (>70% of the source sample) from commonly used dust modified blackbody (MBB) models, characterized by either low spectral indices or low brightness temperatures. Conclusions. Three possible explanations are discussed for the deviation. (1) Optically thick class 0/I young stellar objects (YSOs) with a very small beam filling factor can lead to lower brightness temperatures than what MBB models predict. (2) Large dust grains with millimeter or centimeter in size have more significant self-scattering, and frequency-dependent albedo could therefore cause lower spectral indices. (3) Free-free emission over 30 μJy can severely contaminate dust emission and cause low spectral indices for milliJansky sources, although the number of massive protostars (embedded UCHIIregions) needed is infeasibly high for the normal stellar initial mass function. A reliable measurement of the SFR at low protostellar masses will require future work to distinguish between these possible explanations. 

We present the highest-resolution (~0.04") Atacama Large Millimeter/submillimeter Array 1.3 mm continuum observations so far of three massive star-forming clumps in the Central Molecular Zone (CMZ), namely 20 km/s C1, 20 km/sC4, and Sgr C C4, which reveal prevalent compact millimeter emission. We extract the compact emission with astrodendro and identify a total of 199 fragments with a typical size of ∼370 au, which represent the first sample of candidates of protostellar envelopes and disks and kernels of prestellar cores in these clumps that are likely forming star clusters. Compared with the protoclusters in the Galactic disk, the three protoclusters display a higher level of hierarchical clustering, likely a result of the stronger turbulence in the CMZ clumps. Compared with the mini-starbursts in the CMZ, Sgr B2 M and N, the three protoclusters also show stronger subclustering in conjunction with a lack of massive fragments. The efficiency of high-mass star formation of the three protoclusters is on average 1 order of magnitude lower than that of Sgr B2 M and N, despite a similar overall efficiency of converting gas into stars. The lower efficiency of high-mass star formation in the three protoclusters is likely attributed to hierarchical cluster formation. 

Abstract We present 500 and 700 au resolution 1 and 3 mm Atacama Large Millimeter/submillimeter Array observations, respectively, of protostellar cores in protoclusters Sagittarius B2 (Sgr B2) North (N) and Main (M), parts of the most actively star-forming cloud in our Galaxy. Previous lower-resolution (5000 au) 3 mm observations of this region detected ∼150 sources inferred to be young stellar objects (YSOs) withM> 8M_⊙. With a 10-fold increase in resolution, we detect 371 sources at 3 mm and 218 sources in the smaller field of view at 1 mm. The sources seen at low resolution are observed to fragment into an average of two objects. About one-third of the observed sources fragment. Most of the sources we report are marginally resolved and are at least partially optically thick. We determine that the observed sources are most consistent with Stage 0/I YSOs, i.e., rotationally supported disks with an active protostar and an envelope, that are warmer than those observed in the solar neighborhood. We report source-counting-based inferred stellar mass and the star formation rate of the cloud: 2800M_⊙and 0.0038M_⊙yr⁻¹for Sgr B2 N and 6900M_⊙and 0.0093M_⊙yr⁻¹for Sgr B2 M, respectively. 

Abstract We report the discovery of nine new hot molecular cores in the Deep South (DS) region of Sagittarius B2 using Atacama Large Millimeter/submillimeter Array Band 6 observations. We measure the rotational temperature of CH₃OH and derive the physical conditions present within these cores and the hot core Sgr B2(S). The cores show heterogeneous temperature structure, with peak temperatures between 252 and 662 K. We find that the cores span a range of masses (203–4842M_⊙) and radii (3587–9436 au). CH₃OH abundances consistently increase with temperature across the sample. Our measurements show the DS hot cores are structurally similar to Galactic disk hot cores, with radii and temperature gradients that are comparable to sources in the disk. They also show shallower density gradients than disk hot cores, which may arise from the Central Molecular Zone’s higher density threshold for star formation. The hot cores have properties which are consistent with those of Sgr B2(N), with three associated with Class II CH₃OH masers and one associated with an ultra-compact Hiiregion. Our sample nearly doubles the high-mass star-forming gas mass near Sgr B2(S) and suggests the region may be a younger, comparably massive counterpart to Sgr B2(N) and (M). The relationship between peak CH₃OH abundance and rotational temperature traced by our sample and a selection of comparable hot cores is qualitatively consistent with predictions from chemical modeling. However, we observe constant peak abundances at higher temperatures (T≳ 250 K), which may indicate mechanisms for methanol survival that are not yet accounted for in models. 

Abstract A compact source, G0.02467–0.0727, was detected in Atacama Large Millimeter/submillimeter Array 3 mm observations in continuum and very broad line emission. The continuum emission has a spectral indexα≈ 3.3, suggesting that the emission is from dust. The line emission is detected in several transitions of CS, SO, and SO₂and exhibits a line width FWHM ≈ 160 km s⁻¹. The line profile appears Gaussian. The emission is weakly spatially resolved, coming from an area on the sky ≲1″ in diameter (≲10⁴au at the distance of the Galactic center, GC). The centroid velocity isv_LSR≈ 40–50 km s⁻¹, which is consistent with a location in the GC. With multiple SO lines detected, and assuming local thermodynamic equilibrium (LTE) conditions, the gas temperature isT_LTE= 13 K, which is colder than seen in typical GC clouds, though we cannot rule out low-density, subthermally excited, warmer gas. Despite the high velocity dispersion, no emission is observed from SiO, suggesting that there are no strong (≳10 km s⁻¹) shocks in the molecular gas. There are no detections at other wavelengths, including X-ray, infrared, and radio. We consider several explanations for the millimeter ultra-broad-line object (MUBLO), including protostellar outflow, explosive outflow, a collapsing cloud, an evolved star, a stellar merger, a high-velocity compact cloud, an intermediate-mass black hole, and a background galaxy. Most of these conceptual models are either inconsistent with the data or do not fully explain them. The MUBLO is, at present, an observationally unique object.