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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. 

The central molecular zone (CMZ) of our Galaxy exhibits widespread emission from SiO and various complex organic molecules (COMs), yet the exact origin of such emission is uncertain. Here we report the discovery of a unique class of long (>0.5 pc) and narrow (<0.03 pc) filaments in the emission of SiO 5–4 and eight additional molecular lines, including several COMs, in our ALMA 1.3 mm spectral line observations toward two massive molecular clouds in the CMZ, which we name as slim filaments. However, these filaments are not detected in the 1.3 mm continuum at the 5σlevel. Their line-of-sight velocities are coherent and inconsistent with being outflows. The column densities and relative abundances of the detected molecules are statistically similar to those in protostellar outflows but different from those in dense cores within the same clouds. Turbulent pressure in these filaments dominates over self gravity and leads to hydrostatic inequilibrium, indicating that they are a different class of objects than the dense gas filaments in dynamical equilibrium ubiquitously found in nearby molecular clouds. We argue that these newly detected slim filaments are associated with parsec-scale shocks, likely arising from dynamic interactions between shock waves and molecular clouds. The dissipation of the slim filaments may replenish SiO and COMs in the interstellar medium and lead to their widespread emission in the CMZ. 

Abstract RZ Piscium (RZ Psc) is well known in the variable star field because of its numerous irregular optical dips in the past 5 decades, but the nature of the system is heavily debated in the literature. We present multiyear infrared monitoring data from Spitzer and WISE to track the activities of the inner debris production, revealing stochastic infrared variability as short as weekly timescales that is consistent with destroying a 90 km sized asteroid every year. ALMA 1.3 mm data combined with spectral energy distribution modeling show that the disk is compact (∼0.1–13 au radially) and lacks cold gas. The disk is found to be highly inclined and has a significant vertical scale height. These observations confirm that RZ Psc hosts a close to edge-on, highly perturbed debris disk possibly due to migration of recently formed giant planets that might be triggered by the low-mass companion RZ Psc B if the planets formed well beyond the snowlines. 

Abstract We have comprehensively studied the multiscale physical properties of the massive infrared dark cloud G28.34 (the Dragon cloud) with dust polarization and molecular line data from Planck, FCRAO-14 m, James Clerk Maxwell Telescope, and Atacama Large Millimeter/submillimeter Array. We find that the averaged magnetic fields of clumps tend to be either parallel with or perpendicular to the cloud-scale magnetic fields, while the cores in clump MM4 tend to have magnetic fields aligned with the clump fields. Implementing the relative orientation analysis (for magnetic fields, column density gradients, and local gravity), velocity gradient technique, and modified Davis–Chandrasekhar–Fermi analysis, we find that G28.34 is located in a trans-to-sub-Alfvénic environment; the magnetic field is effectively resisting gravitational collapse in large-scale diffuse gas, but is distorted by gravity within the cloud and affected by star formation activities in high-density regions, and the normalized mass-to-flux ratio tends to increase with increasing density and decreasing radius. Considering the thermal, magnetic, and turbulent supports, we find that the environmental gas of G28.34 is in a supervirial (supported) state, the infrared dark clumps may be in a near-equilibrium state, and core MM4-core4 is in a subvirial (gravity-dominant) state. In summary, we suggest that magnetic fields dominate gravity and turbulence in the cloud environment at large scales, resulting in relatively slow cloud formation and evolution processes. Within the cloud, gravity could overwhelm both magnetic fields and turbulence, allowing local dynamical star formation to happen. 

Context.Traditionally, supersonic turbulence is considered to be one of the most likely mechanisms slowing the gravitational collapse in dense clumps, thereby enabling the formation of massive stars. However, several recent studies have raised differing points of view based on observations carried out with sufficiently high spatial and spectral resolution. These studies call for a re-evaluation of the role turbulence plays in massive star-forming regions. Aims.Our aim is to study the gas properties, especially the turbulence, in a sample of massive star-forming regions with sufficient spatial and spectral resolution, which can both resolve the core fragmentation and the thermal line width. Methods.We observed NH₃metastable lines with the Very Large Array (VLA) to assess the intrinsic turbulence. Results.Analysis of the turbulence distribution histogram for 32 identified NH₃cores reveals the presence of three distinct components. Furthermore, our results suggest that (1) sub- and transonic turbulence is a prevalent (21 of 32) feature of massive star-forming regions and those cold regions are at early evolutionary stage. This investigation indicates that turbulence alone is insufficient to provide the necessary internal pressure required for massive star formation, necessitating further exploration of alternative candidates; and (2) studies of seven multi-core systems indicate that the cores within each system mainly share similar gas properties and masses. However, two of the systems are characterized by the presence of exceptionally cold and dense cores that are situated at the spatial center of each system. Our findings support the hub-filament model as an explanation for this observed distribution. 

We present Atacama Large Millimeter/submillimeter Array Band 6 (1.3 mm) observations of dense cores in three massive molecular clouds within the central molecular zone (CMZ) of the Milky Way, including the Dust Ridge cloud e, Sgr C, and the 20 km s⁻¹cloud, at a spatial resolution of 2000 au. Among the 834 cores identified from the 1.3 mm continuum, we constrain temperatures and linewidths of 253 cores using local thermodynamic equilibrium methods to fit the H₂CO and/or CH₃CN spectra. We determine their masses using the 1.3 mm dust continuum and derived temperatures, and then evaluate their virial parameters using the H₂CO and/or CH₃CN linewidths and construct the core mass functions (CMFs). We find that the contribution of external pressure is crucial for the virial equilibrium of the dense cores in the three clouds, which contrasts with the environment in the Galactic disk where dense cores are already bound, even without the contribution of external pressure. With our new temperature estimates we also find that the CMFs show a Salpeter-like slope in the high-mass (≳3–6M_⊙) end, a change from previous works. Combined with the possible top-heavy initial mass functions (IMFs) in the CMZ, our result suggests that gas accretion and further fragmentation may play important roles in transforming the CMF to the IMF. 

Abstract We use molecular line data from the Atacama Large Millimeter/submillimeter Array, Submillimeter Array, James Clerk Maxwell Telescope, and NANTEN2 to study the multiscale (∼15–0.005 pc) velocity statistics in the massive star formation region NGC 6334. We find that the nonthermal motions revealed by the velocity dispersion function (VDF) stay supersonic over scales of several orders of magnitude. The multiscale nonthermal motions revealed by different instruments do not follow the same continuous power law, which is because the massive star formation activities near central young stellar objects have increased the nonthermal motions in small-scale and high-density regions. The magnitudes of VDFs vary in different gas materials at the same scale, where the infrared dark clump N6334S in an early evolutionary stage shows a lower level of nonthermal motions than other more evolved clumps due to its more quiescent star formation activity. We find possible signs of small-scale-driven (e.g., by gravitational accretion or outflows) supersonic turbulence in clump N6334IV with a three-point VDF analysis. Our results clearly show that the scaling relation of velocity fields in NGC 6334 deviates from a continuous and universal turbulence cascade due to massive star formation activities. 

Abstract We present ALMA dust polarization and molecular line observations toward four clumps (I(N), I, IV, and V) in the massive star-forming region NGC 6334. In conjunction with large-scale dust polarization and molecular line data from JCMT, Planck, and NANTEN2, we make a synergistic analysis of relative orientations between magnetic fields (θ_B), column density gradients (θ_NG), local gravity (θ_LG), and velocity gradients (θ_VG) to investigate the multi-scale (from ∼30 to 0.003 pc) physical properties in NGC 6334. We find that the relative orientation betweenθ_Bandθ_NGchanges from statistically more perpendicular to parallel as column density ( $N_{{\mathrm{H}}_2}$) increases, which is a signature of trans-to-sub-Alfvénic turbulence at complex/cloud scales as revealed by previous numerical studies. Becauseθ_NGandθ_LGare preferentially aligned within the NGC 6334 cloud, we suggest that the more parallel alignment betweenθ_Bandθ_NGat higher $N_{{\mathrm{H}}_2}$is because the magnetic field line is dragged by gravity. At even higher $N_{{\mathrm{H}}_2}$, the angle betweenθ_Bandθ_NGorθ_LGtransits back to having no preferred orientation, or statistically slightly more perpendicular, suggesting that the magnetic field structure is impacted by star formation activities. A statistically more perpendicular alignment is found betweenθ_Bandθ_VGthroughout our studied $N_{{\mathrm{H}}_2}$range, which indicates a trans-to-sub-Alfvénic state at small scales as well, and this signifies that magnetic field has an important role in the star formation process in NGC 6334. The normalized mass-to-flux ratio derived from the polarization-intensity gradient (KTH) method increases with $N_{{\mathrm{H}}_2}$, but the KTH method may fail at high $N_{{\mathrm{H}}_2}$due to the impact of star formation feedback. 

Abstract Characterizing the physical conditions at disk scales in class 0 sources is crucial for constraining the protostellar accretion process and the initial conditions for planet formation. We use ALMA 1.3 and 3 mm observations to investigate the physical conditions of the dust around the class 0 binary IRAS 16293–2422 A down to ∼10 au scales. The circumbinary material’s spectral index,α, has a median of 3.1 and a dispersion of ∼0.2, providing no firm evidence of millimeter-sized grains therein. Continuum substructures with brightness temperature peaks ofT_b∼ 60–80 K at 1.3 mm are observed near the disks at both wavelengths. These peaks do not overlap with strong variations ofα, indicating that they trace high-temperature spots instead of regions with significant optical depth variations. The lower limits to the inferred dust temperature in the hot spots are 122, 87, and 49 K. Depending on the assumed dust opacity index, these values can be several times higher. They overlap with high gas temperatures and enhanced complex organic molecular emission. This newly resolved dust temperature distribution is in better agreement with the expectations from mechanical instead of the most commonly assumed radiative heating. In particular, we find that the temperatures agree with shock heating predictions. This evidence and recent studies highlighting accretion heating in class 0 disks suggest that mechanical heating (shocks, dissipation powered by accretion, etc.) is important during the early stages and should be considered when modeling and measuring properties of deeply embedded protostars and disks.