Taehwa Yoo

The central molecular zone (CMZ) resides in the inner few hundred parsecs of our Galaxy, and despite being the largest reservoir of dense molecular gas in the Milky Way, it has a relatively low present-day star formation rate (SFR) of ∼0.08 M _⊙ yr ⁻¹ . Continuum and spectral line observations from the Atacama Large Millimeter/submillimeter Array (ALMA) CMZ Exploration Survey (ACES) provide the first full-coverage, high-resolution map of the inner 200 pc of the CMZ at 3 mm. In this paper we present the ACES catalog of compact continuum sources, the most complete catalog of potential sites of star formation in the CMZ to date. Using an automated dendrogram-based source extraction procedure in combination with a by-eye morphological classification scheme, we produce a “full” catalog of 1735 detections in total. Additionally, we use spectral index measurements to generate a “filtered” catalog of 567 sources with minimal contamination from nonthermal filaments and extended free–free emission. We find that 359 (∼63%) of the filtered catalog sources are located at column densities 23 cm ⁻² , outside of the densest molecular cloud regions, 195 of which have not been identified in previous surveys. After cross-referencing with various catalogs generated from data at different wavelengths, we consider it likely that many of these newly discovered detections are produced by pre-/protostellar sources or compact H ii regions.

Aims. The gravoturbulent fragmentation of the interstellar medium is expected to create a hierarchical cascade of cloud structures, crossing the scales from core to disk. We aim to predict how the currently observed top-heavy core mass function (CMF) in the massive protocluster W43-MM1 evolves due to core subfragmentation. Methods. We used the getsf algorithm to extract sources in five ALMA images of W43-MM1 at 3 mm, with a spatial resolution ranging from 14 kau to 270 au. Then, we applied FAMILY , a graph-theory-based analysis tool, to create and characterize networks of nested sources in W43-MM1. We compared the hierarchical fragmentation cascade of W43-MM1 to those measured in the NGC 2264 protocluster and in synthetic images of an Orion-like protocluster simulated by magneto-hydrodynamical calculations. Results. Assuming self-similarity, we measured a small fractality index of F _3D = 1.19 ± 0.10 in W43-MM1, which means that, on average, a cloud structure fragments into only 1.19 fragments each time the physical scale decreases by a factor of two. In line with values measured above the core scale in the NGC 2264 and synthetic protoclusters, the W43-MM1 fractality index increases by about 30% at larger scales. We also estimate an imbalanced mass partition between siblings, with two-thirds of the mass of siblings at a given scale belonging to the dominant sibling. The mass transfer efficiency, computed from one physical scale to another, is high and corresponds to a core formation efficiency (CFE) from 2400 au cores to 200 au seeds of ∼16%. Conclusions. Based on the fractality and efficiency values measured in W43-MM1, the gravoturbulent model by Thomasson predicts that its fragmentation below ∼14 kau is not driven by turbulence but by gravity. Using these parameters and the measured mass partition, we demonstrate that the seed mass function, from which the initial mass function (IMF) emerges, has a high-mass end that remains top-heavy. Therefore, based on our current assumptions, core subfragmentation in W43-MM1, and perhaps more broadly in massive Galactic protoclusters, plays a minimal role in shaping the high-mass slope of the IMF.

We report JWST NIRCAM and MIRI observations of Sgr B2, one of the most active sites of star formation in the Galaxy. These observations, using 14 filters spanning 1.5–25 μ m, have revealed a multilayered and highly structured cloud that contains both a revealed, low-extinction and hidden, high-extinction population of massive stars. JWST has detected new candidate H ii regions around massive stars previously missed by radio telescopes. MIRI has detected radiation escaping from the forming massive cluster Sgr B2 N along its outflow cavities, demonstrating that infrared radiation finds geometric escape routes even in the densest, most heavily embedded regions in the universe. JWST further highlights the gas asymmetry in the cloud, showing a sharp, straight cutoff along the eastern cloud edge. Despite the great sensitivity of these observations, no extended population of young stellar objects has been detected, placing a limit on their minimum extinction; this result hints that star formation has only just begun in the cloud. Together, these results suggest that, despite already holding the crown for one of the most actively star-forming clouds, we have underestimated the total star formation in Sgr B2. JWST unveils previously hidden massive stars and ionized structures, offering the clearest view yet of how stars form under some of the most extreme Galactic conditions.

Context . In the solar neighbourhood, the initial mass function (IMF) is canonically described by the Salpeter power-law slope for the high-mass range. As stars inherit their mass from their environment, their IMF may directly result from a core mass function (CMF) through accretion, gravitational collapse, and fragmentation. This inheritance implies that the mass of the gaseous fragments may be connected to the properties of clustered and multiple stellar systems. In these systems, mass and multiplicity are related, and this is supported by the fact that more massive primaries are observed more frequently in multiple systems. Aims . We aim to (i) quantify the influence of the hierarchical fragmentation of cores on the resulting IMF and (ii) determine the consequences of this fragmentation on the multiplicity of the stellar systems. Methods . To do so, we employed a scale-free hierarchical fragmentation model to investigate the stochastic fragmentation of the 2.5 kAU cores of the W43-MM2&MM3 molecular cloud, whose CMF is top-heavy. We also used this model to quantify the influence of deterministic mass-dependent fragmentation processes. Results . The hierarchical fragmentation of gas clumps shifts the CMF towards a lower mass range and can modify its shape. The shift is quantified by both the number of fragments produced at each level of fragmentation and the mass the fragments inherit from their parental core. Starting from the top-heavy power-law CMF observed in W43-MM2&MM3, we show that at least four levels of hierarchical fragmentation are required to generate the turn-over peak of the canonical IMF. Within a radius of 0.2–2.5 kAU, massive stars ( M > 10 M _⊙ ) have on average 0.9 companions, five times fewer than low-mass stars ( M < 0.1 M _⊙ ), which are less dynamically stable and should disperse. We show that a universal IMF can emerge from mass-dependent fragmentation processes provided that more massive cores produce fewer fragments compared to lower mass cores and transfer their mass less efficiently to their fragments. Conclusions . Hierarchical fragmentation alone, however, cannot reconcile a universal IMF with observed stellar multiplicity. We propose that fragmentation is not scale-free but operates in two distinct regimes: a mass-dependent phase establishing the Salpeter slope and a mass-independent phase setting the turn-over. Our framework provides a way to compare core sub-fragmentation in various star-forming regions and numerical simulations.

We present observations of the W51A region, including the massive protoclusters W51-E and W51-IRS2, with JWST in 10 NIRCam and 5 MIRI filters. In this work, we highlight the most novel features apparent in these images and compare them with other multiwavelength images. The broad view of the NIRCam/MIRI images of the W51A region shows that areas dominated by warm dust and ionized gas are distinct from those dominated by polycyclic aromatic hydrocarbons. The high angular resolution of the JWST images resolves dust filaments in high contrast, revealing geometrically converging features feeding W51-E and a cavity around W51-IRS2. This picture adds support to the hypothesis that feedback from W51-IRS2 is suppressing further gas infall onto the protocluster, while by contrast, gas is still accreting onto W51-E. Comparing the NIRCam and MIRI images to Atacama Large Millimeter/submillimeter Array (ALMA) data, we find 24 sources detected by both JWST and ALMA, accounting for only ∼10% of the ALMA sources; the rest are too embedded or too cool to be detected by JWST. A knot of [Fe ii ] and H ₂ emission north of W51-IRS2, previously detected in ground-based images, reveals peculiarly bright and compact peaks detected in all JWST bands. The knot is likely the most energetic example of a protostellar jet driven by a massive star impacting dense interstellar medium. The new images provide a complementary view to the previous long-wavelength perspective on this 4 × 8 pc area of one of the most active star-forming regions in our Galaxy, revealing new mysteries to be further explored.

We present a study of core fragmentation in the W51-E and W51-IRS2 protoclusters in the W51 high-mass star-forming region. The identification of core fragmentation is achieved by the spatial correspondence of cores and compact sources, which are detected in the short- (low-resolution) and the long-baseline (high-resolution) continuum images from the Atacama Large Millimeter/submillimeter Array (ALMA) in Bands 3 (3 mm) and 6 (1.3 mm), respectively. We characterize the compact sources found in the long-baseline images, and conclude that the compact sources are pre-/protostellar objects (PPOs) that are either prestellar dust cores or dust disks or envelopes around protostars. The observed trend of core fragmentation in W51 is that (i) massive cores host more PPOs, (ii) bright PPOs are preferentially formed in massive cores, and (iii) equipartition of flux between PPOs is uncommon. The thermal Jeans masses of the parent cores are insufficient to explain the masses of their fragments, and this trend is more prominent for high-mass cores. We also find that unfragmented cores are large, less massive, and less dense than fragmented cores.

We present a study of core fragmentation in the W51-E and W51-IRS2 protoclusters in the W51 high-mass star-forming region. The identification of core fragmentation is achieved by the spatial correspondence of cores and compact sources which are detected in the short (low resolution) and the long baseline (high resolution) continuum images with the Atacama Large Millimeter/submillimeter Array (ALMA) in Bands 3 (3 mm) and 6 (1.3 mm), respectively. We characterize the compact sources found in the long baseline image, and conclude that the compact sources are pre/protostellar objects (PPOs) that are either prestellar dust cores or dust disks or envelopes around protostars. The observed trend of core fragmentation in W51 is that (i) massive cores host more PPOs, (ii) bright PPOs are preferentially formed in massive cores, (iii) equipartition of flux between PPOs is uncommon. Thermal Jeans masses of parent cores are insufficient to explain the masses of their fragments, and this trend is more prominent at high-mass cores. We also find that unfragmented cores are large, less massive, and less dense than fragmented cores.

Context. High-mass prestellar cores are extremely rare. Until recently, the search for such objects has been hampered by small sample sizes, leading to large ambiguities in their lifetimes and hence the conditions in the cores in which high-mass stars (≳8 M_⊙) form. Aims. Here we leverage the large sample (~580 cores) detected in the ALMA-IMF survey to identify both protostellar and prestellar cores to estimate their relative lifetimes. Methods. We used CO and SiO outflows to identify protostellar cores. We present a new automated method based on aperture line emission and background subtraction to systematically detect outflows associated with each of the 141 most massive cores. Massive cores that are not driving an outflow in either tracer are identified as prestellar. After careful scrutiny of the sample, we derived statistical lifetime estimates for the prestellar phase. Results. Our automated method allows the efficient detection of CO and SiO outflows and has a performance efficiency similar to that of more cumbersome classical techniques. We identified 30 likely prestellar cores with M≳ 8 M_⊙, of which 12 have core masses M≳ 16 M_⊙. The latter group contains the best candidates for high-mass star precursors. Moreover, most of these 12 high-mass prestellar cores are located inside the crowded central regions of the protoclusters, where most high-mass stars are expected to form. Using the relative ratios of prestellar to protostellar cores, and assuming a high-mass protostellar lifetime of 300 kyr, we derive a prestellar core lifetime of 120 kyr to 240 kyr for cores with masses 8 M_⊙ < M < 16 M_⊙. For 30 M_⊙ < M < 55 M_⊙, the lifetimes range from 50 kyr to 100 kyr. The spread in timescales reflects different assumptions for scenarios for the mass reservoir evolution. These timescales are remarkably long compared to the 4 kyr to 15 kyr free-fall time of the cores. Hence, we suggest that high-mass cores live ~10 to 30 free-fall times, with a tentative trend of a slight decrease with core mass. Such high ratios suggest that the collapse of massive cores is slowed down by non-thermal support of turbulent, magnetic or rotational origin at or below the observed scale.

Context. The ALMA-IMF Large Program imaged 15 massive protoclusters down to a resolution of ~2 kau scales, identifying about 10³ star-forming cores. The mass and luminosity of these cores, which are fundamental physical characteristics, are difficult to determine, a problem greatly exacerbated at the distances ≥2 kpc of ALMA-IMF protoclusters. Aims. We combined new datasets and radiative transfer modeling to characterize these cores. We estimated their mass-averaged temperature and the masses these estimates imply. For one-sixth of the sample, we measured the bolometric luminosities, implementing deblending corrections when necessary. Methods. We used spectral energy distribution (SED) analysis obtained with the point process mapping (PPMAP) Bayesian procedure, which aims to preserve the best angular resolution of the input data. We extrapolated the luminosity and dust temperature images provided by PPMAP at 2.5″ resolution to estimate those of individual cores, which were identified at higher angular resolution. To do this, we applied approximate radiative transfer relationships between the luminosity of a protostar and the temperature of its surrounding envelope and between the external heating of prestellar cores and their temperatures. Results. For the first time, we provide data-informed estimates of dust temperatures for 882 cores identified with ALMA-IMF: 17–31 K and 28–79 K (5th and 95th percentiles, up to 127 K) for the 616 prestellar and 266 protostellar cores, respectively. We also measured protostellar luminosities spanning 20–80 000 L_⊙. Conclusions. Dust temperatures previously estimated from SED-based analyses at a comparatively lower resolution validate our method. For hot cores, on the other hand, we estimated systematically lower temperatures than studies based on complex organic molecules. We established a mass-luminosity evolutionary diagram, for the first time at the core spatial resolution and for a large sample of intermediate- to high-mass protostellar cores. The ALMA-IMF data favor a scenario in which protostars accrete their mass from a larger mass reservoir than their host cores.