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Abstract The most direct method of measuring the star formation rate is with young stellar objects (YSOs), but this requires high-resolution observations and high-quality models. Using the latest YSO radiation transfer and stellar evolution models, we have developed a population synthesis code that generates model YSO populations that can be observed by JWST. We combine these model populations with principal component analysis (PCA) and maximum likelihood fitting to create a complete framework for predicting the ages and masses of YSO populations. We dub this combination of population synthesis and PCA as PxP, and show that it is effective at predicting masses and ages with self-fitting tests. We apply PxP to Spitzer Space Telescope–identified YSOs in N44 and find a mass of (1.1 ± 0.1) × 10⁴ M_⊙and an age of $0.74_{-0.03}^{+0.06}$ Myr, consistent with previous work. Next, we identify 112 YSO candidates in the archival JWST observations of NGC 604. Applying PxP to this newly identified population we find a mass of (2.2 ± 0.2) × 10⁴ M_⊙and an age of 0.62 ± 0.01 Myr. This first look at this framework demonstrates its effectiveness with a specific set of models and leaves clear opportunities for future exploration. PxP allows us to directly determine the recent (<3 Myr) star formation history, giving an unprecedented look at the effect of the large-scale environment on individual star formation. 

Abstract 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. 

Abstract Measuring properties of young stellar objects (YSOs) is necessary for probing the pre-main-sequence evolution of stars. As YSOs exhibit complex geometry, measurement generally entails comparing observed radiation to template populations of radiative-transfer model YSO spectral energy distributions (SEDs). Due to uncertainty on the precise mechanics of star formation, the properties inferred for YSOs using these models often depend strongly on the assumed accretion history. We develop a framework for predicting observable properties of YSOs that is agnostic to the underlying accretion history, enabling comparison between theories. This framework links a set of radiative-transfer SEDs with protostellar evolutionary tracks to create models of evolving YSOs. Unlike previous works, we directly relate evolution models to observables through theoretical physical parameters rather than through intermediate, observationally derived analogs. We make flux predictions for YSOs corresponding to stars with birth masses from 0.2 to 50M_⊙during their accretion phase following isothermal-sphere, turbulent-core, and competitive accretion histories, showing that these histories may be observationally distinguished by examining the 100μm and 3 mm fluxes of a YSO. We discuss the impact of dust models and parameter ranges on the output of radiative-transfer simulations through a comparison to another SED model grid. We quantify the degree of confusion between YSO Stages and Classes across a wide range of physical scenarios; for each, we calculate confusion matrices that enable inference of the number of objects of a given Stage from an observed population. Finally, we critically examine the physical significance of various literature Stage and Class definitions. 

Abstract We present near-infraredJHK_sand narrowband H₂(1–0) photometric observations of the W51A region, obtained with GTC EMIR, aiming to characterize its young stellar population, and provide mass estimates for individual cluster members and the protoclusters. Our observations reveal over 3000 new sources, out of which 88 are located in the protoclusters, W51 IRS2 and W51 Main. The average extinction (A_V), measured from theJ–Hcolor, of sources is 19A_Vin W51 IRS2 and 14A_Vin W51 Main. We document 17 new instances of H₂emission in the region by utilizing observations from the H₂(1–0) narrowband filter. Despite limited completeness, we estimated masses for each cluster member and estimated the total cluster mass to be in the range of 900–4700M_⊙for W51 IRS2 and 500–2700M_⊙for W51 Main, using an assumed age range of 1–3 Myr. We measured the initial mass function (IMF) in the proto-clusters assuming a range of ages from 1–3 Myr and found that the IMF slopes for both protoclusters are consistent with the Salpeter IMF in the mass rangeM ≳ 8M_⊙within 1σ–2σ. 

Abstract The Milky Way’s Central Molecular Zone (CMZ) is the largest concentration of dense molecular gas in the Galaxy, the structure of which is shaped by the complex interplay between Galactic-scale dynamics and extreme physical conditions. Understanding the 3D geometry of this gas is crucial, as it determines the locations of star formation and subsequent feedback. We present a catalog of clouds in the CMZ using Herschel data. Using archival data from the APEX and MOPRA CMZ surveys, we measure averaged kinematic properties of the clouds at 1 and 3 mm. We use archival ATCA data of the H₂CO (1_1,0–1_1,1) 4.8 GHz line to search for absorption towards the clouds, and 4.85 GHz Green Bank Telescope (GBT)C-band data to measure the radio continuum emission. We measure the absorption against the continuum to provide new constraints for the line-of-sight positions of the clouds relative to the Galactic Center, and find a highly asymmetric distribution, with most clouds residing in front of the Galactic Center. The results are compared with different orbital models, and we introduce a revised toy model of a vertically oscillating closed elliptical orbit. We find that most models describe the position–position–velocity structure of the gas reasonably well, but find significant inconsistencies in all cases regarding the near versus far placement of individual clouds. Our results highlight that the CMZ is likely more complex than can be captured by these simple geometric models, along with the need for new data to provide further constraints on the true 3D structure of the CMZ. 

Abstract A comprehensive 3D model of the central 300 pc of the Milky Way, the Central Molecular Zone (CMZ) is of fundamental importance in understanding energy cycles in galactic nuclei, since the 3D structure influences the location and intensity of star formation, feedback, and black hole accretion. Current observational constraints are insufficient to distinguish between existing 3D models. Dust extinction is one diagnostic tool that can help determine the location of dark molecular clouds relative to the bright Galactic Center emission. By combining Herschel and Spitzer observations, we developed three new dust extinction techniques to estimate the likely near/far locations for each cloud in the CMZ. We compare our results to four geometric CMZ orbital models. Our extinction methods show good agreement with each other, and with results from spectral line absorption analysis from Walker et al. Our near/far results for CMZ clouds are inconsistent with a projected version of the Y. Sofue two-spiral-arms model, and show disagreement in position–velocity space with the S. Molinari et al. closed elliptical orbit. Our results are in reasonable agreement with the J. M. D. Kruijssen et al. open streams. We find that a simplified toy-model elliptical orbit that conserves angular momentum shows promising fits in both position–position and position–velocity space. We conclude that all current CMZ orbital models lack the complexity needed to describe the motion of gas in the CMZ, and further work is needed to construct a complex orbital model to accurately describe gas flows in the CMZ. 

Abstract The Central Molecular Zone (CMZ) is the way station at the heart of our Milky Way Galaxy, connecting gas flowing in from Galactic scales with the central nucleus. Key open questions remain about its 3D structure, star formation properties, and role in regulating this gas inflow. In this work, we identify a hierarchy of discrete structures in the CMZ using column density maps from Paper I (C. Battersby et al.) We calculate the physical (N(H₂),T_dust, mass, radius) and kinematic (HNCO, HCN, and HC₃N moments) properties of each structure as well as their bolometric luminosities and star formation rates. We compare these properties with regions in the Milky Way disk and external galaxies. Despite the fact that the CMZ overall is well below the Gao-Solomon dense gas star formation relation (and in modest agreement with the Schmidt–Kennicutt relation), individual structures on the scale of molecular clouds generally follow these star formation relations and agree well with other Milky Way and extragalactic regions. We find that individual CMZ structures require a large external pressure (P_e/k_B> 10⁷⁻⁹K cm⁻³) to be considered bound; however, simple estimates suggest that most CMZ molecular-cloud-sized structures are consistent with being in pressure-bounded virial equilibrium. We perform power-law fits to the column density probability distribution functions of the inner 100 pc, SgrB2, and the outer 100 pc of the CMZ as well as several individual molecular cloud structures and find generally steeper power-law slopes (−9 <α< −2) compared with the literature (−6 <α< −1). 

Abstract We present JWST-NIRCam narrowband, 4.05μm Brαimages of the Sgr C Hiiregion, located in the central molecular zone (CMZ) of the Galaxy. Unlike any Hiiregion in the solar vicinity, the Sgr C plasma is dominated by filamentary structure in both Brαand the radio continuum. Some bright filaments, which form a fractured arc with a radius of about 1.85 pc centered on the Sgr C star-forming molecular clump, likely trace ionization fronts. The brightest filaments form a “π-shaped” structure in the center of the Hiiregion. Fainter filaments radiate away from the surface of the Sgr C molecular cloud. The filaments are emitting optically thin free–free emission, as revealed by spectral index measurements from 1.28 GHz (MeerKAT) to 97 GHz (Atacama Large Millimeter/submillimeter Array). But, the negative in-band 1 to 2 GHz spectral index in the MeerKAT data alone reveals the presence of a nonthermal component across the entire Sgr C Hiiregion. We argue that the plasma flow in Sgr C is controlled by magnetic fields, which confine the plasma to ropelike filaments or sheets. This results in the measured nonthermal component of low-frequency radio emission plasma, as well as a plasmaβ(thermal pressure divided by magnetic pressure) below 1, even in the densest regions. We speculate that all mature Hiiregions in the CMZ, and galactic nuclei in general, evolve in a magnetically dominated, low plasmaβregime. 

Abstract We present James Webb Space Telescope (JWST) Near Infrared Camera observations of the massive star-forming molecular cloud Sagittarius C (Sgr C) in the Central Molecular Zone (CMZ). In conjunction with ancillary mid-IR and far-IR data, we characterize the two most massive protostars in Sgr C via spectral energy distribution (SED) fitting, estimating that they each have current masses ofm_*∼ 20M_⊙and surrounding envelope masses of ∼100M_⊙. We report a census of lower-mass protostars in Sgr C via a search for infrared counterparts to millimeter continuum dust cores found with the Atacama Large Millimeter/submillimeter Array (ALMA). We identify 88 molecular hydrogen outflow knot candidates originating from outflows from protostars in Sgr C, the first such unambiguous detections in the infrared in the CMZ. About a quarter of these are associated with flows from the two massive protostars in Sgr C; these extend for over 1 pc and are associated with outflows detected in ALMA SiO line data. An additional ∼40 features likely trace shocks in outflows powered by lower-mass protostars throughout the cloud. We report the discovery of a new star-forming region hosting two prominent bow shocks and several other line-emitting features driven by at least two protostars. We infer that one of these is forming a high-mass star given an SED-derived mass ofm_*∼ 9M_⊙and associated massive (∼90M_⊙) millimeter core and water maser. Finally, we identify a population of miscellaneous molecular hydrogen objects that do not appear to be associated with protostellar outflows. 

Abstract We present Atacama Large Millimeter/submillimeter Array observations of SiO, SiS, H₂O, NaCl, and SO line emission at ∼30–50 mas resolution. These images map the molecular outflow and disk of Orion Source I (SrcI) on ∼12–20 au scales. Our observations show that the flow of material around SrcI creates a turbulent boundary layer in the outflow from SrcI, which may dissipate angular momentum in the rotating molecular outflow into the surrounding medium. Additionally, the data suggest that the proper motion of SrcI may have a significant effect on the structure and evolution of SrcI and its molecular outflow. As the motion of SrcI funnels material between the disk and the outflow, some material may be entrained into the outflow and accrete onto the disk, creating shocks that excite the NaCl close to the disk surface.