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  1. Abstract

    The HH 24 complex harbors five collimated jets emanating from a small protostellar multiple system. We have carried out a multiwavelength study of the jets, their driving sources, and the cloud core hosting the embedded stellar system, based on data from the Hubble Space Telescope, Gemini, Subaru, Apache Point Observatory 3.5 m, Karl G. Jansky Very Large Array, and Atacama Large Millimeter/submillimeter Array (ALMA) telescopes. The data show that the multiple system, SSV 63, contains at least 7 sources, ranging in mass from the hydrogen-burning limit to proto-Herbig Ae stars. The stars are in an unstable nonhierarchical configuration, and one member, a borderline brown dwarf, is moving away from the protostellar system with 25 km s−1, after being ejected ∼5800 yr ago as an orphaned protostar. Five of the embedded sources are surrounded by small, possibly truncated, disks resolved at 1.3 mm with ALMA. Proper motions and radial velocities imply jet speeds of 200–300 km s−1. The two main HH 24 jets, E and C, form a bipolar jet system that traces the innermost portions of parsec-scale chains of Herbig–Haro and H2shocks with a total extent of at least 3 pc. H2CO and C18O observations show that the core has been churned and continuously fed by an infalling streamer.13CO and12CO trace compact, low-velocity, cavity walls carved by the jets and an ultracompact molecular outflow from the most embedded object. ChaoticN-body dynamics likely will eject several more of these objects. The ejection of stars from their feeding zones sets their masses. Dynamical decay of nonhierarchical systems can thus be a major contributor to establishing the initial mass function.

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  2. Abstract We report high-resolution ALMA observations toward a massive protostellar core C1-Sa (∼30 M ⊙ ) in the Dragon infrared dark cloud. At the resolution of 140 au, the core fragments into two kernels (C1-Sa1 and C1-Sa2) with a projected separation of ∼1400 au along the elongation of C1-Sa, consistent with a Jeans length scale of ∼1100 au. Radiative transfer modeling using RADEX indicates that the protostellar kernel C1-Sa1 has a temperature of ∼75 K and a mass of 0.55 M ⊙ . C1-Sa1 also likely drives two bipolar outflows, one being parallel to the plane of the sky. C1-Sa2 is not detected in line emission and does not show any outflow activity but exhibits ortho-H 2 D + and N 2 D + emission in its vicinity; thus it is likely still starless. Assuming a 20 K temperature, C1-Sa2 has a mass of 1.6 M ⊙ . At a higher resolution of 96 au, C1-Sa1 begins to show an irregular shape at the periphery, but no clear sign of multiple objects or disks. We suspect that C1-Sa1 hosts a tight binary with inclined disks and outflows. Currently, one member of the binary is actively accreting while the accretion in the other is significantly reduced. C1-Sa2 shows hints of fragmentation into two subkernels with similar masses, which requires further confirmation with higher sensitivity. 
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    Free, publicly-accessible full text available June 1, 2024
  3. Abstract We present Atacama Large Millimeter/submillimeter Array observations of the ∼10,000 au environment surrounding 21 protostars in the Orion A molecular cloud tracing outflows. Our sample is composed of Class 0 to flat-spectrum protostars, spanning the full ∼1 Myr lifetime. We derive the angular distribution of outflow momentum and energy profiles and obtain the first two-dimensional instantaneous mass, momentum, and energy ejection rate maps using our new approach: the pixel flux-tracing technique. Our results indicate that by the end of the protostellar phase, outflows will remove ∼2–4 M ⊙ from the surrounding ∼1 M ⊙ low-mass core. These high values indicate that outflows remove a significant amount of gas from their parent cores and continuous core accretion from larger scales is needed to replenish core material for star formation. This poses serious challenges to the concept of cores as well-defined mass reservoirs , and hence to the simplified core-to-star conversion prescriptions. Furthermore, we show that cavity opening angles, and momentum and energy distributions all increase with protostar evolutionary stage. This is clear evidence that even garden-variety protostellar outflows: (a) effectively inject energy and momentum into their environments on 10,000 au scales, and (b) significantly disrupt their natal cores, ejecting a large fraction of the mass that would have otherwise fed the nascent star. Our results support the conclusion that protostellar outflows have a direct impact on how stars get their mass, and that the natal sites of individual low-mass star formation are far more dynamic than commonly accepted theoretical paradigms. 
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  4. Abstract In this paper, we carry out a pilot parameter exploration for the collision-induced magnetic reconnection (CMR) mechanism that forms filamentary molecular clouds. Following Kong et al., we utilize Athena++ to model CMR in the context of resistive magnetohydrodynamics (MHD), considering the effect from seven physical conditions, including the ohmic resistivity ( η ), the magnetic field ( B ), the cloud density ( ρ ), the cloud radius R , the isothermal temperature T , the collision velocity v x , and the shear velocity v z . Compared to their fiducial model, we consider a higher and a lower value for each one of the seven parameters. We quantify the exploration results with five metrics, including the density probability distribution function ( ρ -PDF), the filament morphology (250 μ m dust emission), the B – ρ relation, the dominant fiber width, and the ringiness that describes the significance of the ringlike substructures. The exploration forms straight and curved CMR filaments with rich substructures that are highly variable in space and time. The variation translates to fluctuation in all five metrics, reflecting the chaotic nature of magnetic reconnection in CMR. A temporary B ∝ ρ relation is noticeable during the first 0.6 Myr. Overall, the exploration provides useful initial insights into the CMR mechanism. 
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  5. Abstract The mass distribution of dense cores is a potential key to understanding the process of star formation. Applying dendrogram analysis to the CARMA-NRO Orion C 18 O ( J = 1–0) data, we identify 2342 dense cores, about 22% of which have virial ratios smaller than 2 and can be classified as gravitationally bound cores. The derived core mass function (CMF) for bound starless cores that are not associate with protostars has a slope similar to Salpeter’s initial mass function (IMF) for the mass range above 1 M ⊙ , with a peak at ∼0.1 M ⊙ . We divide the cloud into four parts based on decl., OMC-1/2/3, OMC-4/5, L1641N/V380 Ori, and L1641C, and derive the CMFs in these regions. We find that starless cores with masses greater than 10 M ⊙ exist only in OMC-1/2/3, whereas the CMFs in OMC-4/5, L1641N, and L1641C are truncated at around 5–10 M ⊙ . From the number ratio of bound starless cores and Class II objects in each subregion, the lifetime of bound starless cores is estimated to be 5–30 freefall times, consistent with previous studies for other regions. In addition, we discuss core growth by mass accretion from the surrounding cloud material to explain the coincidence of peak masses between IMFs and CMFs. The mass accretion rate required for doubling the core mass within a core lifetime is larger than that of Bondi–Hoyle accretion by a factor of order 2. This implies that more dynamical accretion processes are required to grow cores. 
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  6. Abstract We present a comprehensive analysis of the evolution of envelopes surrounding protostellar systems in the Perseus molecular cloud using data from the MASSES survey. We focus our attention to the C 18 O(2–1) spectral line, and we characterize the shape, size, and orientation of 54 envelopes and measure their fluxes, velocity gradients, and line widths. To look for evolutionary trends, we compare these parameters to the bolometric temperature T bol , a tracer of protostellar age. We find evidence that the angular difference between the elongation angle of the C 18 O envelope and the outflow axis direction generally becomes increasingly perpendicular with increasing T bol , suggesting the envelope evolution is directly affected by the outflow evolution. We show that this angular difference changes at T bol = 53 ± 20 K, which includes the conventional delineation between Class 0 and I protostars of 70 K. We compare the C 18 O envelopes with larger gaseous structures in other molecular clouds and show that the velocity gradient increases with decreasing radius ( ∣  ∣ ∼ R − 0.72 ± 0.06 ). From the velocity gradients we show that the specific angular momentum follows a power-law fit J / M ∝ R 1.83±0.05 for scales from 1 pc down to ∼500 au, and we cannot rule out a possible flattening out at radii smaller than ∼1000 au. 
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  7. Abstract We use 3 mm continuum NOrthern Extended Millimeter Array and NH 3 Very Large Array observations toward the First Hydrostatic Core (FHSC) candidate CB 17 MMS in order to reveal the dust structure and gas properties to 600–1100 au scales and to constrain its evolutionary stage. We do not detect any compact source at the previously identified 1.3 mm point source, despite expecting a minimum signal-to-noise ratio of 9. The gas traced by NH 3 exhibits subsonic motions, with an average temperature of 10.4 K. A fit of the radial column density profile derived from the ammonia emission finds a flat inner region of radius ∼1800 au and a central density of ∼6 × 10 5 cm −3 . Virial and density structure analysis reveals the core is marginally bound ( α vir = 0.73). The region is entirely consistent with that of a young starless core, hence ruling out CB 17 MMS as an FHSC candidate. Additionally, the core exhibits a velocity gradient aligned with the major axis, showing an arc-like structure in the position–velocity diagram and an off-center region with high velocity dispersion, caused by two distinct velocity peaks. These features could be due to interactions with the nearby outflow, which appears to deflect due to the dense gas near the NH 3 column density peak. We investigate the specific angular momentum profile of the starless core, finding that it aligns closely with previous studies of similar radial profiles in Class 0 sources. This similarity to more evolved objects suggests that motions at 1000 au scales are determined by large-scale dense cloud motions, and may be preserved throughout the early stages of star formation. 
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    ABSTRACT We present ALMA 3 mm molecular line and continuum observations with a resolution of ∼3.5 arcsec towards five first hydrostatic core (FHSC) candidates (L1451-mm, Per-bolo 58, Per-bolo 45, L1448-IRS2E, and Cha-MMS1). Our goal is to characterize their envelopes and identify the most promising sources that could be bona fide FHSCs. We identify two candidates that are consistent with an extremely young evolutionary state (L1451-mm and Cha-MMS1), with L1451-mm being the most promising FHSC candidate. Although our envelope observations cannot rule out Cha-MMS1 as an FHSC yet, the properties of its CO outflow and SED published in recent studies are in better agreement with the predictions for a young protostar. For the remaining three sources, our observations favour a pre-stellar nature for Per-bolo 45 and rule out the rest as FHSC candidates. Per-bolo 58 is fully consistent with being a Class 0, while L1448 IRS2E shows no emission of high-density tracers (NH2D and N2H+) at the location of the previously identified compact continuum source, which is also undetected in our observations. Thus, we argue that there is no embedded source at the presumptive location of the FHSC candidate L1448 IRS2E. We propose instead that what was thought to be emission from the presumed L1448 IRS2E outflow corresponds to outflow emission from a nearby Class 0 system, deflected by the dense ambient material. We compare the properties of the FHSC candidates studied in this work and the literature, which shows that L1451-mm appears as possibly the youngest source with a confirmed outflow. 
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