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1. Star formation in ‘the Brick’: ALMA reveals an active protocluster in the Galactic centre cloud G0.253+0.016

   https://doi.org/10.1093/mnras/stab415  

   Walker, Daniel L ; Longmore, Steven N ; Bally, John ; Ginsburg, Adam ; Diederik Kruijssen, J M ; Zhang, Qizhou ; Henshaw, Jonathan D ; Lu, Xing ; Alves, João ; Barnes, Ashley T ; et al ( March 2021 , Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT G0.253+0.016, aka ‘the Brick’, is one of the most massive (>105 M⊙) and dense (>104 cm−3) molecular clouds in the Milky Way’s Central Molecular Zone. Previous observations have detected tentative signs of active star formation, most notably a water maser that is associated with a dust continuum source. We present ALMA Band 6 observations with an angular resolution of 0.13 arcsec (1000 AU) towards this ‘maser core’ and report unambiguous evidence of active star formation within G0.253+0.016. We detect a population of eighteen continuum sources (median mass ∼2 M⊙), nine of which are driving bi-polar molecular outflows as seen via SiO (5–4) emission. At the location of the water maser, we find evidence for a protostellar binary/multiple with multidirectional outflow emission. Despite the high density of G0.253+0.016, we find no evidence for high-mass protostars in our ALMA field. The observed sources are instead consistent with a cluster of low-to-intermediate-mass protostars. However, the measured outflow properties are consistent with those expected for intermediate-to-high-mass star formation. We conclude that the sources are young and rapidly accreting, and may potentially form intermediate- and high-mass stars in the future. The masses and projected spatial distribution of the cores are generally consistent with thermal fragmentation, suggesting that the large-scale turbulence and strong magnetic field in the cloud do not dominate on these scales, and that star formation on the scale of individual protostars is similar to that in Galactic disc environments. 

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2. ALMA-IMF: I. Investigating the origin of stellar masses: Introduction to the Large Program and first results

   https://doi.org/10.1051/0004-6361/202141677  

   Motte, F. ; Bontemps, S. ; Csengeri, T. ; Pouteau, Y. ; Louvet, F. ; Stutz, A. M. ; Cunningham, N. ; López-Sepulcre, A. ; Brouillet, N. ; Galván-Madrid, R. ; et al ( June 2022 , Astronomy & Astrophysics)

   Aims. Thanks to the high angular resolution, sensitivity, image fidelity, and frequency coverage of ALMA, we aim to improve our understanding of star formation. One of the breakthroughs expected from ALMA, which is the basis of our Cycle 5 ALMA-IMF Large Program, is the question of the origin of the initial mass function (IMF) of stars. Here we present the ALMA-IMF protocluster selection, first results, and scientific prospects. Methods. ALMA-IMF imaged a total noncontiguous area of ~53 pc 2 , covering extreme, nearby protoclusters of the Milky Way. We observed 15 massive (2.5 −33 × 10 3 M ⊙ ), nearby (2−5.5 kpc) protoclusters that were selected to span relevant early protocluster evolutionary stages. Our 1.3 and 3 mm observations provide continuum images that are homogeneously sensitive to point-like cores with masses of ~0.2 M ⊙ and ~0.6 M ⊙ , respectively, with a matched spatial resolution of ~2000 au across the sample at both wavelengths. Moreover, with the broad spectral coverage provided by ALMA, we detect lines that probe the ionized and molecular gas, as well as complex molecules. Taken together, these data probe the protocluster structure, kinematics, chemistry, and feedback over scales from clouds to filaments to cores. Results. We classify ALMA-IMF protoclusters as Young (six protoclusters), Intermediate (five protoclusters), or Evolved (four proto-clusters) based on the amount of dense gas in the cloud that has potentially been impacted by H  II region(s). The ALMA-IMF catalog contains ~700 cores that span a mass range of ~0.15 M ⊙ to ~250 M ⊙ at a typical size of ~2100 au. We show that this core sample has no significant distance bias and can be used to build core mass functions (CMFs) at similar physical scales. Significant gas motions, which we highlight here in the G353.41 region, are traced down to core scales and can be used to look for inflowing gas streamers and to quantify the impact of the possible associated core mass growth on the shape of the CMF with time. Our first analysis does not reveal any significant evolution of the matter concentration from clouds to cores (i.e., from 1 pc to 0.01 pc scales) or from the youngest to more evolved protoclusters, indicating that cloud dynamical evolution and stellar feedback have for the moment only had a slight effect on the structure of high-density gas in our sample. Furthermore, the first-look analysis of the line richness toward bright cores indicates that the survey encompasses several tens of hot cores, of which we highlight the most massive in the G351.77 cloud. Their homogeneous characterization can be used to constrain the emerging molecular complexity in protostars of high to intermediate masses. Conclusions. The ALMA-IMF Large Program is uniquely designed to transform our understanding of the IMF origin, taking the effects of cloud characteristics and evolution into account. It will provide the community with an unprecedented database with a high legacy value for protocluster clouds, filaments, cores, hot cores, outflows, inflows, and stellar clusters studies. 

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3. Living on the edge of the Milky Way's central molecular zone: G1.3 is the more likely candidate for gas accretion into the CMZ

   https://doi.org/10.1051/0004-6361/202244870  

   Busch, Laura A. ; Riquelme, Denise ; Güsten, Rolf ; Menten, Karl M. ; Pillai, Thushara G. ; Kauffmann, Jens ( December 2022 , Astronomy & Astrophysics)

   Context. The 1°.3 (G1.3) and 1°.6 (G1.6) cloud complexes in the central molecular zone (CMZ) of our Galaxy have been proposed to possibly reside at the intersection region of the X1 and X2 orbits for several reasons. This includes the detection of co-spatial low- and high-velocity clouds, high velocity dispersion, high fractional molecular abundances of shock-tracing molecules, and kinetic temperatures that are higher than for usual CMZ clouds. Aims. By investigating the morphology and deriving physical properties as well as chemical composition, we want to find the origin of the turbulent gas and, in particular, whether evidence of an interaction between clouds can be identified. Methods. We mapped both cloud complexes in molecular lines in the frequency range from 85 to 117 GHz with the IRAM 30 m telescope. The APEX 12m telescope was used to observe higher frequency transitions between 210 and 475 GHz from selected molecules that are emitted from higher energy levels. We performed non-local thermodynamic equilibrium (non-LTE) modelling of the emission of an ensemble of CH 3 CN lines to derive kinetic temperatures and H 2 volume densities. These were used as starting points for non-LTE modelling of other molecules, for which column densities and abundances were determined and compared with values found for other sources in the CMZ. Results. The kinematic structure of G1.3 reveals an ‘emission bridge’ at intermediate velocities (~150 km s −1 ) connecting low-velocity (~100 km s −1 ) and high-velocity (~180 km s −1 ) gas and an overall fluffy shell-like structure. These may represent observational evidence of cloud-cloud interactions. Low- and high-velocity gas components in G1.6 do not show this type of evidence of an interaction, suggesting that they are spatially separated. We selected three positions in each cloud complex for further analysis. Each position reveals several gas components at various peak velocities and of various line widths. We derived kinetic temperatures of 60–100 K and H 2 volume densities of 10 4 –10 5 cm −3 in both complexes. Molecular abundances relative to H 2 suggest a similar chemistry of the two clouds, which is moreover similar to that of other GC clouds and, especially, agrees well with that of G+0.693 and G−0.11. Conclusions. We conclude that G1.3 may indeed exhibit signs of cloud-cloud interactions. In particular, we propose an interaction of gas that is accreted from the near-side dust lane to the CMZ, with gas pre-existing at this location. Low- and high-velocity components in G1.6 are rather coincidentally observed along the same line of sight. They may be associated with either overshot decelerated gas from the far-side dust line or actual CMZ gas and high-velocity gas moving on a dust lane. These scenarios would be in agreement with numerical simulations. 

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4. The ALMOND survey: molecular cloud properties and gas density tracers across 25 nearby spiral galaxies with ALMA

   https://doi.org/10.1093/mnras/stad424  

   Neumann, Lukas ; Gallagher, Molly J. ; Bigiel, Frank ; Leroy, Adam K. ; Barnes, Ashley T. ; Usero, Antonio ; den Brok, Jakob S. ; Belfiore, Francesco ; Bešlić, Ivana ; Cao, Yixian ; et al ( February 2023 , Monthly Notices of the Royal Astronomical Society)

   We use new HCN(1–0) data from the ACA Large-sample Mapping Of Nearby galaxies in Dense gas (ALMOND) survey to trace the kpc-scale molecular gas density structure and CO(2–1) data from the Physics at High Angular resolution in Nearby GalaxieS–Atacama Large Millimeter/submillimeter Array (PHANGS–ALMA) to trace the bulk molecular gas across 25 nearby star-forming galaxies. At 2.1 kpc scale, we measure the density-sensitive HCN/CO line ratio and the star formation rate (SFR)/HCN ratio to trace the star formation efficiency in the denser molecular medium. At 150 pc scale, we measure structural and dynamical properties of the molecular gas via CO(2–1) line emission, which is linked to the lower resolution data using an intensity-weighted averaging method. We find positive correlations (negative) of HCN/CO (SFR/HCN) with the surface density, the velocity dispersion, and the internal turbulent pressure of the molecular gas. These observed correlations agree with expected trends from turbulent models of star formation, which consider a single free-fall time gravitational collapse. Our results show that the kpc-scale HCN/CO line ratio is a powerful tool to trace the 150 pc scale average density distribution of the molecular clouds. Lastly, we find systematic variations of the SFR/HCN ratio with cloud-scale molecular gas properties, which are incompatible with a universal star formation efficiency. Overall, these findings show that mean molecular gas density, molecular cloud properties, and star formation are closely linked in a coherent way, and observations of density-sensitive molecular gas tracers are a useful tool to analyse these variations, linking molecular gas physics to stellar output across galaxy discs.

    

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5. Large-scale magnetic field in the Monoceros OB 1 east molecular cloud

   https://doi.org/10.1051/0004-6361/202039065  

   Alina, D. ; Montillaud, J. ; Hu, Y. ; Lazarian, A. ; Ristorcelli, I. ; Abdikamalov, E. ; Sagynbayeva, S. ; Juvela, M. ; Liu, T. ; Carrière, J.-S. ( February 2022 , Astronomy & Astrophysics)

   Context. The role of large-scale magnetic fields in the evolution of star-forming regions remains elusive. Its investigation requires the observational characterization of well-constrained molecular clouds. The Monoceros OB 1 molecular cloud is a large complex containing several structures that have been shown to be engaged in an active interaction and to have a rich star formation history. However, the magnetic fields in this region have only been studied on small scales. Aims. We study the large-scale magnetic field structure and its interplay with the gas dynamics in the Monoceros OB 1 east molecular cloud. Methods. We combined observations of dust polarized emission from the Planck telescope and CO molecular line emission observations from the Taeduk Radio Astronomy Observatory 14-metre telescope. We calculated the strength of the plane-of-sky magnetic field using a modified Chandrasekhar-Fermi method and estimated the mass-over-flux ratios in different regions of the cloud. We used the comparison of the velocity and intensity gradients of the molecular line observations with the polarimetric observations to trace dynamically active regions. Results. The molecular complex shows an ordered large-scale plane-of-sky magnetic field structure. In the northern part, it is mostly orientated along the filamentary structures, while the southern part shows at least two regions with distinct magnetic field orientations. Our analysis reveals a shock region in the northern part right between two filamentary clouds that, in previous studies, were suggested to be involved in a collision. The magnetic properties of the north-main and north-eastern filaments suggest that these filaments once formed a single one, and that the magnetic field evolved together with the material and did not undergo major changes during the evolution of the cloud. In the southern part, we find that either the magnetic field guides the accretion of interstellar matter towards the cloud or it is dragged by the matter falling towards the main cloud. Conclusions. The large-scale magnetic field in the Monoceros OB 1 east molecular cloud is tightly connected to the global structure of the complex. In the northern part, it seems to serve a dynamically important role by possibly providing support against gravity in the direction perpendicular to the field and to the filament. In the southern part, it is probably the most influential factor governing the morphological structure by guiding possible gas inflow. A study of the whole Monoceros OB 1 molecular complex at large scales is necessary to form a global picture of the formation and evolution of the Monoceros OB 1 east cloud and the role of the magnetic field in this process. 

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