More Like this

1. Probing fragmentation with ALMA continuum and spectral line observations of the dense clumps in the ℓ = 224° region

   https://doi.org/10.1093/mnras/stac3030  

   Olmi, Luca ; Brand, J. ; Elia, D. ( October 2022 , Monthly Notices of the Royal Astronomical Society)

   We report observations, performed with the Atacama Large Millimeter/submillimeter Array (ALMA), of 1 mm dust continuum emission and molecular line emission in 13CO(2–1) and C18O(2–1), towards a sample of starless and protostellar clumps selected from a region, towards the ℓ = 224° field, of the Herschel Infrared GALactic Plane Survey (Hi-GAL). Using the ALMA images and a source extraction algorithm we have analysed the small-scale (∼1000 AU) structure of the clumps and their population of cores (or fragments). We find in general multiple cores in each Hi-GAL clump, both in the continuum and spectral lines, but we do not find a dominant fragmentation mode and the morphologies are very different among the various sources. Our results suggest that during the transition phase from clump to core, those sources with a higher core formation efficiency are also associated with parent clumps that are more likely to convert a higher fraction of their initial mass into a single or a few cores. We were able to obtain a core mass function, or CoMF, covering masses in the range ∼2 × 10−3 to ∼1 M⊙ for the C18O cores, and ∼4 × 10−2 to ∼10 M⊙ for the continuum cores. We find that the CoMF in our sample is much shallower than the higher mass ($\gtrsim 1$ M⊙) IMF, thus indicating that while approaching the final phase of fragmentation the mass function does not resemble the IMF more closely.

    

   more » « less
2. ALMA-IMF: VI. Investigating the origin of stellar masses: Core mass function evolution in the W43-MM2&MM3 mini-starburst

   https://doi.org/10.1051/0004-6361/202244776  

   Pouteau, Y. ; Motte, F. ; Nony, T. ; González, M. ; Joncour, I. ; Robitaille, J.-F. ; Busquet, G. ; Galván-Madrid, R. ; Gusdorf, A. ; Hennebelle, P. ; et al ( June 2023 , Astronomy & Astrophysics)

   Context. Among the most central open questions regarding the initial mass function (IMF) of stars is the impact of environment on the shape of the core mass function (CMF) and thus potentially on the IMF. Aims. The ALMA-IMF Large Program aims to investigate the variations in the core distributions (CMF and mass segregation) with cloud characteristics, such as the density and kinematic of the gas, as diagnostic observables of the formation process and evolution of clouds. The present study focuses on the W43-MM2&MM3 mini-starburst, whose CMF has recently been found to be top-heavy with respect to the Salpeter slope of the canonical IMF. Methods. W43-MM2&MM3 is a useful test case for environmental studies because it harbors a rich cluster that contains a statistically significant number of cores (specifically, 205 cores), which was previously characterized in Paper III. We applied a multi-scale decomposition technique to the ALMA 1.3 mm and 3 mm continuum images of W43-MM2&MM3 to define six subregions, each 0.5–1 pc in size. For each subregion we characterized the probability distribution function of the high column density gas, η -PDF, using the 1.3 mm images. Using the core catalog, we investigate correlations between the CMF and cloud and core properties, such as the η -PDF and the core mass segregation. Results. We classify the W43-MM2&MM3 subregions into different stages of evolution, from quiescent to burst to post-burst, based on the surface number density of cores, number of outflows, and ultra-compact HII presence. The high-mass end (>1 M ⊙ ) of the subregion CMFs varies from close to the Salpeter slope (quiescent) to top-heavy (burst and post-burst). Moreover, the second tail of the η -PDF varies from steep (quiescent) to flat (burst and post-burst), as observed for high-mass star-forming clouds. We find that subregions with flat second η -PDF tails display top-heavy CMFs. Conclusions. In dynamical environments such as W43-MM2&MM3, the high-mass end of the CMF appears to be rooted in the cloud structure, which is at high column density and surrounds cores. This connection stems from the fact that cores and their immediate surroundings are both determined and shaped by the cloud formation process, the current evolutionary state of the cloud, and, more broadly, the star formation history. The CMF may evolve from Salpeter to top-heavy throughout the star formation process from the quiescent to the burst phase. This scenario raises the question of if the CMF might revert again to Salpeter as the cloud approaches the end of its star formation stage, a hypothesis that remains to be tested. 

   more » « less
3. 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. 

   more » « less
4. Infall and outflow towards high-mass starless clump candidates

   https://doi.org/10.1093/mnras/stad1073  

   Pillai, T. G. S. ; Urquhart, J. S. ; Leurini, S. ; Zhang, Q. ; Traficante, A. ; Colombo, D. ; Wang, K. ; Gomez, L. ; Wyrowski, F. ( April 2023 , Monthly Notices of the Royal Astronomical Society)

   The evolutionary sequence for high-mass star formation starts with massive starless clumps that go on to form protostellar, young stellar objects and then compact H ii regions. While there are many examples of the three later stages, the very early stages have proved to be elusive. We follow-up a sample of 110 mid-infrared dark clumps selected from the ATLASGAL catalogue with the IRAM telescope in an effort to identify a robust sample of massive starless clumps. We have used the HCO+ and HNC (1-0) transitions to identify clumps associated with infall motion and the SiO (2-1) transition to identity outflow candidates. We have found blue asymmetric line profile in 65 per cent of the sample, and have measured the infall velocities and mass infall rates (0.6–36 × 10−3 M⊙ yr−1) for 33 of these clumps. We find a trend for the mass infall rate decreasing with an increase of bolometric luminosity to clump mass, i.e. star formation within the clumps evolves. Using the SiO 2-1 line, we have identified good outflow candidates. Combining the infall and outflow tracers reveals that 67 per cent of quiescent clumps are already undergoing gravitational collapse or are associated with star formation; these clumps provide us with our best opportunity to determine the initial conditions and study the earliest stages of massive star formation. Finally, we provide an overview of a systematic high-resolution ALMA study of quiescent clumps selected that allows us to develop a detailed understanding of earliest stages and their subsequent evolution.

    

   more » « less
5. H II regions and high-mass starless clump candidates: II. Fragmentation and induced star formation at ~0.025 pc scale: an ALMA continuum study

   https://doi.org/10.1051/0004-6361/202038421  

   Zhang, S. ; Zavagno, A. ; López-Sepulcre, A. ; Liu, H. ; Louvet, F. ; Figueira, M. ; Russeil, D. ; Wu, Y. ; Yuan, J. ; Pillai, T. G. ( February 2021 , Astronomy & Astrophysics)

   Context. The ionization feedback from H  II regions modifies the properties of high-mass starless clumps (HMSCs, of several hundred to a few thousand solar masses with a typical size of 0.1–1 pc), such as dust temperature and turbulence, on the clump scale. The question of whether the presence of H  II regions modifies the core-scale (~0.025 pc) fragmentation and star formation in HMSCs remains to be explored. Aims. We aim to investigate the difference of 0.025 pc-scale fragmentation between candidate HMSCs that are strongly impacted by H  II regions and less disturbed ones. We also search for evidence of mass shaping and induced star formation in the impacted candidate HMSCs. Methods. Using the ALMA 1.3 mm continuum, with a typical angular resolution of 1.3′′, we imaged eight candidate HMSCs, including four impacted by H  II regions and another four situated in the quiet environment. The less-impacted candidate HMSCs are selected on the basis of their similar mass and distance compared to the impacted ones to avoid any possible bias linked to these parameters. We carried out a comparison between the two types of candidate HMSCs. We used multi-wavelength data to analyze the interaction between H  II regions and the impacted candidate HMSCs. Results. A total of 51 cores were detected in eight clumps, with three to nine cores for each clump. Within our limited sample, we did not find a clear difference in the ~0.025 pc-scale fragmentation between impacted and non-impacted candidate HMSCs, even though H  II regions seem to affect the spatial distribution of the fragmented cores. Both types of candidate HMSCs present a thermal fragmentation with two-level hierarchical features at the clump thermal Jeans length λ J,clump th and 0.3 λ J,clump th . The ALMA emission morphology of the impacted candidate HMSCs AGAL010.214-00.306 and AGAL018.931-00.029 sheds light on the capacities of H  II regions to shape gas and dust in their surroundings and possibly to trigger star formation at ~0.025 pc-scale in candidate HMSCs. Conclusions. The fragmentation at ~0.025 pc scale for both types of candidate HMSCs is likely to be thermal-dominant, meanwhile H  II regions probably have the capacity to assist in the formation of dense structures in the impacted candidate HMSCs. Future ALMA imaging surveys covering a large number of impacted candidate HMSCs with high turbulence levels are needed to confirm the trend of fragmentation indicated in this study. 

   more » « less