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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A compendium of distances to molecular clouds in the Star Formation Handbook
Accurate distances to local molecular clouds are critical for understanding the star and planet formation process, yet distance measurements are often obtained inhomogeneously on a cloud-by-cloud basis. We have recently developed a method that combines stellar photometric data with Gaia DR2 parallax measurements in a Bayesian framework to infer the distances of nearby dust clouds to a typical accuracy of ∼5%. After refining the technique to target lower latitudes and incorporating deep optical data from DECam in the southern Galactic plane, we have derived a catalog of distances to molecular clouds in Reipurth (2008, Star Formation Handbook, Vols. I and II) which contains a large fraction of the molecular material in the solar neighborhood. Comparison with distances derived from maser parallax measurements towards the same clouds shows our method produces consistent distances with ≲10% scatter for clouds across our entire distance spectrum (150 pc−2.5 kpc). We hope this catalog of homogeneous distances will serve as a baseline for future work.
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- NSF-PAR ID:
- 10189725
- Date Published:
- Journal Name:
- Astronomy & Astrophysics
- Volume:
- 633
- ISSN:
- 0004-6361
- Page Range / eLocation ID:
- A51
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Kinematic distance determinations are complicated by a kinematic distance ambiguity (KDA) within the Solar orbit. For an axisymmetric Galactic rotation model, two distances, a "near" and "far" distance, have the same radial velocity. Formaldehyde (H2CO) absorption measurements have been used to resolve the KDA toward Galactic HII regions. This method relies on the detection of H2CO absorption against the broadband radio continuum emission from HII regions. H2CO absorption at velocities between the HII region velocity and the maximum velocity along the line of sight (the tangent point velocity) implies that the HII region lies at the far kinematic distance whereas a lack of absorption implies that it lies at the near kinematic distance. The reliability of KDA resolutions using H2CO is unclear, however, as disagreements between distances derived using H2CO absorption and those derived using other methods are common. Here we use new H2CO and radio recombination line data from the Green Bank Telescope (GBT) Diffuse Ionized Gas Survey (GDIGS) to test whether H2CO absorption measurements can accurately resolve the KDA for 44 Galactic HII regions that have known distances from maser parallax measurements. For each of the 44 HII regions we determine whether the parallax distance is consistent with either the near or the far kinematic distance. We find that the Galactic distribution of H2CO is too sparse to reliably determine whether an HII region is at its near kinematic distance. The H2CO method also incorrectly resolves the KDA for 80% of HII regions that it places at the far kinematic distance; in such cases H2CO absorption may be caused by other sources of radio continuum emission (possibly the CMB, diffuse free-free, or synchrotron). Our results indicate that the H2CO method is unsuitable to resolve the KDA toward Galactic HII regions.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1051/0004-6361/201936145
