Context. High-mass stars form in clusters, but neither the early fragmentation processes nor the detailed physical processes leading to the most massive stars are well understood. Aims. We aim to understand the fragmentation, as well as the disk formation, outflow generation, and chemical processes during high-mass star formation on spatial scales of individual cores. Methods. Using the IRAM Northern Extended Millimeter Array (NOEMA) in combination with the 30 m telescope, we have observed in the IRAM large program CORE the 1.37 mm continuum and spectral line emission at high angular resolution (~0.4″) for a sample of 20 well-known high-mass star-forming regions with distances below 5.5 kpc and luminosities larger than 104 L⊙. Results. We present the overall survey scope, the selected sample, the observational setup, and the main goals of CORE. Scientifically, we concentrated on the mm continuum emission on scales on the order of 1000 AU. We detect strong mm continuum emission from all regions, mostly due to the emission from cold dust. The fragmentation properties of the sample are diverse. We see extremes where some regions are dominated by a single high-mass core whereas others fragment into as many as 20 cores. A minimum-spanning-tree analysis finds fragmentation at scales on the order of the thermal Jeans length or smaller suggesting that turbulent fragmentation is less important than thermal gravitational fragmentation. The diversity of highly fragmented vs. singular regions can be explained by varying initial density structures and/or different initial magnetic field strengths. Conclusions. A large sample of high-mass star-forming regions at high spatial resolution allows us to study the fragmentation properties of young cluster-forming regions. The smallest observed separations between cores are found around the angular resolution limit which indicates that further fragmentation likely takes place on even smaller spatial scales. The CORE project with its numerous spectral line detections will address a diverse set of important physical and chemical questions in the field of high-mass star formation.

The nearby open cluster NGC 752 presents a rare opportunity to study stellar properties at ages >1 Gyr. However, constructing a membership catalog for it is challenging; most surveys have been limited to identifying its giants and dwarf members earlier than mid-K. We supplement past membership catalogs with candidates selected with updated photometric and proper-motion criteria, generating a list of 258 members, a >50% increase over previous catalogs. Using a Bayesian framework to fit MESA Isochrones & Stellar Tracks evolutionary models to literature photometry and the Tycho-Gaia Astrometric Solution data available for 59 cluster members, we infer the age of, and distance to, NGC 752: 1.34 ± 0.06 Gyr and 438 +8 −6 pc. We also report the results of our optical monitoring of the cluster using the Palomar Transient Factory. We obtain rotation periods for 12 K and M cluster members, the first periods measured for such low-mass stars with a well-constrained age >1 Gyr. We compare these new periods to data from the younger clusters Praesepe and NGC 6811, and to a theoretical model for angular-momentum loss, to examine stellar spin down for low-mass stars over their first 1.3 Gyr. While on average NGC 752 stars are rotating more slowly than their younger counterparts, the difference is not significant. Finally, we use our spectroscopic observations to measure Hα for cluster stars, finding that members earlier than ≈M2 are magnetically inactive, as expected at this age. Forthcoming Gaia data should solidify and extend the membership of NGC 752 to lower masses, thereby increasing its importance for studies of low-mass stars.

The physical and chemical structure of high-mass star-forming regions. Unraveling chemical complexity with the NOEMA large program "CORE"

Context. The fragmentation mode of high-mass molecular clumps and the properties of the central rotating structures surrounding the most luminous objects have yet to be comprehensively characterised. Aims. We study the fragmentation and kinematics of the high-mass star-forming region W3(H2O), as part of the IRAM NOrthern Extended Millimeter Array (NOEMA) large programme CORE. Methods. Using the IRAM NOEMA and the IRAM 30 m telescope, the CORE survey has obtained high-resolution observations of 20 well-known highly luminous star-forming regions in the 1.37 mm wavelength regime in both line and dust continuum emission. Results. We present the spectral line setup of the CORE survey and a case study for W3(H2O). At ~0.′′35 (700 AU at 2.0 kpc) resolution, the W3(H2O) clump fragments into two cores (west and east), separated by ~2300 AU. Velocity shifts of a few km s−1 are observed in the dense-gas tracer, CH3CN, across both cores, consistent with rotation and perpendicular to the directions of two bipolar outflows, one emanating from each core. The kinematics of the rotating structure about W3(H2O) W shows signs of differential rotation of material, possibly in a disk-like object. The observed rotational signature around W3(H2O) E may be due to a disk-like object, an unresolved binary (or multiple) system, or a combination of both. We fit the emission of CH3CN (12K−11K), K = 4−6 and derive a gas temperature map with a median temperature of ~165 K across W3(H2O). We create a Toomre Q map to study thestability of the rotating structures against gravitational instability. The rotating structures appear to be Toomre unstable close to their outer boundaries, with a possibility of further fragmentation in the differentially rotating core, W3(H2O) W. Rapid cooling in the Toomre unstable regions supports the fragmentation scenario. Conclusions. Combining millimetre dust continuum and spectral line data toward the famous high-mass star-forming region W3(H2O), we identify core fragmentation on large scales, and indications for possible disk fragmentation on smaller spatial scales.