ALMA-IMF

III. Investigating the origin of stellar masses: top-heavy core mass function in the W43-MM2&MM3 mini-starburst^★

¹ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
e-mail: yohan.pouteau@univ-grenoble-alpes.fr
² Instituto de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México, Morelia, Michoacán 58089, México
³ AIM, IRFU, CEA, CNRS, Université Paris-Saclay, Université Paris Diderot, Sorbonne Paris Cité, 91191 Gif-sur-Yvette, France
⁴ Laboratoire d’astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
⁵ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
⁶ Department of Astronomy, University of Florida, PO Box 112055 USA
⁷ Institut de RadioAstronomie Millimétrique (IRAM), Grenoble, France
⁸ Laboratoire de Physique de l’École Normale Supérieure, ENS, Université PSL, CNRS, Sorbonne Université, Université de Paris, 75005 Paris, France
⁹ Observatoire de Paris, PSL University, Sorbonne Université, LERMA, 75014 Paris, France
¹⁰ National Astronomical Observatory of Japan, National Institutes of Natural Sciences, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹¹ Department of Astronomical Science, SOKENDAI (The Graduate University for Advanced Studies), 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹² Departamento de Astronomía, Universidad de Concepción, Casilla 160-C, 4030000 Concepción, Chile
¹³ Max-Planck-Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
¹⁴ S. N. Bose National Centre for Basic Sciences, Block JD, Sector III, Salt Lake, Kolkata 700106, India
¹⁵ Departament de Física Quàntica i Astrofísica, Institut de Ciències del Cosmos, Universitat de Barcelona (IEEC-UB), c/ Martí i Franquès 1, 08028, Barcelona, Catalonia, Spain
¹⁶ Instituto Argentino de Radioastronomía (CCT-La Plata, CONICET; CICPBA), C.C. No. 5, 1894 Villa Elisa, Buenos Aires, Argentina
¹⁷ Department of Astronomy, Yunnan University, Kunming 650091, PR China
¹⁸ Institute of Astronomy, National Tsing Hua University, Hsinchu 30013, Taiwan
¹⁹ Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO 80389, USA
²⁰ Universidad Internacional de Valencia (VIU), C/Pintor Sorolla 21, 46002 Valencia, Spain
²¹ Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, PR China
²² Max Planck Institute for Radio Astronomy, Auf dem Hügel 69, 53121 Bonn, Germany
²³ Nobeyama Radio Observatory, National Astronomical Observatory of Japan, National Institutes of Natural Sciences, Nobeyama, Minamimaki, Minamisaku, Nagano 384-1305, Japan
²⁴ University of Connecticut, Department of Physics, 196A Auditorium Road, Unit 3046, Storrs, CT 06269, USA
²⁵ School of Physics and Astronomy, Cardiff University, Cardiff, UK

Received: 19 December 2021
Accepted: 8 February 2022

Abstract

Aims. The processes that determine the stellar initial mass function (IMF) and its origin are critical unsolved problems, with profound implications for many areas of astrophysics. The W43-MM2&MM3 mini-starburst ridge hosts a rich young protocluster, from which it is possible to test the current paradigm on the IMF origin.

Methods. The ALMA-IMF Large Program observed the W43-MM2&MM3 ridge, whose 1.3 mm and 3 mm ALMA 12 m array continuum images reach a ~2500 au spatial resolution. We used both the best-sensitivity and the line-free ALMA-IMF images, reduced the noise with the multi-resolution segmentation technique MnGSeg, and derived the most complete and most robust core catalog possible. Using two different extraction software packages, getsf and GExt2D, we identified ~200 compact sources, whose ~100 common sources have, on average, fluxes consistent to within 30%. We filtered sources with non-negligible free-free contamination and corrected fluxes from line contamination, resulting in a W43-MM2&MM3 catalog of 205 getsf cores. With a median deconvolved FWHM size of 3400 au, core masses range from ~0.1 M_⊙ to ~70 M_⊙ and the getsf catalog is 90% complete down to 0.8 M_⊙. Results. The high-mass end of the core mass function (CMF) of W43-MM2&MM3 is top-heavy compared to the canonical IMF. Fitting the cumulative CMF with a single power-law of the form N(> log M) ∝ M^α, we measured α = −0.95 ± 0.04, compared to the canonical α = −1.35 Salpeter IMF slope. The slope of the CMF is robust with respect to map processing, extraction software packages, and reasonable variations in the assumptions taken to estimate core masses. We explore several assumptions on how cores transfer their mass to stars (assuming a mass conversion efficiency) and subfragment (defining a core fragment mass function) to predict the IMF resulting from the W43-MM2&MM3 CMF. While core mass growth should flatten the high-mass end of the resulting IMF, core fragmentation could steepen it.

Conclusions. In stark contrast to the commonly accepted paradigm, our result argues against the universality of the CMF shape. More robust functions of the star formation efficiency and core subfragmentation are required to better predict the resulting IMF, here suggested to remain top-heavy at the end of the star formation phase. If confirmed, the IMFs emerging from starburst events could inherit their top-heavy shape from their parental CMFs, challenging the IMF universality.