1. Department of Astronomy, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan
2. Subaru Telescope, National Astronomical Observatory of Japan, National Institutes of Natural Sciences, 650 North A'ohoku Place, Hilo, Hawaii 96720-2700, USA
3. Institute of Astronomy, The University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan
4. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan
5. Institute of Astrophysics & Planetary Sciences, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki 310-8512, Japan
6. Purple Mountain Observatory, National Astronomical Observatories, Chinese Academy of Sciences, Nanjing 210008, China
7. Centre for Frontier Science, Chiba University, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan

Correspondence to: Shigeyuki Sako^1,2,3 Correspondence and requests for materials should be addressed to S.S. (Email: sako@ioa.s.u-tokyo.ac.jp).

Abstract

The birth of very massive stars is not well understood^{1, 2, 3}, in contrast to the formation process of low-mass stars like our Sun^{4, 5}. It is not even clear that massive stars can form as single entities; rather, they might form through the mergers of smaller ones born in tight groups^{6, 7}. The recent claim of the discovery of a massive protostar in M17 (a nearby giant ionized region) forming through the same mechanism as low-mass stars⁸ has therefore generated considerable interest. Here we show that this protostar has an intermediate mass of only 2.5 to 8 solar masses (M), contrary to the earlier claim of 20M (ref. 8). The surrounding circumstellar envelope contains only 0.09M and a much more extended local molecular cloud has 4–9M.

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