Accretion outbursts from high-mass young stellar objects

B. Stecklum1, A. Caratti o Garatti2, R. Garcia Lopez2, J. Eislöffel1,T. P. Ray2, A. Sanna3, R. Cesaroni4, C. M. Walmsley2,4, R. D. Oudmaijer5, W. J. de Wit6, L. Moscadelli4, J. Greiner7, A. Krabbe8, C. Fischer8, R. Klein9 and J. M. Ibañez10
1Thüringer Landessternwarte Tautenburg
2Dublin Institute for Advanced Studies
3Max-Planck Institut für Radioastronomie, Bonn
4INAF - Osservatorio Astrofisico di Arcetri
5School of Physics and Astronomy, University of Leeds
6ESO, Santiago de Chile
7Max-Planck Institut für Extraterrestrische Physik, Garching
8Deutsches SOFIA Institut, Stuttgart
9NASA Ames Research Center, Moffett Field
10Instituto de Astrofísica de Andalucía, Granada

The question whether high-mass stars form like their low-mass siblings has been debated for quite some time. Two pathways are considered to produce such stars - disk-mediated accretion and (proto-)stellar mergers. During recent years evidence for circumstellar disks around high-mass young stellar objects (HMYSOs) mounted. Since disk instabilities of low-/intermediate-mass YSOs cause episodic accretion outbursts, similar events are expected for HMYSOs too. In 2015 we discovered the first burst of this kind from S255IR-NIRS3, a \sim20M_\odot HMYSO, which was signaled by flaring of its Class II 6.7 GHz masers. Incidentally, another burst of a HMYSO was detected by submm/mm observations at about the same time. We will summarize the observational findings of these discoveries and their implications for our understanding of massive star formation, the physics and evolution of circumstellar disks as well as the connection between accretion and ejection of matter.