Abstract

The formation process of massive stars (M 8 M) is still poorly understood (e.g., Zinnecker and Yorke 2007; Beuther et al. 2007; Tan et al. 2014).Dueto their short formation timescale (1045 yr) and the severe extinction (AV 10100 mag) by the surrounding gas and dust, observations of massive young stellar objects (MYSOs)are challenging. Additionally, massive stars are rare and therefore they are typically located at larger distances. Already before reaching the zero-age main sequence (ZAMS), a MYSO is expected to produce significant amounts of ultraviolet (UV) radiation, creating an expanding hyper- or ultra-compact H ii region (Churchwell 2002). Despite the strong UV radiation counteracting the accretion process through, for example, radiation pressure or photo-ionization (e.g., Wolfire and Cassinelli 1987; Krumholz et al. 2009; Kuiper et al. 2011; Kuiper and Hosokawa 2018), the current belief is that mass accretes onto the central (proto-)star via an accretion disk that is similar to low-mass stars.