Disks in high-mass stars

Accretion disks are one of the key ingredients of the star and planet formation process. However, the process of disk formation in the presence of the strong braking induced by the magnetic field of the collapsing cloud still presents severe challenges to theoretical modelling (see, e.g., Lizano & Galli 2015). More important, while disks around nearby solar-type stars have been studied to great extent and detail, our knowledge of the properties of disks around more distant, massive O and B-type stars is comparatively less certain. Our group has been involved for many years in the search and study of such disks with state-of-the-art millimeter and submillimeter telescopes, in particular with ALMA. As a result of these studies, clear signatures of rotating Keplerian disks around stars with masses up to 20–30 M_☉, which would correspond to early B-type or late O-type stars (see Fig. 1), have been found (e.g., Sánchez-Monge et al. 2013; Beltrán et al. 2014). In recent years, the quest for Keplerian disks has been extended to stars beyond a mass limit of 30 M_☉, that is, to early O-type stars, thanks to observations carried out at subarsecond angular resolution (Cesaroni et al. 2017). These studies have identified possible accretion disks candidates that, once observed at the highest angular resolution available, show substructure in the form of spirals and rings (e.g., Sanna et al. 2019; Maud et al. 2019). For a review on the properties of disks around high-mass stars, see Beltrán & de Wit (2016).

Figure 1. A candidate circumbinary Keplerian disk in the high-mass star forming region G35.20-0.74N: A study with ALMA, by Sánchez-Monge, Cesaroni, Beltrán et al. (2013)

The next step of our investigation will push to the limit the resolving power of sub-mm interferometers to image a limited number of disks on scales of a few astronomical units, comparable to those already observed with the VLBI at centimeter wavelengths. The goal is to study the interplay between the physical structure and the chemical composition as a function of disk radius, as well as to investigate the launching mechanism of the jet at the interface with the disk.

Physics of the disks

The physical properties of the disks, estimated thanks to spatially resolved observations of high-density molecular (typically CH₃CN and isotopologues) and continuum tracers from millimeter to centimeter wavelengths, and maser emission (see Fig. 2), indicate that these disks are massive (a few M_☉) and large (sizes of a few hundreds to a thousand au), if compared to those around low-mass protostars. Detailed studies of the kinematics of the disks have revealed velocity gradients perpendicular to molecular outflows that are consistent with rotation, in some cases Keplerian. In addition, the material in the disk is not only rotating but is also infalling towards the central star, suggesting that the formation mechanism of high-mass stars is similar to that of their lower-mass counterparts.

Chemistry of the disks

The wide bandwidth of ALMA allows us to study the disks in different species, from simple diatomic molecules to more complex organic molecules (COMs). The chemistry in such disks is in fact very rich, with many transitions of multiple species with up to 10 atoms. Some of these molecules are of prebiotic interest, and they could be the chemical ingredients that gave rise to Life in our planet, such as glycolaldehyde, the simplest sugar, or formamide, which is related to the formation of amino acids.

Figure 2. Maser emission in the high-mass star-forming region NGC7538 IRS1. The colored dots show the positions and line of sight velocities of the methanol masers, Maser positions and velocities are determined with accuracies of ~1 mas and a few 0.1 km/s, respectively. Each of the two linear distributions traces an edge-on, rotating disk around a massive protostar. The star symbols, labeled IRS1a and IRS1b, mark the protostars' positions derived through a fit of the maser kinematics. The dotted contours represent the 1.3~cm continuum emission of the ionized gas observed with the Very Large Array. From Moscadelli & Goddi (2014).