Accretion disks in luminous young stellar objects

High-mass stars, of spectral types O and B, are very important for galaxy formation and evolution, for the production of heavy elements, and for the injection of energy in the interstellar medium. However, their formation mechanism represents a challenge from both a theoretical and an observational point of view and, as a consequence, our knowledge of their formation mechanism has lagged behind that of solar-type stars. From a theoretical point of view, the main problem related to the radiation pressure of the newly formed OB star that would stop the accretion preventing further growth seems to have been more or less solved and all theoretical models converge to a disk-mediated accretion scenario.

In a recent article,”Accretion disks in luminous young stellar objects” published on Astronomy & Astrophysics Review (2016, 24, 6), M. T. Beltrán (INAF-Arcetri Observatory) and W. J. de Wit (ESO-Chile) discuss the observational evidence of the existence of such accretion disks around young luminous stars in embedded and optically revealed phases. Figure 1 summarizes the current knowledge of such accretion disks as acquired with radio/(sub)millimeter and optical/infrared interferometry.

Figure 1. Schematic view of the circumstellar structures around intermediate- (A-type) and high-mass (B- and O-type) (proto)stars as observed at different spatial scales and wavelengths. (From Beltran & de Wit, A&AR 2016, 24,6)

Spatially resolved molecular line observations have revealed circumstellar disks in Keplerian rotation around embedded luminous young stellar objects (YSOs) with masses up to 30 M_⊙, that correspond to late-O spectral type stars with luminosities up to ∼10⁵ L_⊙. These disks have typical radii of a few hundreds to a few thousands of AU (tens to hundreds billion km) and masses that range from a few M_⊙ to a few tens of M_⊙. The basic properties of the disks around embedded intermediate-mass (proto)stars appear as scaled-up versions of those found for disks around low-mass protostars.

Circumstellar disks have not (yet) been found around more massive early-O type (proto)stars, with luminosities >10⁵ L_⊙. What has been found instead are huge and massive structures called toroids, with radii of 10³–10⁴ AU and masses of a few 100 M_⊙. These toroids are probably pseudo-disks that will never reach equilibrium and are susceptible to gravitational collapse and fragmentation. Whether circumstellar disks are embedded in these massive toroids is a question that only higher angular resolution and sensitivity observations can answer.

The comparison of the accretion rates estimated for intermediate- and high-mass (proto)stars with those of less massive objects and more evolved T Tauri and Herbig Ae/Be stars has revealed a dependence of the accretion rate on the square of the mass of the star. That such correlation holds for such a broad range of luminosities (from low- to high-mass) and evolutionary stages (from embedded to optically revealed YSOs) suggests continuity in the accretion process from the low- to the high-mass regime and that the accreting mechanism could be the same for all luminosities.