A Keplerian Disc Around a Massive Young Star
A proposed sketch of the Keplerian disk (and the outflow) around a massive young star, as viewed from Earth. A new study using the SMA finds that this massive star, still in its early stages of life, is accreting material through a disk in a manner analogous to that used in lower mass stars.
A spinning solid disk, like a CD in its player, obviously moves faster at its outer edge than at its inner radii. A Keplerian disk, made up not of a single solid body but of many bodies orbiting under the rule of gravity, as deduced by Kepler, is the opposite: bodies at the outer edge move more slowly than those closer in. Thus the Earth orbits the Sun in 365 days, while Mercury does so in only 176 days. Keplerian motions are always predominant in disks around stars, including pre-planetary systems, unless winds or other physical processes interfere with the simple gravitational dominance of the central star.
Stars more massive than about eight solar-masses have a very short pre-main-sequence lifetimes, maybe only a few hundred thousand years. This means that they spend the entirety of their formation stages deeply embedded in their obscuring, parent molecular clouds. Such short formation time-scales also mean that there are far fewer nearby examples of young, massive stars when compared with their lower mass counterparts. All these factors contribute to the extreme difficulty of observing young massive stars and their disks directly. As a result, their formation mechanisms are poorly understood, and in particular the process by which young massive stars accrete their high masses is not known.
CfA astronomer Qizhou Zhang and six colleagues used the Submillimeter Array and the Very Large Array to obtain high spatial resolution images (as small as about 1500 astronomical units) of the distribution of molecules in the disk around the massive star G11.92-0.61MM1. This young star is known to have a bipolar outflow, but earlier measurements had found emission from molecules that appeared to becoming not from the outflow winds, but from elsewhere in the vicinity of the star. The astronomers conclude from their new work that the dense gas and dust reside in a disk perpendicular to the outflow that contains about two or three solar-masses of material. An analysis of the motions of the gas indicates that the disk is Keplerian, and that it also has a component on gas that is falling inward: accreting onto the star. The results show that at least this massive star is following a growth process that resembles the one more commonly seen in lower mass stars.
"G11.92−0.61 MM1: A Keplerian Disc around a Massive Young Proto-O Star," J. D. Ilee, C. J. Cyganowski, P. Nazari, T. R. Hunter, C. L. Brogan, D. H. Forgan and Q. Zhang, MNRAS 462, 4386, 2016.