CO bandhead emission of massive young stellar objects: determining disc
  properties
  Massive stars play an important role in many areas of astrophysics, but
numerous details regarding their formation remain unclear. In this paper we
present and analyse high resolution (R ~ 30,000) near-infrared 2.3 micron
spectra of 20 massive young stellar objects from the RMS database, in the
largest such study of CO first overtone bandhead emission to date. We fit the
emission under the assumption it originates from a circumstellar disc in
Keplerian rotation. We explore three approaches to modelling the physical
conditions within the disc - a disc heated mainly via irradiation from the
central star, a disc heated mainly via viscosity, and a disc in which the
temperature and density are described analytically. We find that the models
described by heating mechanisms are inappropriate because they do not provide
good fits to the CO emission spectra. We therefore restrict our analysis to the
analytic model, and obtain good fits to all objects that possess sufficiently
strong CO emission, suggesting circumstellar discs are the source of this
emission. On average, the temperature and density structure of the discs
correspond to geometrically thin discs, spread across a wide range of
inclinations. Essentially all the discs are located within the dust sublimation
radius, providing strong evidence that the CO emission originates close to the
central protostar, on astronomical unit scales. In addition, we show that the
objects in our sample appear no different to the general population of MYSOs in
the RMS database, based on their near- and mid-infrared colours. The
combination of observations of a large sample of MYSOs with CO bandhead
emission and our detailed modelling provide compelling evidence of the presence
of small scale gaseous discs around such objects, supporting the scenario in
which massive stars form via disc accretion.
