A near-infrared view of the evolution, environment and multiplicity of Massive Young Stellar Objects

This thesis presents near-infrared spectroscopy and imaging on large samples of Massive Young Stellar Objects (MYSOs) drawn from the Red MSX (RMS) survey. Low resolution spectra of 92 MYSOs in the near-infrared (NIR) H and K spectral bands taken at the Anglo-Australian Telescope (AAT) are presented. The targets are classified under the (Cooper, 2013) MYSO evolutionary scheme. Based on the presence or absence of H2, HI and FeII lines, MYSOs are placed in an evolutionary sequence ranging from Type Is, which are embedded and show strong shocks to the evolved Type IVs, which are the least embedded. Bolometric luminosity and distance are excluded as drivers of the evolutionary sequence through Kolmogoroff-Smirnoff tests. The different MYSO classes segregate in mid-IR colour space, with the youngest sources being the most embedded, and the more evolved sources bluer. The results from the spectroscopy are compared with radio data on ionised jets in MYSOs and with submilimetre data on star-forming clumps. The lowest jet detection rates are found in evolved MYSOs. The youngest sources are located in the most massive clumps. Next, medium-resolution spectra from Gemini are presented for 36 MYSOs. This is the largest sample of MYSOs observed at this resolution in the NIR. One MYSO showed intrinsic stellar absorption lines, and was assigned a spectral type of an early A giant/supergiant with added continuum dust excess emission. This is consistent with the swollen up MYSOs found in the simulations of (Hosokawa et al, 2010), and is the first time observational evidence for swollen up MYSOs has been found. Line luminosities and accretion rates deduced from Br gamma emission are consistent with values from low-mass YSOs, supporting theories of scaled-up high mass star formation. The luminosities of various lines show strong correlations with each other, even when accounting for distance and line flux biases. This may be due to the luminosities being proportional to the stellar mass. The detection rates of P Cygni and inverse P Cygni are lower in MYSOs than in low-mass YSOs. This is consistent with a change in the accretion mechanism from magnetospheric accretion in T Taus/HAe stars to disc accretion in HBes/MYSOs. Velocity-resolved Br gamma/Br12 ratios are analysed, revealing that the observed ratio features do not correlate with bolometric luminosity, NIR colour or the (Cooper, 2013) evolutionary sequence. The mechanism that causes the variation in profile ratio features is not clear from this data, but it may be a consequence of the variety in inclination with respect to the line of sight of the MYSOs. Finally, a pilot adaptive optics survey in the K band has been performed on 32 MYSOs in order to search for binary companions. This is the first ever systematic study of multiplicity in MYSOs. 18 new companions are found within 3", corresponding to a raw multiplicity fraction of 31% and a companion fraction of 53%. MYSOs have larger multiplicity fractions than their lower mass or more evolved counterparts at these scales. This indicates that multiplicity increases with mass and decreases with evolutionary stage, similar to trends seen for field stars. Lower limits to companion masses and mass ratios are obtained from the K band magnitudes, correcting only for foreground extinction. Results indicate that a larger fraction than expected of the companions have high mass ratios (q>0.5), in disagreement with the idea that the companions are randomly captured field sources. In summary, this thesis presents a large amount of spectra of MYSOs, forming a census of massive star formation in different environments throughout the Galaxy, probing the circumstellar region and evolution throughout the MYSO phase. Accretion rates from Br gamma support high mass star formation theories based on scaling up from the low mass case. The first ever detection of a swollen up MYSO is found in this sample. The first systematic search for multiplicity in the MYSO phase is reported, and results from this investigation prove that many massive stars form as part of multiple systems.