The RMS survey: near-IR spectroscopy of massive young stellar objects

Near-infrared H- and K-band spectra are presented for 247 objects, selected from the Red MSX Source (RMS) survey as potential young stellar objects (YSOs). 195 (∼80 per cent) of the targets are YSOs, of which 131 are massive YSOs (L_BOL > 5 × 10³ L_⊙, M > 8 M_⊙). This is the largest spectroscopic study of massive YSOs to date, providing a valuable resource for the study of massive star formation. In this paper, we present our exploratory analysis of the data. The YSOs observed have a wide range of embeddedness (2.7 < A_V < 114), demonstrating that this study covers minimally obscured objects right through to very red, dusty sources. Almost all YSOs show some evidence for emission lines, though there is a wide variety of observed properties. The most commonly detected lines are Brγ, H₂, fluorescent Fe II, CO bandhead, [Fe II] and He I 2-1 ¹S-¹P, in order of frequency of occurrence. In total, ∼40 per cent of the YSOs display either fluorescent Fe II 1.6878 μm or CO bandhead emission (or both), indicative of a circumstellar disc; however, no correlation of the strength of these lines with bolometric luminosity was found. We also find that ∼60 per cent of the sources exhibit [Fe II] or H₂ emission, indicating the presence of an outflow. Three quarters of all sources have Brγ in emission. A good correlation with bolometric luminosity was observed for both the Brγ and H₂ emission line strengths, covering 1 < L_BOL < 3.5 × 10⁵ L_⊙. This suggests that the emission mechanism for these lines is the same for low-, intermediate- and high-mass YSOs, i.e. high-mass YSOs appear to resemble scaled-up versions of low-mass YSOs.