Optical surveys within our Galaxy are severely hampered by dust obscuration; complete samples therefore must be obtained with longer wavelength studies. Here we describe a survey procedure for evolved massive stars in the Galactic plane at K-band wavelengths, where the extinction is 10 magnitudes lower than for traditional V-band surveys. Our scientific driver is the discovery of young stellar populations in our Galaxy through the detection of evolved massive stars. These stars have strong emission lines, which makes them relatively easy to detect using narrow-band filters.
Massive stars drive the evolution of galaxies through powerful stellar winds, large mass ejections, and explosive deaths. These mechanisms are the dominant source of energy input into the ISM; thus the structure and composition of the ISM in most galaxies is largely determined by the massive star population (Leitherer et al. 1992; Oey & Clarke 1997; Heckman et al. 1998; Martin 1999; Oey et al. 2001; Heckman et al. 2001). Massive stars are also essential contributors to the chemical evolution of their host galaxy, ejecting material enriched in helium, carbon, and nitrogen during their lives, and depositing elements heavier than nitrogen in their final eruption as SNe.
As a massive star evolves, its spectrum becomes dominated by emission
lines, arising either in a dense stellar wind, or in circumstellar
material produced by mass loss. The presence and strength of
individual lines are clues to the star's evolutionary state and
atmospheric structure. Among evolved massive stars with such spectra,
emission lines are most pronounced in Wolf-Rayet
(WR) stars and in the Luminous Blue Variable (LBV, or S Dor variable)
stage, where they shed large (1 to 10 
)
amounts of chemically enriched matter in a relatively small amount of
time (
104 yrs) (Pasquali et al. 1997; Smith et al. 1998; Langer et al. 1999).
WR stars have lifetimes <10 Myr, and thus are excellent tracers of recent star formation, and so also Galactic structure. They are also important in our quest to understand how star formation proceeds. For example, most of the previously known WRs are relatively isolated or in OB associations, but recent searches in the IR have found a plethora of these objects in compact clusters near the Galactic center (Blum et al. 1995; Krabbe et al. 1995; Nagata et al. 1995; Figer et al. 1999a; Blum et al. 2001).
Copyright ESO 2003