Astronomical Spectroscopy - Physics - University of Cincinnati

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500 stars. Figure 20 shows the Tonry & Davis (1979) r parameter (a measure of how well
the cross-correlation worked) versus the radial velocity of each star. There are clearly two
distributions, one centered around a velocity of zero (expected for foreground stars) and one
centered around 160 km s −1 , the radial velocity of the SMC. All together 176 certain and
16 possible SMC supergiants were found, and their numbers in the H-R diagram were used
to show that the current generation of stellar evolutionary models greatly over estimate the
duration of this evolutionary phase (Neugent et al. 2010). Since a similar finding had been
made in the higher metallicity galaxy M31 (using radial velocities from Hectospec; Drout et
al. 2009) the SMC study established that uncertainties in the mass-loss rates on the mainsequence
must not be to blame. The OI λ7774 line proved to be a useful luminosity indicator
even at relatively low metallicities.
150
100
50
-100 0 100 200 300
Fig. 20.— The radial velocities of SMC F/G supergiant candidates. The Tonry & Davis
(1979) r parameter is plotted against the radial velocity of approximately 500 stars observed
with Hydra at the CTIO 4-m telescope. The stars with radial velocities