Title:  A new extragalactic distance determination method using
                    spectroscopy of blue supergiants


Rolf-Peter Kudritzki
Institute for Astronomy


Abstract:


The best established stellar distance indicators, Cepheids and RR Lyrae,
suffer from two major problems, extinction and metallicity dependence,
both of which are difficult to determine for these objects with sufficient
precision. We introduce a new method, which uses quantitative spectroscopy
of blue supergiants, the brightest "normal" stars in the universe. The
method is based on the determination of effective temperature and gravity
from the spectra, which then yields absolute magnitude with high
precision. Since the method involves the spectroscopic determination of
stellar parameters and thus the intrinsic energy distribution, reddening
and extinction law towards each stellar source can easily be measured.
In addition, metallicity can also be derived from the spectra.
 
We use medium resolution spectra of supergiants in two spiral galaxies,
NGC 300 (2Mpc) and NGC 3621 (7Mpc), to demonstrate that the method works.
We discuss the potential of the method and conservatively conclude that
once properly calibrated it has the potential to allow for measurements of
distance moduli out to 31 mag with an accuracy of 0.1 mag or better.