An HST Determination of the Distance to M81

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Session 67 -- Galaxy Clusters and the Extragalactic Distance Scale
Oral presentation, Thursday, 10:30-12:00, Zellerbach Auditorium Room

[67.06] An HST Determination of the Distance to M81

W.L. Freedman, M.G. Lee, R. Hill (OCIW), S. M. Hughes, B.F. Madore, J. R. Mould (CIT), P. Stetson (DAO), R.C. Kennicutt, A. Turner (U.Arizona), L. Ferrarese, H. Ford (JHU), J.A. Graham (CIW), J.G. Hoessel (U. Wisc.), S. Faber, G.D. Illingworth (UCSC), J. Huchra (Harvard/CfA), J. Gunn (Princeton)

As part of the Hubble Space Telescope Key Project on the Extragalactic Distance Scale, a new distance to the nearby galaxy M81 has been determined using Cepheids. M81 is an important galaxy for the determination of H$_0$ because it serves as a calibrator and a test for several independent secondary methods: the Tully-Fisher relation, the surface brightness fluctuation method, the planetary luminosity function method, and (very recently) type II supernovae (the expanding photosphere method). Two fields in M81 were monitored for Cepheids. Eighteen $V$ and six $I$ observations were made in HST Cycles 1 and 2. The observations were spaced over time in an optimal way to ensure that good coverage of the light curves (and hence accurate mean magnitudes and periods) would be achieved for variables with periods ranging from 10 to 60 days. Mean $V$ and $I$ magnitudes and periods have been determined from which apparent period-luminosity relations are constructed. These period-luminosity relations are used to determine apparent distance moduli relative to the LMC. The two-color data are then used to measure the reddening-corrected (true) modulus of M81.

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