Modern Observations of Hubble’s First-discovered Cepheid in M31

Abstract

ABSTRACTWe present a modern ephemeris and modern light curve of the first-discovered Cepheid variable in M31, Edwin Hubble’s M31-V1. Observers of the American Association of Variable Star Observers (AAVSO) undertook these observations during the latter half of 2010. The observations were in support of an outreach program by the Space Telescope Science Institute’s Hubble Heritage project, but the resulting data are the first concentrated observations of M31-V1 made in modern times. AAVSO observers obtained 214 V-band, Rc-band, and unfiltered observations from which a current ephemeris was derived. The ephemeris derived from these observations is JDMax = 2,455,430.5( ± 0.5) + 31.4( ± 0.1)E. The period derived from the 2010 data is in agreement with the historic values of the period, but the single season of data precludes a more precise determination of the period or measurement of the period change using these data alone. However, using an ephemeris based upon the period derived by Baade and Swope, we are able to fit all of the observed data acceptably well. Continued observations in the modern era will be very valuable in linking these modern data with data from the 1920s–1930s and 1950s and will enable us to measure period change in this historic Cepheid. In particular, we strongly encourage intensive observations of this star around predicted times of maximum to constrain the date of maximum to better than 0.5 days.