Alert Notice 422: Observing Campaign on Hubble's First Variable in M31: M31_V1

July 16, 2010: An observing campaign is being carried out on M31_V1, the first variable star discovered in M31 by Edwin Hubble. The Hubble Heritage Team, with Dr. Keith Noll, Space Telescope Science Institute (STScI), as Principal Investigator, plans to observe M31_V1 with the Hubble Space Telescope (HST), and needs to know the phase of this Cepheid variable. Although basic parameters are known for this star, no recent photometry exists, so observations are required to generate current phase information.

In 1925 Edwin Hubble published a note in The Observatory (vol. 48, 139) on "Cepheids in Spiral Nebulae." Then in 1929, Hubble published a seminal paper in the Astrophysical Journal (vol. 69, 103), "A Spiral Nebula as a Stellar System, Messier 31." This paper discussed in detail the galaxy and the 50 variable stars he found in its outer regions. Hubble remarked that the 40 Cepheids found showed the period-luminosity relationship in a conspicuous manner, enabling distance to the galaxy to be calculated. Furthermore, he said that the results of his calculations supported the value determined by Harlow Shapley of the zero point of the period-luminosity relation. This confirmation of the zero point had significant implications for future extragalactic distance determinations. As the first of the variables on Hubble's list, V1, a Cepheid, is a historical curiosity.

Dr. John Grunsfeld, NASA astronaut and mission specialist veteran of the last three HST servicing missions (and now Deputy Director, STScI), carried the original glass photographic plate (view here) showing Hubble's identification of V1 on the M31 image to space on the final HST servicing mission in May 2009 (and returned it safely to Earth)!

M31_V1 is magnitude 19.4V, so unfortunately it is not suitable for visual or photoelectric observers, but is a good target for CCD observers. The B-V = +1.28, period is 30.41 days, and amplitude ~ 1.2 magnitudes in B, likely smaller in V. Five nights of data obtained by Arne Henden, AAVSO, show that the variable appears to have peaked on JD 2455367 (2010 June 19) at about R=18 and as of July 2 was on its way down.

It is recommended that observers use either an Rc filter or observe unfiltered. About an hour or more of exposure per integration will be required to reach S/N = 20, depending on your equipment and sky brightness; multiple exposures and stacking might be necessary to avoid saturating the background.

The field is not crowded, and the variable itself is not blended; there are several 15-16V supergiants in the region. Contamination from the M31 background, while present, should not be prohibitive.

Coordinates:  00:41:27.30 +41:10:10.4  (J2000.0)

An R-band finder chart from the Isaac Newton 2.5-m telescope, provided by Arne Henden and showing North up, East left, and a 4.44x4.44 field of view, may be seen here.

AAVSO charts for M31_V1 may be plotted in VSP.

Observers should use the table of photometry to find the Rc magnitudes of the comparison stars. Note that the magnitudes go down only to Rc magnitude 16.672 (17.477V).

The entry for M31_V1 is not publicly viewable in VSX, but charts may be generated and data submitted to the AAVSO International Database with the name "M31_V1" (note the underscore). The AUID is 000-BJV-425.

Your observations of this historically significant but neglected Cepheid are essential and will be greatly valued. Many thanks for your contributions!

This campaign is being conducted at AAVSO Headquarters by Elizabeth O. Waagen.

This AAVSO Alert Notice was compiled by Elizabeth O. Waagen.

Information on submitting observations to the AAVSO.