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. Dr. John
Grunsfeld, Deputy Director of the Space Telescope Science
Institute, plans to observe M31_V1 with the Hubble Space
Telescope (HST), and needs to plan for 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. Dr. Grunsfeld, NASA
astronaut and mission specialist veteran of the last three
HST servicing missions, carried the original glass
photographic plate (http://www.aavso.org/images/M31_V1plate.gif)
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)!
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.
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 found at:
http://www.aavso.org/images/m31_v1_int_r.jpg
AAVSO charts for M31_V1 may be plotted in VSP at
http://www.aavso.org/observing/charts/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 AAVSO Alert Notice was prepared by Elizabeth O. Waagen.
SUBMIT OBSERVATIONS TO THE AAVSO
Information on submitting observations to the AAVSO may be found at:
http://www.aavso.org/observing/submit/
US $76.00
