The AAVSO is currently working with Dr. Robert Zavala of the US Naval Observatory, Flagstaff to provide photometric support of optical interferometry of beta Geminorum, better known as Pollux, a cool, K0 giant star. Pollux is very bright, with a V-band magnitude of about 1.15, and is thus a challenging target even for our Photoelectric observers. It is known to be only slightly variable -- in fact the level of variability is so low it is currently one of our PEP-IR standard stars. What we are searching for is evidence for starspots. The star is known to be an X-ray source and is also rotating, so it is possible there may be some starspot activity. Your photometric observations will go hand in hand with imaging of the star's surface by NPOI to to see whether starspots exist, and whether the star's brightness changes as it slowly rotates.

The Navy Prototype Optical Interferometer (NPOI), situated a few miles southwest of Flagstaff, Arizona.

Dr. Zavala has provided the following background statement for the campaign, explaining the purpose of the observations:

The Navy Prototype Optical Interferometer (NPOI) will shortly begin an observing campaign to image the surface of Pollux. Pollux has a diameter of approxiamtely 8 milliarcseconds which is well suited to resolution of surface features on this red giant star with the NPOI. We would like to have simultaneous monitoring of the IR and V band photometry of Pollux to help interpret our interferometric images. Photometric variations may reveal the rotation of stellar surface features (star spots) which the NPOI detects in the interferometric images. Pollux is an xray source detected by both the Einstein and ROSAT orbiting xray telescopes, and has other observational signatures of a magnetically active star, and thus we expect some spot activity may be present on the stellar surface.

Pollux's rotation velocity is 4.8 km/sec and it has a radius of approximately 15 solar radii. If the rotation axis of Pollux is inclined at 45 degrees to the line of sight the rotation period is ~ 110 days. Due to the length of the rotation period we ask that Pollux be observed as long as is possible for the current observing season.

NPOI is located on Anderson Mesa, southwest of Flagstaff, Arizona, and is a joint project of the US Naval Observatory, the Naval Research Laboratory, and Lowell Observatory. It is an experimental project to combine the light from several, widely spaced telescopes to produce optical interferometric images in much the same way that radio interferometers like the Very Large Array make radio images. NPOI is capable of seeing details on scales of around one milliarcsecond, enabling astronomers to directly image the surfaces of some stars, and to split very close binaries. It is hoped that NPOI will detect starspots on the surface of Pollux, and your observations will be crucial to interpreting any variability seen. In addition to the optical interferometry, Dr. Christopher Tycner (also of USNO-FS) will obtain high-resolution spectroscopy of the hydrogen Hα emission line, which is also an indicator of stellar magnetic activity.

The AAVSO is requesting PEP observations in both the optical and near-infrared, and we hope that both the PEP and PEP-IR communities will contribute! Observations should commence immediately, and continue for at least the next 30 days. Observations after this point would also be appreciated to track any variations observed during the window of interferometric observations. PEP-IR committee chair Doug West has selected Castor and Procyon as comparison and check stars, respectively. A table with RA/Dec and magnitudes is below.

PEP-IR observers should submit their reduced data to the AAVSO; optical PEP observers may submit their raw data which will be reduced at HQ. You may submit all of your data at once at the end of the campaign if you prefer.

On behalf of Bob Zavala & the AAVSO, good luck, clear skies, & thanks!

Matthew Templeton
AAVSO