CY Aqr, an extraordinary pulsating variable


CY Aqr, a pulsating variable variously described as dwarf Cepheid, Delta Scuti, or SX Phoenicis type, is an ideal object for students to observe for a fall semester CCD photometry project. The visual magnitude ranges from 10.4 to 11.1 over a period of just 88 minutes! The spectral type of CY Aqr varies from A2 to A8.


On September 28, 2005 UT, I observed CY Aqr with an SBIG ST-402ME CCD on a Meade 10" LX6 Schmidt-Cassegrain telescope using an f/3.3 focal reducer. Since the BVI-Clear filter wheel for this camera was not yet available on the date of observation, CY Aqr was observed using the unfiltered CCD. Each exposure was of duration 15 seconds, and CY Aqr was observed continuously from 1:38:50 - 3:43:24 UT from Dodgeville, Wisconsin. Any images suffering from poor tracking, wind, etc. were removed from the analysis. The images were bias subtracted and flat-fielded using twilight flats obtained earlier in the evening. Aperture photometry was performed using Maxim DL software.


The brightest comparison star in the field, TYC 567-2036-1, was used for the differential photometry. TYC 567-2036-1 has a visual magnitude of 10.7, and its B-V of +0.5 indicates a spectral type around F6, a little cooler and redder than CY Aqr. Taking the difference in instrumental magnitude between the two stars takes care of the first-order extinction effects, but second-order extinction effects were not corrected for. The altitude of the stars ranged from 37° to 48° during the observations, giving an airmass of 1.66 at the beginning of the observations and 1.34 at the end. As one gets closer to the horizon, blue light is extinguished (due to scattering) to a much greater extent than red light, so when doing differential photometry over a wide range of airmasses it is desirable to pick a comparison star whose spectral characteristics are as similar as possible to the variable star. Also, using a narrow-band filter (e.g. B, V, R, or I) will also help to minimize the second-order extinction effects.


The UT mid-observation times are shown along the x-axis. These are observed times, and have not been transformed to heliocentric Julian date. The difference in instrumental magnitudes (comparison - CY Aqr), called "delta magnitude" or "delta m", are shown along the y-axis.

One remarkable aspect of CY Aquarii's lightcurve is the extremely rapid growth phase from minimum to maximum brightness, lasting only about 20 minutes! In this unfiltered, broad-band view, the change in brightness is nearly 0.7 magnitude. The 88-minute period can be easily discerned through simple inspection of the light curve.


The spectral range and sensitivity of the unfiltered SBIG ST-402ME CCD detector are shown in the diagram below.

David Oesper