Astronomy 350 Au2002

Lab 4:
CCD Differential Photometry of CY Aquarii

Please re-acquaint yourselves with the "Laboratory Rules of Operation" before proceeding.



In this lab you will be analyzing B-band CCD images of the high-amplitude SX Phoenicis variable star CY Aquarii that were acquired with the YALO 1-meter Telescope at the Cerro Tololo InterAmerican Observatory in Chile. YALO is a queue-scheduled telescope operated by a consortium of Yale, AURA (the body that runs KPNO and CTIO for the NSF), Lisbon University in Portugal, and Ohio State (hence the name, YALO, an acronym of the 4 partners). The data were taken with the ANDICAM, a dual CCD/IR Array imager built by Ohio State (PI: Darren DePoy).

During 14 nights from UTC 2000 September 25 through October 12, CY Aquarii was observed in the Johnson B-band using the CCD camera channel of ANDICAM. For approximately 20 minutes each clear night during this time, we acquired 25 images of 20-seconds duration each using the CCD binned 2x2 pixels. The data were downloaded from CTIO the following morning and stored on disk at OSU, along with calibration images (flat fields). Conditions during the observations varied from night-to-night, we began during new moon and ended during full moon, so the background level was steadily increasing. On some nights we had good seeing, others it was worse. Some nights had cirrus clouds passing over and others were clear. The observing conditions were noted in the YALO Observer's Logs that you will be examining.

About CY Aquarii

CY Aquarii is a large-amplitude, short-period pulsating variable star of the SX Phoenicis class (named for the prototype of this class). SX Phe stars are 1-2 solar mass Population II blue stragglers in their post-main-sequence phase of evolution that have entered the pulsational "instability strip" of the H-R diagram. This is the same instability strip that gives rise to the supergiant Cepheid variables important for measuring cosmic distances, and so SX Phe stars are often referred to as "dwarf cepheids". SX Phe stars are characterized by short periods (0.03-0.08 days) and high amplitudes of variability (0.3-0.7 magnitudes). They have been seen in globular clusters and nearby dwarf galaxies as well as in the field (CY Aqr is a field star).

Among the SX Phe stars, CY Aquarii has been a popular object of study by small telescopes because of its relative brightness (V=10.48mag max), large amplitude of variability (0.88 mag at B, 0.74 mag at V), and rapid period (about 88 minutes). Our goal here is to measure the period and amplitude of variability in the B band using the data that were acquired at CTIO.

Because we only had about 20 minutes of observing time with the Yale 1-m telescope each night, each night of data covers about a quarter of a complete cycle of variability. Since CY Aqr undergoes at ~16.4 cycles of variability in 24 hours, by combining data from different 20 minute observing windows across a number of different nights, we can reconstruct a complete "light curve" for CY Aquarii during this period.

To reconstruct the complete cycle, we need to "fold" (or "phase") the light curve (brightness plotted as a function of Heliocentric Julian Date) over one or two periods so that the different cycles we observed overlap. For this purpose we can either adopt the previously reported period (0.061038612 days; Powell et al. 1995, PASP, 107, 225), or we can compute it from the data using a time-series analysis. The latter calculation is outside the scope of this lab, but I did it for the entire data set and found P=0.0610378 days using the Schwarzenberg-Czerny method. This is sufficiently close to the value from Powell et al. that we'll adopt their number.

I have provided web tools to take care of all the dirty work of phasing and merging your individual data sets into a grand light curve for the entire class. This is done in Part 4 of the lab.

Lab Goals

In this lab, each of you has been assigned data from a different night. Each data set for a given night consists of 25 CCD images taken one after the other over the course of about 20 minutes. Because there were different conditions of transparency (e.g., clouds), airmass, and sky brightness on each night, we will use the technique of Differential Photometry to reduce the data for a given night, and to combine the data from all the nights. To do this, we will establish a common brightness system based on a set of "comparison stars" common to all of the images.

The goals of this lab are as follows:

  1. Examine the images and measure the sky values during the observations, as well as to look for systematics such as changes in the sky brightness, seeing, or transparency during the course of the observations.

  2. Measure the apparent brightness of CY Aquarii and fainter non-variable comparison stars on the same images, deriving instrumental and differential magnitudes and their uncertainties. This also involves computing the times of each measurement based on information from the data logs and the image FITS headers.

  3. Construct a differential light curve of the data for each night using the comparison stars, and then merge all of the light curves together and "phase" with the period of CY Aquarii to create a "folded" light curve. From this we will recover the full cycle of variability, and permit you to measure the amplitude of variability in the B-band during our observations.



This lab is divided into 5 parts, meant to be done in series:

Part 1: Finding Catalog Data

Part 2: Retrieve your CCD Data

Part 3: Measure the Images

Part 4: Differential Photometry of CY Aquarii

Part 5: Lab Write Up


To guard against people getting badly stuck, I've set a couple of deadlines, whereby you must pass certain milestones among the above 5 parts before certain dates.

Thursday, December 5, 5pm.

By this date, you need to have finished the measurement phases of the lab through at least part 3 above, and submit a table of your preliminary photometric results via email. Details are given at the end of Part 3.

Tuesday, December 10, 5pm.

On or before this date, you need to have submitted your final differential photometry data using the Photometry Data Entry Form. Having these done by this date allows all of the students to have the final, full light curve in hand to use for writing up the final parts of the lab report.

Thursday, December 12, 5pm.

Submit your lab notebooks containing your final reports for Lab 4 to the professor by 5pm. There are no extensions except for medical or family emergencies.

Return to the Lab Index
Return to the Astronomy 350 Main Page
Updated: 2002 November 20 [rwp]

Copyright Richard W. Pogge, All Rights Reserved.