Measuring RR Lyrae Stars in NGC 3201

Globular Cluster NGC 3201 in the constellation Vela. Image by J Newland using Skynet Robotic Telescope Network.

Star clusters are great targets of study because the stars formed more or less at the same time from more or less the same material and the stars are more of less at the same distance. I decided to attempt to take a series of images of a globular cluster all in a single night so I could find some RR Lyrae variable stars. These variables often have a period of less than a day and they are all about the same absolute magnitude.

If you can get a good measurement for apparent magnitude for an RR Lyrae, then you can use the distance modulus calculation to find the distance to the star. In this way, I was hoping to measure the distance to globular cluster NGC 3201.

The first thing was to take images. I decided to use the Skynet Robotic Telescope Network because of the ease of scheduling a repeating observations. I help manage a group of youth as part of the Skynet Junior Scholars program. I’ve included some of the original (reduced) FITS files here for others to try the same project. The files are listed in chronological order over a night.

NGC 3201 Time Series (green filter)

The first task was to load the images in a format that allowed them to be “blinked” so that variable stars might become obvious. The next thing I did was to confirm the type of star I found. I used the Aladin desktop program from CDS to handle both tasks.

Aladin with multiple FITS files loaded as multiple planes.

Once all images are loaded, click the blink/movie generator and select the files in chronological order. Here is a more aligned version of the same series of images as animation and a video version as well.

Animated GIF of variable stars in NGC 3201 over a single night.

You can have Aladin use the RA/Dec information in the FITS files to automatically look for related data and overlay the records simply by clicking Simbad at the top of the image. This is how I confirmed which stars were RR Lyrae variables. Then I selected one I thought I could isolate in my photometry program meaning it wasn’t in a crowded area of the image. By clicking on the star, a data table is created. From here you can click the row in the table and go to the Simbad entry on that particular star. There is a LOT of information here including links to papers where this star was a topic of research.

Aladin with Simbad overlaid on FITS image

Now I need to measure the light from the star I’ve decided to study. I used Aperture Photometry Tool. I learned how to use the program from this very thorough and helpful tutorial video. The learning curve was steep but not too bad. The hardest bit was being sure I was selecting the same star in each image.

APT aperture selection.
Aperture attributes

Next I wrote down the magnitudes for all the files using APT. Since these are relative apparent magnitudes, any distance measurement I do from this data is likely to have some uncertainty. I opted to apply a calibration step. I selected a non-variable star in the field and measured the magnitude in all the images to be sure it wouldn’t change. Next I looked up THAT star in Simbad and used the known apparent magnitude. Then I created a scaling factor from these two data points and applied that to my variable star magnitudes. This I used to find the distance to the cluster by applying the distance modulus. The units for distance modulus are parsecs. The distance from literature was around 5 Kpc and I measured 4.7 Kpc. Close enough!

Distance modulus with distance measured in parsecs and m and M measured in magnitude units.I also made a rough light curve for this star and could clearly see the period was less than a day. From literature I found the measured period to be half a day. Below are some measured light curves for RR Lyrae stars in NGC 3201 from the literature. Note that phase in the diagram below refers to how far along the cycle the star is at that point.

From Layden & Sarajedini The Astronomical Journal, 125:208–223, 2003 January

This project was a great way to use FITS images to see how astronomers do science with images. I was able to replicate a known result and a lot of other cool and useful stuff can be done with data like this. I hope this encourages others to do their own research. All you need is some FITS files, some patience, and effort!