Create and Inspect the Preliminary Light Curves

Overview: The observational end product of our project is to create light curves of each star in V and I. This will require many nights of observation. It is very useful (and satisfying, I think) to check periodically how many observations have been obtained for a given star, and how the light curve is looking.

We could plot up the magnitude of each star as a function of the time (HJD) it was observed. However, given the rarity of clear nights in BG, these plots tend to have long gaps where no data were taken, and so are difficult to analyze. Fortunately, most RR Lyrae stars have very regular periods, so we can "fold" the time axis of the plot by the known period of the star in question to produce a light curve: a plot of magnitude as a function of phase in the star's pulsation cycle.

Nick Pearson and I have developed a script that will automatically create the light curve for a given star. At this stage, we use for the star's magnitude the difference between the magnitude of the variable and that of a specified comparison star. It is also useful to determine the magnitude difference between pairs of comparison stars -- if the difference is small, it means the observational errors in our measurements are small, which is good!

1) Start up IRAF, and change directories to the star you want to study -- say its the star "U Pic". You should see a lot of TXDUMP files. For example, Jane Schmoe might do the following:

baade% cd
baade% cl
cl> cd /data/jschmoe/PHOT/ALL/
cl> ls
RU_Lup     UY_Boo      (etc) 

2) If Jane doesn't have a directory for U__Pic, she must create one:

cl> mkdir U__Pic

If there was a directory U__Pic, and if it contained *.txd files (maybe she had inspected the light curve a month ago), she would delete the existing *.txd files there in preparation for the updated files:

cl> cd U__Pic
cl> ls *.txd    (she sees lots of *.txd files)  
cl> del *.txd ver-


3) All the data for U Pic has been compiled into the directory /data/layden/BGTEL/ALL/U__Pic, and Jane will now copy all these .txd files into her new directory.

cl> cp /data/layden/BGTEL/ALL/U__Pic/*.txd .

4) Jane will need several files to make the plot. If they are not in this directory from a previous creation/inspection, copy them fron the Repository.

cl> cp /data/layden/BGTEL/PHOTOM/MAGDIFF/ .
cl> cp /data/layden/BGTEL/PHOTOM/MAGDIFF/magdiff.U__Pic .

5) The file called magdiff.U__Pic contains the known period of this star, and the default choices for which comparison stars to use. The file called is the script which creates the plot. Run it, and you will get asked several questions. (1) Accept the default choices for which comparison stars to use, or enter new choices. For now, accept the defaults: type "1". (2) Plot only V data, only I data, or both: type 1, 5, or 9. In this example, we will use V only: type "1".

cl> cl <
   Enter name of data file (e.g., magdiff.AW_Ser): magdiff.U__Pic
   Compare mags using default stars or chose new ones (1 or 9): 1
   Use V data, I data, or both (1, 5, or 9): 1
   S1 S2   <Dmag>      SD    TypErr  Npts
 2  0   -1.169    0.220    0.156   19
 3  1    0.836    0.004    0.003    8
 6  1    2.379    0.019    0.014    8
 9  5    1.036    0.013    0.009   19

6) The program has spat out some numbers, which we will talk about in Step 7, and created a SuperMongo macro which you will now use to plot the light curve. Start an XTERM window, and start up Supermongo in that window. In the XTERM window, read the macro and execute it. You should see four plots.

cl> !xterm &
% sm
macro read

If you used the default choices for the comparison stars, the upper left plot will show the light curve of the star. Hopefully, it has the characteristic shape of an RR Lyrae light curve, with fairly little scatter. The different symbols (squares, triangle, cross, ...) indicate different observing years.


7) The other three plots show differences between comparison stars. Since these stars are non-variable, ideally, the difference would be constant, with no scatter. Observational errors (inevitable) cause there to be some scatter. The table of numbers spat out tells us how large the scatter is. Look at the lower three rows. Each row is the difference between two comparison stars. In the case of Row 2, it is Comp3 - Comp1. The average magnitude difference between these stars is 0.836 mag. The scatter around this average (technically, the standard deviation) is 0.004 mag. Since the observation of each of the two stars contributes to the scatter, we can estimate the error in the magnitude of a individual stars as SD/sqrt(2), which is shown in the TypErr (typical error) column. Npts indicates the number of observations used.

Our goal is to get 1% photometry, which translates to typical errors of 0.01 mag or less. In this example, we seem to be doing pretty well. Don't get concerned if one of the TypErr values is larger than 0.01 mag. Fainter stars tend to have larger errors, and some of our comparison stars are fairly faint.

If you have questions or concerns, contact ACL <layden(at)baade(dot)bgsu(dot)edu> .

Updated 2002 July 11 - ACL