Estimate the age of Ruprecht 119

Revised Oct 22, 2002

A recent issue of the journal Astronomy and Astrophysics contains an article describing observations of three young open clusters: "BVI photometry and integrated spectroscopy of the very young open clusters Ruprecht 119, NGC 6318 and BH 245", by Piatti, Bica and Claria. Your job is to estimate the age of the cluster Ruprecht 119 by comparing its color-magnitude diagram to theoretical color-magnitude diagrams.

Part I. Print out the observed color-magnitude diagram of Ruprecht 119

You can find the abstract and links to the paper in NASA's Astrophysics Data Service Abstract Server. Go to

http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2000A%26A...362..959P&db_key=AST&high=3dad60e77823685 with your browser. You will see a page with the title, authors, the abstract of the paper, etc. Near the top of the page is a list of links.

Electronic Refereed Journal Article will take you to the WWW site of the journal; at that point, you may choose an HTML or PDF version of the paper
Full Refereed Journal Article will download a PDF copy of the article

By hook or by crook, find and print out a hardcopy version of Figure 2.

There will be several panels. The top-left panel is a color-magnitude diagram of the stars in Ruprecht 119. Make sure you have a nice, clear printout of this diagram.

What is the quantity plotted on the horizontal axis? What is the quantity plotted on the vertical axis?

Part II. Create your own theoretical color-magnitude diagram

Go to Siess' WWW site which allows you to calculate theoretical HR diagrams:

http://www-laog.obs.ujf-grenoble.fr/activites/starevol/FILES/iso.html

Scroll down the page, accepting all the defaults until you reach the "Isochrone" item. Check the "Enter the age" box, and type "10.0d6" into the box. Then go to the bottom of the form and click on "Submit Form". You will be given a new page which contains a table with information on stars of different masses. Save the table into a file on your local computer.

Repeat this process with ages "500.0d6" and "2000.0d6". You should now have three data files on your computer. Check the data files: any entries with the "flag" field set to "1" means that stars of that mass have died. Edit the files by hand to remove any such entries.

Now, make a graph of these theoretical stellar models.

You can use graph paper and a pencil, or Excel, or some other plotting software. It just has to be neat and reasonably accurate. Your graph must have

color (B-V) running on the horizontal axis, with limits (-0.5, 2.5)
absolute magnitude Mv running on the vertical axis, with limits +18 on the bottom and -3 at the top

Place the data from all three files, with ages 10, 500 and 2000 Myr, on the same graph.

Estimate the age of Ruprecht 119

Compare your graph to the observed color-magnitude diagram.

What is the age of the stars in the cluster? Explain your reasoning.

Astronomers believe that there is a significant amount of dust and gas between us and the cluster. That scatters the light from the cluster, making it dimmer than it ought to be, and also redder. The arrow in the published diagram shows the estimated distance by which stars would be moved lower (fainter) and to the right (redder) by dust. We could correct for the presence of dust by moving all the points upwards and to the left by the size of the arrow.

After you correct the measurements for reddening, estimate the age of the cluster again. Are the stars younger or older than in your previous estimate? Explain.

If you read the abstract to the paper, you will see the authors' estimate of the cluster's age. How does it compare with yours?