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(White dwarfs in the Globular Cluster M4)
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GOAL: This tutorial shows you how to use the MAST Discovery Portal and CasJobs to make a Color-Magnitude Diagram to search for outliers. In the process, you will learn how to download a table from CasJobs, and upload it into the Discovery Portal.
SCIENCE CASE: The science case is searching for white dwarfs (i.e. in the Globular Cluster M4; Richer et al. 1997, ApJ 484, 741).
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Step 1 - Determine which object(s) you wish to study. |
In the Inventory tab, scroll over to the PropID field and click on the ID (for example proposal 12311 (blue)). This will take you to the proposal page where you can see the abstract for the proposal, as well as papers that use this dataset.
Step 2 - Go to the MAST Discovery Portal. |
At this point, you may want to look directly at the HST data. This can be done by clicking on the icon (yellow) under Actions, and then clicking on the Preview image (see Use Case 1 for details).
Note that while the HSC can be used directly for some science projects, in many cases (like this) you would need to make a much deeper catalog to get the most out of the data. In these cases the HSC provides: 1) a quick look at what data exists, 2) a feasibility study, 3) a dataset to compare with at the brighter wavelengths, and 4) the ability to link with other instruments or surveys.
Step 3 - Make a Color-Magnitude Diagram (CMD) Make a plot by clicking the | |
Next, select X = U-I and Y = W3_F275W (blue). Click on the Update Series button (green) to plot the CMD. Reverse the Y axis to have magnitudes go from bright to faint (orange). The white dwarfs are blueward of the main band of stars (yellow). |
In some cases, you may find that there are objects in the CMD where you don't expect them. If you look at the details for these objects, you may find that they have particular high CI values (possibly due to a cosmic ray or other defect in the image) or large magnitude errors (due to the inclusion of a saturated measurement). While we attempt to screen these out when building the source lists, we are not always successful. In these cases, you should use the Filtering options to screen out the bad data.
Step 4 - Expand the dataset |
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Submit (orange) the query for processing.
Step 5 - Export the CasJobs data After the query has finished, go to the MyDB (blue) page. Find the dataset (M4) in the list on the left and select it (green). Note that there are ~381 objects. We need to download the data so we can import it into the Discovery Portal for plotting. Select the Download option (orange). Select your format (yellow) and click on Go. | |
After hitting Go, you are taken to the Output page. Once the dataset is available, click on Download (blue). |
Step 6 - Import the catalog into the Discovery Portal |
After importing, you will see the catalog and the objects overlaid on the DSS image.
Step 7 - Make the Color-Magnitude Diagram | |
The white dwarfs are blueward of the main band of stars. Of the 8 objects in the WD area, 6 are in Richer, 1 is not in the WFPC2 FOV (green) used by Richer and hence is a new white dwarf candidate, and 1 is not visible on the WFPC2 F814W image (but is in the WFC3 data so is in the HSC; orange). |
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