The nature of a cluster of X-ray sources near the Chamaeleon star-forming region (originally developed by S. Murphy, RSAA/ANU; further developed by M. Allen and C. Bot; it can be found in its original form here)
The goal of this tutorial is to identify pre-main sequence stars, detected in X-ray, that belong to a young cluster near the Chamaeleon star-forming region. To do so, we will use different properties: these sources are spatially clustered, they have similar proper motions, we expect them to be brighter than field stars, and they lie on the same isochrone in a color-magnitude diagram. The stars we will find during this process are members of the "Eta Chamaeleontis" cluster, an 8 Myr-old cluster of stars 97 pc from the Sun. The 18 cluster known members were discovered over the course of many years by a very similar process.
Get the tables:
- launch VODesktop, TOPCAT and Aladin
- in VODesktop, click on File -> "New Smart List"
- give the list a name
- any main field contains RASS
- any main field contains Chamaeleon
- when the query is over, i.e. the red moving status bar at the bottom of the page becomes green, press "Create"
- a new smart list will appear at the bottom of the "Resource Lists" in the main VODesktop window
- from the four resources in the list, select "ROSAT All-Sky Survey: Chamaeleon Star Forming Region Study"; the green cone next to the name of the resource indicates a cone search capability available for this resources; check the available resource information at the lower part of the VODesktop window
- select "Query" under Actions in the lower left part of the window; this will launch an Astroscope window
- in order to obtain the full catalogue, set RA, Dec and radius to 0.0,90.0 and 180.0 degrees, respectively and "Search"
- when the query has finished, right-click on the result VOTable and send it to TOPCAT
Explore the catalogue using TOPCAT:
- Double click on the table in the Table List to see the table. You can also explore the table metadata or column metadata (units, datatypes, descriptions)
- The first thing we can ask is where on the sky are our sources. Do a 3D spherical plot.
- Covino et al. (1997) identified 4 sources in a cluster, highlighted in yellow in the following figure:
Can you identify these sources in your plot?
- Perhaps a Cartesian projection would be better. Make a scatter plot of RA vs Dec and flip the RA axis to get the right orientation on the sky
Create a subsample:
- create a subsample of just the T Tauri stars in the catalogue with contains(class,"T TAURI STAR")
- from the spatial distribution, select the 4 clustered sources and create a subset
- Outside X-ray luminosity, all cluster members also share similar space motions. Without the knowledge of radial velocities and distances, we will asume that proper motions (angular motion on the sky) should be the same. We will use the US Naval Observatory NOMAD catalogue
- in TOPCAT, select "Cone Search Capability" in the top bar menu VO -> Cone Search (look for the green cone), type "NOMAD" in the keywords field and "Submit Query"; from the resulting catalogues, select the one hosted by vo.astronet.ru
- make a query around the position of the four clustered stars (RA=130.5, Dec=-79.0, radius=0.5deg); this should return a table with 27 columns and some 16500 rows
- Make a histogram of the 2MASS J magnitude 'jmag' and switch to a logarithmic Y-axis. What can you say about the completeness of the J detections?
- We now need to find our cluster stars amongst the 16522 NOMAD sources. In the main TOPCAT window select the two table cross match tool
- There are various cross matching methods available. 'Sky' is the most common. Given the poor positional accuracy of the X-ray data (check the offset column in the RASS table) a 1 arcmin max error is probably appropriate. Select the two tables and cross match away (keep the Output Rows set to 'Best Match Only')
- Examine the new table (it should only have 4 rows but now 16+27+1=44 columns). The new Separation column shows the distance in arcsec between the cross matched positions. Note that only 1 of the stars has a NOMAD proper motion and Vmag. The other 3 are presumably too faint on the DSS plates NOMAD uses
- Plot a scattergram of all the NOMAD sources: pm_ra versus pm_dec. Most stars have small proper motions (<100 mas/yr in each direction). Have a look at the error bars; add the 4 cross match results to the plot
- zoom into the region around the one good X-ray star: there seems to be a few points clustered around that proper motion
- We expect nearby, pre-main sequence stars to be brighter than field stars. To visualise this in conjunction with the proper motion add an auxiliary axis of jmag. Notice that the stars with similar proper motions are systematically brighter than many others
- Select the Draw Region tool from the top menu of the Scatter Plot. Given the largish errors in the proper motions draw a wide region around the clustered points and click the draw region icon again to confirm. This creates a row subset of source.
- Keep the proper motion plot open while performing the next steps.
- We will now plot a colour-magnitude diagram (vmag versus vmag-kmag) in a new window.
- Select the NOMAD table again
- Create a new column vmag-kmag.
- Make the proper motion subset visible on the color-magnitude diagram. Several of the stars lie in a line, elevated above the bulk of the stars. This is the cluster isochrone in the V, V-K colour space
- We can now select cluster members having appropriate proper motions and photometry. Deselect the red points (Row Subset: "All"), leaving only the proper motion subset. Draw a region around the cluster isochrone
- Add all the stars back to the plot. Bring up the table of the NOMAD sources. Select points on the CMD that lie close to the cluster isochrone. TOPCAT automatically selects the same object in all open plot and table windows. Given the errors in the data some objects have proper motions and photometry that could still be consistent with membership in the cluster.
Overlay the candidates on images in Aladin:
- Now send the cluster candidates to Aladin: Select the NOMAD table in the main TOPCAT window, with the cluster candidates row subset selected. Go to Interop -> Send table to Aladin.
- In Aladin, select Overlay -> Grid; this overlays a coordinate grid. Click on one source. The table information for this object is shown at the bottom of the window. By selecting all the sources, all table information is shown. Also notice that the objects are highlighted in your TOPCAT table when you select them in Aladin.
- Overlay images of the cluster candidates using Aladin:
- Select one of the stars in the centre of the field, then go to File -> Load from the Virtual Observatory. Deselect Catalogs and Spectra and click 'SUBMIT'
- Select the SIA Service for ROSAT Archive, Soft X-Ray, and choose one of the 'RASS CHA WTTS' images. Click 'SUBMIT' again.
- Move the NOMAD table plane above the ROSAT plane, if it's not already. Now we can see the candidates overlaid on the ROSAT image. Four of the candidates are clearly seen in the ROSAT images. Select Image -> Pixel Contrast and map, and try changing the contrast in the image.
- Go to the server selector window and choose some images in a different wavelength range, for example, 2MASS J, ESO Digitized Sky Survey. Also try File -> Load Astronomical Image -> Others -> SuperCOSMOS SSS images and VO-Paris MAMA Atlas.
- A table of the known members can be found here: http://www.mso.anu.edu.au/~murphysj/topcat/ Load this table into TOPCAT and Aladin and compare it with the NOMAD sources. Why couldn't we find the other cluster members?