Brown Dwarf Tutorial

Discovery of Brown Dwarfs mining the 2MASS and SDSS databases

Uses AladinVizieR and TOPCAT.


 

Background: 
Brown dwarfs are objects occupying the gap between the least massive stars and the most massive planets. They are intrinsically faint objects so their detection is not straighforward and, in fact, was almost impossible until the advent of global surveys at deep optical and near-infrared bands like SDSS, 2MASS or DENIS among others. Here we propose to mine the SDSS and 2MASS databases to identify T-type brown dwarfs through an appropriate combination of colours in the optical and the infra-red.

With this use case, we explore different ways to do the same tasks (cross-match, sources selection,...) with different VO tools. Given the capabilities of each VO tool, there can be indeed several ways to execute the same workflow (series of steps in the data processing)

First workflow:

  • Launch Aladin
  • Search 2MASS and SDSS sources around RA:08h 30m DEC:01d 30m (default radius: 14 arcminutes)
    • 2MASS: 686 sources
    • SDSS: 9855 sources
  • Find common sources in 2MASS and SDSS catalogues (use the Aladin catalogue cross match tool with default threshold (4 arcsec) and bestmatch option): 680 sources (677 point sources)
  • Select points sources using the SDSS flag (cl=6) creating a filter plane (you need to edit a filter in advanced mode to perform this selection - see the Filter manual for detailed help): 642 sources (639 point sources)
  • Create a new plane with the filtered sources
  • Select sources with no detection in the u,g SDSS filters (u > 22.0 & g >22.2) using a new filter plane: 6 sources
  • Select sources following the criteria provided by Burgasser et al. (2000, Apj, 531, L57)
    • (J-H)<0.3 && (H-K)<0.3 : 1 object --> RA:127.703265deg; DEC:1.475320deg
    • To do so, you can either use arithmetic operations between columns directly in the filter syntax, or you can create new columns in the x-match catalog that are an arithmetic combination of 2 columns (e.g. Jmag & Hmag)
  • Confirmation of the brown dwarf nature of this object by searching through VO services using the Load from the "allVO" option

Second workflow: repeat the same analysis using TOPCAT

  • Launch Aladin and TOPCAT
  • earch the 2MASS and SDSS sources with a cone search from TOPCAT around RA:08h 30m DEC:01d 30m

    • Go to the File menu and "Load Table"
    • Go to the DataSources menu and select "Cone Search"
    • Give "2MASS" as a Keyword to query available services and select the 2MASS point sources catalog
    • Give the coordinates and a radius of 14'
    • Repeat for the SDSS search (select the DR6 SDSS calatog for example)
    • Alternatively, you could load the 2MASS and SDSS catalogs in Aladin and broadcast the planes to TOPCAT using SAMP
  • Create a new table by matching raws in the two tables with 4" maximum error and the best match option. 680 pairs are found
  • Visualise the cross-matched raws and define a raw subset for which the SDSS class is equal to 6
  • Select sources with no detection in the u,g SDSS filters (u > 22.0 & g >22.2) by defining a new raw subset (6 remaining sources)
  • Update the selection criteria to add the criteria:
    • (J-H)<0.3 && (H-K)<0.3 : 1 object --> RA:127.703265deg; DEC:1.475320deg
    • As in the previous flow, you can either give selection criteria that are an arithmetic combination of columns, or you can create new columns in the catalog. For this, go to the main TOPCAT window -> Display column meta data -> Columns menu -> New synthetic column
  • To visualise the information on the brown dwarf candidate, you can select the name of your subset in the "Row Subset" menu in the main TOPCAT window and broadcast it to Aladin (if Aladin is opened) or any other VO application compatible with the SAMP interoperability standard

Third workflow: Do the query and selection with Vizier, cross-match with TOPCAT or Aladin

  • Vizier enables you to query catalogs with selection criteria. Go to the Vizier webpage: http://vizier.u-strasbg.fr/viz-bin/VizieR
  • Type "SDSS" in the description, give the RA:08h 30m and DEC:01d 30m coordinates as well as target radius of 14 arcminutes, and press "Find Catalogues"
  • Select the SDSS Photometric Catalogue
  • Give constraints on the class of objects (=6), the u magnitude (>22.0) and g magnitude (>22.2) columns
  • Change the number of entries per table to a large number (in order to see the 5829 sources) and the output to VOTable (doing so, the table will be readable by all VO tools)
  • Read the VO table file with Aladin or Topcat and proceed as described above (first flow for Aladin, second flow for TOPCAT)

Scripting with Aladin: Aladin has a script mode, where you can build a list of commands to be processed (Tool > Macro Controller). The workflow can be executed automatically for a list of targets.

  • Open the Tool > Macro Controller in Aladin
  • Load the script Aladin_workflow_script
  • Load the parameters Aladin_workflow_params.txt (note that the coordinates columns need be separate by TABS)
  • Execute the script for one or all the parameters
  • Modify the script to save some results for example (Help > Help on script commands)