Discovery of Brown Dwarfs mining the 2MASS and SDSS databases

  • Tools: Aladin, 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 among others.
  • Workflow: In this use case, we will explore different ways to do the same tasks (cross-match, sources selection,...)


First flow (Aladin): ALADIN is used to

  • Search 2MASS PSC and SDSS sources around RA:08 30 00; DEC: +01 30 00 (change radius to: 14 arcminutes).
    • 2MASS: 683 sources
    • SDSS: 9855 sources
  • Load a background image : from Image > DSS > DSS from ESO and choose 30'x30' size
  • 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.
  • We will then use Aladin Filters to make selections in the resulting catalogue plane. A Filter in Aladin can be used to select some sources, and/or change the symbols used to display them. In our case, we are interested in filtering, and we will :
    • Create a new filter plane. We will edit the filter expression to display only sources matching some criteria. The basic syntax of a filter is condition {action}
    • Select the filtered sources (right click on plane > Select all objects in the selected plane)
    • Catalog > Create a new plane with the selected sources
  • Select points sources using the SDSS flag (cl=6) creating a filter plane
    • Create a filter whose expression is something like ${cl_tab2}==6 { draw }
    • Select filtered sources and export as a new plane containing 643 sources
  • With another filter plane, select sources with no detection in the u,g SDSS filters (u > 22.0 && g >22.2), and again export as a new 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 using Simbad. (Load SIMBAD from the Catalogue Servers, try to use the Grab Coo). You can also try the Tool > Simbad automatic pointer.


Second flow (TOPCAT) : TOPCAT is used to

  • Search the 2MASS and SDSS sources with a cone search from TOPCAT around RA:08 30 00 DEC:+01 30 00
    • 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 (from the Vizier service)
    • Give the coordinates and a radius of 14'
    • Repeat for the SDSS search (select the DR7 SDSS calatog from the Vizier service)
    • 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. Select the coordinates given in each catalogue (RAJ2000, DEJ2000)
  • Visualise the cross-matched raws and define a raw subset for which the SDSS class is equal to 6 --> 644 sources are found.
  • Select sources with no detection in the u,g SDSS filters (u > 22.0 & g >22.2) by defining a new raw subset (7 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


If you have time, try to understand why we have 643 objects in the first flow and 644 in the second.

Try to load in Aladin images from various surveys, and use different ways to compare them :

  • multiview
  • RGB composition
  • blinking
  • contours

Third flow (Advanced scripting / 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.

  • If you still have Aladin, delete all planes (or start a new Aladin session)
  • Open the Tool > Macro Controller in Aladin
  • Save the file Aladin_workflow_script.ajs and load it as script
  • Save the file parameters and load it as parameters
  • Execute the script for one or all the parameters
  • Try to figure out the operations that are performed, and modify the script
  • More information can be found at: "Help > Help on script commands"


-- EnriqueSolano - 11 Jan 2010

-- MarkAllen - 18 Dec 2009


This topic: EuroVOAIDA/VOSchool10 > WebHome > ThematicTutorials > ThematicTutorial4
Topic revision: r3 - 26 Jan 2010 - SebastienDerriere
 
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