Spanish Virtual Observatory


Primer Curso Práctico de la Red Temática SVO.

27-28 Octubre 2009

Case 1: Discovery of Brown Dwarfs mining the 2MASS and SDSS databases

  • 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.

We propose here 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, an approach that perfectly fits into the Virtual Observatory.

  • Workflow:

With this use case, we explore different ways to do the same tasks (cross-match, sources selection,...) with different VO tools.

  • First flow (Aladin): ALADIN is used to
  • Search 2MASS PSC and SDSS sources around RA:08h 30m DEC:01d 30m (default radius: 14 arcminutes).
  • 2MASS: 683 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.
  • Select points sources using the SDSS flag (cl=6) creating a filter plane: 643 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
  • (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 Virtual Observatory option.

  • Second flow (TOPCAT): TOPCAT is used to
  • Search 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 (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 --> 643 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 (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

  • 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.
  • Open the Tool > Macro Controller in Aladin
    • 2mass = get VizieR(2MASS) $1 $2 14'
    • sync
    • sdss = get VizieR(II/282/sdss6) $1 $2 14'
    • sync
    • 2massdss= xmatch 2mass sdss 4 bestmatch
    • sync
    • hide 2mass
    • hide sdss
    • sync
    • filter stars_SDSS { ${cl_tab2}==6 {draw} }
    • sync
    • select 2massdss
    • sync
    • cplane stars
    • sync
    • hide 2massdss
    • hide stars_SDSS
    • sync
    • filter candidates1 {${umag_tab2}>22.0 && ${gmag_tab2}>22.2 && ${Jmag_tab1}-${Hmag_tab1}<0.3 && ${Hmag_tab1}-${Kmag_tab1}<0.3 {draw}}
    • hide stars_SDSS
    • sync
    • select stars
    • cplane candidatos
    • sync
    • hide stars
    • hide candidates1
    • sync
  • The parameters
    • $1=08 30 00
    • $2=+01 30 00
  • Execute the script for one or all the parameters
  • More information can be found at: "Help > Help on script commands"