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

LAST UPDATE 11/07/2016 18 :00h

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

• VO-Tools: ALADIN, TOPCAT

• Scientific 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 2MASS-PSC and SDSS-DR9 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:

In this use case, we will explore different ways to do the same tasks (cross-match, sources selection, filtering,...) using ALADIN and TOPCAT.

• First flow (Aladin): 

• Launch Aladin: Open a terminal and type: java -jar Aladin.jar &

• Search 2MASS-PSC and SDSS-DR9 sources around RA:08h 30m DEC:01d 30m with a 14 arcmin. radius.

• File --> Open --> Surveys
• Write 08:30 01:30 in the "Target" box
• Click on the 2MASS-PSC catalogue. "2MASS-PSC" will appear in the "Survey" box.
• Click on "Submit". Aladin will load the 2MASS-PSC plane with 683 sources.
• Repeat the same steps for the SDSS-DR9 catalogue. A new plane will be loaded with 12404 sources.

• Find common sources in 2MASS-PSC and SDSS-DR9 catalogues.
  • Catalog --> Cross match objects 
  • First catalogue: 2MASS-PSC. Second catalogue: SDSS-DR9.
  • Default threshold (4 arcsec) and bestmatch option
  • Click on "Perform cross-match". A new plane ("X-match") with 679 sources will be loaded.

• Select sources fulfilling the following conditions:
  
  • Points sources in the SDSS catalogue (cl=6)
  • Sources with no detection in the u,g SDSS filters (u > 22.0 && g >22.2). Brown dwarfs are cool objects so they are not detected in the blue SDSS filter.
  • Sources following the brown dwarf criteria provided by Burgasser et al. (2000, Apj, 531, L57).(J-H)<0.3 && (H-K)<0.3
    
    • Hide (cursor on the name of the plane and then click mouse right button) the 2MASS-PSC and the SDSS-DR9 planes
    • Click on the "Xmatch" plane
    • Catalogue --> Create a filter (or click on the "Filter" button on the right panel of Aladin main window).
    • Advanced mode --> Columns --> Columns in loaded catalogue. Select the appropriate columns from the Xmatch catalogue
    • Complete the filter condition. It should be ${cl_tab2}=6 && {umag_tab2}>22.0 && {gmag_tab2}>22.2 && ${Jmag_tab1}-${Hmag_tab1}<0.3 && ${Hmag_tab1}-${Kmag_tab1}<0.3 {draw}
    • Click on "Apply". Then click on "Export" to create a new plane with the filtered sources.
    • A new plane "Filter.src" will be loaded in the Aladin main window. It will contain 1 source. (RA:127.703265deg; DEC:1.475320deg).

 We can now use Simbad to confirm the brown dwarf nature of this object

• File --> Open --> Simbad
• Click on "Grab coord" and click on the source. The coordinates of the source will be copied to the "Target (ICRS, Name)" box.
• Select a 30arcsec radius. Click "Submit".
• A new plane "Simbad" will be loaded in the Aladin main window.
• Click on the "Simbad" plane. Select all sources in this plane (mouse right button). A table containing one row will appear at the bottom of the Aladin main window.
• Click on the Main ID. Simbad will be launched in your browser with information on the source.

• Second flow (TOPCAT): TOPCAT is used to

• Launch TOPCAT: Open a terminal and type: java -jar topcat-full.jar &

• Search 2MASS-PSC and SDSS-DR9 sources around RA:08h 30m 00s  DEC:01d 30m 00s with a 14 arcmin. radius. Change RA, DEC and radius units to “hh:mm:ss”, “dd:mm:ss” and “arcmin”, respectively.

• In the main window: VO--> Vizier Catalogue Service.
• Cone selection: Give the coordinates and a radius of 14'.
• Catalogue selection: Surveys --> 2MASS PSC. --> OK
• Repeat previous steps for SDSS-DR9.
• Alternatively, you could load the 2MASS and SDSS DR9 catalogs in Aladin and broadcast the planes to TOPCAT using SAMP.

• Find common sources in 2MASS-PSC and SDSS-DR9 catalogues.
  • Joins --> Pair Match
  • In the "Match Table" windows
    • Max error: 4 arcsec
    • Table1: II_246_out (2MASS_PSC). RA/Dec columns: RA_J2000, DE_J2000
    • Table2: V_139_sdss9 (SDSS-DR9). RA/Dec columns: RA_J2000, DE_J2000
    • A new plane "match(1,2)" with 679 sources is loaded.

•  Select points sources using the SDSS flag (cl=6) 
  • View --> Row subsets
  • In the "Row subsets" window: Subsets --> New subset
  • Fil in the "Subset name" box (for instance, filt1)
  • Expression: cl = =6. Click OK.
  • In the TOPCAT Main Window select "filt1" in the "Row Subset" box. 649 sources are selected.

• Select sources with no detection in the u,g SDSS filters (u > 22.0  & g >22.2). Brown dwarfs are cool objects so they are not detected in the blue SDSS filter. 
  • Update the selection criteria adding umag>22.0 & gmag>22.2. 6 sources are selected.

• Select sources following the brown dwarf criteria provided by Burgasser et al. (2000, Apj, 531, L57).(J-H)<0.3 & (H-K)<0.3.
  • Update the selection criteria adding (Jmag-Hmag)<0.3 & (Hmag-Kmag)<0.3. 1 source is selected. RA:127.703265deg; DEC:1.475320deg.