Cool white dwarfs in the Sloan& SuperCOSMOS Sky Surveys

Nigel Hambly, Wide Field Astronomy Unit, IfA, University of Edinburgh

Why study cool white dwarfs?

• most numerous remnants of their progenitor population

• mass-to-light ratio >10,000 makes truly “dark” matter

• WD LFs can be employed in cosmochronometry

• individual cool WDs can be very interesting

Multicolour imaging surveys yield stellar CMDs

• hot (blue) WDs relatively easily identified in sparsely populated regions of colour space

• cool WDs unidentifiable by colour alone (although …)

would ideally like HR diagram to determine luminosity class of all stars to identify the WDs.

Multi-epoch data provide the means via proper motions

Following Hertzsprung:

dvT 74.4/

5log5 10 dmM

cmM 5log5 10

where constant c is a function of tangential velocity. Thequantity on the RHS is called H, the Hertzsprung parameter or ‘reduced proper motion’. A plot of H versus colour yieldsan HR-ish diagram.

SuperCOSMOS

• high mechanical stability

• controlled environment

• fast

enables a whole-sky multi-epoch digitisation programme:

SuperCOSMOS Sky Survey

SuperCOSMOS Sky Survey: Hambly et al., 2001, MNRAS, 326, 1279

et seq.• Schmidt photographic imaging survey

• B,R,I (22.0, 20.5, 18.5); R at second epoch

• Currently Dec < +3.0; whole sky end 2005

• Absolute calibrations: m +/- 0.3; RA, Dec +/- 0.3” typically

• Relative calibrations: m +/- 0.05; RA, Dec +/- 0.03” at best

• Proper motions: typically < +/- 10 mas/yr

eg. SSS astrometry and SDSS photometry

• very clear separation of different populations, even with a small colour baseline

• this work accomplished by wholesale download of EDR data and heavy processing by user

• entire SSS & DR1 now incorporated in one-stop shop: the SuperCOSMOS Science Archive (SSA)

(work by Andrew Digby: MNRAS, 2003, 344, 583)

SuperCOSMOS Science Archive: features• catalogue data in a commercial DBMS: - SQL interface ( + JDBC/ODBC, …) - highly flexible and configurable - ease of indexing (eg. spatial & other) to expedite common queries

• high performance hardware - trawl 1 billion merged records in 15min - trawl at the server side; ship results only

• Virtual Observatory compatibility: - output formats include XML VOTable - Unified Content Descriptors (UCDs)

• basic interfaces with push-button functionality as well as generalised SQL

SSA: homepage

• ~4 TB catalogue data, including SSS, DR1, EDR 2MASS & USNOB

• comprehensive online documentation

• flexible user interface - basic (eg. cone search) - advanced (eg. free-form SQL for datamining)

http://surveys.roe.ac.uk/ssa

SSA : schema browser

http://surveys.roe.ac.uk/ssa

• extragalactic example:“select top 5 * from ReliableGalaxies where gCorMagB < 16.5”

bright galaxy catalogue made to order.

Flexible, (relatively!) simple SQL syntax:

• process (~10 min) at the server (no large downloads reqd.)

Combine SSS astrometry and SDSS DR1 photometry from the SSA:

• very clear separation of different populations, even with a small colour baseline

• high degree of completeness: - 15.0 < r < 20.0 - 40.0 < mu < 600.0 (mas/yr)

• see also: Gould & Kolmeier, ApJS, 152, 103 (2004) Munn et al., AJ, 127, 3034 (2004) (both using USNOB)

~1000 new cool WDs;

• very clear separation of different populations, even with a small colour baseline

• see also Gould et al. astro-pa/04????

• thousands of new, cool white dwarfs

• combined selection using Hr, (g-r) and (r-i)

Further analysis: two-colour diagrams

• very clear separation

• latest cool WD models from Didier Saumon & colleagues

Further analysis: model atmosphere fits

• only pure H fits done at this preliminary stage

Future work:

• DR2 …

• expand comparison with model atmospheres

• decomposition of sample into different kinematic populations

• computation of LFs with strict selection and precise analysis

eg. Digby et al. subdwarf LF calculations