2. HIGH PROPER MOTION WHITE DWARF CANDIDATES GSCII Annual Meeting October 19-20 2000 CBBS, Stevensville (MD) by Daniela Carollo Osservatorio Astronomico di Torino M.G. Lattanzi, B. McLean, R.L.Smart, A. Spagna

3. Why look for WD in the Milky Way? Dark Matter problem: halo WD could explain the recent results of microlensing events Galactic evolution: the oldest (than coolest) WD give an estimation of the limit age of the galactic disk Stellar evolution comprehension: new experimental points are needed to add to the theoretical cooling sequences

4. Dark Matter Problem Spiral Galaxy Rotation Curves show a flat disk rotation curve which is a strong evidence of a massive “ Halo of Dark Matter” surrounding the Galaxy Several types of dark matter are candidate: remnants from early epochs of galactic star formation (white dwarf, neutron stars), remnants from the early epochs Universe (subatomic particles, primordial black-holes) MACHO project observations suggest that 10%-20% of the dark halo is composed from compact objects having masses of ~ 0.5M 

5. Cool White Dwarfs MACHO favorite candidates are very old, cool white dwarf (the evolutionary end state of all stars having masses m 10 4 M  /L  ) Recently new models predict “unusual” colors and magnitudes for the oldest (coolest) WD. Hydrogen atmosphere WD with ages > 10 Gyr have suppressed red and near infrared fluxes, and they look blue (Hansen, 1998)

8. State of the Art in the Halo WD search Survey Material Limit Magnitude Area Covered (deg2) Number of Objects Found Ibata Photographic Plates R = 19 790 2 Halo WD Spectra conf. Monet Photographic Plates R = 191378 1 Halo WD Spectra conf. De Jong CCDR = 232.5 3 High Proper Motion Objects No spectra EROS CCDI = 20.5250 0 Halo WD Super Cosmos Photographic Plates R = 195000 Few candidates Spectra next year

10. Expected number of halo WDs Using GSCII Data Area covered  1  ·10 -4 (  1 =  2  /5)  2  7.0 ·10 -4 (Ibata)  3  3.5 ·10 -3 (  3  = 5 ·  2 ) 1000 deg 2 < 1 ~ 5 ~ 20 5000 deg 2 ~ 3 ~ 20 ~ 100 6000 deg 2 ~ 5 ~ 27 ~ 135

11. The observative parameters GSC2 data can provide All sky observations (>1 billion objects, mostly faint) J (blue) magnitude, F (red) magnitude, N magnitude Colors: J-F, F-N Proper motions Object classification The selection of WD candidate can be performed by means of all these parameters. In any case, spectroscopic follow-up is required in order to confirm the nature of these candidates.

12. Spectra can provide: Effective temperature Metallicity Radial Velocity

13. Object selection criteria Halo WDs are difficult to identify, due to their faint magnitude (Mv > 15, and the small number of these objects. An efficient methods is to select: High proper motion stars (  > 0.5 “ /yr) Faint target: R>18 Color: cooling track inversion point fall in V-I ~ 1.2, 1.5 (late K) which correspond to J – F ~ 1.5 – 1.8 (indicatively), then we search high proper motion objects with J-F < 1.8 Plates with epoch difference  T = [1,10] yr High galactic latitude field: low crowding

14. Some Advantages Residual astrometric systematic errors are not a problem because in any case these are much smaller than the high PM of WD For the same reason, relative proper motions which sufficient for this search (cor. to absolute reference frame ~ 0.01 as/yr)

15. Operative Selection Matching algorithm (three POSSII plate) Proper motion algorithm Color-Magnitude Diagram Color-Color Diagram Vector Point Diagram Total PM error vs magnitude Reduced PM Diagram (H = J + 5log(  ) + 5) A short list of high proper motion candidates, which can include other peculiar objects (eg. M dwarf.) Visual inspection and cross correlation with other catalogues (2MASS, Luyten, etc)

16. Future Observing Proposal In order to take spectra, recently we submit an observative proposal (PATT collaboration) for the 4.2m William Herschel Telescope (Roque de Los Muchachos Observatory, La Palma, Spain). Involved Institutes: STScI, Institute of Astronomy of Cambridge and Torino Astronomical Observatory The observing semester will be February-July The TAG meeting for the final decision will be in November 24

18. Plate selected in the GSCII archive We select 25 fields in the GSCII archive (some of them are processed) They have RA fall in the range [8, 20] Area covered: 1000 square degree

19. Summer Candidates Spectra Ask to Brian

20. LHS Stars We insert also in the PATT proposal a list of 14 LHS stars without spectroscopy. These objects are selected from the RPM diagram from Luyten’s catalogue and have properties consistant with cool white dwarfs, we expect to “ rediscover” many of these stars in the rest of our survey