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
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
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
Cool White Dwarfs
• MACHO favorite candidates are very old, cool
white dwarf (the evolutionary end state of all stars
having masses m < 8 M) which have mean
4
masses of 0.5 M (m/L > 10 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)
State of the Art in the Halo
WD search
Material
Limit
Magnitude
Area Covered
(deg2)
Number of
Objects
Found
Survey
R = 19
790
Ibata
Photographic
Plates
2 Halo WD
Spectra conf.
R = 19
1378
Monet
Photographic
Plates
1 Halo WD
Spectra conf.
CCD
R = 23
2.5
3 High Proper
Motion Objects
No spectra
CCD
I = 20.5
250
0 Halo WD
Photographic
Plates
R = 19
5000
Few candidates
Spectra next year
De Jong
EROS
Super Cosmos
Expected number of halo WDs
Using GSCII Data
Area
covered
1 = 1.4 ·10-4
(1 = 2 /5)
2 = 7.0 ·10-4
(Ibata)
3 = 3.5 ·10-3
(3 = 5 · 2)
1000 deg2
<1
~5
~ 20
5000 deg2
~3
~ 20
~ 100
6000 deg2
~5
~ 27
~ 135
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.
Spectra can provide:
• Effective temperature
• Metallicity
• Radial Velocity
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 (m > 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 DT = [1,10] yr
• High galactic latitude field: low crowding
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)
Operative Selection
•
•
•
•
•
•
•
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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(m) + 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)
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
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
Summer Candidates Spectra
• Ask to Brian
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