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Dark Matters in Torino
- Villa Gualino, February 21, 2000
Baryonic Dark Matter: Search for Ancient
Cool White Dwarfs in the Galactic Halo
Daniela Carollo, Alessandro Spagna
(Osservatorio Astronomico di Torino)
Thanks to the contributions of:
- M. Lattanzi (OATo)
- R. Smart (OATo)
- B. McLean (STScI, Baltimore)
- S. Hodgkin (IA, Cambridge - UK)
- A. Zacchei (TNG)
Baryonic DM
Component
Fraction
Total baryons
0.04*
Cold dark matter
~0.30
Dark energy
~0.66
* Where:
 Stars and cold gas: 0.01
 Hot gas:
0.03
From: S. van den Bergh, 2000, Proc. of "Development of Modern
Cosmology", astro-ph/005314
Evidence of Dark Matter in galactic halos
The Milky Way and most other
galaxies possess halos of dark
matter that extend well beyond the
the visible components of the
systems. These are evidenced by:
• Rotation curve of galactic disks.
The flatness of velocity rotation
need to be supported by a
dominant invisible component.
• Microlensing events: the
observed frequency is 3-4 times
that expected because of the
known stellar populations of the
Milky Way (MACHO, EROS,
OGLE collaborations)
Rotation curves of galactic disks
Stars and gas in the galactic disks
follow circular orbits whose
velocity depends on the inner
mass only:
v2(r) = G M(<r) / r
A flat rotation curve means that
the total M(<r) increases linearly
with r, while the total luminosity
approaches a finite asymptotic
limit as r increases. Clearly a large
amount of invisible gravitating
mass (more than 90% of the total
mass in the case of the Milky Way
and other examples) is needed to
explain these flat rotation curves.
No evidence exists of disk DM in
the solar neighborhood (from
analysis of stellar velocity
dispersions).
Rotation curve of the spiral galaxy NGC
6503 as established from radio observations
of hydrogen gas in the disk (K Begeman et al
MNRAS 249 439 (1991)). The dashed curve
shows the rotation curve expected from the
disk material alone, the chain curve from the
dark-matter halo alone.
Gravitational Microlensing
This effect (Pacynski 1986) permits the detection of invisible compact and massive obiects
(MACHOs) which transit near the line of sight to a background star. The distortion is too
weak to produce multiple resolved images. The event can be revealed by the photometric
signature which produces a temporary increase of apparent brightness due to the light
being deflected by the gravitational field of the dark MACHOs. An astrometric signature
(variation of position) is also predicted.
1/ 2
 4GM 
 E  2 
 c 
A
u2  2
u u2  4

2E

Einstein Radius
,u 

E
Magnification
Time scale
Microlensing results
~20% of the galactic halo is made
of compact objects of ~ 0.5 M
MACHO: 11.9 million stars toward the
LMC observed for 5.7 yr  13-17 events
 8%-50% (C.L. 95%) of halo made of
0.15-0.9 M compact objects.
EROS-2: 17.5 million stars toward LMC for
2 yr  2 events (+2 events from EROS-1)
 less that 40% (C.L. 95%) of standard
halo made of objects < 1 M
Candidate MACHOs:
• Late M stars, Brown Dwarfs, planets
• Primordial Black Holes
• Ancient Cool White Dwarfs
Limits for 95% C.L. on the halo mass fraction in the form of
compact objects of mass M, from all LMC and SMC EROS
data 1990-98 (Lassarre et al 2000). The MACHO 95% C.L.
accepted region is the hatched area, with the preferred value
indicated by the cross (Alcock et al. 1997)
Brown Dwarfs and Low Mass Stars
Low mass objects:
•Late M dwarfs:
0.07-0.08* < M/M < 0.6
(* H burning limit)
• Brown dwarfs:
0.01** < M/M < 0.075
(** D burning limit)
•Planetary objects (jupiters, M/M ~ 1/1000)
These objects do not seem to
constitute a substantial fraction of the
dark matter, in fact:
 BD’s mass density ~ 15% of the stellar
mass density. (Reid et al 1999)

BD ( M )  M ,  1.0  1.5
 No short duration microlensing events
 M
H-R diagram. Burrows et al. (1993,
1997) models for masses from 0.015
to 0.1 M. Solid points: VLM
dwarfs; open circles: four L dwarfs
with trigonometric parallax.
(Reid et al, 1999, 521,613)
Ancient Halo White Dwarfs
•MACHOs favored candidates are very old, cool white dwarf (the
evolutionary end state of all stars having masses < 8 M ) which
have mean masses of 0.5 M  (m/L > 104M /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)
• A few cool and faint WDs having kinematics consistent with halo
population have been discovered in wide photographic surveys
(Hambly, Smartt & Hodgkin, 1997) and in deep HST fields (Ibata et al
1999).
Ancient WDs as cool blue objects
Recent models of white-dwarf atmospheres point out the
dramatic effect of collision-induced absorption by molecular
hydrogen on the spectra of very cool, hydrogen-rich white
dwarfs.
At effective temperatures below 4,000 K, H2 molecules become
abundant in the atmosphere, and, as the collision-induced
absorption bands deepen, the peak of the resultant energy
distribution shifts to the blue.
References:
• Hansen, 1998, Nature, 394, 860
• Saumon & Jacobsen, 1999, AJ, 511
• Chabrier et al, 2000, ApJ, 543,
WD cooling tracks
Cooling sequences for different masses for the reference model DA WDs of
Chabrier et (2000). The green triangles correspond to the Leggett et al.
(1998) WDs identied as H-rich atmosphere WDs.
Spectra of cool WD
Spectrum of the very cool degenerate WD 0346+246 (Hodgkin et al 2000). This
WD was discovered by Hambly et al. 1997. They measured an absolute
parallax of 36±5 mas , yielding a distance estimate of 28±4 pc. The resulting
absolute visual magnitude of the object is MV=16.8±0.3.
HST
Faint blue objects
toward the
HDF North
and South
Surveys in progress
Survey
Limit
Mag
Solid
Angle
Volume
( pc 3 )
Ibata
R = 19
(deg 2 )
790
De Jong
I = 23.5
2.5
16000
EROS
I = 20.5
250
25500
SSS
R = 19
5000
30000
Monet
R = 19
1378
9000
GSC2
R = 19
2000
13000
5000
GSC-2
The Second Guide Star Catalogue
• The GSC-2 project is a collaborative effort between the Space
Telescope Science Institute (STScI) and the Osservatorio
Astronomico di Torino (OATo) with the support of the European
Space Agency (ESA) - Astrophysics Division, the European
Southern Observatory (ESO) and GEMINI.
• Based on about 7000 photographic Schmidt plates (POSS and
AAO) with a large field of view (6º x 6º) digitized by STScI (DSS)
• Astronomical catalogue containing classifications, colors,
magnitudes, positions and proper motions of ~ 1billion
objects up to visual magnitude V = 19 covering all the sky. (The
largest stellar catalog!!!)
The observative parameters of GSC-2
•All sky observations (>1 billion objects, mostly faint)
•J (blue), F (red), N (infrared) magnitudes
•Proper motions, , based on multi-epoch
observations (19502000)
•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.
Object selection criteria
Halo WDs are difficult to identify, due to their faint magnitude (Mv >
15) and the small number of these objects. We select:
•High proper motion stars,  > 0.5 ”/yr, derived from
plates with epoch difference T = [1,10] yr
•Faint targets: R>18
•Color J-F < 1.8 (corresponding to the turn-off of the
cooling tracks at V-I ~ 1.2, 1.5)
•High galactic latitude field: low crowding
• Visual inspection and cross correlation with other
catalogues (2MASS, Luyten’s LHS, etc)
Expected number of halo WDs
Using GSCII Data
Area
covered
r1  1.4 ·10-4
(r1 = r2 /5)
r2  7.0 ·10-4
(Ibata)
r3  3.5 ·10-3
(r3 = 5 · r2)
1000 deg2
<1
~5
~ 20
5000 deg2
~3
~ 20
~ 100
6000 deg2
~5
~ 27
~ 135
Reduced Proper Motion Diagram
The reduced proper motions
(Luyten 1922) is defined as:
H = 5 log  + m + 5
which corresponds to
H = M +5 log VT - 3.379
High values of H mean:
“faint & fast moving objects”
(We are interested in H>22
objects)
Halo WD candidate
(NGP Region 443)
First Epoch
Second epoch
Spectroscopic follow-up: first results
Low resolution spectroscopy
performed at:
•4.2 m William Herschel
Telescope+ISIS
specrograph (La Palma) •3.5 m TNG+DOLORES (La
Palma)
• 3.5 m APO (Apache Point
Obs., USA)
New discover: coolish WD, observed at
WHT on 27 January, 2001.