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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)
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Baryonic DM
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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)

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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(ltr) / r A flat
rotation curve means that the total M(ltr)
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.
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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.
Einstein Radius
Magnification
Time scale
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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 lt 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)
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Brown Dwarfs and Low Mass Stars

• Low mass objects
• Late M dwarfs 0.07-0.08 lt M/M? lt 0.6
  ( H burning limit)
• Brown dwarfs 0.01 lt M/M? lt 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
• ? BDs mass density 15 of the stellar mass
  density. (Reid et al 1999)
• ? No short duration microlensing events

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)
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Ancient Halo White Dwarfs

• MACHOs favored candidates are very old, cool
  white dwarf (the evolutionary end state of all
  stars having masses lt 8 M? ?) which have mean
  masses of 0.5 M? ? (m/L gt 104M? /L? )
• Recently new models predict unusual colors and
  magnitudes for the oldest (coolest) WD. Hydrogen
  atmosphere WD with ages gt10 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).

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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,

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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.
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Spectra of cool WD
Spectrum of the very cool degenerate WD 0346246
(Hodgkin et al 2000). This WD was discovered by
Hambly et al. 1997. They measured an absolute
parallax of 365 mas , yielding a distance
estimate of 284 pc. The resulting absolute
visual magnitude of the object is MV16.80.3.
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HST Faint blue objects toward the HDF North
and South
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Surveys in progress
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GSC-2The 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!!!)

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The observative parameters of GSC-2

• All sky observations (gt1 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.

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Object selection criteria

• Halo WDs are difficult to identify, due to their
  faint magnitude (Mv gt 15) and the small number of
  these objects. We select
• High proper motion stars, ? gt 0.5 /yr, derived
  from plates with epoch difference ?T 1,10 yr
• Faint targets Rgt18
• Color J-F lt 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, Luytens LHS, etc)

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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 Hgt22 objects)
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Spectroscopic follow-up first results

• Low resolution spectroscopy performed at
• 4.2 m William Herschel TelescopeISIS specrograph
  (La Palma) -
• 3.5 m TNGDOLORES (La Palma)
• 3.5 m APO (Apache Point Obs., USA)

New discover coolish WD, observed at WHT on 27
January, 2001.