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VII GSC2 ANNUAL MEETING BAROLO, 2001, 22-23
OCTOBER
Search for Ancient Cool White Dwarfs in the
Galactic Halo using GSC2 material Daniela
Carollo (Osservatorio Astronomico di
Torino) Thanks to the contributions of
- M. Lattanzi (OATo) - B. McLean (STScI,
Baltimore) - A. Spagna (OATo) - R.
Smart (OATo) - S. Hodgkin (IA, Cambridge -
UK) - A. Zacchei (TNG)
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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 which are evolving rapidly

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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(rotation curve means that the total M(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 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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Ancient Halo White Dwarfs

• MACHOs favored candidates are very old, cool
  white dwarf (the evolutionary end state of all
  stars having masses 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).

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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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DA 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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Cooling sequence for DA and non-DA WDs

• MV vs. (V-I) color- magnitude diagram for a data
  set of cool WDs drawn from a Yale catalogue
  parallax and a proper motion survey in the
  southern hemisphere. In the sample are present DA
  (filled circle) and non-DA stars (open circle)
• Left panel data set with a
• superimposed pure hydrogen
• model sequences
• Right panel data set with
• a superimposed pure helium
• model sequence
• Temperatures are indicated in units of 1000 K,
  and M 0.4, 0.6, 0.8, 1.0, and 1.2 M from top to
  bottom

Bergeron et al., April 2001, ApJ
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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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Main Results from other surveys

• Ibata et al. 2 Halo WDs in 790 deg2 which
  correspond to the 10 of the local density of the
  standard dark matter halo model (MACHO
  collaboration, Alcock et al, 1997)
• The most extensive survey to date (Oppenheimer et
  al 2001a) 38 Halo WDs in 5000 deg2. They
  estimate the lower limit of the space density to
  1 of the expected local halo density

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Oppenheimer et al survey a matter of debate

• Reid et al.(2001) most of the WDs identified by
  Oppehimer et al. are member of the disk
  population (thick disk) and then their evaluation
  of the local density is not correct
• Cèline et al.(2001) most high velocity WDs in
  the Oppenheimer sample can be interpreted as disc
  and thick disc stars. This is due to a bias
  introduced by the selection of high proper motion
  which change the velocity dispersion curve. Thick
  disc population could be a non-negligible part of
  high proper motion selected sample, as
  consequence thick disc WDs could be not easily
  distinguished from halo WDs

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WD Velocity distribution from the Oppenheimer et
al. survey

• U and V components of the sample
• Dashed ellipses indicate the velocity
• dispersions of the galaxy halo (left)
• and thick disk (right), while solid
• ellipses shown the 2s dispersions.
• Most of the new Halo WDs fall in 1s
• or 2s velocity dipersion of the halo
• The space density of Halo WDs is

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Aims of the Project

• Find more nearby Halo WDs and improve the
  measurements of its space density
• Confront WD models with a well defined sample of
  cool ancient objects. In fact, the cooling tracks
  of WDs with Teff

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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.

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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, ? 0.3 /yr, derived
  from plates with epoch difference ?T 1,10 yr
• Faint targets R16
• Color J-F 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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Expected number of halo WDs

• Area covered r 1 10-4 r 710-4
• (SSS)
  (Ibata)
• 1000 deg2 1
  5
• 5000 deg2 3
  20

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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 H22 objects)
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Project Status

• Number of fields processed 16
• Number of square degree covered 550
• Number of selected candidates 89
• Number of spectra follow-up 30
• (including some interesting LHS stars)

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Preliminary results

• New Cool WDs discovered 6
• Four of these shown H-alpha line (5000-6000 K)
• One is a binary system WDdM and probably the WD
  component is cool
• One is a very cool Helium WD stars

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While we look for WDs ...we are discovering
other very interesting objects

• Cdwarf in a binary system (Carollo et al 2001, in
  preparation)
• Magnetic WDs with a very high temperature

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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.
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Very Cool He-WD

• Low resolution spettroscopy
• No evidence of H-alpha line
• Temperature from B-B fit
• is 2800 K.
• We got UBVRIZ photometry
• at TNG telescope
• If the temperature is confirmed, this star will
  be the coolest He-WD till now discovered

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Cdwarf in a probably binary system

• Cdwarf are a very rare class of objects
• Peculiar spectral features which includes strong
  C2 absorption bands similar to C giant, high
  proper motion, low luminosity
• and dwarf-like-near infrared JHK colors
• Only a dozen of this stars are known and five of
  them are in a binary system

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Cdwarf Spectrum

• Object classified as QSO due to his UV excess!
• Spectroscopy points out the UV excess, which an
  indication of the presence of a hot WD companion

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Probably Magnetic WD
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Proposals for low resolution spectroscopy

• In progress AOT4
• Telescope 3.5 m TNGDOLORES (La Palma)
• Next Run November 17-18
• Submitted AOT5
• First semester 2002
• Submitted PATT2-First semester 2002
• Telescope 4.2 m William Herschel
  TelescopeISIS specrograph (La Palma)

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Other proposals

• Submitted PATT2-First semester 2002
• Halo WDs Photometry
• Telescope 1 m JKT
• Submitted He-WD Optical Photometry
• JKT Cassegrain Imaging - UBVRIZ
• Submitted He-WD Near Infrared Photometry UKIRT
  service program - JHK