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)

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

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 MicrolensingThis 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.

2/1

2

4

c

MGE Einstein Radius

E

uuu

uA

,

4

22

2

Magnification

E2

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)

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 becomeabundant 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,

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.

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 asuperimposed 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

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.

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

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

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 2dispersions.

• Most of the new Halo WDs fall in 1

or 2 velocity dipersion of the halo

• The space density of Halo WDs is:

3sun

4 pc M 103.1 -

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 < 4000 K need to be improved

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 (19502000)•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, > 0.3 ”/yr, derived from plates with epoch difference T = [1,10] yr

•Faint targets: R>16

•Color J-F < 2.0 (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

Area covered ·10-4 ·10-4

(SSS) (Ibata)

1000 deg2 ~ 1 ~ 5

5000 deg2 ~ 3 ~ 20

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

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)

Preliminary results

• New Cool WDs discovered : 6

• Four of these shown H-alpha line (5000-6000 K)

• One is a binary system WD+dM and probably the WD component is cool

• One is a very cool Helium WD stars

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

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.

Very Cool He-WDLow 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

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

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

Probably Magnetic WD

Proposals for low resolution spectroscopy

• In progress: AOT4

Telescope: 3.5 m TNG+DOLORES (La Palma)

Next Run: November 17-18• Submitted: AOT5

First semester 2002• Submitted: PATT2-First semester 2002

Telescope: 4.2 m William Herschel Telescope+ISIS specrograph (La Palma)

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