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)

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 ModernCosmology", astro-ph/005314

Baryonic DM

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)

Brown Dwarfs and Low Mass StarsLow 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)

No short duration microlensing events

5.10.1,)( MMBD

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 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,

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

SS uu rr vv ee yy LL ii mm ii ttMM aa gg

SS oo ll ii ddAA nn gg ll ee

)(deg 2

VV oo ll uu mm ee)( 3pc

II bb aa tt aa RR == 11 99 77 99 00 55 00 00 00

DD ee JJ oo nn gg II == 22 33 .. 55 22 .. 55 11 66 00 00 00

EE RR OO SS II == 22 00 .. 55 22 55 00 22 55 55 00 00

SS SS SS RR == 11 99 55 00 00 00 33 00 00 00 00

MM oo nn ee tt RR == 11 99 11 33 77 88 99 00 00 00

GG SS CC 22 RR == 11 99 22 00 00 00 11 33 00 00 00

Surveys in progress

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!!!)

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, > 0.5 ”/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 WDsUsing GSCII Data

~ 135 ~ 27 ~ 56000 deg2

~ 100 ~ 20 ~ 35000 deg2

~ 20 ~ 5 < 11000 deg2

r3 3.5 ·10-3

(r3 = 5 · r2)

r2 7.0 ·10-4

(Ibata)

r1 1.4 ·10-4

(r1 = r2 /5)Area

covered

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.