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The new view on the population of Galactic open clusters
A.E.Piskunov1, N.V.Kharchenko2, S.RÆser3, E.Schilbach3, R.-D.Scholz4
1 2 3 4

Institute of Astronomy, Moscow, Russia Main Astronomical Observatory, Kiev, Ukraine Astronomisches Rechen-Institut, Heidelberg, Germany Astrophysikalisches Institut Potsdam, Germany


What are open clusters and what do they teach us
Groups of Population I stars having formed together; Excellent empirical reference sequences: basis of various scales and calibrations (distances, reddening, age, abundance); Traditional laboratories well suited for studies of stellar evolution (tracks, isochrones); Star formation: population of young clusters, luminosity functions, IMF; Relation to large scale structure and evolution of galaxies (spiral structure, rotation curve)


Previous studies of the open cluster population


Numerous dedicated studies restricted to a specific problem and/or sample (generally non-uniform and with unknown completeness):


cluster age scale (Loktin et al., 2004) cluster sizes (Danilov & Seleznev, 1993) many others...

- 500 objects - 100 objects

statistics of CCD-observed clusters (Tadross at al., 2002) - 160 objects



A few comprehensive studies based on the Lund catalogue of Open Cluster Data


Lynga (1982)

over 400 objects with data from the literature

Janes et al. (1988) over 400 objects with data from the literature (weighted).


Open clusters and present-day all-sky catalogs
Hipparcos/Tycho: 2.5 mln stars up to 12 ­ 14 mag, BT VT ; ASCC-2.5: 2.5 mln stars up to 12 ­ 14 mag, B V, PM compiled; 2MASS: 470 mln stars up to 17 mag, J H KS;

Open clusters in the Post Hipparcos era:
Dias et al. (on-line): compilation of <1700 open cluster data; Platais et al. 1998: 15 new clusters/stellar groups, Hipparcos; Alessi et al. 2003, 2004: 11,~50 new clusters, Tycho-2; Bica et al. 2003: ~350 new IR clusters/stellar groups, 2MASS; Current project 2004-2005: parametrs of 520 known and 130 new clusters, ASCC-2.5.


Aims of the current project:
To benefit from Hipparcos and other All-Sky surveys in a systematic study of the stellar population in open clusters; To identify in ASCC-2.5 all known open clusters and associations and find new clusters; To derive a uniform set of parameters of these clusters (membership,
coordinates, radii, proper motions, radial velocities, ages, reddenings, distances);

To determine the parameters of the galactic disk population of open clusters; To study selected star groups.


Cluster sample: the unified processing scheme
Known cluster lists
Member selection pipeline Coordinate selection Proper motion selection Photometric selection

Verification and preliminary parameter evaluation

Yes Is 1-member list changed? No

New cluster search pipeline
seed selection 221,000 stars, V<9.5 clustering search 2,500 objects (8 kinem.members, in areas r 0.3 deg)

Cluster parameters: Final member list
1-members: P >61% 2-members: P >14% 3-members: P > 1% coordinates of the center core and corona radii proper motion age distance and color excess


Radial Velocity

Membership probabilities
P=Pcmin{P,PPh}


Cluster sample: summary
The Sample

The sources
Stellar Data:


·



ASCC-2.5 (Kharchenko,2001): 2.5 mln compiled BV and PM 0.5 mln Spectral Classes CRVAD (Kharchenko, Piskunov, Scholz, 2004):
36,000 RVs

511 9 130 2

known clusters known associations newly detected clusters large nearby clusters

Cluster Lists and Data:



PM-CMD membership, radii, coordinates, distances, reddenings, proper motions, radial velocities, ag es

Cluster parameters

Dias et al.(on-line) Lynga (1987) Ruprecht et al. (1981)
Loktin et al. (2004) WEBDA (on-line) Robichon et al. (1999) De Zeeuw et al. (1999)


Example of known young open cluster

P re - M S


Example of a newly-discovered open cluster


Cluster radii determination
Spatially and instrumentally unbiased!
Previous radii estimates are systematically 2-3 times lower! No gap between clusters and associations
Comaprison with published values Linear radii (pc)

N o sp a t i a l b i a s d e t e c t e d


Cluster kinematical parameters
Comparison of proper motions with
Hipparcos Hipparcos Tycho-2 Robichon et al. Baumgardt et al. Dias et al.

Comparison of radial velocities with published data

Clusters with PM the Hipparcos sys New PM: Source:

directly in tem: 650 301+130 ASCC-2.5

Clusters with RV : 359 New RV : 94 +69 Source: CRVAD


Age determination of open clusters
s tar s ld ie tf h rig b of n tio lec e r-s e nd U ers mb me t gh i br of ing t lec g Ne

Technique: average of individual ages of evolved stars at turn-off point Isochrones: Padova,2002 Comparison: Loktin,2004 Sample: 650 clusters

New ages: 195 +130


Spatial distribution:
the surface density density profiles at different ages
log t <7.9 (269 clusters)

Completeness limit

completeness Density enhancement Background level ~800 missing clusters background 552 clusters

log t =7.9-8.3 (101 clusters)

log t =8.3-8.6 (132 clusters)

log t >8.6 (150 clusters)


Spatial distribution: clusters and
interstellar extinction dust concentration
< 0 1 2 > 0 .5 .5 .5 3 .5 -1.5 -2.5 -3.5 .5

AV, mag
He avy ob scu rat ion

He av y

ob sc ur at io

n

He avy ob scu rat ion
He av y ob sc ur at io n

< 0 1 2 >

0.5 .5--1.5 .5--2.5 .5--3.5 3.5

aV = AV/d, mag/kpc


Spatial distribution: the view in the galactic plane
Symbols: logt <7.9 logt =7.9-8.3 logt =8.3-8.6 logt>8.6 Dias et al.(comp
:

°
â

lementary)

Logt < 7.9 a ll


.

Grand design with pitch angle=-6°


Spatial distribution: Z-profile parameters
D(Z) = D0 â exp{-| Z ­ Z 0| / h Z }
log t <7.9

259 clusters

d<850pc
log t =7.9-8.3

Z0 Zst hZ D0

-22 pc 74 pc .div. 56 pc 1015 kpc-3 114 kpc-2 (4-5 times larger!) Predicted number of open clusters within RG=15 kpc 93,000
The Sun is 22 pc above the symmetry plane of the cluster population

log t =8.3-8.6

log t >8.6


Kinematics of open clusters: general
Oort constants [km/s/kpc]: A = +14.5±0.8 B = -13.0±1.1 Solar motion [km/s]: (U,V,W)= (+9.4, +11.9, +7.2) Velocity ellipsoid [km/s]: (U,V ,W ) = (13.9, 8.8, 5.0)

New kinematic (Hyades) group

(U : V : W )=
(1 : 0.63 : 0.36) Cosmic dispersion [km/s]:

Tl = 1.44 (log t - 6) +3.93 Tb = 1.33 (log t - 6) +1.65

C B B R

yan curves ­ nearer limit of distance bin lue curves ­ farther limit of the distance bin lack points ­ known clusters ed points ­ new clusters


Kinematics of open clusters:
tangential velocity VPDs and the OCC membership
Kinematic data Proper motions RV (log t<8.3) RV (log t>8.3) : 100% 75% 31%

OCC-1

Perseus-Auriga OCC-2


Tnagential velocity probabilities

log t < 7.9

log t = 8.3...8.6 Hyades

Probabilities: 1 : P t> 6 1 % 2 : Pt=14 - 61% 3 : Pt= 1- 13 %

log t = 7.9...8.3

log t > 8.6

Does kinematics support spatial clustering?


Cluster complexes: OCC-1 = Gould's Belt cluster complex?
Membership Surface density profile (d<500pc): total 37 background 16±4 excess 20±4 Kinematics: 1 22 OCC-1 parameters: (X,Y)c = (-78,-53) pc (X, Y, Z) = (0.9, 0.75, 0.35)kpc

Z = -46 pc i = 19° Rotation: = 8.6±2.2 km/s/kpc log age = 6.7...7.8 groups: Orion Vela Sco


Cluster complexes: Newly identified OCC-2 and OCC-3
Membership Density profile (d<650pc): total 53 background 33-20 excess 20-33 Kinematics: 1 -27 OCC-2 parameters: (X,Y)c = (0,0)? kpc X,Y,Z = (1.2,1.2,0.2) kpc log age = 8.48±0.09 groups: OCC-2a OCC-2b Perseus-Auriga group parameters: kinematic members: 9 (X,Y)c = (-0.4,0.2) kpc X,Y,Z = (0.3,0.3,0.2) kpc log age = 8.52±0.14

OCC-3 OCC-2a

OCC-2b


Cluster complexes: ho

w do they look like in the general environment ? Contours ­ interstellar clouds Blue triangles - OCC-1 Red circles ­ OCC-2 Magenta diamonds Perseus-Auriga group Black crosses - Hyades moving group
Small circles "field" clusters


Evolution of the cluster population:
age distribution and lifetime
P=1/ ­ decay probability a ­ cluster age, a = t1 ­ t, t1 present, t at birth

­ rate of cluster formation N(a) = (t1-a) e -a/
t1/2 0.7
All clusters Complete sample Wielen 1971

= 364±32 Myr = 0.24± 0.02 1/Myr

Literature: = 140-240 Myr = 0.1-0.25 1/Myr


Evolution of the cluster population:
clusters and field stars Number of stars ever born in open clusters n* = ng â no* â

ng = t1/ = 10-12 Gyr/0.36 Gyr = 30-40 no* = 1000 stars (for typical open cluster) = 114 kpc-2 N* = 7â107 kpc-2, field stars

n* = 3.6â106 n*/N* = 6-10%

Optical open clusters still do not provide all Galactic disk stars!


Future work

Studies related to cluster dynamical status (radii, masses...); Systematic search of cluster groups and their relation to moving groups of field stars; Going fainter and deeper; Study of large area star forming regions.