Massive Galactic Stars in Isolation

1
Massive Galactic Stars in Isolation

Willem-Jan de Wit (1) Leonardo Testi
(2) Francesco Palla (2) Hans Zinnecker (3)
1. CNRS, LAOG (Grenoble, France) 2. INAF-Arcetri
(Florence, Italy) 3. AIP (Potsdam, Germany)
2
Birth location of high mass stars
2/13

ONC 0.45kpc 103 /pc3
500pc
Do all high mass stars form in stellar clusters?
We may thus say that the great majority (if not
all) O stars are formed (...) in clusters and
associations Roberts 1957
HST, Yang Hester
.
NGC 604 (M33) 840 kpc 200 O stars (M gt 17.5 Msol)
3
Some O star statistics
3/13
e.g. Poster 125 Pflamm Kroupa

• 70 in clusters/OB associations
• 10 known runaways (SN binary / dynamical
  ejection)
• 20 field stars

Garmany et al. (1982) Gies (1987) Mason et al.
(1998) Maiz-Apellaniz et al. (2004) Blaauw
(1961) Gies Bolton (1986) Poveda (1967) Van
den Heuvel (1983) Hoogerwerf et al. (2001), etc.

• 43 O type field stars
• V lt 8m ( 2.5 kpc)

3
4
K-band imaging survey
4/13

• Deep (subsolar) imaging survey with NTT/SOFI and
  TNG/NICS
• FoV 5 x 5 arcminutes ( 2 x 2 pc)
• Sensitive to scales of 0.1pc
• 2MASS catalogue (less deep) scales 1 pc

RESULT

• 5/43 harbour a small cluster
• ISOLATED STARS

HD52533 O8.5V
3
5
5/13
HD15137
O9.5II
HD1337
O9.5III
HD120678
O8IIInep
HD117856
O9.5III
6
The runaway cases
6/13

Observables

• High space velocities gt 40 km/s
• Large distances (gt 500 pc) from the Galactic
  plane
• Nearly always single (not binary) objects
  
• D lt 65 pc to young clusters with age lt 10Myr

Kaper et al. 1997
3
7
The runaway cases (I)
7/13

• Using Hipparcos proper motion vrad (Gies 1987)
  to derive spatial velocities
• 7 field O stars have high (gt40km/s) spatial
  velocities.
• All 7 are single objects

Spatial velocities
3
8
The runaway cases (II)
8/13

• OB association (lt 3kpc) distribution from Mel'Nik
  Efremov (1995)
• Z(min) OB-assoc. max(Z) /- 65pc.
• 11 target stars with Z gt Z(min)
• 3 of which also have large Vpec

gt additional 8 field O stars were found lt 65pc
near a 10Myr cluster.
3
9
9/13
Summary

• 8 (20) field O stars to be part of newly
  discovered clusters (5) or existing OB
  associations (3).
• 24 (60) field O stars are found to be plausible
  OB-runaway candidates.

• 11 (20) O type field stars are isolated and not
  associable kinematically or spatially with
  clusters.

gt For the complete census of O type stars in the
solar neighbourhood (Vlt 8m) we thus find a
fraction of 4.
3
10
10/13
Galactic extragalactic examples
e.g. Poster 32 Deharveng et al. Poster 96
Marco Negueruela

LMC (52kpc)
M51 (8Mpc)
500 pc
150 pc
O3e
3
Lamers et al. 2002
Massey et al. 1998
11
11/13

Expected number of isolated O stars adopting 3
assumptions

• All stars are born in clusters.
• Clusters follow a powerlaw, downto
  clusters of a single star.
• Each cluster is populated by an IMF.

3
12
Stellar MF and cluster MF
12/13
N(O-star) per cluster

• MC calculations combining IMFs and CMFs
• Compute number of O-stars per cluster

N(star) per cluster
Cumulative distribution
13
13/13
Conclusions

• The large majority of the field O stars are
  isolated stars, and are plausible runaway stars.
  About a quarter of all O stars are runaways,
  reconfirming the importance of dynamics in early,
  dense phases of a massive stars cluster (see
  e.g. Clarke Pringle 1992).
• The large majority ( 95) of the O-type stars
  are born in a clustered environment.
• A fraction of 5 of the O-type stars can
  currently not be associated with
  clusters/OB-association.
• This observed number of field stars and the
  observed number of N(O stars)/cluster are
  consistent with the assumption of a powerlaw
  cluster number distribution (down to a single
  star) with slope -1.7 combined with a standard
  stellar IMF.

3
14
HD112244

• Visual binary with a K0III star (Lindroos 1986)
• Suspected single line spectr. binary
• O8.5Iab(f) --gt 50 Msol
• Associated with IRAS 60 micron emission

15
The situation in the Galaxyintermediate mass
HAeBe stars

• PMS stars
• Increase in cluster richness
• Constant cluster radius of 0.25pc

Testi et al. 1999
.. just the IMF ( cluster MF)
Bonnel Clarke 1999