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MODEST 6 -- Chicago, August 31, 2005
study of the environment
effect on “canonical”
evolutionary sequences
the “creation” of
ARTIFICIAL sequences as
Blue Stragglers Stars
and exotic objects
by-products of
binary systems
UVE
BSS have been detected for the first time by Sandage (1953)
according to their position in the CMD,
BSS should be more massive than
normal stars
merger of 2 low-mass stars
unevolved, massive star
primordial
binaries
BSS
direct
collisions
crucial link between
stellar evolution & stellar dynamics
loose GGCs
low c, low r0
high resolution studies
BSS also in the inner region
of high density GGCs
natural habitat
for BSS
• NGC6397 Auriere et al. 1990
• 47 Tuc Paresce et al. 1991
• M15
Ferraro & Paresce 1993
<1990
>1990
Catalogs:
Fusi Pecci et al. 1992
Sarajedini et al. 1992
Ferraro, Fusi Pecci, Bellazzini 1995
Guhathakurta et al. 1994, 1998
Piotto et al 2004
BSS are a normal population of GGCs,
found in each cluster properly observed
BSS are more concentrated than RGB stars
exceptions: M3, M55,47 Tuc..
UV sensitivity , high resolution
HST
systematic studies of hot SPs
in the core of high density GGCs
UV-plane ideal to study
the photometric properties
of the BSS population:
- the distribution is almost vertical
- span more than 3 magnitudes
M3
PHOT. PLATES
(Buonanno et al 1994)
HIGH RES. CCD
(Ferraro et al 1993)
HST IMAGES
(Ferraro et al 1997)
M3 :
The first
surprise
The very first complete
coverage of the entire
the cluster extension
BSS are more concentrated
in the central region
BSS are less concentrated
in the external region
R
=
RBSS
RGB=
Highly peaked in the
center rapidly decreasing
at intermediate radii and
finally rising again at
larger radii
Is this distribution
really “peculiar”
& unique ?
TOT
TOT
N
/
N
NBSS
/
N
RGB
L
LSS/L
LTOT
TOT
Theany
BSS
radial distribution
For
not-segregated
pop.
is
R BIMODAL
=1
47 Tuc: another surprise!!!!
The BSS radial distribution in
47 Tuc is quite similar to that
observed in M3
Central peak
&
the Decreasing at
intermediate radii
?
collisional BSS kicked off
from the cluster core
or
BSS generated by
primordial binaries
Dynamical simulations performed
in the case of 47 Tuc (Mapelli et
al 2004) showed that a pop. of
PB is needed to reproduce the
bimodal distribution
Mass segregation
&
dynamical friction
collisional BSS @ center
& PB-BSS in the outer regions?
Is this the “natural” BSS radial
distribution?
M55
Ferraro et al (1993,1997)
47 Tuc
Ferraro et al (2004)
Zaggia et al (1997)
NGC6752
Sabbi et al (2004)
Omega Centauri
Ferraro et al, (2004),ApJ, 603,L81
v Centauri: NO evidence
of mass segregation!!!!
A pure population of
non-collisional BSS?
New catalog containing 3000 BSS in 56 GGCs from
HST optical observations
Piotto et al (2004) + Davies et al (2004)
N(BSS) varies only a factor 10 !!!
BSS are produced by
both channels (collisions
& binary evolution)
Accordingly with previous
suggestions by Fusi Pecci et
al (1993), Baylin (1995), etc…
The total number of BSS is independent of
cluster mass and collision rate
In more massive systems
exchange encounters with a third
more massive MS star “used up”
PB early in the cluster history
1-2 : A more massive MS star
exchanges into a MS-MS binary
3 : the primary evolves off the MS
and fills its Roche lobe
4: the secondary gains mass from
the primary becoming a BSS
Davies et al (2004) suggested:
Davies et al (2004)
1. This process was particulary
active in the past in high-density
clusters
2. most of the BSS generated
by this channel have now evolved
The number of BSS produced
over the last 1 Gyr
The number of BSS produced
from primordial binaries
decrease with cluster
luminosity because they are
already evolved
The number of Collisional
BSS increases with the
cluster luminosity
Davies et al (2004)
Interesting working hypothesis
but the scenario seems
more complex….
Since the dynamical history
of each cluster plays an
important role
NBSS must be normalized to the
cluster population
F=
BSS specific frequency
F = NBSS /
NHB
305 BSS !!
One of the largest population
ever observed in a GGC
The most concentrated
BSS population
ever found in a GGC
Why M80 has such a huge population of BSS ?
M80 is much more concentrated than M3
3
(Log r0 = 5.8 M</pc )
Could the dynamical evolution
of the cluster play a role in the
formation of BSS?
BUT other clusters with similar concentration like
3
47 Tuc (Log r0 = 5.1 M</pc )
3
NGC2808 (Log r0 = 5.0 M</pc )
3
NGC6388 (Log r0 = 5.7 M</pc )
have many fewer BSS
M80 is not a PCC
but it should be !!!!
its dynamical time scale
is much shorter than its age !
BUT
even the PCC state cannot explain
such a huge BSS population
(NBSS< 100)
Are collisions delaying
the core collapse and
generating the BSS?
twin clusters
M3
M 13
3
3
Log r0 = 3.5 Ms/pc
Log r0 = 3.4 Ms/pc
Log M = 5.8 Ms
Log M = 5.8 Ms
NBSS = 72
NBSS = 16
F
F
= 0.28
clusters in different
dynamical phases ?
?
= 0.07
different primordial
binary population ?
Collapsing?
Not-yet coll.
NBSS = 17
NBSS = 129
F = 0.16
F = 0.44 – 1.0
binaries are preventing
core collapse ?
?
are binaries destroyed
during the collapse ?
Log r0 = 5.8 Ms/pc
Log r0 = 2.1 Ms/pc
NBSS = 129
NBSS = 24
3
F
F
= 0.44
= 0.92 !!!
the largest specific frequency
ever observed in one of the
lowest density cluster
F = 1 if only the PC
is considered
different types of BSS ?
3
?
NGC288 has a large
fraction of binaries !
Bellazzini et al. 2002
AJ, 123, 1509
M3 & M92
M3, M13 & M10
• 2 GGCs without
• 3 GGCs with
HB tails
long HB blue tails
• similar b-BSS LF
extending 2.5 mag
brighter than
• similar b-BSS LF
extending <1.5 mag
brighter than
m255=19
m255=19
?
are the BSS photometric properties
and HB morphology linked?
M80
• b-BSS
distribution
in the UV-CMD
Models by A. Sills:
All Collisional BSS:
generated
by s-b interactions
Binary fraction 20%
BSS formation rate
is constant or zero
BSS distribution in the CMD depends on when the BSS are created
Models are still too
rough to properly
reproduce the
observations
BSS formation has lasted over a relatively
long period (5-2 Gyr)
The existence of bright BSS could indicate
triple systems ?
Theoretical Predictions
Models give controversial
predictions on the resulting
properties of a BSS formed via
collision process:
Collisional BSS show
high rotational
velocities
Collisional BSS are not
fast rotators
(Leonard & Livio 1995)
(Benz & Hills 1987)
•Negligible mixing between
inner cores and outer
envelopes of colliding stars
is expected (Lombardi et al. 1995)
•Binary mass transfer is
likely to create a fast
rotating BSS and to lead an
abundance pattern
indicative of mixing with
regions of incomplete CNburning (Sarna and de Greve 1996)
These hypothesies have
never been checked with
observations!!!
Looking for abundance
signatures of the formation
process……