Download Ferraro et al. 2006

Another Non-segregated Blue
Stragglers Population in a
Globular Cluster:
the Case of NGC2419
(Dalessandro et al. 2008b, ApJ accepted)
Emanuele Dalessandro
Università di Bologna - Dipartimento di Astronomia
52° CONGRESSO SAIT
TERAMO 2008
The Blue Stragglers
population
BSS have been detected for the first time by Sandage (1953)
 They are a typical population of GC
 According to their position in CMD,BSS
should be more massive than normal stars
(Shara et al. 1997)
 They are particulary concentrated in
the central regions because of
Mass Segregation
 BSS are one one of the most warm
population in Globular Clusters
(5000°K<T<10000°K)
HST
UV sensibility and high resolution
Bologna-Dipartimento di
Astronomia
POSSIBLE SCENARIOS of
FORMATION
(I) mass-transfer or coalescence
(Fusi Pecci et al. 1992; Davies, Piotto & de Angeli 2004)
BSS in loose GCs might be
produced from coalescence
or
mass-transfer of
primordial
Binaries (MT-BSS)
In 47Tuc has been discovered a sub-population of BSS with significant
depletion of Carbon and Oxygen with respect the dominant population.
This evidence is interpreted as the presence of CNO burning products on the
BSS surface coming from a deeply peeled parent star, as expected in the case
of mass-transfer formation channel
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(Ferraro et al. 2006)
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(II) stellar collisions
In high density GCs
(depending on survivaldestruction rates for
primordial
binaries) BSS might arise
mostly
from stellar collisions
(COL-BSS),
particulary that involve
binaries
The two formation channels can have comparable efficency in producing BSS
(Ferraro et al. 1999; Bellazzini et al. 2002)
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The BSS radial distribution
Radius of avoidance
Radius at which all
objects with a mass
similar to BSS have
been sunk into the core
in a time comparable
to the cluster age
Important signatures
of the dynamical
evolution of the parent
cluster is imprinted in the
BSS properties
Is this the “natural” BSS radial
distribution?
The case of OMEGA CENTAURI
(Ferraro et al. 2006)
v Centauri: NO evidence
of mass segregation!!!!
This is the cleanest evidence that the
system is not relaxed even in the
central region.
What we know about NGC2419
• This remote objetc (d=81Kpc; Harris et al. 1997) is one
of the most lominous globular cluster (MV= −9.4; see Bellazzini 2007)
similar to OmegaCen
• Given its high luminosity (Mv=-9.4) and and half-mass radius
(rh=25pc; Bellazzini 2007) NGC2419 lies (together with OmegaCen)
Ripepi et al. 2007
in the (rh, MV )plane well above the locus defined by all the
other Galactic GCs thus suggesting it might be the stripped core
of a former dwarf galaxy (see van den Bergh & Mackey 2004; Mackey &
van den Bergh 2005)
• Newberg et al. (2003) suggested
that NGC2419 could be somehow connected with the Sagittarius (Sgr)
dwarf spheroidal
• There is no evidence of multiple stellar population (see for example the
high quality CMD published by Ripepi et al. 2007)
• The study of the RRlyrae stars (Ripepi et al. 2007) shows that NGC2419
Mackey&Van den Bergh 2005
is an Osterhoff II cluster
The peculiar Galactic Globular Cluster NGC2419
ACS-WFC@HST
(Prop GO9666, P.I. Gilliland)
F435W: 2 images with texp=800sec
F555W: 2 images with texp=720sec
F814W: 2 images with texp=676sec
Suprime-Cam@SUBARU
Combination of long (texp=180sec)
and median (texp =30sec) exposures
both in V and I filters
STRUCTURAL PARAMETERS AND DISTANCE MODULUS
(m  M )  19.7  0.1
0
d  (87  4) Kpc
Good agreement both with the structural parameters and distance (d) obtained
in precendent works with indipendent methods (see for example Bellazzini 2007
and references therein, Ripepi et al. 2007 , Harris et al. 1997)
Population Selection
•B=23.6 (1 mag above the TO)
•B−I < 0.75
• 183 BSS in the HST sample
• V ≃ 23.3
• V − I < 0.48
• 49 BSS in the SUBARU sample
(r<500”)
THE BSS RADIAL DISTRIBUTION (I)
• One of the largest BSS population
ever obeserved: more than 230 BSS
in the brightest portion
• a Kolmogorov-Smirnov test gives
a 70% and 50% probabilities that
the BSS is extracted from the same
Population as HB and RGB stars
• This is a first evidence that the
radial distribution of BSS is
indistinguishable from that of the
“normal” cluster population,
in contrast to what found in most
of the typical GCs (see references in
Dalessandro et al. 2008a)
THE BSS RADIAL DISTRIBUTION(II)
NO SIGNATURES OF MASS
SEGREGATIONS
ARE VISIBLE FOR THE BSS
POPULATION OF NGC2419
CONCLUSION AND DISCUSSION
• NGC2419 and OmegaCen (Ferraro et al. 2006) are the only 2 GCs
analyzed so far to show a FLAT BSS RADIAL DISTRIBUTION
• This is the cleanest evidence that these two systems are not relaxed yet
THE BSS ARE AN USEFULL MEAN TO UNDERSTAND THE
DYNAMICAL HISTORY OF STAR CLUSTERS
• In NGC2419 stellar collisions played a minor role (if any)
modifying the radial distribution of massive objects and probably also in
generating exotic binary systems
THE BSS POPULATION OBSERVED IN NGC2419 COULD BE A PURE
POPULATION OF PB-BSS
• NGC2419, we find S4PB−BSS = 3.1 ± 0.6. This result should be strongly
related with the binary–fraction of the cluster….(Sollima et al. 2008)
THE END
SPECIFIC FREQUENCY
(Ferraro et al. 1993)
N pop
SPECIFIC
N B (t ) L t
FREQUENCY
j 
R
T j
The Consumption Theorem
(Renzini&Buzzoni 1986; Renzini&FusiPecci 1988)
N
tot
pop
samp
L
samp
tot
L
Lj   Li
i j
Bologna-Dipartimento di
Astronomia
Nj  B(t ) LTtj