Another Non-segregated Another Non-segregated Blue Stragglers Population Blue Stragglers Population

in a Globular Cluster: in a Globular Cluster: the Case of NGC2419the Case of NGC2419

(Dalessandro et al. 2008b, ApJ accepted)

Emanuele DalessandroUniversità di Bologna - Dipartimento di Astronomia

52° CONGRESSO SAIT

TERAMO 2008

Bologna-Dipartimento di Astronomia

The Blue Stragglers The Blue Stragglers populationpopulation

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)

BSS have been detected for the first time by Sandage (1953)

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 ofPOSSIBLE SCENARIOS of FORMATIONFORMATION

(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(Ferraro et al. 2006)

Bologna-Dipartimento di Astronomia

(II) stellar collisions

In high density GCs(depending on survival-destruction rates for primordialbinaries) 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)

Radius at which all objects with a masssimilar to BSS have been sunk into the core in a time comparable to the cluster age

Radius of avoidance

Is this the “natural” BSS radialIs this the “natural” BSS radial distribution?distribution?

Important signaturesof the dynamical evolution of the parentcluster is imprinted in theBSS properties

The BSS radial distributionThe BSS radial distribution

Centauri: NO evidence Centauri: NO evidence of mass segregationof mass segregation!!!!!!!!

This is the cleanest evidence that the This is the cleanest evidence that the system is not relaxed even in the system is not relaxed even in the central region.central region.

The case of OMEGA CENTAURIThe case of OMEGA CENTAURI (Ferraro et al. 2006)

What we know about NGC2419What 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) 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)

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 is an Osterhoff II cluster

Ripepi et al. 2007

The peculiar Galactic Globular Cluster NGC2419The peculiar Galactic Globular Cluster NGC2419

ACS-WFC@HST (Prop GO9666, P.I. Gilliland)

F435W: 2 images with texp=800secF555W: 2 images with texp=720secF814W: 2 images with texp=676sec

Suprime-Cam@SUBARUSuprime-Cam@SUBARU

Combination of long (texp=180sec) Combination of long (texp=180sec) and median (texp =30sec) exposures and median (texp =30sec) exposures both in V and I filtersboth in V and I filters

STRUCTURAL PARAMETERS AND DISTANCE MODULUSSTRUCTURAL PARAMETERS AND DISTANCE MODULUS

Good agreement both with the structural parameters and distance (d) obtainedin precendent works with indipendent methods (see for example Bellazzini 2007 and references therein, Ripepi et al. 2007 , Harris et al. 1997)

KpcdMm

)487(1.07.19)(

0

Population SelectionPopulation 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)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)THE BSS RADIAL DISTRIBUTION(II)

NO SIGNATURES OF MASS SEGREGATIONSARE VISIBLE FOR THE BSSPOPULATION OF NGC2419

• 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)

CONCLUSION AND DISCUSSIONCONCLUSION AND DISCUSSION

THE ENDTHE END

Bologna-Dipartimento di Astronomia

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FREQUENCY

SPECIFIC FREQUENCY(Ferraro et al. 1993)

The Consumption Theorem(Renzini&Buzzoni 1986; Renzini&FusiPecci 1988)

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