Emanuele Dalessandro Università di Bologna - Dipartimento di Astronomia

1. 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

2. 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

3. POSSIBLE SCENARIOS ofFORMATION(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

4. (II) stellar collisions In high density GCs (depending on survival- destruction 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) Bologna-Dipartimento di Astronomia

5. Is this the “natural” BSS radial distribution? 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

6. 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.

7. 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) 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) Ripepi et al. 2007 • 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 Mackey&Van den Bergh 2005

8. The peculiar Galactic Globular Cluster NGC2419

9. 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

10. STRUCTURAL PARAMETERS AND DISTANCE MODULUS 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)

11. Population Selection • V ≃ 23.3 • V − I < 0.48 • 49 BSS in the SUBARU sample • (r<500”) • B=23.6 (1 mag above the TO) • B−I < 0.75 • 183 BSS in the HST sample

12. 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)

13. THE BSS RADIAL DISTRIBUTION(II) NO SIGNATURES OF MASS SEGREGATIONS ARE VISIBLE FOR THE BSS POPULATION OF NGC2419

14. 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)

15. THE END

16. SPECIFIC FREQUENCY (Ferraro et al. 1993) SPECIFIC FREQUENCY The Consumption Theorem (Renzini&Buzzoni 1986; Renzini&FusiPecci 1988) Bologna-Dipartimento di Astronomia