MODEST 6 -- Chicago, August 31, 2005

UVE by-products of binary systems study of the environment study of the environment effect on “canonical” effect on “canonical” evolutionary sequences the “creation” of the “creation” of ARTIFICIAL sequences as Blue Stragglers Stars Blue Stragglers Stars and exotic objects and exotic objects

according to their position in the CMD, according to their position in the CMD, BSS should be more massive than BSS should be more massive than normal stars BSS have been detected for the first time by Sandage (1953) merger of 2 low-mass stars merger of 2 low-mass stars unevolved, massive star unevolved, massive star primordialbinaries directcollisions BSS crucial link between BSS crucial link between stellar evolution & stellar dynamics

BSS are a normal population of GGCs, BSS are a normal population of GGCs, found in each cluster properly observed BSS are more concentrated than RGB stars BSS are more concentrated than RGB stars exceptions: M3, M55,47 Tuc.. exceptions: M3, M55,47 Tuc.. <1990 loose GGCs natural habitat low c, low   for BSS >1990 high resolution studies BSS also in the inner region of high density GGCs NGC6397 Auriere et al NGC6397 Auriere et al Tuc Paresce et al Tuc Paresce et al M15 Ferraro & Paresce 1993 M15 Ferraro & Paresce 1993Catalogs: Fusi Pecci et al Fusi Pecci et al Sarajedini et al Ferraro, Fusi Pecci, Bellazzini 1995 Guhathakurta et al. 1994, 1998 Piotto et al 2004

UV sensitivity, high resolution UV sensitivity, high resolution systematic studies of hot SPs in the core of high density GGCs HST

UV-plane ideal to study UV-plane ideal to study the photometric properties the photometric properties of the BSS population: of the BSS population: - the distribution is almost vertical - span more than 3 magnitudes

PHOT. PLATES (Buonanno et al 1994) HIGH RES. CCD (Ferraro et al 1993) HST IMAGES (Ferraro et al 1997 ) M3

M3 : M3 : The first surprise The very first complete coverage of the entire the cluster extension BSS are more concentrated in the central region in the central region BSS are less concentrated in the external region in the external region

For any not-segregated pop. R = 1 R = 1 N RGB / N TOT N RGB / N TOT R RGB = L S / L TOT L S / L TOT N BSS / N TOT N BSS / N TOT R BSS = L S / L TOT L S / L TOT The BSS radial distribution The BSS radial distribution is BIMODAL is BIMODAL Is this distribution really “peculiar” really “peculiar” & unique ? & unique ? Highly peaked in the center rapidly decreasing at intermediate radii and finally rising again at larger radii

47 Tuc: another surprise!!!!

The BSS radial distribution in 47 Tuc is quite similar to that observed in M3

Mass segregation & dynamical friction Central peak Central peak & the Decreasing at intermediate radii collisional BSS kicked off from the cluster core from the cluster core BSS generated by primordial binaries or 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 collisional center & PB-BSS in the outer regions? ?

Ferraro et al (1993,1997) M55 Zaggia et al (1997) 47 Tuc Ferraro et al (2004) Sabbi et al (2004) NGC6752 Is this the “natural” BSS radial distribution? distribution?

Omega Centauri Ferraro et al, (2004),ApJ, 603,L81

 Centauri: NO evidence of mass segregation!!!!

A pure population of non-collisional BSS?

Piotto et al (2004) + Davies et al (2004) The total number of BSS is independent of cluster mass and collision rate N(BSS) varies only a factor 10 !!! New catalog containing 3000 BSS in 56 GGCs from HST optical observations HST optical observations BSS are produced by both channels (collisions & binary evolution) & binary evolution) Accordingly with previous suggestions by Fusi Pecci et al (1993), Baylin (1995), etc…

Davies et al (2004) Davies et al (2004) 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: 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 In more massive systems exchange encounters with a third more massive MS star “used up” PB early in the cluster history PB early in the cluster history

Davies et al (2004) Davies et al (2004) The number of Collisional BSS increases with the cluster luminosity The number of BSS produced from primordial binaries decrease with cluster luminosity because they are already evolved The number of BSS produced over the last 1 Gyr over the last 1 Gyr Interesting working hypothesis but the scenario seems more complex…. more complex…. Since the dynamical history of each cluster plays an important role

N BSS N BSS must be normalized to the cluster population F = BSS specific frequency F = N BSS / N HB

305 BSS !! 305 BSS !! One of the largest population ever observed in a GGC The most concentrated BSS population BSS population ever found in a GGC

Could the dynamical evolution of the cluster play a role in the formation of BSS? Are collisions delaying the core collapse and the core collapse and generating the BSS? generating the BSS? Why M80 has such a huge population of BSS ? M80 is much more concentrated than M3 (Log   = 5.8 M < /pc 3 ) BUT other clusters with similar concentration like 47 Tuc (Log   = 5.1 M < /pc 3 ) NGC2808 (Log   = 5.0 M < /pc 3 ) NGC6388 (Log   = 5.7 M < /pc 3 ) have many fewer BSS (N BSS < 100) M80 is not a PCC 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

M 3 Log   = 3.5 M s /pc 3 Log   =  5.8 M s N BSS = 72 F = 0.28 M 13 Log   = 3.4 M s /pc 3 Log   =  5.8 M s N BSS = 16 F = 0.07 twin clusters different primordial binary population ? clusters in different dynamical phases ? ?

Not-yet coll. N BSS = 129 F = 0.44 – 1.0 Collapsing? N BSS = 17 F = 0.16 binaries are preventing core collapse ? core collapse ? are binaries destroyed are binaries destroyed during the collapse ? during the collapse ? ?

Log   = 2.1 M s /pc 3 N BSS = 24 F = 0.92 !!! the largest specific frequency ever observed in one of the lowest density cluster different types of BSS ? NGC288 has a large fraction of binaries ! Bellazzini et al Bellazzini et al AJ, 123, 1509 AJ, 123, 1509 Log   = 5.8 M s /pc 3 N BSS = 129 F = 0.44 F = 1 if only the PC is considered ?

M3 & M92 2 GGCs without HB tails similar b-BSS LF extending 2.5 mag brighter than m 255 =19 are the BSS photometric properties and HB morphology linked? and HB morphology linked? M3, M13 & M10 3 GGCs with long HB blue tails similar b-BSS LF extending <1.5 mag brighter than m 255 =19 ?

M80 b-BSS distribution in the UV-CMD BSS distribution in the CMD depends on when the BSS are created BSS distribution in the CMD depends on when the BSS are created Models by A. Sills: All Collisional BSS: generated by s-b interactions Binary fraction 20% BSS formation rate is constant or zero

BSS formation has lasted over a relatively BSS formation has lasted over a relatively long period (5-2 Gyr) Models are still too Models are still too rough to properly rough to properly reproduce the reproduce the observations observations The existence of bright BSS could indicate The existence of bright BSS could indicate triple systems ? 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 (Benz & Hills 1987) Collisional BSS are not fast rotators (Leonard & Livio 1995) 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 CN- burning (Sarna and de Greve 1996) These hypothesies have never been checked with observations!!! Looking for abundance signatures of the formation process……