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Observations of BSS in

MODEST 6 -- Chicago, August 31, 200 5. Observations of BSS in. Galactic Globular Clusters. Francesco R. Ferraro. Dipartimento di Astronomia - Univ. di Bologna. by-products of binary systems. study of the environment effect on “canonical” evolutionary sequences.

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Observations of BSS in

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  1. MODEST 6 -- Chicago, August 31, 2005 Observations of BSS in Galactic Globular Clusters Francesco R. Ferraro Dipartimento di Astronomia - Univ. di Bologna

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

  3. merger of 2 low-mass stars unevolved, massive star BSS crucial link between stellar evolution & stellar dynamics Blue Straggler Stars 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 primordial binaries direct collisions

  4. loose GGCs natural habitat low c, low r0 for BSS • high resolution studies • BSSalso in the inner region • of high density GGCs • NGC6397 Auriere et al. 1990 • 47 Tuc Paresce et al. 1991 • M15 Ferraro & Paresce 1993 • 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 BSSare more concentrated than RGB stars exceptions: M3, M55,47 Tuc.. Blue Straggler Stars <1990 >1990

  5. UV sensitivity , high resolution systematic studies of hot SPs in the core of high density GGCs The "classical" plane The UV plane HST

  6. BSS in the UV: UV-planeideal to study the photometric properties of theBSS population: - the distribution is almost vertical - span more than 3 magnitudes

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

  8. 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 M3 : The first surprise

  9. NBSS/NTOT RBSS= LS/LTOT The BSS radial distribution is BIMODAL For any not-segregated pop. R = 1 Is this distribution really “peculiar” & unique ? NRGB/NTOT RRGB= LS/LTOT Blue Straggler Stars: M3 Highly peaked in the center rapidly decreasing at intermediate radii and finally rising again at larger radii

  10. BSS radial distribution: 47 Tuc 47 Tuc: another surprise!!!!

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

  12. 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 The BSS bimodal distributions ? collisional BSS @ center & PB-BSS in the outer regions?

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

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

  15. BSS radial distribution v Centauri: NO evidence of mass segregation!!!!

  16. BSS radial distribution A pure population of non-collisional BSS?

  17. Central-BSS catalogs 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

  18. BSS from primordial binaries in high-density clusters 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: 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 Davies et al (2004)

  19. BSS: collisions & binary evolution 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 Interesting working hypothesis but the scenario seems more complex…. Since the dynamical history of each cluster plays an important role Davies et al (2004)

  20. NBSSmust be normalized to the cluster population F= BSS specific frequency F = NBSS / NHB Direct comparison of BSS populations

  21. Blue sequences in the UV: The large population of BSS in M80 The most concentrated BSS population ever found in aGGC 305 BSS !! One of the largest population ever observed in a GGC

  22. The large population of BSS in M80 Why M80 has such a huge population of BSS ? M80 is much more concentrated than M3 (Log r0 = 5.8 M</pc3) BUT other clusters with similar concentration like 47 Tuc (Log r0 = 5.1 M</pc3) NGC2808 (Log r0 = 5.0 M</pc3) NGC6388 (Log r0 = 5.7 M</pc3) have many fewer BSS(NBSS< 100) Could the dynamical evolution of the cluster play a role in the formation of BSS? M80is 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 Are collisions delaying the core collapse and generating the BSS?

  23. Blue sequences in the UV: direct comparison of BSS populations twin clusters M 3 Log r0 = 3.5 Ms/pc3 Log M =5.8 Ms NBSS = 72 F = 0.28 M 13 Log r0 = 3.4 Ms/pc3 Log M =5.8 Ms NBSS = 16 F = 0.07 ? different primordial binary population ? clusters in different dynamical phases ?

  24. Blue sequences in the UV: direct comparison of BSS populations Collapsing? NBSS = 17 F = 0.16 Not-yet coll. NBSS = 129 F = 0.44 – 1.0 ? binaries are preventing core collapse ? are binaries destroyed during the collapse ?

  25. Blue sequences in the UV: direct comparison of BSS populations Log r0 = 5.8 Ms/pc3 NBSS = 129 F = 0.44 F = 1 if only the PC is considered Log r0 = 2.1 Ms/pc3 NBSS = 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. 2002 AJ, 123, 1509

  26. Blue sequences in the UV: direct comparison of BSS populations • M3 & M92 • 2 GGCs without • HB tails • similar b-BSSLF • extending 2.5 mag • brighter than • m255=19 • M3, M13 & M10 • 3 GGCs with • long HB blue tails • similar b-BSSLF • extending <1.5 mag • brighter than • m255=19 ? are the BSS photometric properties and HB morphology linked?

  27. BSS: comparison with models • 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 BSSdistribution in the CMD depends on when the BSS are created

  28. BSS: comparison with models Models are still too rough to properly reproduce the observations BSSformation has lasted over a relatively long period (5-2 Gyr) The existence of bright BSS could indicate triple systems ?

  29. Negligible mixing between inner cores and outer envelopes of colliding stars is expected (Lombardi et al. 1995) Theoretical Predictions Models give controversial predictions on the resulting properties of a BSS formed via collision process: • 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) Collisional BSS show high rotational velocities (Benz & Hills 1987) Collisional BSS are not fast rotators (Leonard & Livio 1995) These hypothesies have never been checked with observations!!! Looking for abundance signatures of the formation process……

  30. The End

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